LIBRARY 

UNIVERSITY  OF  CALIFORNIA 
DAVIS 


HUMAN  EMBRYOLOGY 


BY   THE  SAME   AUTHOR 

A  BIBLIOGRAPHY  OF  VERTEBRATE  EMBRYOLOGY. 
Containing  over  three  thousand  titles  classified  by  subjects  and  indexed 
by  authors.  4to.  Boston,  1892.  PUBLISHED  AND  FOR  SALE  BY  THE 
BOSTON  SOCIETY  OF  NATURAL  HISTORY 


HUMAN 


EMBRYOLOGY 


BY 

CHARLES  SEDGWICK  MINOT 

PROFESSOR  OF  HISTOLOGY  AND  HUMAN  EMBRYOLOGY 
HARVARD  MEDICAL  SCHOOL,  BOSTON 


FOUR   HUNDRED  AND  SIXTY-THREE   ILLUSTRATIONS 


NEW  YORK 

WILLIAM    WOOD   AND    COMPANY 
1892 


LIBRARY 

.UNIVERSITY  OF  CALIFORNIA 
DAVIS 


COPYRIGHT 
BY  CHARLES  SEDGWICK  MINOT 

1892 


TO 

CARL   LUDWIG 

PROFESSOR  OF  PHYSIOLOGY 
AT    THE    UNIVERSITY    OF    LEIPSIC 

IN  TOKEN  OF 

RESPECT,  GRATITUDE,  AND  AFFECTION 

THIS  WORK  IS  DEDICATED 

BY  THE  AUTHOR 


PREFACE. 


THE  following  attempt  to  present  a  comprehensive  summary  of 
Embryology,  as  it  bears  upon  the  problems  of  human  development, 
is  the  result  of  ten  years'  labor.  I  have  endeavored  to  become  famil- 
iar with  the  principal  facts  by  my  own  observation,  and  with  the 
results  of  the  principal  numerous  investigations,  working  over  the 
material  into  satisfactory  form.  The  reader  will  find,  nevertheless, 
imperfections  of  which  I  am  conscious,  and  perhaps  errors,  for 
which  I  must  be  responsible.  There  is  probably  not  a  page  which 
might  not  be  enriched  with  facts  already  recorded  by  investigators ; 
certainly  not  a  page  which  would  not  be  improved  by  further  revis- 
ion. Notwithstanding  these  defects,  I  have  the  hope  that  the  book 
will  be  a  useful  contribution  toward  that  final  and  exhaustive  colla- 
tion of  embryological  facts  which  the  future  alone  can  give  us. 

I  have  sought  to  form  an  unbiased  judgment  upon  each  ques- 
tion, to  accept  facts  of  observation  without  regard  to  their  supposed 
theoretical  bearings ;  and  to  pay  due  attention  to  both  Schools  of  Em- 
bryology, the  Phylogenetic  and  the  Anatomical,  in  the  belief  that 
both  are  justified.  Whenever  I  have  inserted  a  new  observation  or 
opinion,  it  is  indicated  as  such  by  the  use  of  the  first  person.  In 
making  my  compilation,  I  have  drawn  constantly  from  the  embryo- 
logical  manuals  of  Kolliker,  Oskar  Hertwig,  Balfour  and  Duval; 
from  the  researches  of  W.  His,  and  from  the  writings,  especially 
the  "  Entwickelungsgeschichte  der  Unke, "  of  Alexander  Goette. 

In  regard  to  the  technical  terms,  I  have  made  certain  innovations. 

It  seems  to  me  important  to  make  the  number  of  terms  as  small  as 
is  compatible  with  clearness,  and  to  avoid  duplication.  Accordingly 
I  have  discarded  the  words  "  epiblast,  mesoblast,  and  hypoblast." 
Further  it  has  seemed  to  me  that,  as  a  thorough  knowledge  of  Ger- 
man is  indispensable  to  the  student  of  embryology,  it  is  justifiable, 


Vlll  PREFACE. 

where  no  English  equivalent  is  to  be  found,  to  adopt  such  unaltered 
German  terms  as  have  been  fully  established  in  embryological  liter- 
ature. Where  there  has  occurred  an  accepted  term  in  English, 
French,  or  German,  I  have  used  it  in  preference  to  a  Greek  or  Latin 
derivative. 

Whatever  merit  this  work  may  possess  should  be  attributed  to 
the  training  in  scientific  research  which  I  received  in  Germany  and 
France.  I  cannot  too  gratefully  acknowledge  the  unlimited  kindness 
shown  me  while  a  student  in  Leipzig  under  Professor  Carl  Lud- 
wig  and  Professor  Rudolph  Leuckart ;  in  Paris  under  Professor  Leon 
Ranvier ;  and  in  Wurzburg  under  Professor  Carl  Semper.  I  would 
also  here  express  my  gratitude  to  Professor  Wilhelm  His,  to  whom  I 
am  particularly  indebted  for  his  great  generosity  in  permitting  me  to 
study  his  unique  embryological  collection  in  Leipzig ;  also  to  the  large 
number  of  physicians,  both  in  Europe  and  America,  who  have  sup- 
plied me  with  material  to  carry  on  my  investigations  in  human  em- 
bryology. 

CHARLES  SEDGWICK  MINOT. 

HARVARD  MEDICAL  SCHOOL, 

BOSTON,  MASS.,  July  26,  1892. 


TABLE  OF  CONTENTS. 


CHAPTER  PAGE 

INTRODUCTION, 1 

I.     The  Uterus, 1 

II.    General  Outline  of  Human  Development,       ....  28 

PART  I. 

THE  GENITAL  PRODUCTS. 

III.  History  of  the  Genoblasts  and  the  Theory  of  Sex,          .        .  39 

PART   II. 
THE  GERM-LAYERS. 

IV.  Segmentation  ;  Formation  of  the  Diaderm,    ....  93 
V.     Concrescence  :  the  Primitive  Streak, 115 

VI.     The  Mesoderm  and  the  Coelom, 144 

VII.     General  Remarks  on  the  Germ-Layers, 159 

PART  III. 

THE  EMBRYO. 

VIII.     The  Medullary  Groove,  Notochord  and  Neurenteric  Canals,  173 

IX.     Divisions  of  the  Coelom  ;   Origin  of  the  Mesenchyma,   .        .  192 

X.     Origin  of  the  Blood,  Blood- Vessels  and  Heart,       .        ,        .  211 

XI.     Origin  of  the  Urogenital  System, 230 

XII.     The  Archenteron  and  the  Gill  Clefts, 254 

XIII.  The  Germinal  Area,  the  Embryo  and  its  Appendages,        .  271 

PART  IV. 
THE  FCETAL  APPENDAGES. 

XIV.  The  Chorion, 317 

XV.     The  Amnion  and  Proamnion, 333 

XVI.     The  Yolk  Sack,  Allantois  and  Umbilical  Cord,       ...  346 

XVII.    The  Placenta,  3G4 


TABLE   OF  CONTENTS. 


PART  V. 

THE  FCETUS. 

CHAPTER  PAGE 

XVIII.    Growth  and  External  Development  of  the  Embryo  and 

Foetus, 381 

XIX.    The  Mesenchyuial  Tissues, 397 

XX.     The  Skeleton  and  Limbs, .422 

XXI.     The  Muscular  System, 470 

XXII.     The  Splanchnocoele  and  Diaphragm, 480 

XXIII.  The  Urogenital  System, 490 

XXIV.  Transformations  of  the  Heart  and  Blood- Vessels,  ...  521 
XXV.     The  Epidermal  System,      .                548 

XXVI.  The  Mouth  Cavity  and  Face, 567 

XXVII.  The  .Nervous  System, 593 

XXVIII.  The  Sense  Organs, 706 

XXIX.    The  Entodermal  Canal, 743 


LIST  OF  ILLUSTEATIOKS. 


FIG.  PAGE 

1.  Connective  tissue  of  mucosa,  uterus  of  pig, 3 

2.  Vertical  section  of  the  mucosa  corpus  uteri  of  the  first  day  of  men- 

struation,     ............  5 

3.  Mucous  membrane  of  a  virgin  uterus  during  the  first  day  of  men- 

struation,        7 

4.  Semi-diagrammatic  outline  of  an  antero-posterior  section  of  the 

gravid  uterus  and  ovum  of  five  weeks, 8 

5.  Uterus  about  forty  days  advanced  in  pregnancy,        ....  9 

6.  Uterus  one  month  pregnant ;  outlines  of  the  glands  from  a  vertical 

section, 14 

7.  Uterus  one  month  pregnant ;  portion  of  the  compact  layer  of  the 

decidua  seen  in  vertical  section, 15 

8.  Uterus  one  month  pregnant ;  section  of  gland  from  cavernous  layer, 

with  the  epithelium  partly  adherent  to  the  walls,        .        .        .16 

9.  Uterus  one  month  pregnant ;    section    of   gland   from   cavernous 

layer  with  the  epithelium  loosened  from  the  walls,      .        .     '  .  16 

10.  Section  of  the  decidua  serotina,  near  the  margin  of  the  placenta  ; 

normal  uterus  about  seven  months  pregnant,        ....  17 

11.  Decidual  cells  from  the  section  represented  in  Fig.  10;  stained  with 

alum  haemotoxylin,  and  eosin, 18 

12.  Section  of  human  decidua  reflexa  at  two  months,        ....  20 

13.  Uterus  twelve  hours  after  artificial  delivery  at  six  months'  preg- 

nancy,  .............  22 

14.  Section  of  the  placental  area  of  the  uterus  three  weeks  post  partum,  23 

15.  Vertical  section  through  the  wall  of  a  uterus  about  seven  months 

pregnant,  with  the  foetal  membranes  in  situ,        ....  29 

16.  Human  embryo,  4.2  mm.  long, 32 

17.  Embryo,  2.15  mm.  long, 33 

18.  Diagram  of  an  embryo  of  fifteen  to  sixteen  days,         ....  34 

19.  Generalized  diagram  of  an  amniote  vertebrate  embryo,      ...  34 

20.  Generalized  diagram  of  an  amniote  vertebrate  embryo  before  the 

separation  of  the  amnion, 35 

21.  Structure  of  a  rat's  spermatozoon, 41 

22.  Human  spermatozoa 42 

23.  Peripheral  layer  of  the  seminiferous  tubule  of  a  rat,  ....  43 

24.  Column  of  spermatocytes  from  the  rat, 44 

25.  Developing  spermatoblasts  of  the  rat, 45 

26.  Developing  spermatozoa  of  a  marsupial 46 

27.  Human  spermatoblasts,  to  illustrate  the  rupture  of  the  membrane,  46 

28.  Sertolf  s  column,  with  a  basal  nucleolated  nucleus  and  a  cluster  of 

developing  spermatoblasts, 47 


Xil  LIST   OF   ILLUSTRATIONS. 

FIG.  PAGE 

29.  Part  of  a  cross-section  of  a  seminiferous  tubule  of  a  rat,     ...  47 

30.  Egg  of  Tendra  zostericola, 49 

31.  Primary  follicles  from  the  ovary  of  a  woman  thirty-one  years  old,    .  51 

32.  Ovary  of  cat, 52 

33.  Egg-cell  of  Tengenaria  domestica, 54 

34.  Full-grown  human  ovum  before  maturation, 56 

35.  Part  of  the  ovum  of  a  mole 57 

36.  Ovum  of  a  sea  urchin,  Toxopneustes  lividus, 58 

37.  Ovarion  egg  of  hsemops, 63 

38.  Egg  of  a  leech  (nephelis),  three-quarters  of  an  hour  after  being  laid,  63 

39.  Ovum  of  nephelis  (a  leech),  three  hours  after  laying,  ....  64 

40.  Ovum  of  a  rabbit ;  taken  from  the  middle  of  the  oviduct  about 

eighteen  hours  after  coitus,       ........  71 

41.  Anterior  pole  of  the  ovum  of  the  petromyzon,  with  a  spermatozoon,  72 

42.  Egg  of  nephelis,  three  hours  after  laying 74 

43.  Ovum  of  sagitta  with  two  pronuclei,      . 75 

44.  Two  ova  of  the  land-snail,  arion, .76 

45.  Ovum  of  a  rabbit  seventeen  hours  after  coitus  with  the  pronuclei 

about  to  conjugate,    ..........  76 

46.  Ovum  of  Limax  campestris  during  the  first  cleavage,          ...  96 

47.  Blastula  stage  of  Echinocardium  cordatum   twenty  hours  after 

impregnation, 97 

48.  Segmentation  of  the  egg  of  the  common  frog, 98 

49.  Section  of  the  segmented  ovum  of  axolotl, 99 

50.  Four  stages  of  the  segmentation  of  the  hen's  ovum,    .        .        .        .100 

51.  Ovum  of  a  flounder  in  transverse  vertical  section,       ....  102 

52.  Ovum  of  a  rabbit  of  twenty-four  hours,         ......  103 

53.  Rabbit's  ovum  of  about  seventy  hours,          ......  104 

54.  Ovum  of  a  bat,  Vespertilis  murina,  with  four  segmentation  spheres,  105 

55.  Ovum  of  Virginian  opossum,  with  four  segments,       ....  105 

56.  Young  blastodermic  vesicle  of  a  mole, 105 

57.  Sections  through  the  inner  mass  of  the  blastodermic  vesicle  of  the 

mole^  at  three  successive  stages,       .        .        .        .        .        .        .  106 

58.  Ovum  of  a  rabbit,  ninety-four  hours  after  coitus,        ....  107 

59.  Diagram  of  a  segmented  mammalian  ovum,         .....  108 

60.  Ovum  of  Amphioxus  lanceolatus  during  segmentation  stage,  with 

eighty-eight  cells, 110 

61.  Section  of  a  gastrula  of  Toxopneustes  lividus, 113 

62.  Diagrams  of  the  principal  modifications  of  the  gastrula,     .        .        .114 

63.  Longitudinal  section  of  an  early  stage  of  the  gecko,    .        .        .        .115 

64.  Diagram  illustrating  the  growth  of  the  blastoderm  and  concrescence 

of  its  rim  to  form  the  primitive  axis,       ......  116 

65.  Diagram  of  concrescence  in  a  teleostean  egg,        .        .        .        .        .118 

66.  Diagram  of  an  elasmobranch  blastoderm  to  illustrate  the  formation 

of  the  marginal  groove, 119 

67.  Diagram  of  a  vertebrate  blastoderm  a  little  more  advanced  than 

Fig.  96, 120 

68.  Ovum  of  axolotl, 121 

69.  Ovum  of  petromyzon  in  longitudinal  section, 121 

70.  Longitudinal  section  of  the  ovum  of  a  sturgeon  after  the  formation 

of  the  entodermic  cavity, 122 

71.  Formation  of  the  blastoporic  canal  in  Lacerta  muralis,      .        .        .  123 


LIST   OF   ILLUSTRATIONS.  Xlll 

FIG.  PAGE 

72.  Hen's  ovum  ;  incubated  six  hours, 124 

73.  Diagrammatic  cross-section  of  a  vertebrate  ovum,  in  which  con- 

crescence is  supposed  to  have  been  arrested,          ....  126 

74.  Dog-fish  embryo,  nearly  in  Balfour's  stage  C, 126 

75.  Germinal  area  of  a  guinea-pig  at  thirteen  days  and  twenty  hours,  127 
70.  Diagram  showing  the  relations  of  a  vertebrate  ovum  with  an  em- 
bryo in  cross-section  and  a  large  yolk 128 

77.  Sections  of  axolotl  eggs,    ..........  130 

78.  Area  pellucida  of  a  hen's  egg,  with  completed  primitive  furrow,      .  131 

79.  Longitudinal  section  of  the  region  of  the  primitive  streak  of  a  hen's 

ovum  incubated  six  hours, 132 

80.  Transverse  sections  of  a  germinative  area,  with  half-formed  primi- 

tive streak,  of  a  hen's  egg, 133 

81.  Transverse  section  of  theanterior  region  of  a  fully  developed  primi- 

tive streak  of  a  hen's  ovum,      ........  135 

82.  Blastoderm ic  vesicle  of  a  rabbit  of  seven  days, 136 

83.  Transverse  section  of  the  embryonic  shield  of  the  blastodermic 

vesicle  of  a  sheep, 137 

84.  Central  portion  of  a  sheep's  blastodermic  vesicle  of  twelve  to  thir- 

teen days 138 

85.  Embryonic  shield  of  a  rabbit's  ovum  of  five  days,       ....  139 

86.  Section  of  the  primitive  streak  of  the  mole, 140 

87.  Blastodermic  vesicle  of  Mus  sylvaticus, 142 

88.  Axolotl  embryo  ;  transverse  section  of  an  early  stage,         .        .        .  146 

89.  Diagrams  of  the  embryonic  area  of  the  chick,      .        .        .  r  .        .  150 

90.  Diagram  of  the  embryonic  area  of  a  chick, 150 

91.  The  mesodermal  cavities  of  the  germinal  area  of  a  chick  of  the 

third  day 151 

92.  Section  of  a  chicken  embryo  of  about  thirty-six  hours,               .        .  152 

93.  Transverse  section  of  an  amphioxus  embryo,        .....  156 

94.  Amphioxus  embryo,  ...........  157 

95.  Opossum  embryo  of  seventy-three  hours  ;  transverse  section  at  the 

level  of  the  heart, 173 

96.  Blastoderm  of  rabbit's  ovum 174 

97.  Chicken  embryo  with  seven  primitive  segments,          ....  175 

98.  Part  of  a  transverse  section  of  a  young  mole  embryo,         .        .        .  176 

99.  Surface  view  of  a  young  mole  embryo, 176 

100.  Transverse  section  of  a  mole  embryo,     .        .        .        .        .        .        .176 

101.  Early  stage  of  Ainblystoma  punctatum, 177 

102.  Part  of  a  transverse  section  of  an  axolotl  embryo,       ....  178 

103.  Transverse  section  of  a  rabbit  embryo  of  eight  days  and  two  hours,  179 

104.  Part  of  a  transverse  section  of  an  embryo  of  Lumbricus  trapezoides,  180 

105.  Transverse  section  of  a  mole  embryo, 183 

106.  Longitudinal  section  of  the  head  end  of  a  mole  embryo,     .        .        .  183 

107.  Rabbit  embryo  of  6  mm.;  median  longitudinal  section  of  the  head,  185 

108.  Longitudinal  sections  of  the  notochord  of  bombinator,     .        .        .  186 

109.  Degenerating  notochord  tissue,  from  the  central  portion  of  the  in- 

tervertebral  disc  of  a  cow's  embryo, 187 

110.  Longitudinal  section  of  a  frog's  ovum,  shortly  after  closure  of  the 

medullary  groove, 188 

111.  Transverse  section  of  an  embryo  paroquet  (melopsittacus)  to  show 

the  anterior  or  true  neurenteric  canal, 189 


LIST   OF   ILLUSTRATIONS. 

FIG.  PAGE 

112.  Chicken  embryo  with  one  segment, 193 

113.  Area  vasculosa  and  embryo  with  eight  segments  of  a  hen's  egg,       .  193 

114.  Rabbit  embryo  with  eight  segments, 194 

115.  Transverse  section  of  a  pristiurus  embryo  with  fourteen  segments, 

through  the  centre  of  the  fourth  segment,     .        .  .      .        .        .194 

116.  Transverse  section  through  a  recently  formed  primitive  segment  of 

a  chick  with  eighteen  and  twenty  segments,         ....  196 

117.  Section  of  a  chick  with  about  twenty  segments,  ....  198 

118.  Head  of  an  embryo  of  Torpedo  ocellata,  in  Balfour's  stage1  J,   .        .  200 

119.  Longitudinal  vertical  section  through  five  primitive  segments  of  a 

rabbit  embryo  of  nine  days  and  seventeen  hours,         .        .        .     204 

120.  Longitudinal  horizontal  section  through  a  segment  of  a  rabbit 

embryo  of  ten  and  one-half  days,     .......  205 

121-  Transverse  section  through  the  upper  part  of  a  myotome  of  a  chick 

of  about  seventy  hours, 206 

122.  Pristiurus  embryo  with  forty-five  to  forty-six  segments,    .        .        .  208 

123.  Diagram  of  a  cross-section  of  a  young  amphioxus,       ....  209 

124.  Surface  view  of  a  small  part  of  the  vascular  network  of  an  embryo 

chick  of  two  days, 212 

125.  Vascular  anlages  of  the  area  vasculosa  of  a  chick  of  forty  hours,      .  213 

126.  Section  of  the  area  vasculosa  of  a  chick, 214 

127.  Corpuscles  from  rabbits,  from  acanthias,  from  a  chick,  from  a  hu- 

man embryo, 219 

128.  Salamandra  maculosa  ;  larva,  very  young ;  transverse  section  to 

show  the  formation  of  the  coelom  in  the  heart  region,          .        .  224 

129.  Salamandra  maculosa,  larva  with  branchial  arches,  ....  225 
129A.  Embryo  chick  ;   section  through  the  anlage  of  heart,      .        .        .  226 

130.  Chick  embryo, 227 

131.  Diagrammatic  cross-section  of  a  vertebrate  to  show  the  fundamen- 

tal relations  of  the  urogenital  system, 230 

132.  Rana  temporaria.     Tadpole  of  12  mm.     Cross-section  through  the 

pronephros,          ...        .        .        .        .        .        .        .        .     231 

133.  Nephridium  (or  Wolfflan  tubule)  of   an  acanthias  embryo  of  28.2 

mm.,  seen  from  the  caudal  side  ;  reconstructed  from  the  sections,    236 

134.  Section    through  a  Wolfflan  tubule   of    a  chick  with    primitive 

segments, '.  2:58 

135.  Wolfflan  tubule  of  a  sheep  embryo  of  9  mm., 238 

136.  Coste's  embryo  of  thirty-five  days, 240 

137.  Transverse  section  of  the  Wolffian  body  or  primitive  kidney  of  a 

rabbit  of  thirteen  days,      .........     241 

138.  Longitudinal  vertical  section  of  the  Wolffian  body  of  a  rabbit  em- 

bryo of  thirteen  days, 242 

139.  Section  through  the  testis  of  a  human  embryo  of  sixty-three  to  sixty- 

eight  days, 244 

140.  Transverse  section  through  an  advanced  embryo  of  a  shark,  sycm- 

nuslichia;  from  the  abdominal  region  (dots  represent  nuclei),  .     247 

141.  Section  of  the  urogenital  fold  of  a  chick  embryo  of  the  fourth  day,     248 

142.  Diagrammatic  section  of  the  yellow  of  a  hen's  egg  at  an  early  stage 

to  show  the  relations  of  the  archenteron  to  the  yolk-sac,    .        .     255 

143.  Diagrams  to  illustrate  the  separation  of  the  embryo  from  the  yolk,    250 

144.  Cross-section  of  a  rabbit  embryo  of  eight  days  and  two  hours,        .    257 


LIST   OF   ILLUSTRATIONS.  XV 

FIG.  PAGE 

145.  Longitudinal  section  of  the  posterior  end  of  a  sheep  embryo  of  six- 

teen days, 258 

146.  Longitudinal  median  section  of  young  chick  embryo,       •  .        .        .261 

147.  Transverse  section  of  the  head  of  a  chick  embryo  with  seven 

segments, 262 

148.  Two  views  of  a  wax  model  of  the  cavity  of  the  pharynx  of  a  rabbit 

embryo  of  eleven  days, 264 

149.  Acanthins  embryo  of  17  mm.   Horizontal  section  of  the  anterior  half,  266 

150.  Chicken  embryo  of  sixty-eight  hours,     .......  266 

151.  Acanthias  of  17  mm. 267 

152.  Cross-section  of  a  branchial  arch  of  an  advanced  shark  embryo,      .  267 
15:5.  Longitudinal  section  of  an  embryo  of  Petroniyzon  planeri,  four 

days  old,  reared  at  Naples, 268 

154.  Diagrams  to  indicate  the  fundamental  relations  of  the  archenteron,  270 

1 55.  Chicken  embryo  and  germ  area  after  twenty-seven  hours'  incubation,  272 

156.  Embryonic  area  of  a  rabbit  of  eleven  days,  with  the  placental  area 

partly  torn  off 273 

157.  Diagram  of  the  circulation  in  a  chick  at  the  end  of  the  third  day, 

as  seen  from  the  under  or  ventral  side,  .        .        .        .         .        .  274 

15s.  Area  vasculosa  and  embryo  of  a  rabbit, 275 

1-V.».  Transverse  section  of  the  rump  of  a  dog-fish  embryo  14  mm.  long,  .  280 

160.  Section  through  the  rump  of  a  rabbit  embryo  of  eight  days  and 

three  hours. 282 

161.  Transverse  section  of  the  rump  of  an  embryo  chick  of  the  third  day,  283 

102.  Diagrams  r<>  illustrate  II  i>'  theory  of  the  origin  of  the  human  amnion,  285 

103.  Reichcrt's  ovum.     Two  views  engraved  from  the  original  plate,       .  288 

Kit.  Cross-section  of  Spec's  embryo 292 

105.  Section  passing  through  the  blastopore  of  Spec's  embryo.         .        .  292 
100.  Diagram  of  His'  embryo  E  :  age  fourteen  (?)  days;  length  about  2.3 

nun 293 

107.  Thomson's  second  ovum,  ..........  294 

10S.  Human  embryo  of  thirteen  to  fourteen  days,        .....  295 

101).    Embryo  of  the  beginning  of  third  week,         ......  296 

170.  Human  embryo  of  2.15mm.;  anatomy  reconstructed  from  the  sections  297 

171.  His1  embryo  L,  2.4  mm.  long, 299 

17:2.  Ovum  supposed  to  be  from  fifteen  to  eighteen  days  old,     .        .        .  301 

173.  Embryo  supposed  to  be  from  fifteen  to  eighteen  days  old,         .        .  302 

174.  Fragment  of  the  chorion  of  fig.  4,  highly  magnified,    ....  302 
175    His' embryo  M, ". 304 

176.  Digestive  canal  of  His'  embryo,       .        .        .        .        .        .        .         .  305 

177.  Anterior  wall  of  the  pharynx  of  His'  embryo  BB,  3.2  mm.  long,       .  305 

178.  W.  His'  embryo  M, 306 

17'.).  Reconstruction  of  His'  embryo  BB,  3.2  mm.  long,        .         .        .        .306 

ISO.  Reconstruction  of  His' embryo, 307 

181.  Isolated  terminal  branch  of  a  villus  from  the  chorion  of  an  embryo 

of  twelve  weeks, 320 

182.  Villous  stem  from  a  placenta  of  the  fifth  month,          ....  320 

183.  Terminal  villi  of  a  placenta  at  full  term, 320 

184.  Section  of  the  chorion  at  three  weeks, 321 

1*5.  Aborting  villus  from  a  chorion  of  the  second  month,          .        .        .  321 

186.  Placental  chorion  of  an  embryo  of  seven  months,        ....  324 


XVI  LIST   OF   ILLUSTRATIONS. 

FIG.  PAGE 

187.  Section  of  the  chorionic  membrane  of  an  ovum  supposed  to  belong 

to  the  third  week 328 

188.  Section  of  the  chorionic  membrane  of  an  embryo  of  three  weeks,     .  329 

189.  Section  of  the  amnion  and  placental  chorion  of  the  fifth  month,      .  329 

190.  Adenoid  tissue  of  a  villus  from  a  placenta  of  four  months,         .        .  330 

191.  Section  of  the  amnion  covering  the*  placenta  of  a  two  months  em- 

bryo,        334 

192.  Two  sections  of  the  placental  amnion, 334 

193.  A  natural  group  of  nuclei  from  the  mesoderm  of  the  amnion  of  a 

foetus  of  the  fifth  month,   .        . 335 

194.  Mesodermic  nuclei  of  the  amnion  of  an  embryo  of  about  four  months,  335 

195.  Surface  view  of  the  amniotic  epithelium  of  an  embryo  of  144  days,  336 

196.  Diagram  of  the  development  of  the  foetal  adnexa  in  the  rabbit,        .  343 

197.  Longitudinal  median  section  of  a  petromyzon  larva,          .        .        .  346 

198.  Wall  of  the  yolk-sac  in  the  area  opaca  of  a  chick  of  the  second  day,  347 

199.  Section  of  the  yolk-sac  of  a  human  embryo, 350 

200.  Diagram  of  the  embryo  and  yolk-sac  of  a  rabbit,         ....  351 

201.  Vertical  section  of  the  wall  of  the  yolk-sac  of  a  rabbit  embryo  of 

thirteen  days, 351 

202.  Diagram  of  an  opossum  embryo  and  its  appendages,  ....  351 

203.  Section  of  the  allantois  from  the  umbilical  cord  of  an  embryo  of 

three  months,       ...........  355 

204.  Diagrammatic  section  of  the  bauchstiel  of  a  human  embryo,  modi- 

fied from  W.  His, 356 

205.  Sections  of  human  umbilical  cords, 357 

206.  Connective  tissue  of  the  umbilical  cord  of  an  embryo  of  21  mm.,      .  358 

207.  Connective  tissue  of  the  umbilical  cord  of  a  human  embryo  of  about 

three  months,       ...........  359 

208.  Epithelial  covering  of  the  umbilical  cord  of  an  embryo  of  three 

months, 360 

209.  Cross-section  of  an  umbilical  cord  at  term. 362 

210.  Placenta  at  full  term,  doubly  injected  by  Dr.  H.  P.  Quincy  to  show 

the  distribution  of  the  vessels  upon  the  surface,  ....  365 

211.  Placenta  at  full  term, 367 

212*  Mesenchymal  tissue  of  a  villus,  from  a  placenta  of  four  months,      .  368 

213.  Section  through  a  normal  placenta  of  about  seven  months,  in  situ^  370 

214.  Portion  of  an  injected  villus  from  a  placenta  of  about  five  months,  371 

215.  Placenta  of  about  five  months  ;  portion  of  a  small  villus,  .        .        .  371 

216.  His'  embryo  a,  age  probably  twenty-three  days,          ....  385 

217.  Fol's  embryo  of  5.6  mm.,  probably  twenty-five  days  old,    .        .        .  386 

218.  His1  Embryo  A,  7.5  mm.  long, 387 

219.  Embryo  of  9.8  mm.,    .        .        .     * 388 

220.  Embryo  of  about  14mm., 389 

221.  Dorsal  view  of  an  embryo  of  about  14  mm., 389 

222.  Embryo  of  about  thirty-five  days, 390 

223.  His' embryo  XXXIV, 391 

224.  Embryo  of  22  mm., 392 

225.  Embryo  of  28  mm.,     ...........  :J92 

226.  Embryo  of  32mm., 393 

227.  Embryo  of  34mm., 393 

228.  Embryo  of  55  mm., 393 

229.  Embryo  of  78  mm., 394 


LIST   OF   ILLUSTRATIONS.  Xvii 

FIG.  PAGE 

230.  Front  view  of  the  head  and  face  of  the  embryo 394 

231.  Embryo  of  about  120  mm 394 

232.  Embryo  of  118  mm., 395 

233.  Embryo  of  155  mm., 396 

234.  Mesenchyina  of  a  chick  embryo  of  the  third  day  from  close  to  the 

otocyst, 399 

235.  Oinentum  of  a  human  embryo  of  five  months 400 

236.  Parietal  bone  of  a  human  embryo  of  fourteen  weeks,         .        .        .  408 

237.  Transverse  section  of  the  mandible  of  a  human  embryo  of  the  tenth 

week 408 

238.  From  a  section  of  an  ossifying  vertebra  of  a  human  embryo  of  four 

months,         ............  411 

239.  Section  of  a  vertebra  of  the  same  embryo  at  right  angles  to  the 

plane  of  tig.  238,  and  corresponding  in  level  to  the  lower  part  of 

the  bracket  L,  fig.  238 412 

240.  Artery  from  the  allautois  of  a  chick,  surrounded  by  a  network  of 

lymphatics, 414 

241.  Section  of  the  spleen  of  a  human  embryo  of  six  months,    .        .        .  416 

242.  Fat  island  from  the  skin  of  a  human  embryo  of  five  months,    .        .  418 

243.  Reconstruction  of  the  last  occipital,  and  first  two  cervical  vertebra 

of  a  cow  embryo  of  8.8  mm.,      ........  425 

244.  Cross-section  of  the  anlage  of  second  cervical  vertebra  of  a  cow 

embryo  of  8.8  mm 425 

245.  Longitudinal  median  section  of  the  upper  portion  of  the  vertebral 

column  of  a  cow  embryo  of  22.5  mm., 4*27 

246.  Frontal  projection  of  the  cephalic  part  of  a  vertebral  column  of  a 

cow  embryo, 431 

247.  Embryo  pig  of  about  16  mm., 435 

248.  Embryo  pig,  one  and  one-third  inch  long, 436 

249.  Section  of  the  anterior  portion  of  the  snout  of  an  embryo  pig,          .  437 

250.  Embryo  pig,  six  inches  long, 439 

251.  Chondrocranium  of  an  insectivorous  mammal  (Tatusia),   .         .        .  442 
25:2.  Pectoral  fin  of  a  young  embryo  of  sycllium  in  longitudinal  and 

horizontal  section, 449 

253.  Scapula  of  a  human  embryo  of  five  and  one-half  inches,  dorsal  view,  453 

254.  Vertical  section  of  the  ankle  of  a  human  embryo  of  nearly  six  months,  459 

255.  Isolated  muscle  fibres  of  a  frog  embryo, 471 

256.  A,  transverse  section  ;  B,  longitudinal  section  of  muscle  fibres  in  the 

neck  of  a  human  embryo  of  sixty-three  to  sixty-eight  days,        .  472 

257.  Chick  embryo,  transverse  section  of  the  upper  part  of  a  myotome,  476 

258.  Transverse  section  of  a  branchial  arch  of  a  selachian  embryo,  .        .  478 

259.  His1  embryo  R,  5  mm.    Reconstruction  to  show  the  septum  trans- 

versum, 480 

260.  Head  of  a  rabbit  embryo,  with  segments  seen  from  the  under  side,  481 

261.  Rabbit  embryo,  eight  and  a  half  days,  with  eleven  or  twelve  somites, 

cross-section, 482 

262.  Model  of  part  of  the  pleural  and  abdominal  cavities  of  a  rat  embryo 

at  a  stage  corresponding  to  a  rabbit  at  fifteen  days,     .        .        .  484 

263.  Section  of  the  supra-renal  body  of  a  rabbit  embryo  of  twenty-six 

days, 487 

264.  Supra-renal  capsule  of  a  four  months'  human  embryo,       .        .        .  488 

265.  Diagram  of  the  indifferent  stage  of  the  urogenital  system  of  amniota,  490 


XV111  LIST  OF  ILLUSTRATIONS. 

FIG.  PAGE 

266.  Diagram  to  illustrate  the  homologies  of  the  sexual  apparatus,         .  491 

267.  Section  of  the  testis  of  a  human  embryo  of  sixty-three  to  sixty-eight 

days, 493 

268.  Section  of  the  ovary  of  a  human  embryo  of  7  cm.,        ....  495 

269.  To  illustrate  the  decensus  testiculorum, 498 

270.  Cross-section  of  the  ovary  and  Wolffian  body  of  a  human  embryo 

of  the  third  month, 499 

271.  Cross-section  of  the  rectum,  genital  cord,  and  allantois  of  a  male 

human  embryo  of  about  two  months,     ......  502 

272.  Section  of  broad  ligament  of  a  female  human  embryo  of  four  months,  504 

273.  Cross-section  through  the  hind  end  of  the  left  AVolffian  body  of  a 

crocodile  embryo  of  12  mm., 508 

274.  Section  of  a  kidney,  human  embryo  of  about  five  months,        .        .  505) 

275.  Seniidiagrammatic  figures  of  developing  renal  tubules  of  a  mammal,  510 

276.  Section  parallel  to  the  medullary  rays  of  the  kidney  of  a  human 

foetus  of  about  five  months, 511 

277.  Cross-section  of  the  medullary  tubules  of  the  kidney  of  a  human 

embryo  of  about  five  months, 512 

278.  Longitudinal  median  section  of  the  cloaca  of  a  sheep  embryo  of  18 

mm 517 

279.  Longitudinal  section  of  the  penis  of  a  human  embryo  of  about  five 

months, 518 

280.  External  genitalia,  female  embryo  of  105  mm., 518 

281.  Section  of  the  clitoris  and  labia  niajora  of  a  human  embryo  of  about 

four  and  one-half  months, 519 

282.  External  genitalia  of  the  female  human  foetus  at  about  four  months,  519 

283.  Head  of  chick  of  thirty-eight  hours,  seen  from  the  under  side,         .  522 

284.  Reconstruction  of  the  heart  and  veins  of  a  human  embryo  of  2. 15  mm. ,  522 

285.  Endothelial  heart  of  a  human  embryo  of  2.15  mm.;  seen  from  the 

left  side, 522 

286.  Reconstructed  side  view  of  the  endothelial  heart  of  a  human  em- 

bryo of  4.2  mm., .  523 

287.  Model  of  the  muscular  heart  of  a  rabbit  embryo  of  nine  to  nine  and 

one-half  days,  seen  from  the  left  side,     ......  523 

288.  Endothelial  heart  of  a  human  embryo  of  5  mm.,          ....  524 

289.  Inner  surface  of  the  heart  of  a  human  embryo  of  10  mm.,          .        .  524 

290.  Section  of  the  heart  and  pericardial  cavity  of  a  rabbit  embryo  of  ten 

and  one-half  days,       ..........  526 

291.  Section  in  the  frontal  plane  through  the  heart  of  a  rabbit  embryo 

of  thirteen  days, 529 

292.  Oblique  section  of  the  heart  of  a  human  embryo  of  8.5  mm.,     .        .  530 

293.  Sections  at  different  levels  through  the  cardiac  aorta  of  a  human 

embryo  of  11.5  mm., 531 

294.  A  diagram  of  pharynx  of  an  amniote  vertebrate,          ....  535 

295.  Anterior  wall  of  the  pharynx  of  a  human  embryo  of  3.2  mm.  length,  536 

296.  Aortic  system  of  His1  embryo  Bl.,  4.25mm., 536 

207.  Aortic  system  of  His'  embryo  Si,  12.5  mm.;  seen  from  the  front,       .  537 

-Mis.  Aortic  system  of  W.  His' embryo  Rg,  11.5  mm., 539 

299.  Reconstruction  of  the  arteries  of  the  head  and  neck  of  a  rabbit  em- 

bryo at  the  end  of  the  eleventh  day, 540 

300.  His'  embryo  Lr  (4.2  mm.).     Reconstruction  to  show  the  course  of  the 

blood-vessels, 542 


LIST   OF   ILLUSTRATIONS.  XIX 

FIG.  PAGE 

301.  Cross-section  through  the  hinder  part  of  Hits'  embryo  H  (5  mm.),     .  542 

302.  Three  diagrams  to  illustrate  the  transformation  of  the  venous  system,  543 

303.  Reconstruction  of  a  human  embryo  (His'  Bl.)  of  4.25  mm.,         .        .  545 

304.  Reconstruction  of  the  venous  trunks  and  liver  of  His1  embryo  R, 

5  mm., 546 

305.  Reconstruction  of  the  venous  system  of  His'  embryo  Rg,  11.5  mm.,  .",17 
:!or>.  Section  of  the  skin  of  a  human  embryo  of  sixty-three  to  sixty-eight 

days 548 

:!<>;.  Epidermis  from  the  occiput  of  the  human  embryo  of  two  and  one- 
half  months 550 

:><>*.  Section  of  the  skin  of  the  under  side  of  the  right  second  toe  of 

four  months' embryo, 551 

309.  Epitrichiuin  of  a  human  embryo  of  the  fifth  month,  ....  55:3 

310.  Vertical  section  of  the  skin  of  a  human  embryo  of  the  fifth  month,  554 

311.  Longitudinal  section  of  the  nail  of  the  great  toe  of  a  human  embryo 

of  five  months, 556 

312.  Development  of  hairs  in  a  human  embryo  of  about  seven  months,  558 
31:;.  Isolated  epidermis  of  a  human  embryo  of  five  to  six  months,    .        .  550 

314.  Section  of  the  sole  of  the  foot  of  a  foetus  of  the  fifth  month,  to  show 

the  sweat  glands, 563 

315.  Development  of  the  mammary  gland  in  the  rabbit,    ....  5»54 
31G.  Acanthias  embryo  of  17  mm.,  under  side, 569 

317.  Blastoderm  of  a  dog-fish,  acanthias,  with  commencing  Concrescence,  570 

318.  Longitudinal  median  section  of  a  recently  hatched  larva  of  petro- 

myzon, 572 

319.  Longitudinal  section  of  an  acanthias  embryo  of  13.2  mm.,          .        .  572 

320.  Median  section  of  the  head  of  a  rabbit  embryo  of  thirteen  and  one- 

half  days, 573 

321.  His' embryo  A,  7.5  mm., 576 

322.  Facial  region  of  a  human  embryo  of  8  mm.,  front  view,      .         .         .  576 

323.  Reconstruction  of  the  face  of  His1  embryo  Sch, 577 

324.  View  of  the  roof  of  the  mouth  of  a  human  embryo,     ....  578 

325.  Frontal  section  of  the  oral  and  nasal  chambers  of  a  young  cow 

embryo, 579 

320.  Frontal  section  of  the  nasal  arid  oral  cavities  of  a  human  embryo  of 

three  months, r  581 

327.  Dental  papilla  of  a  dermal  tooth  of  an  acanthias  embryo  of  10  cm.,  582 

328.  Section  of  the  lower  jaw  of  an  acanthias  embryo  of  10  cm.,        .        .  582 
320.  Section  of  part  of  the  lower  jaw  of  a  human  embryo  of  40  mm.,       .  583 

330.  Explanation  in  text, 584 

331.  Vertical  section  of  a  molar  tooth-germ  of  a  human  emTbryo  of  160  mm.,  585 
•332.  Part  of  the  enamel  organ  of  a  new-born  child,  incisor  germ,      .        .  586 
3:):}.  Odontoblasts  from  cow  embryos.     A,  of  30  cm.;  B,  of  24  cm.,    .        .  588 
334.  Section  of  the  submaxillary  gland  of  a  human  embryo  of  sixty- 
three  to  sixty-eight  days, 591 

33.">.  Reconstruction  of  the  pharynx  of  a  human  embryo,  ....  592 

33< '..  Chick  embryo  of  twenty-nine  hours, 594 

337.  Cross-section  through  the  fore-brain  and  optic  vesicles  of  a  lepidos- 

teus  embryo  of  eight  days, 595 

338.  Brain  of  embryo  No.  22,  p.  297, 5'. Hi 

339.  Reconstruction  of  the  brain  of  His1  embryo  Ko 597 

340.  Reconstructed  median  view  of  the  fore-brain  of  His1  embryo  Ko,     .  59 7 


XX  LIST   OF   ILLUSTRATIONS. 

FIG.  PAGE 

341.  Brain  of  a  human  embryo  of  five  weeks, 599 

342.  Hind-brain  of  a  human  embryo, 600 

343.  Dorsal  view  of  the  hind-brain  of  a  human  embryo  of  one  month,    .  600 

344.  Sections  through  the  cervical  part  of  the  medulla  of  a  human 

embryo  with  thirteen  segments, 602 

345.  Longitudinal  horizontal  section  of  the  wall  of  the  hind-brain  of  a 

young  embryo  of  a  lizard  (Anolis  Sagraei), 605 

346.  Diagrammatic  section  of  the  embryonic  spinal  cord,  ....  607 

347.  Section  of  the  medulla  and  otocysts, 608 

348.  Sections  through  the  regions  3  and    5  of  the   hind-brain  of  His' 

embryo, 608 

349.  Sections  through  the  region  3  of  the  hind-brain  of  His1  embryo  A,  609 

350.  Four  sections  of  the  brain  of  a  human  embryo  of  about  five  weeks,  609 

351.  Brain  of  His'  embryo  Br.  3, 610 

352.  Neuroglia  of  the  dorsal  zone  of  the  spinal  cord  of  a  human  embryo 

of  about  three  and  one-half  weeks, 613 

353.  Cross-section  of  the  spinal  cord  of  a  human  embryo  of  14  mm.,  to 

show  the  neuroglia  cells,    .        .        .        .        .        .        .        .        .614 

354.  Part  of  a  transverse  section  of  the  spinal  cord  of  a  human  embryo  of 

23cm., 615 

355.  From  a  section  of  the  medulla  oblongata  of  His' embryo  Br3.,           .  617 

356.  Group  of  motor  neuroblasts  and  nerve  fibres  from  a  transverse  sec- 

tion of  the  spinal  cord  of  a  cat  embryo  of  6  mm.,          .        .        .  618 

357.  Bipolar  cells  from  a  spinal  ganglion  of  an  embryo,      ....  619 

358.  Transverse  section  of  the  dorsal  cord  and  ganglion  of  a  chick  of  nine 

days, 620 

359.  Isolated  nerve  fibres  from  the  sciatic  nerve  of  a  sheep  embryo  of  150 

mm., 621 

360.  Part  of  the  nerves  of  a  human  embryo  of  13.8  mm.,     ....  623 

361.  Cells  and  nuclei  from  the  cervical  region  of  the  spinal  cord  of  a 

human  embryo  of  one  hundred  and  sixty  days,      ....  624 

362.  Spinal  ganglion  cells  from  a  longitudinal  horizontal  section  of  a 

human  embryo  of  the  tenth  week, 626 

363.  Peripheral  nervous  system  of  a  human  embryo  of  about  10  mm.,    .  628 

364.  Transverse  section  of  a  mouse  embryo  of  about  seventeen  to  eighteen 

days  through  the  lumbar  region, 629 

365.  Transverse  section  of  the  sympathetic  cord  from  the  lower  dorsal 

region  of  a  rat  embryo  of  about  thirteen  days,      ....  631 

366.  Sympathetic  ganglia  of  one  side  of  a  human  embryo  of  the  fifth 

month, 632 

367.  Transverse  section  through  the  posterior  part  of  the  mid-brain  of  a 

human  embryo  of  five  weeks, <>:'><) 

368.  Section  of  the  brain  of  a  five  weeks'  embryo, 640 

369.  Section  of  the  brain  of  a  five  weeks'  embryo, 643 

370.  Section  of  the  brain  of  a  human  embryo  of  five  weeks,      .        .        .  645 
871.  Otocysis  and  nerves  of  a  human  embryo  of  four  and  one-half  weeks,  647 
372    Acoustic  ganglia  of  a  human  embryo  of  two  months,        .        .        .  647 

373.  Torpedo  embryo  of  12  mm., 653 

374.  Section  of  the  medulla  oblongata  of  a  five  weeks'  human  embryo,  055 

375.  Lower  end  of  the  spinal  cord  of  a  human  embryo  of  three  months,  658 

376.  Section  of  the  spinal  cord  of  a  human  embryo  of  sixty-three  to 

sixty-eight  days, 660 


LIST   OF   ILLUSTRATIONS.  XXI 

FIG.  PAGE 

377.  Transverse  section  of  the  spinal  cord  from  the  upper  dorsal  region 

of  a  human  embryo  of  six  weeks, 661 

;)?*.  Lower  cervical  cord  of  a  human  embryo  of  about  five  months,          .  663 

31!).  Transverse  section  of  the  medulla  oblongata  of  His' embryo  Ru,      .  <i(i(> 

380.  Transverse  section  of  the  medulla  oblongata  of  His'  embryo  Mr,      .  <>r>7 

381.  Section  through  the  medulla  oblongata  of  His' embryo  CR,      .        .  <><><) 

382.  Median  section  of  the  brain  of  a  chick  embryo  of  about  four  days,  673 

383.  Longitudinal  median  section  of  the  cerebellum  of  a  chick  of  about 

twelve  days, 673 

384.  Section  through  the  cerebellum  and  medulla  oblongata  of  a  human 

embryo  of  one  hundred  and  sixty  days, 674 

385.  Section  of  the  cerebellum  of  a  human  embryo  of  one  hundred  and 

sixty  days,    ............  C>75 

386.  Median  section  of  the  head  of  a  sheep  embryo  of  .36  mm.,         .        .  678 

387.  Brain,  human  foetus,  five  months, 678 

388.  Part  of  the  brain  of  His' embryo  CR,  13.6mm.,  .        .        .        .        .  679 
3su.  Section  of  the  thalamencephalon  of  an  embryo  of  five  weeks,            .  681 

390.  Section  of  the  fore-brain  of  a  sheep  embryo  of  27  mm.,       .        .        .  681 

391.  Brain  of  a  human  embryo  of  about  three  months,       ....  683 

592.  Brain  of  a  human  embryo  of  the  fourth  month, 683 

393.  Median  view  of  a  frog's  brain,           ........  684 

;>'.i4.  Section  through  the  fore-brain  of  a  foetal  guinea-pig,          .        .        .  685 

395.  Reconstruction  of  the  brain  of  an  embryo  of  about  seven  and  one- 

half  weeks 686 

396.  Brain  of  a  chick  embryo,  fourth  day, 688 

397.  Human  embryo  of  about  four  months  ;  brain  in  situ,        .        .        .  691 

398.  Section  through  the  lateral  wall  of  the  cerebral  hemisphere  of  a 

human  embryo  of  four  months,        .......  694 

399.  View  of  the  hemisphere  of  a  human  embryo  from  the  early  part  of 

the  third  month 695 

400.  Outlines  of  the  fissure  of  Sylvius  of  human  embryos  at  successive 

lunar  months 696 

401.  Median  view  of  the  fore-brain  of  a  human  embryo  from  the  begin- 

ning of  the  third  month, 696 

402.  Brain  of  human  embryo  of  the  fifth  month  after  removal  of  the 

right  hemisphere,        ..........  698 

403.  Right  hemisphere,  natural  size  of  a  foetus  of  nearly  seven  months,  699 

404.  Under  side  of  the  brain  of  a  human  embryo  of  the  fifth  month,        .  700 

405.  Section  of  the  fore-brain  of  a  human  embryo  of  nearly  five  weeks,  704 

406.  Horizontal  section  of  the  ciliary  ganglion  of  a  young  torpedo  embryo,  707 

407.  Reconstruction  to  show  the  cephalic  ganglia  of  a  petromyzon  larva 

4  mm.  long,  ............  707 

408.  Rabbit  embryo  of  ten  and  one-half  days  ;  section  of  head,        .        .711 

409.  Rabbit  embryo  of  thirteen  days  ;  section  of  the  eye,  ....  712 

410.  Reconstruction  from  His' embryo  Sch,  13.8mm.,          .        .        .        .713 

411.  Section  through  the  iris  region  of  the  eye  of  a  chick  of  thirteen  days,  714 

412.  Rabbit  embryo  of  ten  and  one-half  days  ;  section  of  the  lens  anlage,  715 
41:5.  Vertical  section  of  the  eye  of  a  chick  embryo  of  the  third  day,         .  715 

414.  Section  of  the  distal  portion  of  the  optic  nerve  of  a  rabbit  embryo 

of  thirteen  days,          ..........  718 

415.  Surface  view  of  the  membraiia  limitans  externa  with  the  develop- 

ing: rods  and  cones  of  a  chick  of  fifteen  to  sixteen  days,       .        .  720 


XX11  LIST   OF   ILLUSTRATIONS. 

FIG.  PAGE 

416.  Injected  vascular  membrane  of  the  retina  of  the  eye  of  a  pig  embryo, 

16  cm.  long 721 

417.  Section  through  the  iris  region  of  the  eye  of  a  chick  of  thirteen 

days, .        .        .        .        .     725 

418.  United  eyelids  of  a  human  embryo  of  about  four  months,  seen  in 

vertical  section, 726 

419.  Sections  of  human  embryos  showing  the  otocyst ;  A,  embryo  of  2.4 

mm.;  B,  embryo  of  4  mm.,        ........     728 

420.  Horizontal  section  of  the  otocyst  of  a  chick  of  the  third  day,    .        .     729 

421.  Left  otocyst  of  a  human  embryo  of  about  four  weeks  ;  A,  from  the 

inner,  B,  from  the  outer  side, 729 

422.  Transverse  section  of  the  head  of  a  rabbit  embryo  of  ten  and  one- 

half  days,     730 

423.  Left  otocyst  of  a  human  embryo  of  about  five  weeks,  seen  from  out- 

side and  below,    730 

424.  Transverse  section  of  the  semicircular  canal  of  an  embryo  rabbit  of 

twenty-four  days,       ..........     731 

425.  Left  otocyst  of  a  human  embryo  of  about  two  months,      .        .        .    732 

426.  Transverse  section  of  scala  media  cochleae  of  a  rabbit  embryo  of  55 

mm., 733 

427.  Section  through  Corti's  organ  of  the  lower  coil  of  the  cochlea  of  a 

rabbit  embryo  of  75  mm.,  .        .        .        .        .        .        .        .    734 

428.  Section  through  the  internal  ear  of  a  sheep  embryo,  28  mm.,    .        .     736 

429.  Isolated  right  membranous  labyrinth  of  human  embryo  of  six 

months,  seen  from  in  front  and  outside, 737 

430.  Section  through  the  region  of  the  ear  of  a  human  embryo  of  three 

months 739 

431.  Development  of  the  human  external  ear  ;  A,  embryo  of  one  month  ; 

B,  six  weeks  ;  C,  eight  weeks  ;  D,  ten  weeks ;  E,  fourteen  weeks,     741 

432.  Reconstruction  of  the  pharyngeal  region  of  a  human  embryo  of  11.5 

mm., 744 

433.  From  a  section  of  a  tonsil  of  a  human  embryo  of  five  months,  .        .     745 

434.  Section  through  the  third  gill-cleft  of  a  human  embryo  from  the 

beginning  of  the  third  week,     ........     746 

435.  Reconstruction  of  the  pharyngeal  region  of  a  human  embryo  of  9.1 

mm., 748 

436.  Reconstructions  to  show  the  development  of  the  thyroid  gland  in  the 

pig;  A,  embryo  of  15  mm.;  B,  of  16  mm.;  C,  of  20  mm.;  D,  of 
22.5  mm., 749 

437.  A,  section  of  the  thyroid  gland  of  a  human  embryo  of  about  four 

months  ;  B,  a  single  acinus,  more  highly  magnified,    .        .        .751 

438.  Reconstruction  of  His'  embryo  B  ;  the  head  is  drawn  as  if  erected,     752 

439.  Transverse  section  of  the  oesophagus  of  a  human  embryo  of  four 

months, 752 

440.  Highly  magnified  view  of  a  small  portion  of  the  epithelium  of  fig.  439,  753 

441.  Reconstruction  of  Fol's  embryo, 753 

442.  Epithelium  of  the  greater  curvature  of  the  stomach  of  an  embryo 

cat  of  85  mm., 754 

443.  Peptic  glands  from  the  greater  curvature  of  stomach  of  a  human 

embryo  from  the  end  of  the  eighth  lunar  month,        .        .        .     754 

444.  Digestive  tracts  of  four  human  embryos.     A,  embryo  of  4.2  mm. ;  B, 

euibryoof  7mni.;  C,  embryo  of  13.8  inni.;  D,  embryo  of  12.5  mm.,     756 


LIST   OF  ILLUSTRATIONS.  XXlll 

FIG.  PAGE 

445.  Two  front  views  of  the  entoderinal  canal.     A,  embryo  Sch.  1  of  H  is1 ; 

B,  His' embryo  Sch.  2, 757 

440.  Part  of  the  intestine  of  a  human  embryo  of  about  six  months,  .  758 
447.  Section  of  the  small  intestine  of  a  human  embryo  of  sixty-three  to 

sixty-eight  days,          ..........  759 

41s.  Section  of  the  small  intestine  of  a  human  embryo  of  three  ]nonth>,  759 

44!».  Portion  of  a  section  of  the  liver  of  an  acanthias  embryo  of  29  mm..  7<il 

450.  Section  through  the  liver  of  a  rabbit  embryo  of  thirteen  days,        .  763 

451.  Section  of  a  rabbit  embryo  of  thirteen  days  through  the  region  of 

the  fore  limbs  and  liver, 764 

452.  Section  of  the  pancreas  of  a  human  embryo  of  four  months,     .        .  767 
45:>.  Two  diagrams  to  illustrate  morphological  relations  of  the  vertebrate 

mesentery  ;  A,  earlier  ;  B,  later  condition, 768 

454.  Diagram  to  illustrate  the  relations  of  the  mesentery,          .        .        .  768 

455.  Diagram  of  the  human  mesentery  in  its  primitive  relations,      .        .  769 

456.  Diagrams  to  illustrate  the  history  of  the  human  mesentery.     A. 

earlier  ;  B,  later  condition, 770 

457.  Two  diagrams  to  illustrate  the  history  of  the  mesentery;  A,  earlier  ; 

B,  later  stages, Tt\ 

458.  Outline  of  the  entodermal  canal  of  His'  embryo  Lr.,   .        .     '    .        .  773 

459.  Three  views  of  the  lungs  of  a  human  embryo  of  10.5  mm.,          .        .  775 

460.  Lungs  of  a  human  embryo  of  five  months, 776 

461.  Cross-section  of  the  bronchial  tube  of  a  human  embryo  of  sixty- 

three  to  sixty-eight  days, 777 

462.  Section  through  the  lung  of  a  human  embryo  of  the  fourth  month,  1 1 : 

463.  Epithelium  and  gland  of  the  trachea  of  a  four  months'  embryo,       .  778 


ERRATA. 


PAGE 

44.  Explanation  of  Fig.  24,  for  "after  Binodi "  read  "after  Biondi." 
54.  Line  21  from  bottom,  for  u yolk-plates,  or"  read  "yolk-plates  of." 
62.  Line  18  from  top,  for  "37.1,  4  and  9,"  read  "27.1,  4  and  9." 
79.  Line  19  from  top,  for  "small"  read  "  large." 
88.  Line  7,  for  "85.1"  read  "84.2." 

188.  Line  2  from  top,  after  "proboscis"  insert  "sheath." 

349.  Line  8  from  bottom,  for  "  somatopleure  "  read  "  splanchnopleure." 

356.  Line  5  below  table,  for  "water"  read  "urates." 

449.  Fig.  252  is  from  Balfour. 

464.  Line  16,  p.  469,  lines  2,  4,  and  10,  for  "  Goette"  read  "  Goethe." 

471.  Fig.  255  is  after  Calberla. 

474.  Line  18  from  bottom,  for  "fusion"  read  "fission.' 

505.  Line  5  from  bottom,  for  "amniota"  read  "anamnia." 

523.  Fig.  287,  transpose  reference  letters  Au  and  V. 

599,  Lines  27  and  28,  transpose  "  metencephalon  "  and  "epencephalon." 

615.  Fig.  354  is  after  von  Lenhosse"k. 

689.  Line  3,  for  "fourth"  read  "third." 


HUMAN    EMBRYOLOGY 


INTRODUCTION. 


CHAPTER   I. 

THE   UTERUS. 

THE  uterus  enters  in  the  mammalia  into  such  intimate  relations 
with  the  embryo,  that  a  thorough  knowledge  of  its  structure  is  nec- 
cessary  to  the  embryologist.  The  treatment  of  the  uterus  in  the  text- 
books of  human  anatomy  is  usually  too  brief  for  the  requirements  of 
embryology.  These  considerations  make  it  desirable  to  give  a  some- 
what detailed  account  of  the  human  uterus. 

The  uterus  is  the  most  variable  organ  within  normal  limits  of 
the  body,  both  as  to  size  and  structure.  The  virgin  uterus  is  about 
three  inches  long  and  two  inches  wide  at  the  upper  part,  where  it  is 
broadest ;  it  weighs  about  40  grammes.  At  the  end  of  pregnancy  it 
is  about  ten  inches  long  and  nine  wide,  and  weighs  about  1,000 
grammes .  The  walls  of  the  virgin  or  resting  uterus  are  tense  and 
mainly  muscular;  those  of  the  pregnant  organ  are  more  spongy  in 
texture  and  extremely  vascular,  yet  at  the  same  time  the  muscular 
layers  are  greatly  increased,  though  relatively  less  than  the  vascular 
layer.  After  a  pregnancy  the  uterus  never  returns  to  its  primitive 
condition,  and  its  weight  does  not  fall  below  two  or  three  ounces ; 
from  the  gradual  effects  of  advanced  age,  however,  and  independent 
of  pregnancy,  the  uterus  shrinks,  becomes  paler  in  color,  and  harder 
in  texture.  Finally  at  each  successive  recurrence  of  menstruation  a 
complete  removal  of  the  superficial  part  of  the  mucous  membrane 
takes  place  by  a  process,  which  we  can  describe  but  not  explain. 
The  removal  is  said  to  commence  close  to  the  cervix  or  at  the  os 
internum,  and  to  progress  toward  the  fundus  during  the  remaining 
days  of  the  flow  of  blood.  As  the  shape  and  topographical  relations 
are  sufficiently  described  in  the  standard  Anatomies,  we  confine  our- 
selves principally  to  the  histology.  The  descriptions  are  arranged  in 
the  following  order : 

1.  Muscularis.  2.  Mucosa  corpus  uteri. 

3.  Mucosa  cervicis.  4.  Blood-vessels. 

5.   Lymphatics. 

1 .  Muscular  Coat.  — The  volume  of  the  muscularis  varies  greatly 
with  the  condition  of  the  uterus,  for  during  pregnancy  the  muscles 
1 


2  INTRODUCTION. 

undergo  a  progressive  hypertrophy,  which  is  so  great  that  not  only 
is  there  an  enormous  expansion  corresponding  to  the  dilatation  of  the 
uterus,  but  also  a  great  thickening  of  the  coat.  The  increase  in 
volume  is  due — 1,  to  the  growth  of  the  single  fibres  (in  length  from 
44-68  /*  to  220-560  /j)  ;  2,  it  is  said  also  by  the  development  of  new 
muscle  cells  from  small  granular  cells.  After  parturition  the  fibres 
in  part  return  to  their  original  size,  in  part  undergo  fatty  Regenera- 
tion (Kolliker,  "Gewebelehre,"  1867,  p.  566). 

The  disposition  of  the  fibres  is  most  readily  elucidated  in  uteri 
near  the  end  of  gestation.  Having  made  no  original  observations 
on  this  subject,  I  transcribe  the  following  passage  from  Quain's 
"  Anatomy  "  :  "  The  external  layer  of  the  muscular  coat  forms  a  thin 
superficial  sheet  immediately  beneath  the  peritoneum,  and  incomplete 
strata  situated  more  deeply.  A  large  share  of  these  fibres,  beginning 
as  longitudinal  bands  at  the  cervix,  arch  transversely  and  obliquely 
over  the  fundus  and  adjoining  part  of  the  body  of  the  organ,  and 
pass  on  each  side  into  the  broad  ligament.  Of  these  some  at  either 
side  run  toward  the  commencement  of  the  round  ligaments,  along 
which  they  are  in  part  prolonged  to  the  groin ;  others  pass  off  to  the 
Fallopian  tubes,  and  strong  transverse  bands  from  the  anterior  and 
posterior  surfaces  are  extended  into  the  ovarian  ligaments.  Other 
fibres  run  back  from  the  cervix  uteri  beneath  the  recto-uterine  folds 
of  the  peritoneum.  The  inner  layer  of  the  muscular  coat,  which  is 
also  thin,  is  composed  of  fibres  which  are  found  chiefly  on  the  back 
of  the  uterus,  and  stretch  over  the  fundus  and  toward  the  sides, 
running  somewhat  irregularly  between  the  ramifications  of  the  blood- 
vessels. "  On  the  inner  boundary  the  mucosa  is  quite  sharply  set  off 
from  the  muscularis ;  an  erroneous  contrary  statement  is  frequent  in 
English  and  American  works. 

It  is  commonly  asserted  that  the  muscular  coat  of  the  uterus  is 
largely  made  up  of  the  hypertrophied  muscularis  mucosce.  The 
evidence  for  this  view  is  not  to  be  found  either  in  the  anatomy  or 
in  the  developmental  history  of  the  uterus,  but,  so  far  as  I  can  ascer- 
tain, solely  in  the  preconception  that  every  mucosa  must  have  a 
special  muscularis  to  itself,  as  is  the  case  in  the  intestine,  for  exam- 
ple. Comparative  anatomy,  however,  is  conclusive  on  this  point; 
for  it  is  not  rare  to  find  a  mucosa  without  the  special  muscle  layer. 
The  true  morphological  relations  are  probably  the  reverse  of  those 
which  have  been  assumed  by  the  view  here  criticised ;  the  primitive 
form  is  probably  a  mucosa  composed  of  epithelium  and  sub-epithelial 
connective  tissue  resting  on  a  muscular  layer,  as  in  the  uterus ;  the 
secondary  form,  that  in  which  other  muscular  fibres  have  been  dif- 
ferentiated to  form  a  special  layer,  the  muscularis  mucosae. 

The  muscle  fibres  have  been  shown  by  Elischer,  76.1,  to  differ 
somewhat  from  the  forms  known  in  other  organs.  They  are 
elongated  cells,  often  spindle-shaped,  but  frequently  broad  and 
stumpy ;  in  the  pregnant  uterus  they  are  enlarged  and  flattened ;  in 
length  they  increase  from  40-60  i>-  (virgin  uterus)  to  300-600  /* 
(uterus  at  term) ;  in  transverse  section  they  are  seen  to  be  more  or 
less  distinctly  polyhedral ;  their  ends  and  sometimes  their  sides  bear 
branching  processes ;  they  have  one,  sometimes  two,  or  even  more 
nuclei,  which  are  usually  oval,  sometimes  round,  and  usually  nucleo- 


THE   UTERUS. 


lated;  the  nucleolus  is  eccentric.  The  nucleus  is  surrounded  by 
granular  matter,  which  stretches  out  toward  each  end  of  the  cell; 
often  the  granules  are  separated  by  a  clear  space  from  the  nucleus. 
This  space  has  been  observed  by  various  authors.  Eimer  has  found 
it  in  several  sorts  of  cells  and  gives  it  the  name  of  hyaloid.  It  is  a 
peculiarity  of  the  uterus  that  its  muscle  cells  vary  greatly  among 
themselves  in  appearance. 

2.  Mucosa  Corporis  Uteri.  (A).  Virginalis.— At  birth  the 
mucosa  of  the  body  of  the  uterus  is  about  0.2  mm.  thick,  soft,  pale 
gray  or  reddish-gray;  it  consists  of  a  covering  ciliated  cylinder 
epithelium  and  a  connective-tissue  layer;  it  is  without  glands,  the 
glands  not  appearing  usually  until  the  third  or  fourth  year,  and  de- 
veloping very  slowly  up  to  the  age  of  puberty.  Wyder,  78.1,  has 
shown  that  the  time  of  the  appearance  of  the  glands  is  extremely 
variable. 

In  the  virgin  resting  uterus  after  puberty  the  mucosa  is  about  1 
mm.  in  thickness.  It  is  sharply  marked  off  from  the  muscularis. 
The  glands  are  tubular,  often  bifurcated  in  their  lower  third,  round 
or  oval  in  transverse  sec- 
tion; they  run  more  or 
less  perpendicularly  to  the 
surface  of  the  membrane, 
upon  which  they  open; 
yet,  strictly  speaking,  this 
is  true  of  the  glands  in 
their  upper  half  only,  and 
even  in  that  part  their 
course  is  not  straight  but 
wavy .  In  their  lower  half 
they  deviate  much  more, 
being  more  irregular  and 
tortuous,  the  f  u  n  d  u  s 
curved  sometimes  even  so 
much  as  to  run  parallel 
to  the  muscular  layer  (G. 
J .  EngelnTann,  75.1). 
These  differences  between 
,the  upper  and  lower  parts 
of  the  glands  are  accentu- 
ated during  menstruation 
and  gravidity.  The 
glands  are  invaginations 
of  the  uterine  epithelium, 
are  accordingly  lined  by 
ciliated  cylinder  cells,  and 
have  a  nucleated  basement  membrane  (Fig.  1,  d),  formed  by  a  layer 
of  anastomosing  connective-tissue  cells  (Leopold,  74.1).  Overlach, 
85.1,  however,  expressly  denies  the  existence  of  any  such  membrane 
in  the  human  uterus  examined  by  him.  The  glands  reach  to,  and 
may  even  slightly  penetrate,  the  muscularis. 

Between  the  glands  is  found  a  somewhat  embryonic  connective 
tissue,  consisting  of  elongated  cells  with  oval  nuclei  and  branching 


FIG.  1. — Connective  tissue  of  mucosa,  uterus  of  pig;  a  a, 
capillaries;  66,  sheath  of  the  same;  c,  uterine  gland;  d, 
gland-sheath.  After  Leopold. 


4  INTRODUCTION. 

processes,  which  anastomose  with  one  another  *  (Fig.  1) ;  the  spaces 
of  the  cellular  network  communicate,  according  to  Leopold,  Z.s.c., 
with  the  lymphatic  vessels  of  the  muscularis  and  external  serosa,  and 
may  therefore  be  regarded  as  lymph  roots  or  lymph  spaces.  The 
branching  spindle-cells  resemble  somewhat  those  found  in  the  um- 
bilical and  other  embryonic  structures,  and  known  under  the  name  of 
mucous  tissue.  They  tend  to  crowd  together  around  the  blood- 
vessels and  glands.  There  do  not  appear  to  be  any  fibres  in  this 
layer,  although  some  observers  have  so  stated. 

Between  the  spindle-cells  are  small,  round  cells,  probably  wander- 
ing cells  (leucocytes) ,  which  vary  greatly  in  number. 

The  blood-vessels  enter  as  veins  and  arteries  from  the  muscularis, 
and  take  a  winding  course  toward  the  surface ;  the  capillaries  form 
a  network  around  the  glands  and  under  the  surface  of  the  mucosa. 

(B).  Decidua  Menstrualis. — The  function  of  menstruation  in- 
volves great  changes  in  the  mucosa  of  the  body  of  the  uterus.  We 
distinguish  three  periods:  1,  tumefaction  of  the  mucosa,  with  accom- 
panying structural  changes,  taking  5  days,  or,  according  to  Heusen, 
10  days ;  2,  menstruation  proper,  about  4  days ;  3,  restoration  of  the 
resting  mucosa,  about  7  days.  The  times  given  are  approximative 
only.  The  whole  cycle  of  changes  covers  about  16  days;  as  the 
monthly  period  is  about  four  weeks,  the  period  of  rest  as  thus  calcu- 
lated is  only  about  12  days. 

1.  TUMEFACTION. — A   few  days  before   the   menstrual  flow  the 
mucosa   gradually   thickens;    the   surface   becomes   irregular;    the 
openings  of  the  glands  lie  in  depressions.     The   connective-tissue 
cells  are  increased  in  number,  and  it  is  said  by  some  authors  in  size, 
but  the  increase  in  size  I  doubt;  the  number  of  round  cells  increases; 
the  glands  expand  and  become  more  irregular  in  their  course;  a 
short  time  before  hemorrhage  begins,  the  blood-vessels,  especially 
the  capillaries   and  veins,  become    greatly  distended.     We  must 
assume  that  the  connective-tissue  cells  proliferate,  but  we  have  no 
satisfactory   observations   upon   their   division.       It   was   formerly 
asserted  that  the  menstrual  decidua  contains  decidual  cells,  but  in 
all  the  specimens  I  have  studied  there  are  none  present. 

2.  MENSTRUATION. — When  the  changes  just  described  are  com- 
pleted, the  decidua  menstrualis  is  fully  formed,  and  its  partial  dis- 
integration begins.     The  process  commences  with  an  infiltration  of 
blood  into  the  subepithelial  tissues:   this  infiltration  has  hitherto 
been  commonly  explained  as  due  to  the  rupture  of  the  -capillaries ; 
but  as  no  ruptures  at  this  period  have  been  observed,  Overlach, 
85.1,   very  justly  regards   this   explanation  as    inadmissible    and 
thinks  the  infiltration  occurs  per  diapedesin.     It  lasts  for  a  day  or 
two,  and  is  apparently  the  immediate  cause  of  a  very  rapid  molecular 
disintegration  of  the  superficial  layers  of  the  mucosa,  which  in  con- 
sequence are  lost ;  the  superficial  blood-vessels  are  now  exposed,  and 
by  rupturing  cause  the  well-known  hemorrhagia  of  menstruation ; 
by  the  disappearance  of  its  upper  portion  the  mucosa  is  left  without 
any  lining  epithelium,  and  very  much   (and  abruptly)   reduced  in 
thickness.     Its  surface  is  formed  by  connective  tissue  and  exposed 

*  Compare  also  Schmidt,  Arner.  Journal  Obstet.,  Jan.,  1884. 


THE   UTERUS.  5 

blood-vessels.  The  third  stage  is  the  restoration  of  lost  parts.  Sigus 
of  fatty  degeneration  are  found  during  the  above-mentioned  disinte- 
gration. Kundrat  and  Engelmann,  73.1,  supposed  this  degenera- 
tion to  precede  and  cause  the  hemorrhage ;  but  this  view  has  not  been 
confirmed  by  subsequent  investigation,  it  having  been  found  that 
the  degeneration  begins  later  than  the  bleeding.  Overlach,  85.1, 
suggests  that  the  hemorrhage  is  caused  by  the  gorging  of  the 
veins  and  capillaries,  which  in  its  turn  is  caused  by  the  contraction 
of  the  muscles  of  the  uterus  compressing  the  thin-walled  veins. 
Against  this  view  I  would  urge  that  it  is  not  shown  that  marked 
contraction  of  the  muscles  precedes  the  bloody  discharge,  and  that  if  it 
does  occur  it  cannot  be  assumed  that  it  would  cause  sufficient  com- 
pression of  the  veins  to  produce  capillary  ruptures. 

It  is  desirable  to  add  a  few  words  as  to  Williams'  view,  75.1, 
75.2.     This   author  has  maintained  that  the  whole,  or  nearly  the 


Msc. 


FIG.  2. — Vertical  section  of  the  mucosa  corpus  uteri  oi  the  first  day  of  menstruation;  after 
Leopold.  Mac,  muscularis;  Mnc,  mucosa;  the  blood-vessels  (shaded  dark)  are  much  distended; 
the  glands  much  contorted;  there  is  a  subepithelial  blood  infiltration,  in  consequence  of  which 
the  epithelium  is  partly  lost. 

whole,  of  the  mucosa  disappears  from  the  body  of  the  uterus  during 
menstruation.  This  opinion  is  often  cited  as  authoritative,  espe- 
cially by  English  and  American  writers,  but  it  is  now  definitely 
known  to  be  erroneous  (Leopold,  77.1,  Underbill,  75.1,  et  al.). 
It  was  based  upon — 1,  failure  to  consider  the  effects  of  disease  upon 
the  uteri  observed  (cf.  Wyder,  78.1,  24);  2,  erroneous  observa- 
tions ;  3,  erroneous  interpretations,  involving  a  total  disregard  of  the 
elementary  laws  of  histogenesis. 

Minot,  98,  413-410,  describes  and  figures  a  normal  virgin  uterus 
near  the  close  of  menstruation.  "  The  mucous  membrane  is  from 
1.1-1.3  mm.  thick;  its  surface  is  irregularly  tumefied;  the  gland 
openings  lie  for  the  most  part  in  the  depressions.  In  the  cavity  of 
the  uterus  there  was  a  small  blood-clot.  The  mucosa  is  sharply 
limited  against  the  muscularis,  Fig.  3.  In  transverse  sections 
one  sees  that  the  upper  fourth  of  the  mucosa  is  very  much  broken 
down  and  disintegrated,  Fig.  3,  d;  the  cells  stain  less  than  those 


6  INTRODUCTION. 

of  the  deep  portions  of  the  membrane ;  as  represented  in  the  figure 
the  tissue  is  divided  into  numerous  more  or  less  separate  small 
masses;  some  of  the  blood-vessels  appear  torn  through,  but  it  is 
difficult  to  make  sure  observation."  Overlach,  85.1,  considers  it 
probable  that  the  infiltration  of  blood  takes  place  by  diapedesin,  not 
by  rupture  of  the  capillaries.  The  superficial  epithelium,  ep,  is 
loosened  everywhere ;  in  places  fragments  of  it  have  fallen  off,  and 
in  some  parts  it  is  gone  altogether ;  it  stains  readily  with  cochineal 
and  its  nuclei  color  well,  the  epithelium  differing  in  this  respect  from 
the  underlying  connective  tissue,  which  does  not  stain  well ;  the 
blood-vessels  in  the  disintegrated  layer  are  for  the  most  part  small. 
The  deeper  layer  of  the  mucosa  is  dense  with  crowded,  well-stained 
cells,  which  lie  in  groups  separated  by  clearer  lines ;  in  the  figure 
this  grouping  shows  less  plainly  than  in  the  preparation ;  the  lighter 
channels  are  perhaps  lymph  vessels — a  suggestion  which  occurs  to  me 
because  in  so-called  "  moulds"  one  sometimes  finds  similar  channels 
crowded  with  leucocytes.  The  cells  appear  to  be  the  proliferated 
interglandular  tissue ;  there  are  very  few  leucocytes,  so  far  as  I  can 
distinguish ;  the  cells  have  small,  oval  or  elongated,  darkly  stained 
nuclei,  with  a  very  small  granular  protoplasmatic  body  each ;  there 
is  certainly  no  noticeable  enlargement  of  the  cells,  but  only  a  remark- 
able multiplication. 

3.  KESTORATION  OF  THE  MUCOSA. — At  the  close  of  menstruation 
the  mucosa  is  2-3  mm.  thick ;  the  regeneration  of  the  lost  layers  be- 
gins promptly  and  is  completed  in  a  variable  time,  probably  five  to 
ten  days.  The  hyperaamia  rapidly  disappears;  the  extravasated 
blood  corpuscles  are  partly  resorbed,  partly  cast  off ;  the  spindle-cell 
network  grows  upward,  while  from  the  cylinder  epithelium  of  the 
glands  young  cells  grow  up  and  produce  a  new  epithelial  covering ; 
new  subepithelial  capillaries  appear.  The  details  of  these  changes 
are  imperfectly  known ;  they  effect  the  return  of  the  mucosa  to  its 
rest  ing- stage. 

(C).  The  decidua  graviditatis  is  the  decidua  menstrualis  pre- 
served in  situ,  and  considerably  metamorphosed  in  consequence  of 
pregnancy.  The  preservation  is  initiated  by  the  presence  of  a  fertil- 
ized ovum  in  the  upper  end  of  the  Fallopian  tube,  as  is  shown  for 
various  mammals  by  observation,  and  for  man  by  conclusive  infer- 
ence ;  and  the  preservation  is  dependent  for  its  continuance  upon  the 
further  development  of  the  ovum  in  utero.  In  the  very  youngest 
gravidity  yet  studied  (twelve  days)  very  great  alterations  have  oc- 
curred, and  we  are  reduced  to  hypotheses  to  explain  how  these  alter- 
nations are  effected.  The  ovum  at  this  stage  is  already  attached  to 
the  wall  of  the  uterus,  and  is  completely  enclosed  by  a  special  cover- 
ing known  as  the  decidual  reflexa.  The  arrangement  of  the  parts 
can  also  be  followed  in  older  ova,  and  is  illustrated  by  the  accom- 
panying woodcut,  Fig.  4,  which  represents  a  median  section  of  a 
uterus  about  five  weeks  pregnant.  The  whole  uterus  is  considerably 
enlarged ;  the  mucosa  lining  the  uterus  is  very  greatly  thickened ; 
to  one  part  of  it  the  ovum  is  attached;  the  mucosa  also  rises  all 
around  the  ovum,  completely  covering  it  in,  so  as  to  make  a  closed 
bag.  The  ovum  itself  is  a  sack,  known  as  the  chorionic  vesicle, 
which  is  covered  on  all  parts  by  snaggy  villi,  and  encloses  the  small 


THE   UTERUS. 


i  .., 


3 


INTRODUCTION. 


P 


a 


embryo  in  its  interior ;  it  is  very  important  to  note  that  only  the  tips 
of  the  chorionic  villi  come  in  contact  with  the  mucosa.  The 
mucosa,  we  thus  learn,  is  divided  into  three  parts :  1,  the  decidua 
serotina,  the  area  of  the  uterine  wall,  6'  s,  to  which  the  ovum  is 
attached ;  2,  the  decidua  vera,  comprising  all  the  remaining  portions 
of  the  mucosa  forming  part  of  the  walls  of  the  body  of  the  uterus ; 
3,  the  decidua  reflexa,  the  arching  dome  of  maternal  tissue,  r  r, 

which  rises  from  the  walls 
of  the  uterus  and  completely 
encapsules  the  ovum. 

If  the  walls  of  the  uterus 
are  cut  through  and  simply 
reflected,  leaving  the  reflex 
intact,  the  appearances  will 
be  found  essentially  as  in 
Fig.  5.  The  mucosa  is 
enormously  hypertrophied, 
and  contains  a  great  many 
dilated,  irregular  blood  sin- 
uses. From  one  part  hangs 
down  a  large  bag,  the  de- 
cidua reflexa,  D.  ref., 
nearly  filling  the  cavity  of 
the  uterus.  The  reflexa 
presents  the  same  general 
appearance  as  the  surface  of 
the  uterus ;  if  the  reflexa  be 
opened  we  come  upon  the 
villous  chorion  of  the  ovum, 
and  find  as  previously  stated 
that  only  the  tips  of  the  villi 
are  united  with  the  surface 
of  the  reflexa  or  serotina. 

To  form  the  placenta  the 
serotina  and  the  parts  of 
the  villi  and  chorion  con- 
nected with  it  (chorion  f  ron- 
dosum  of  later  stages)  un- 
dergo synchronous  hyper- 
trophy and  metamorphoses 

j    i  i         i  •>     j 

and  become  closely  united, 
compare  Chapter  XVII. 
In  gross  appearance  the  decidua  is  reddish-gray,  spongy  or  pulpy, 
soft  and  very  moist ;  after  the  fourth  month  it  acquires,  especially 
in  the  superficial  layers,  a  duller  brownish  color,  which  subsequently 
becomes  more  marked ;  this  coloration  is  due  to  the  decidual  cells. 
The  vera  and  serotina  are  divided  each  into  an  upper  or  superficial 
more  compact  layer,  and  a  deeper  cavernous  or  spongy  layer,  Fig. 
6;  the  two  layers  are  usually  of  about  equal  thickness,  but  the 
cavernous  layer  sometimes  encroaches  upon  the  compact  layer.  After 
the  fifth  month,  they  are  found  very  distinctly  differentiated.  The 
lumina  of  the  deep  layer  are  the  cavities  of  the  enlarged  and  irregu- 


Fio.  4.— Semi-diagrammatic  outline  of  au  autero- 
posterior  section  of  the  gravid  uterus  and  ovum  of  five 
weeks;  a,  anterior  surface;  p,  posterior  surface;  g, 
inner  margin  of  metamorphosed  mucosa :  s  to  s,  area 
of  the  decidua  serotina  ; — all  the  parts  or  the  mucosa 
adherent  to  the  uterine  walls  and  not  included  in  the 
area  of  the  serotina  constitute  the  decidua  vera;  ch, 
chorion,  within  which  is  the  embryo  enclosed  in  the 
amnion,  and  attached  to  the  walls  of  the  chorion ;  ap- 

ick; 


pended  to  the  embryo  is  the  long-stalked  yolk-sac 
"  y  the  arching  extension 
lecidua  reflexa,  r  r.    After 


the  chorion  is  covered  in  by  the  arching  extension  of 
de 


the  mucosa,  which  is  the 
Allen  Thompson. 


THE    UTERUS. 


0 


lar  uterine  glands.     During  the  first  two  or  three  months  the  scat- 
tered openings  of  the  uterine  glands  can  still  be  distinguished  over 


lr 


00 

•c  c 


~  o 

p 

-•  p. 


l! 


rf 
5 


—  z. 
K.  - 


the  surface  alike  of  the  vera  and  serotina  and  over  both  surfaces  of 
the  reflexa.  The  surfaces  of  the  vera  and  reflexa,  though  somewhat 
irregular,  remain  more  or  less  smooth ;  the  inner  surface  of  the  reflexa 


10  INTRODUCTION. 

is  more  irregular,  and  the  protuberant  parts  are  united  with  the 
tips  of  the  foetal  chorionic  villi. 

The  surface  of  the  decidua  serotina  becomes  very  irregular  dur- 
ing the  progress  of  pregnancy.  Rohr,  89.  1,  has  distinguished 
three  kinds  of  projections  in  a  uterus  of  the  eighth  month,  viz. :  1, 
Hillocks,  1-4  mm.  high,  and  with  broad  bases,  their  summits 
pointed,  irregular  or  even  branching ;  2,  columns,  beginning  with  a 
slightly  expanded  base,  narrow  stalk  and  often  enlarged  ends ;  the 
columns  are  long  and  stretch  up  toward  the  chorion,  which  they  act- 
ually reach  at  least  in  the  peripheral  parts  of  the  placenta ;  in  the 
central  region  they  rise  more  or  less  vertically,  but  obliquely  in  the 
peripheral  region ;  in  sections  of  the  placenta  they  are  often  cut  across, 
and  give  rise  then  to  the  appearance  of  islands  of  decidual  tissue  in 
the  midst  of  the  villi ;  3,  septa,  with  wide  bases  rising  irregularly  to 
the  height  of  0.5  to  1.5  cm. ;  it  is  by  these  septa  that  the  placenta  is 
divided  into  the  so-called  cotyledons,  compare  Chapter  XVII. 

THE  ORIGIN  OF  THE  DECIDUA  REFLEXA  is  uncertain,  there  being 
no  actual  observations  upon  its  genesis.  The  only  view  which  has 
hitherto  commanded  attention  is  the  following :  When  the  ovum  at- 
taches itself  to  the  wall  of  the  uterus,  the  mucosa  (decidua)  is  sup- 
posed to  form  an  annular  upgrowth  around  it ;  the  upgrowth  con- 
tinues making  first  a  high  wall,  then  arching  over,  and  finally  clos- 
ing at  the  top,  dome-like.  I  do  not  know  with  whom  this  hypothesis 
originated. 

In  certain  rodents  also  there  is  a  decidua  reflexa.  Selenka  has 
shown  that  in  them  the  ovum  becomes  completely  buried  in  the  uter- 
ine mucosa,  and  that  the  part  of  the  mucosa  covering  in  the  ovum 
is  converted  into  the  reflexa  as  the  ovum  expands.  In  the  hedgehog 
a  reflexa  is  formed,  according  to  Hubrecht,  in  a  similar  manner. 

DISAPPEARANCE  OF  THE  DECIDUA  REFLEXA. — A  very  important 
change  in  the  disposition  of  the  parts  takes  place  usually  during  the 
fifth  month,  viz. :  the  reflexa,  which,  by  its  own  expansion,  corre- 
sponding to  the  growth  of  the  ovum  it  encloses,  is  pressed  close 
against  the  vera,  disappears.  Its  disappearance  has  long  been 
known,  but  until  recently  was  unexplained ;  it  seems  safe  now  to  say 
that  it  degenerates  and  is  resorbed,  compare  p.  19.  In  consequence 
of  the  disappearance  of  the  reflexa  the  outermost  layer  (chorion  Iceve) 
of  the  ovum  comes  into  direct  contact  with  the  decidua  vera.  Be- 
fore the  fifth  month,  if  we  cut  through  the  uterine  wall  in  the  region 
of  the  vera,  we  come  upon  the  decidua  reflexa ;  after  the  fifth  month 
a  similar  cut  brings  us  upon  the  chorion  of  the  foetus. 

THE  GLANDS  are  already  dilated  in  the  menstrual  mucosa;  in 
pregnancy  the  dilatation  is  continued,  but  is  still  chiefly  confined  to 
the  deeper  parts  of  the  glands.  In  the  same  proportion  as  the  uterus 
expands  the  deep  portions  of  the  glands  become  stretched  in  their 
transverse  diameter  and  appear  during  the  latter  half  of  pregnancy 
in  sections  of  the  decidua,  Fig.  10,  as  narrow  fissures;  by  the  fifth 
month  the  glands  can  no  longer  be  traced  in  the  upper  compact  layer, 
their  ducts  being  obliterated.  The  partitions  left  between  the  glands 
are  quite  thin,  Fig.  10;  they  carry  the  blood-vessels  and  contain 
spindle  cells,  and,  it  is  said,  also  multinucleate  giant-cells  after  the 
fourth  month.  Compare  the  description  below  of  the  serotina  of  the 


THE    UTERUS.  11 

eighth  month.  The  spindle  cells,  as  stated  by  Langhaus,  resemble 
smooth  muscle  cells  in  appearance,  but  when  isolated  are  seen  rather 
to  be  broad,  round,  and  fiat ;  they  ought  probably  to  be  regarded 
rather  as  true  decidual  cells  than  as  merely  enlarged  connective-tissue 
cells. 

The  epithelium  of  the  glands  very  early  breaks  down,  as  described 


by  Minot,  compare  below,  p.  16.     The  epithelial  cells  at  first  lie 

id  cavity,  although  patches  of  them  still 
adhere  to  the  walls;   the  cells  disintegrate.     I  have  observed  this 


scattered  singly  in  the  glan< 


degeneration  in  every  one  of  a  large  number  of  specimens  which  I 
have  examined  of  all  ages  up  to  seven  months.  The  degree  of  break- 
ing down  may  be  said  in  a  general  way  to  advance  with  the  duration 
of  pregnancy,  but  even  at  term  patches  of  intact  epithelium  and 
groups  of  single  cells  are  always  recognizable.  The  openings  of  the 
glands  have  been  shown  by  Mogilowa,  91.1,  to  be  closed  by  the 
growth  of  decidua ;  this  fact  is  important,  for  it  shows  that  the 
glands  cannot  discharge  any  secretion,  and  shows  further  that  we 
must  discard  the  suggestion  made  by  Minot,  98,  420,  that  some  of 
the  persistent  openings  on  the  surface  of  the  placental  decidua  are 
glandular  and  not  vascular. 

THE  BLOOD-VESSELS  of  the  mucosa  are  all  enlarged,  those  in  the 
deeper  parts  to  a  lesser  degree  than  the  superficial  capillaries  and 
veins,  which  are  enormously  dilated,  forming  huge,  sinus-like  cavi- 
ties in  the  upper  stratum  of  the  decidua.  During  the  latter  part  of 
pregnancy  the  vessels  are  less  conspicuous.  The  remarkable  arrange- 
ment of  the  blood-vessels  in  the  decidua  serotina  is  fully  described 
in  Chapter  XVII. ;  it  will  suffice,  therefore,  to  state  now  merely  that 
the  arteries  and  veins  both  open  upon  the  surface  of  the  decidua,  so 
that  the  maternal  blood  circulates  in  the  spaces  between  the  villi  of 
the  placental  chorion. 

The  following  changes  in  the  blood-vessels  must  be  noted,  beside 
those  already  mentioned  in  describing  the  gross  appearances.  The 
vessels  of  the  vera  arid  reflexa  reach  their  maximum  development 
at  the  end  of  the  second  month,  when  they  begin  to  atrophy,  prepar- 
atory to  finally  disappearing.  Apparently  in  the  serotina,  also,  the 
blood-vessels  are  reduced  in  volume  and  number  toward  the  end  of 
pregnancy ;  but  this  alteration  needs  very  much  to  be  further  inves- 
tigated. 

GROWTH  OF  THE  DECIDUA. — With  the  growth  of  the  foetus  and 
the  consequent  dilatation  of  the  uterus,  the  deciduse,  of  course, 
must  increase  rapidly  in  superficial  extension.  In  fact  there  goes 
on  a  steady  growth  of  the  tissues,  which  however  is  not  sufficient  to 
effect  the  expansion  of  the  membrane  throughout  the  whole  period 
of  pregnancy  in  both  superficies  and  thickness.  The  growth  begins 
by  a  thickening  of  the  mucosa  within  the  area  of  the  uterine  wall  to 
which  the  ovum  is  attached,  so  that  during  the  third  and  perhaps 
fourth  week  this  area  (serotina}  is  the  thickest  portion  of  the  de- 
cidua (Kollmann,  79.1);  but  the  vera  and  reflexa  also  thicken, 
the  former  much  the  most,  and  soon  outdo  the  serotina.  By  the  end 
of  the  fifth  week  the  reflexa  measures  nearly  2  mm.  and  the  vera 
fully  1  cm.  The  absolute  thickness  of  the  serotina  does  not  change 
much  after  this  period,  remaining  3  mm.  or  a  little  less  up  to  the 


12  INTRODUCTION. 

end  of  pregnancy.  On  the  other  hand,  by  the  eighth  month  the 
reflexa  has  entirely  disappeared,  and  the  vera  is  reduced  to  about  2 
mm.  It  must  be  added — 1,  that  the  reflexa  is  thinner  over  the  poles 
opposite  the  serotina  than  elsewhere,  and  2,  that  the  vera  thins  out 
toward  the  cervix  and  toward  the  opening  of  each  Fallopian  tube. 

The  decidual  cells  are  the  most  striking  of  the  histological  ele- 
ments of  the  decidua.  They  are  very  large,  somewhat  flattened, 
rounded,  oval,  or  branching  cells,  which  assume  a  characteristic 
brownish  color  after  the  fourth  month ;  they  usually  have  a  single, 
often  nucleolated  nucleus,  but  sometimes  two,  three,  or  more  up  to 
thirty  or  forty,  Fig.  1 1 .  They  are  exceedingly  numerous  and  continue 
increasing  in  number  up  to  nearly  if  not  quite  the  termination  of  ges- 
tation. In  size  they  vary  from  0.03-0.1  mm.  Kundrat  and  Engel- 
mann,  73.  1,  and  others  maintain  that  the  cells  undergo  fatty 
degeneration  before  delivery,  and  attribute  the  loosening  of  the  pla- 
centa to  the  very  fact  of  the  fatty  metamorphosis.  This  view  is  at 
best  questionable,  and  it  is  even  doubtful  whether  the  fatty  change  is 
a  constant  phenomenon.  Of  the  decidual  cells,  we  notice  particularly 
the  very  large  ones  (giant  cells  of  Leopold) ,  with  numerous  nuclei 
and  often  with  branching  processes ;  the  number  of  nuclei  varies 
from  ten  to  thirty  and  more.  These  giant  cells  are  said  by  Leopold, 
77.  1,  to  appear  quite  abruptly  and  abundantly  during  the  fifth 
month.  They  lie  at  first  principally  in  the  neighborhood  of  the 
blood-vessels  of  the  deep  parts  of  the  decidua ;  they  do  not  occur  in 
the  reflexa,  and  are  far  less  numerous  in  the  vera  than  in  the  sero- 
tina. The  multinucleate  decidual  cells  are  perhaps  only  interme- 
diate stages  in  the  multiplication  of  the  uninuclear  cells,  each 
nucleus  of  the  large  cells  finally  separating  from  the  parent  with  its 
share  of  the  parent  protoplasm  to  make  a  new  decidual  cell ;  if  this 
is  the  case  it  accounts  for  the  final  disappearance  of  the  giant  cells. 
As  regards  the  function  of  the  multinucleate  cells  we  know  nothing ; 
in  the  rabbit,  however,  the  multinucleate  decidual  cells  have  a  gly- 
cogenic  function  (see  Chapter  XVII.),  but  they  differ  very  much  in 
microscopic  appearance  from  the  human  multinucleate  cells,  and 
perhaps  differ  equally  in  function. 

The  decidual  cells  are  most  abundantly  crowded  together  in  the 
upper  or  compact  layer,  and  contribute  much  to  give  that  layer  its 
main  characteristics.  By  the  eighth  month  they  are  found  to  have 
wandered  into  the  cellular  layer  of  the  placental  chorion,  as  is  more 
fully  described  in  the  chapter  on  the  chorion,  apparently  finding  an 
entrance  at  the  edge  of  the  placenta. 

Scattered  among  the  decidual  cells  may  be  found  a  number  of 
smaller  cells  which  are  more  conspicuous  during  the  earlier  months, 
and  are  usually  regarded  as  wandering  cells  (leucocytes).  Lang- 
haus,  77.  1,  110,  regards  the  leucocytes  as  the  parents  of  the  decid- 
ual cells — a  view  I  cannot  accept. 

THE  ORIGIN  of  the  decidual  cells  was  long  uncertain.  Three 
views  contended  for  acceptance:  1st,  they  are  modified  leucocytes 
(Hennig,  Langhaus  just  cited  above,  Sinety,  76.1,);  2d,  they 
arise  from  the  connective-tissue  cells  of  the  mucosa  (Hegar  und 
Maier,  Leopold) ;  3d,  they  are  produced  by  the  epithelium.  In 
favor  of  the  first  view  there  has  never  been,  to  my  knowledge,  any 


THE    UTERI'-.  13 

evidence  of  importance.  The  second  view  has  been  definitely  estab- 
lished by  Minot,  98,  429. 

The  epithelial  origin  was  first  advocated  by  Frommel  (Aerztliches 
Intelligenzblatt,  Miinchen,  1883,  No.  21)  for  the  mouse ;  by  Overlach, 
85.1,  for  man.  Overlach  traced  the  decidual  cells  to  their  origin 
in  the  epithelium,  but  his  observations  are  restricted  too, single  uterus 
with  pseudo-menstruation  from  acute  phosphorus-poisoning.  In  the 
epithelium  of  the  cervix  of  the  uterus  in  question  the  following 
developmental  stages  of  the  decidual  ceUs  were  found :  1,  cells  with 
a  mother  nucleus  and  one  or  several,  up  to  fifteen  smaUer  daughter 
nuclei ;  2,  ceUs  with  a  little  clearer  though  granular  protoplasm  col- 
lected around  the  daughter  nucleus  (or  nuclei) ;  3,  cells  in  which  the 
protoplasm  about  the  daughter  nuclei  has  increased  and  is  separated 
by  a  clear  vacuole-like  space  from  the  protoplasm  of  the  parent ;  wo 
have  then  a  mother  cell,  much  distended,  with  a  vacuole  partly  filled 
by  a  daughter  cell,  or  by  several  such  huddled  together;  4,  young 
decidual  cells,  lying  just  under  the  epithelium  and  closely  similar  to 
the  endogenous  brood  in  the  cells.  The  observations  of  A.  Walker, 
87. 1,  on  a  case  of  abdominal  pregnancy  maybe  taken  as  confirming 
Overlach.  Walker  found  that  the  peritoneal  epithelium  at  certain 
points  in  contact  with  the  chorion  had  proliferated,  forming  several 
layers  of  cells,  presenting  an  obvious  similarity  to  true  decidual  cells. 
Isolated  cells  of  a  similar  character  were  observed  in  the  underlying- 
connective  tissue  of  the  peritoneum.  It  thus  appears  that  the  ovum 
may  cause  in  other  epithelia  than  the  uterine  a  cell  growth  analogous 
to  that  described  by  Overlach.  Walker,  it  must  be  added,  maintains 
that  in  his  specimen  the  pseudo-decidual  cells  also  arise  in  part  by 
metamorphosis  of  the  connective-tissue  cells.  I  am  inclined  to 
interpret  Overlach  and  Walker's  observations  as  evidence  of  hyper- 
plastic  degeneration,  and  not  of  the  production  of  decidual  cells. 

The  manner  in  which  the  true  decidual  cells  arise  is  described  in 
the  next  section.  For  a  description  of  the  fully  developed  cells  see 
p.  IS. 

Uterus  One  Month  Pregnant. — The  specimen  to  be  described 
came  from  a  woman  who  committed  suicide  by  violence.  The  speci- 
men was  received  in  very  fresh  condition,  but  the  reflexa  was  badly 
torn ;  the  embryo  had  been  removed,  and  I  was  therefore  unable  to 
verify  the  age,  or  investigate  the  attachment  of  the  villi  of  the  chorion 
to  the  uterus.  There  was  a  beautiful  corpus  luteum  in  one  ovary, 
quite  similar  to  that  figured  by  Dalton  in  his  report  on  the  corpus 
luteum  in  the  Transactions  of  the  American  Gynecological  Society 
for  1877,  Fig.  9. 

My  specimen  enables  me  to  confirm  in  most  respects  Turner's 
accurate  description  of  two  uteri  of  about  the  same  age,  79.1, 
546-548.  The  inner  surface  shows  the  hillocks  (Inseln)  described 
by  Reichert  in  the  uterus  of  two  weeks  studied  by  him,  which  have 
been  figured  by  Coste  in  slightly  older  specimens,  and  found  by 
Turner  also,  79.1,  540. 

The  four  illustrations  given  herewith  are  all  from  sections  through 
what  I  suppose  to  be  the  placental  region. 

There  is  an  upper  compact  layer,  Fig.  6,  Z),  and  a  lower  cavernous 
layer  D' ;  the  caverns  being  gland  cavities,  which  appear  as  rounded 


14 


INTRODUCTION. 


areolse  partly  lined  with  epithelium,  partly  filled  with  broken-down 
epithelial  cells.  The  drawing,  reproduced  in  Fig.  C,  was  obtained 
by  drawing  the  outlines  very  carefully,  stippling  the  areas  occupied 

by  the  connective 
tissue,  represent- 
ing the  blood-ves- 
sels by  double  out- 
lines, and  omit- 
ting the  glandular 
epithelium  alto- 
gether. It  will  be 
noticed  that  about 
three  -  fourths  of 
the  diameter  of 
the  mucosa  is  oc- 
cupied by  the  cav- 
ernous layer,  D" . 

The  upper  or 
p|^B;-;:  compact  layer  is 
lltiC-S  shown  in  Fig.  7. 
The  surface  is 
without  any  trace 
of  epithelium,  and 
is  covered  only  by 
a  thin  fibrous  and 
granular  coagu- 
lum,  coagl ;  the 
tissue  itself  con- 
sists almost  exclu- 
sively of  young 
decidual  cells,  d, 
d',  with  a  clear, 
homogeneous 
matrix;  here  and 
there  are  leuco- 
cytes, but  they  are 
nowhere  numer- 

FIG.  ti.— Uterus  one  month  pregnant;  outlines  of  the  glands  from  a  ons:-   thp   rlpridlial 

vertical  section ;  to  show  the  division  of  the  mucosa  into  an  upper  u 

compact  layer  D'and  a  lower  cavernous  layer  D";  gl',  gl",  glands;  art,  cells   are  all  quite 

spiral  artery ;  muse,  muscularis.  large?    ^^    ^^ 

bodies  deeply  stained  by  the  eosin ;  the  nuclei  are  round,  oval,  or 
slightly  irregular  in  shape,  coarsely  granular,  and  sharp  in  outline ; 
the  cells  themselves,  though  irregular  and  variable  in  shape,  are  all 
more  or  less  rounded  with  processes  running  off  in  various  directions ; 
scattered  between  the  cells  are  many  sections  of  their  processes; 
occasionally  it  can  be  seen  that  two  cells  are  connected ;  in  fact,  we 
have  in  this  tissue  evidently  a  modified  embryonic  or  so-called 
anastomosing  connective  tissue.  Now,  as  we  know  through  the  ob- 
servations of  Leopold,  77.1,  which  I  have  verified,  the  connective 
tissue  of  the  uterine  mucosa  consists  of  anastomosing  cells,  and  as 
stated  in  the  previous  section  the  cells  are  found  proliferating  in  the 
menstruating  uterus;  we  have,  therefore,  only  to  imagine  the  cells 


muse 


THE   UTERUS. 


15 


enlarged  with  certain  accompanying  modifications  to  obtain  the 
tissue  figured  in  Fig.  7.  There  is  no  special  formation  of  cells 
around  the  blood-vessels,  where,  according  to  Ercolani,  the  decidual 
tissue  arises  by  new  formation.  In  Turner's  specimens  the  upper 
part  of  the  compact  layer  was  imperfectly  preserved,  but  according  to 
his  description  there  appears  to  have  been  a  coagulum  similar  to 
that  which  I  have  found,  but  thicker.  In  the  deep  part  of  the  layer 
the  cells  are  less  enlarged,  and  when  the  cavernous  layer  is  reached 
there  occurs  a  rapid  transition  in  the  character  of  the  cells,  which 
become  smaller  and  more  fusiform,  and  their  nuclei  more  elongate, 
smaller,  and  deeper  stained  by  alum-cochineal.  The  gland  openings 
upon  the  surface  of  the  uterus  lead  into  tubes,  Fig.  6,  gl ',  which  run 
slightly  obliquely  through  the  compact  layer,  taking  a  more  or  less 
nearly  straight  course  and  joining  the  contorted  gland  tubes,  Fig.  6, 
gl",  of  the  cavernous  layer.  The  gland  ducts  are  completely  devoid 
of  lining  epithe- 
lium, which  has 
disappeared  ex- 
cept for  a  very 
loose  cell,  occa- 
sionally found 
tying  free  in  the 
ducts;  the  cells 
have  not  fallen 
out  from  the  sec- 
tions, but  were 
lost  before  the 
tissue  was  im- 
bedded.* The 
ducts  then  are 
wide  tubes  run- 
n ing  nearly 
straight  through 
the  upper  part  of 
the  decidua  and 
bounded  direct- 
ly by  the  decid- 
ual tissue;  they 
c  o  m  m  u  n  icate 
below  with  the 
contorted  cavi- 
ties. 

T  h  e  cavern- 
ous layers  con- 
tain numerous 

spaces,  the  areolse  of  Turner,  79.1,  547,  who  was  uncertain  as  to 
their  character,  though  he  ascertained  that  many  of  them  belong  to 
the  glandular  system.  In  my  specimen  it  is  perfectly  clear  that  all 
the  larger  areolse  belong  to  the  glands,  which  must  be  extremely  dis- 
torted and  distended  to  give  the  shapes  shown  in  Fig.  G.  The  thin 


•d 


HI! 


FIG.  7. — Uterus  one  month  preguanc;  Bunion  <>i  tin-  rompact  layer 
of  the  decidua  seen  in  vertical  section ;  coayl,  coagulum  upon  the  sur- 
face; rf,  d',  decidual  cells,  x  445diams. 


*  The  blocks  to  be  cut  were  stained  in  into  with  alum -cochineal  and  eosin,  imbedded  in  paraf- 
fin, etc.     The  sections  were  fastened  on  the  side  with  celloidin,  to  keep  the  parts  in  place. 


16 


INTRODUCTION. 


dissepiments  between  the  areolse  are  composed  of  connective  tissue, 
the  long  dark  nuclei  of  which,  Fig.  8,  are  strikingly  different  from 
those  of  the  cells  of  the  compact  layer,  Fig.  7.  The  areolaa  present 
two  extreme  modifications  and  all  intermediate  phases  between 

these  two.  The  smaller  areolse 
are  lined  by  a  well-preserved 
cylinder  epithelium,  or  by  one 
in  which  the  cells  are  separat- 
ed by  small  fissures ;  in  other 
areola3  the  cells  are  a  little 
larger,  Fig.  8,  each  for  the 
most  part  cleft  from  its  fel- 
lows, and  some  of  them  loos- 
ened from  the  wall  and  lying 
free  in  the  cavity.  The  other 
extreme  is  represented  in  Fig. 
9;  the  size  of  the  areolse  is 
much  increased  —  compare 
Figs.  8  and  9 — both  drawn  on 
the  same  scale ;  the  epithelium 
is  entirely  loosened  from  the 
wall,  and  the  cells  lie  separate- 
ly in  the  cavity  which  they  fill ;  the  cells  are  greatly  enlarged,  their 
bodies  having  three  or  four  times  the  diameter  of  the  cells  in  the 
small  areola3;  they  have  not  the  cylinder  shape,  but  are  irregular  in 
outline;  their  protoplasm  is  finely  granular  and  stains  rather  lightly ; 
the  nuclei  are  large,  rounded,  granular,  and  with  sharp  outlines; 
they  are  less  darkly  stained  than  the  nuclei  of  the  epithelium  of  Fig. 
S.  The  obvious  interpretation  of  the  appearances  described  is  that 


FIG.  8.—  Uterus  one  month  pregnant;   section  of 
land  from  cavernous  layer,   with 
partly  adherent  to  the  walls, 


. 

gland  from  cavernous  layer,   with  the  epithelium 
x  445  diams. 


FIG.  9.  — Uterus  one  month  pregnant ;  section  of  gland  from  cavernous  layer  with  the  epithe- 
lium loosened  from  the  walls.     X  445  diams. 

the  glandular  epithelium  is  breaking  down,  being  lost  altogether 
from  the  ducts,  but  is  still  present  in  the  deep  portions  of  the  glands ; 
in  breaking  down  the  cells  separate  from  one  another,  and  then  from 


THE   UTERUS. 


17 


the  wall,  and  falling  into  the  gland  cavity  there  enlarge,  the  cavity 
enlarging  also.  Similar  appearances  are  also  found  in  "  moulds"  of 
the  second  month ;  very  likely  they  have  been  often  observed  and 
mistaken  for  pathological  changes. 

The  blood-vessels  of  course  lie  in  the  dissepiments  between  the 
glands.  I  observed  nothing  to  correspond  with  the  "  colossal  capil- 
laries dilated  into  small  sinuses,"  mentioned  by  Turner,  79.1,  548. 
Were  not  these  supposed  capillaries  gland  cavities,  from  which  the 
epithelium  had  fallen  out?  Occasionally  the  sections  pass  through 
a  spiral  artery,  Fig.  (>,  art,  which  is  cut  again  and  again  as  it 
twists  around  in  its  characteristic  separate  column  of  connective 
tissue. 

Decidua  Serotina  at  Seven  Months. — In  a  normal  uterus  about 
eight  months  pregnant  I  find  the  following  relations :  The  serotina 


FIG.  10.— Section  of  the  decidua  serotina,  near  the  margin  of  the  placenta;  normal  uterus 
about  seven  months  pregnant,  me,  Muscularis ;  D',  D",  decidua  serotina;  Z>',  cavernous  or 
spongy  layer,  the  spaces  in  which  are  the  glands;  Z>",  compact  layer;  Vi,  scattered  chorionic 
villi ;  the  intervillous  spaces  were  filled  with  blood,  which  is  not  represented  in  the  figure. 

is  about  1.5  mm.  thick,  and  contains  an  enormous  number  of  decidual 
cells,  Fig.  10 ;  the  cavernous,  D\  and  compact  layers,  D",  are  very 
clearly  separated ;  the  mucosa  is  sharply  marked  off  from  the  mus- 
cularis,  although  scattered  decidual  cells  have  penetrated  between 
the  muscular  fibres.  The  muscularis  is  about  10  mm.  thick  and  is 
characterized  by  the  presence  of  quite  large  and  numerous  venous 
thrombi,  especially  in  the  part  toward  the  decidua.  The  decidua 
itself  contains  few  blood-vessels.  Upon  the  surface  of  the  decidua 
can  be  distinguished  a  special  layer  of  denser  decidual  tissue,  which 
in  many  places  is  interrupted  by  the  ends  of  the  chorionic  villi  which 
2 


INTRODUCTION. 


have  penetrated  it,  as  is  well  shown  in  tho  accompanying  Figure  10. 
The  gland  cavities  of  the  spongy  layer,  D',  are  long  and  slit-like ; 
they  are  filled  for  the  most  part  with  fine  granular  matter,  which 
stains  light  blue  with  hsematoxylin ;  they  also  contain  a  little  blood, 
and  sometimes  a  few  decidual  cells.  I  have  also  seen  in  them  a  few 
oval  bodies  several  times  larger  than  any  of  the  decidual  cells,  and 
presenting  a  vacuolated  appearance.  What  these  bodies  are  I  have 
not  ascertained;  in  a  number  of  uteri  over  two  months  pregnant 
I  have  found  them  invariably  present.  In  many  places  tho  glandu- 
lar epithelium  is  perfectly  distinct ;  its  cells  vary  greatly  in  appear- 
ance, neighbors  being  often  quite  dissimilar;  nearly  all  are  cuboidal, 
but  some  are  flattened  out ;  of  the  former  a  number  are  small  with 
darkly  stained  nuclei,  but  the  majority  of  the  cells  are  enlarged, 
with  greatly  enlarged  hyaline,  very  refringent  nuclei.  There  are 
also  in  many  of  the  gland  spaces  isolated  enlarged  cells,  which  have 
detached  themselves  from  the  wall,  and  in  some  cases  the  detached 
cells  nearly  fill  the  gland  cavity,  very  much  as  in  Fig.  9. 

The  decidual  cells  of  the  cavernous  layer,  Fig.  10,  Z>,  are  smaller 
and  more  crowded  than  most  of  those  of  the  compact  layer.  The 
largest  cells  are  scattered  through  the  compact  layer,  but  are  most 
numerous  toward  the  surface.  They  extend  around  the  margin  of 

the  placenta  and  have 
penetrated  the  chorion,  in 
the  cellular  layer  of  which 
they  are  very  numerous 
(compare  on  this  point  the 
chapter  on  the  Chorion) ; 
the  immigration  has  im- 
parted to  the  chorionic 
layer  in  question  some- 
what the  appearance  of  a 
decidual  membrane.  Mis- 
led by  this  peculiarity, 
Kolliker  and  others  have 
held  this  layer  to  be  mater- 
nal in  origin,  and  accord- 
ingly have  described  it  as 
a  "  decidua  subchorialis. " 
The  error  was,  so  far  as  I 
am  aware,  first  definitely 
corrected  by  Langhaus, 
77.1.  The  decidual  cells 
exhibit  great  variety  in 
their  features,  Fig.  11.  They  are  nearly  all  oval  discs,  so  that  their 
outlines  vary  according  as  they  are  seen  lying  in  the  tissue  turned 
one  way  or  another ;  they  vary  greatly  in  size ;  the  larger  they  are, 
the  more  nuclei  they  contain;  the  nuclei  are  usually  more  or  less 
elongated ;  the  contents  of  the  cell  granular.  Some  of  the  cells  pre- 
sent another  type,  c  ;  these  are  more  nearly  round,  are  clear  and 
transparent;  the  nucleus  is  round,  stains  lightly,  and  contains 
relatively  few  and  small  chromatin  granules;  such  cells  are  most 
numerous  about  the  placental  margin. 


FIG.  11.— Decidual  cells  from  the  section  represented  in 
Fig.  10 ;  stained  with  alum  haematoxylin,  and  eosin ;  a,  6, 
d  and/,  various  forms  of  cells,  from  the  serotina;  c.  giant 
cell,  from  the  margin  of  the  placenta;  e.  clear  cells  from 
the  chorion.  At  a,  seven  blood  globules  have  been  drawn 
in  to  scale  to  afford  a  ready  measure  of  size. 


THE    UTERUS.  19 

Fate  of  the  Decidua  Reflexa.— The  decidua  reflexa  is  a  dis- 
tinct membrane  up  to  the  end,  it  is  said,  of  the  fifth  month  of  gesta- 
tion, and  after  that  period  it  can  no  longer  be  found.  Exactly  at 
what  time  it  disappears  is  not  established  by  observation,  though  the 
fact  of  the  disappearance  has  long  been  known,  nor  have  we  had 
hitherto  any  definite  knowledge  as  to  how  it  disappears,  although  its 
gradual  attenuation  and  increasing  transparency  during  the  first  four 
or  five  months  have  been  familiar  to  us  since  the  publication  of 
Coste's  magnificent  atlas.  The  view  most  generally  accepted  has 
been  that  it  fused  with  the  decidua  vera,  and  that  accordingly  the 
layer  of  decidua  nearest  the  chorion  during  the  latter  half  of  preg- 
nancy represents  the  decidua  reflexa. 

I  have  had  opportunity  to  study  four  well-preserved  normal  preg- 
nant uteri  of  two,  three,  five  to  six,  and  seven  months'  gestation 
respectively.  These  show  that  at  two  months  the  decidua  reflexa  is 
undergoing  hyaline  degeneration,  that  at  three  months  the  degener- 
ation is  considerably  more  advanced,  and  that  by  the  sixth  and 
seventh  month  the  reflexa  can  no  longer  be  found.  These  obser- 
vations justify  the  theory  that  the  reflexa  degenerates  and  is  com- 
pletely resorbed. 

I  will  review  briefly  the  actual  observations : 

First,  the  reflexa  at  two  months.  It  starts  from  the  edge  of  the 
placenta!  area  with  considerable  thickness,  which  is  rapidly  lost, 
m<  >st  of  the  reflexa  being  a  thin  membrane  and  the  thinnest  point 
being  opposite  the  placenta.  The  examination  of  sections  snows  that 
the  entire  reflexa  is  undergoing  degeneration,  which  is  found  to  be 
the  more  advanced  the  more  remote  the  part  examined  is  from  the 
placenta.  The  chorion  laeve  lies  very  near  the  reflexa,  being 
separated  only  by  chorionic  villi,  which  are  very  much  altered  by 
degeneration,  their  ectoderm  having  become  a  hyaline  tissue,  which 
stains  darkly,  and  their  mesoderm  showing  clearly  the  partial  loss  of 
its  cellular  organization.  In  the  region  half-way  between  the  base 
and  the  apex  of  the  reflexa  dome  the  tissue  of  the  decidual  membrane 
shows  only  vague  traces  of  its  original  structure ;  only  here  and  there 
can  a  distinct  cell  with  its  nucleus  be  made  out,  for  most  of  the  cells 
have  broken  down  and  fused  into  irregular  masses  without  recogniz- 
able organization.  Ramifying  through  the  fused  detritus  there  are 
two  layers  of  so-called  "fibrin,"  or,  in  other  words,  of  a  hyaline  sub- 
stance, which  like  the  "  canalized  fibrin"  of  the  chorion  stains  very 
deeply  with  the  ordinary  histological  dyes,  carmine  and  logwood. 
The  fibrin  is  much  more  developed  upon  the  inner  or  chorionic  than 
upon  the  outer  side  of  the  reflexa.  It  forms  on  the  inner  side  a  dense 
network,  which  on  the  one  hand  fuses  with  the  degenerated  ectoderm 
of  the  chorionic  villi  wherever  the  villi  are  in  contact  with  the 
decidua ;  and  on  the  other  hand  ramifies  more  than  half-way  through 
the  decidua,  the  ramifications  being  easily  followed,  owing  to  the 
hyaline  character  and  deep  staining  of  the  "fibrin."  Upon  the  out- 
side the  fibrin  forms  a  thinner  layer,  and  shows  its  network  structure 
in  many  sections  much  less  clearly.  All  of  these  points  are  illus- 
trated by  the  accompanying  figure. 

In  the  uterus  three  months  pregnant  I  find  essentially  the  same 
conditions,  except  that  the  degeneration  is  farther  advanced,  since 


20  INTRODUCTION. 

\ 

the  traces  of  cellular  structure  in  the  reflexa  are  still  more  vague  and 
the  fibrin  is  more  developed.  The  membrane  is  much  thinner  than 
at  two  months;  the  thickness  is  about  two-thirds  of  what  it  was. 
In  the  fresh  specimen  the  membrane  appeared  much  more  transpar- 
ent than  before.  In  all  the  parts  examined  I  found  leucocytes  pres- 
ent, and  in  the  region  of  the  reflexa  near  the  placenta  they  are  very 
numerous  and  conspicuous;  it  is  natural  to  conclude  that  they  are 
concerned  in  the  resorption  of  the  degenerated  tissue.  In  a  section 
not  far  from  the  base  of  the  reflexa  the  three  layers  are  distinct  as 
at  two  months,  there  being  a  thicker  inner  and  a  thinner  outer 
fibrin  layer,  while  between  them  is  a  stratum  in  which  remains  of 
cells  are  seen ;  occasionally  is  an  appearance  which  suggests  a  sur- 
viving decidual  cell,  and  nearer  the  placenta  the  phantoms  of  cells 


i  I:   : 


FIG.  1J}.—  Section  of  human  decidua  reflexa  at  two  mouths. 

become  distinctly  cells,  and  true  decidual  cells  can  be  made  out.  The 
inner  fibrin  layer  is  much  denser  and  its  meshes  smaller  than  in 
the  two  months  specimen,  the  trabeculaa  of  fibrin  having  become 
thicker  during  the  month  elapsed. 

Those  who  conceive  that  there  is  a  fusion  between  the  reflexa  and 
vera,  are  forced  to  seek  for  traces  of  the  former  membrane  next  the 
chorion.  They  may  assume  either  that  the  epithelioid  layer  (cho- 
rionic  ectoderm)  is  the  remnant  of  the  decidua,  which  forces  them  to 
leave  the  fate  of  the  chorionic  epithelium  unexplained,  or  that  the 
upper  stratum  of  the  decidua  is  the  reflexa  which  is  fused  with  and 
acquired  the  same  structure  as  the  underlying  vera.  If  my  obser- 
vations on  the  degeneration  of  the  reflexa  are  correct,  and  corre- 
spond, as  there  is  sufficient  ground  to  believe  they  do,  to  normal  con- 
ditions, then  both  assumptions  as  to  the  persistence  of  the  reflexa 
involve  the  further  and  very  improbable  assumption  that  the  degen- 
erated tissue  is  removed  and  replaced  by  fully  organized  cellular 
decidual  tissue.  It  is  obviously  more  in  accordance  with  our  knowl- 
edge of  degenerative  changes  to  assume  that  the  hyaline  metamor- 
phosis is  necrotic  and  is  succeeded  by  the  disintegration  and  removal 


THE   UTERUS.  21 

of  the  tissue.  This  accounts  in  a  satisfactory  manner  for  the  absence 
of  the  decidua  reflexa  during  the  sixth  and  seventh  month.  The 
relations  of  the  membranes  at  this  period  have  been  well  described 
and  figured  by  an  admirable  observer,  Dr.  G.  Leopold,  whose  views 
and  one  of  whose  drawings  have  been  incorporated  by  Prof.  O.  Hert- 
wig,  in  his  i%  Entwickelungsgeschichte"  (third  edition,  pp.  21G-217, 
fig.  14  T).  Leopold  holds  that  the  epithelioid  layer  is  the  reflexa ;  but 
what  has  just  been  said  suffices,  I  think,  to  show  that  this  view  is 
untenable. 

That  the  membrana  decidua  reflexa  should  degenerate  and  disap- 
pear no  longer  seems  strange,  since  recent  investigations  have  shown 
that  in  many  placental  mammals  there  occurs  an  extensive  pseudo- 
pathological  destruction  of  the  mucosa  uteri  during  gestation.  These 
changes,  which  are  best  known  in  the  rabbit  (cf.  Minot,  Biol.  Cen- 
IralhL,  x.,  114)  vary  considerably  in  character  and  are  exceedingly 
remarkable  both  for  their  extent  and  for  their  numerous  modifica- 
tions, so  that  we  need  feel  no  surprise  at  the  entire  destruction  of 
the  decidua  reflexa  in  man,  nor  at  the  form  of  the  destruction  being 
unlike  the  forms  hitherto  found  in  other  mammals. 

As  to  the  purpose  or  advantage  of  the  sacrifices  of  maternal  tissue 
we  are  in  the  dark.  The  same  is  true  of  the  causation  of  the  degen- 
eration, although  we  must  regard  it  as  the  result  of  a  reflex  nervous 
activity.  It  is  becoming  more  and  more  evident  that  the  nerves 
have  a  profound  influence  upon  organization,  and  it  is  no  strained 
hypothesis  which  places  the  structure  of  the  mucosa  uteri  under  the 
immediate  control  of  the  nervous  system. 

The  changes  in  the  decidua  at  parturition  require  special 
description.  During  labor  a  split  occurs  in  the  decidua  serotina  and 
vera ;  all  the  parts  within  the  split — that  is,  toward  the  chorion — are 
expelled,  their  expulsion  being  part  of  the  act  of  delivery ;  the  term 
decidua  or  c<idit<-<i  refers  to  the  fact  that  the  membranes  are  cast 
off;  they  are  discharged  after  the  foetus,  and,  together  with  the  vera 
and  foetal  envelopes,  constitute  the  so-called  after-birth.  There  are 
thus  removed  the  superficial  portions  of  the  vera  and  serotina.  The 
split,  according  to  Friedlander,  70.1,  76.1,  usually  occurs  in 
the  upper  or  compact  layer  just  above  the  cavernous  layer,  leaving 
the  surface  of  the  uterus  smooth  and  glistening,  but  the  surface  of 
the  pla,cental  area  is  thrown  into  irregular  hills  and  valleys.  Some- 
times the  split  occurs  at  or  just  below  the  upper  limit  of  the  cavernous 
kyer,  in  which  case  the  surface  of  the  uterus  after  parturition  is 
jagged  and  irregular.  In  rarer  cases  the  split  occurs  higher  up  in 
the  compact  layer,  leaving  consequently  by  far  the  greater  part  of 
the  decidua  in  situ  quo  ante.  In  all  normal  cases,  however,  more 
of  the  mucosa  is  lost  than  in  menstruation,  and  a  considerable  portion 
is  always  left  in  utero;  this  latter  portion  contains  the  remnants  of 
the  uterine  glands,  and  is  the  organ  of  regeneration  for  the  entire 
mucosa ;  it  has,  of  course,  no  epithelium  upon  its  surface,  which  in- 
stead is  formed  by  connective  tissue  and  ruptured  blood-vessel  (and 
lymphatics?).  The  layer  of  vera  left  on  the  uterus  is  usually  about 
1  mm.  thick;  that  of  the  serotina  may  be  considerably  less. 

The  post-partum  regeneration  of  the  mucosa  begins  very  soon, 
but  varies  greatly  in  the  rate  with  which  it  progresses,  being  very 


22 


INTRODUCTION. 


rapid  in  vigorous,  healthy  women  and  slow  in  weakly  women.     The 
region  of  the  vera  is  restored  more  rapidly  than  the  placental  area. 

The  first  step  is  the  thickening  of  the  mucosa  to  about  2  mm., 
owing  to  the  contraction  of  the  uterus,  which  of  course  reduces  the 
superficial  extent  without  altering  the  volume  of  the  mucosa.  In 
consequence  of  this  change  also  the  gland  spaces  become  rounder  and 


m. 


FIG.  13.— Uterus  twelve  hours  after  artificial  delivery  at  six   months  pregnancy;  cuuyl,   blood 
clot;  D,  decidua;  m,  muscularis.     X  22  diams. 

the  course  of  the  glands  straighter.  I  will  here  interpolate  a  descrip- 
tion of  a  human  uterus,  twelve  hours  after  abortion,  see  Minot,  98, 
428.  The  uterus  was  apparently  normal;  it  was  already  very  much 
contracted;  the  mucosa  measured  about  1  mm.  in  thickness;  the 
surface  was  ragged  and  more  or  less  covered  with  clotted  blood,  pre- 
senting very  much  the  appearance  so  superbly  figured  by  Coste 
("Devel.  corps  organises,"  pi.  x.  Espece  humaine).  Vertical 
sections,  Fig.  13,  show  that  the  surfaces  of  the  mucosa  are  very  un- 
even ;  on  the  free  surface  there  is  a  thin  layer  of  clotted  blood,  coagl; 
the  upper  or  compact  layer  of  the  decidua  has  entirely  disappeared, 
"leaving  only  the  deep  portion,  Z),  permeated  by  numerous  large 
gland  spaces,  between  which  are  partitions  containing  the  brownish 
and  hyaline  decidual  cells,  and  a  great  many  blood  corpuscles,  which 
lie  in  the  tissues  as  well  as  in  the  blood-vessels.  The  presence  of 
blood  corpuscles  in  the  tissues  is  probably  a  constant  feature  of  the 
decidua  post  partum." 

The  second  step  is  the  restoration  of  the  surface  by  the  resorption 
of  the  blood  and  detritus,  parallel  with  which  advances  the  restora- 
tion of  the  glandular  epithelium.  These  changes  occupy  apparently 


THE    UTERUS.  23 

from  seven  to  fourteen  days.  The  cuboidal  gland  cells  at  this  time 
appear  swollen,  with  indistinct  intercellular  boundaries ;  the  nuclei' 
are  almost  all  enlarged  until  they  nearly  fill  the  cells;  rapid  cell  divi- 
sion is  going  on.  At  this  time  also  venous  thrombi  are  very  con- 
spicuous, especially  in  the  placental  area,  where  they  are  found  fresh 
and  in  various  stages  of  progressing  obliteration,  Fig.  14.  The 
thrombi  persist  for  a  long  period  (Leopold,  77. 1,  xii.,  185). 

The  third  step  is  the  completion  of  the  restoration  of  the  glands  up 
1  >  their  external  openings,  and  the  regrowth  of  the  normal  connec- 
tive tissue  of  the  mucosa.  The  resulting  stage  was  found  by  Leopold, 
77. 1,  xii.,  199,  to  have  been  reached  in  a  normal  uterus  three  weeks 
after  parturition.  Of  this  specimen  he  gives  the  following  descrip- 
tion, which  refers  to  the  placental  region.  "  As  shown  by  the  illus- 
tration (Fig.  14)  the  young  mucosa  is  composed  mainly  of  fine  short 
spindle  cells,  which  form  the  interglandular  tissue.  They  exhibit 
extraordinary  proliferation,  and  are  showing  themselves  in  numerous 
processes  (Zap fen)  into  the  musculature,  but  still  leaving  the  limits  of 
muscularis  and  mucosa  distinct  as  in  every  non-pregnant  and  preg- 
nant uterus.  Secondly,  between  the  young  cells  we  find  manyblood- 
v(  'ssels,  especially  capillaries,  in  the  neighborhood  of  which  are  col- 
lected blood  corpuscles,  luematin  crystals,  and  pigment.  Many  ap- 
pearances indicate  the  new  formation  of  capillaries  from  simple  cords 


FIG.  14.—  Section  of  the  placental  area  of  the  uterus  three  weeks  post  partum.     J/HC,  mucosa: 
Msc.  muscularis.     After  Leopold. 

of  cells,  which  extend  to  the  very  surface.  Thirdly,  and  most  im- 
portant, we  find  the  young  glands,  which  are  short  vertical  follicles, 
imparting  to  the  surface  a  more  definite  sieve-like  appearance. 
Their  cuboidal  epithelium  is  spreading  out  from  their  mouths  to  re- 
cover the  surface  ;  but  at  this  time  the  new  epithelium  is  not  yet 
completed.  The  mucosa  is  still  a  wounded  tissue;  for  its  complete 
restoration  there  is  still  lacking  .  .  .  the  vascular  network.  "  The 
fourth  step  is  a  double  one:  the  restoration  of  1,  the  superficial 


24  INTRODUCTION. 

epithelium,  which  is  accomplished  by  the  spreading  of  the  growing 
epithelium  from  the  mouths  of  the  glands,  and  of  2,  the  subepithelial 
network  of  capillaries.  The  completion  of  this,  the  last  step  in  the 
restoration,  has  been  observed  in  a  normal  uterus  six  weeks  after 
parturition. 

A  very  different  regenerative  process  is  stated  by  Duval,  90.3,  to 
occur  in  rodents ;  he  believes  that  in  these  animals  the  epithelium  is 
reproduced  nearly  simultaneously  over  the  rupture  surface  by  a  direct 
transformation  of  the  connective-tissue  cells  of  the  placental  decidua. . 

4.  Mucosa  Cervicis  Uteri. — The  mucosa  of  the  cervix  has  been 
only  very  imperfectly  investigated.  It  resembles  somewhat  that  of 
the  body  of  the  organ ;  but  is  distinguished  first,  by  the  possession  of 
two  kinds  of  glands,  one  agreeing  with  the  utricular  or  uterine  glands 
proper,  the  other  of  the  "mulberry"  type,  there  being  numerous 
alveolar  branches  of  the  gland  cavity ;  second,  by  the  character  of  its 
lining  epithelium,  composed  of  enormous  cylinder  cells  of  many  shapes, 
in  length  averaging  fully  55  A*  (cf.  Overlach,  85.1,  214,  219ff.). 
The  stratified  epithelium  of  the  vagina  does  not,  it  appears,  nor- 
mally extend  inside  the  os.  The  utricular  glands  are  lined  by  an 
epithelium  like  that  of  the  corpus,  while  the  epithelial  cells  of  the 
"  mulberry"  glands  resemble  those  lining  the  cervix;  the  latter  glands 
are  in  fact  strictly  cervical,  and  apparently  secrete  only  mucous 
matter ;  they  are  very  likely  important  contributors  to  the  plug  of 
mucus  which  closes  the  cervix  during  pregnancy: 

The  cervix,  except  for  this  plug,  remains  open  during  gestation ; 
it  also  preserves  its  covering  epithelium,  and  although  it  becomes 
tumefied  during  gravidity,  and  may,  as  claimed  by  Overlach,  par- 
ticipate in  the  formation  of  decidual  cells,  it  never,  as  far  as  yet  as- 
certained, forms  a  true  deciduous  membrane. 

A  thorough  investigation  of  the  histology  of  the  cervix  in  all 
phases  of  the  uterine  functions  would  be  extremely  valuable. 

4.  Blood- Vessels. — The  uterus  is  supplied  from  four  arteries: 
two,  the  ovarian,  running  along  the  broad  ligaments  and  giving 
each  a  considerable  branch  to  the  f undus ;  two,  the  uterine,  derived 
from  the  internal  iliacs,  running  to  the  cervix,  and  thence  mounting 
by  a  very  tortuous  course  to  ward  the  f  undus  to  there  join  the  ovarian 
arteries.  The  arteries  give  off  very  numerous  branches,  which  take 
a  characteristic  spiral  course  through  the  muscularis,  and  form  fre- 
quent anastomoses  with  one  another.  The  arterial  vessels  of  the 
uterus  are  further  remarkable  for  the  great  development  of  their 
muscular  walls,  all  the  more  striking  because  the  muscular  coat  of 
the  capillaries  and  veins  is  slightly  developed. 

The  capillaries  are  wider  in  calibre  than  usual,  and  form  specially 
distinct  networks  under  the  epithelium  and  around  the  glands  of  the 
mucosa.  The  veins  are  very  wide,  almost  sinus-like,  even  in  the 
resting  uterus. 

During  and  just  before  the  menstrual  flow,  and  still  more  during 
the  first  half  of  pregnancy,  the  vessels  are  all  dilated,  and  it  is 
thought  by  some  actually  increased  in  number ;  this  latter  opinion 
may  be  fairly  doubted.  The  increase  in  the  amount  of  blood  is  very 
obvious;  indeed  Rouget,  58.1,  speaks  of  the  tissue  of  the  uterus 
as  erectile,  but  this  adjective  is  not  applicable  in  the  anatomical 


THE    UTERUS.  25 

sense,  as  Kolliker  has  very  properly  pointed  out.  The  vascular 
enlargement  affects  principally  the  capillaries  and  veins  (Turner). 
It  is  most  marked  during  the  second  and  third  month  of  pregnancy ; 
in  the  fourth  or  fifth  month  the  vessels  begin  to  atrophy,  and  by 
the  eighth  month,  as  previously  stated,  the  vessels  are  far  less  nu- 
merous; these  changes  require  further  investigation.  A  number  of 
large  venous  sinuses  remain,  however,  especially  in  the  inner  portion 
of  the  muscularis,  and  are  highly  characteristic  of  the  latter  half  of 
the  period  of  gestation. 

Large  thrombi  normally  appear  in  these  sinuses,  becoming  first 
noticeable  during  the  eighth  month  and  persisting  several  weeks 
post  partum.  Apparently  they  continue  to  arise  during  the  eighth 
and  ninth  months  and  even  after  delivery  (Leopold).  The  thrombi, 
which  were  first  discovered  by  Friedlander,  70.1,  76.1,  and 
have  been  studied  also  by  Leopold,  are  supposed  by  the  latter  au- 
thor to  be  directly  caused  by  an  immigration  of  giant  cells  into 
the  veins.  Leopold  further  supposes,  77.1,  xi.,  492-500,  the 
presence  of  the  thrombi  to  cause  venous  congestion  of  the  uterus. 
Now,  if  it  is  true,  as  Brown-Sequard  has  maintained  ("  Experim. 
Researches  Applied  to  Physiol.  and  Path.,"  1853,  117,  and  Brown- 
Sequard's  Journ.  P////.S-/O/.,  i.,  1858,  105),  that  carbonic  acid  excites 
toward  the  end  of  gestation  uterine  contractions  very  readily,  then 
it  is  possible  that  the  venous  congestion  above  mentioned  may  be 
one  of  the  proximate  causes  of  parturition. 

Additional  facts  in  regard  to  the  blood-vessels  during  pregnancy 
are  given,  pp.  23,  27. 

5.  Lymphatics. — Our  knowledge  of  this  subject  rests  princi- 
pally upon  the  admirable  memoir  of  Leopold,  74.1.  The  system 
begins  in  the  intercellular  spaces  of  the  connective-tissue  layer  of 
the  mucosa ;  in  this  and  in  the  muscular  layer  are  lymph  capillaries, 
which  communicate  with  the  subserous  (subperitoneal)  network  of 
lymphatics. 

"  Special  Physiology  of  the  Uterus.— Our  anatomical  study 
has  shown  us  that  the  most  remarkable  changes  of  the  uterus  during 
its  menstrual  and  gestative  functions  are:  1,  the  gradual  thickening 
of  the  mucosa ;  2,  the  removal  of  the  superficial  portions  of  the  mu- 
cosa, in  the  one  case  during  the  menstrual  flow  and  in  the  other  dur- 
ing labor;  3,  the  appearance  of  an  enormous  number  of  the  very 
characteristic  and  peculiar  decidual  cells  during  the  thickening  of  the 
mucosa.  The  menstrual  and  gravidital  changes  follow  the  same 
cycle,  and  differ  from  one  another  essentially  only  in  two  points:  1, 
the  time  occupied,  and  2,  the  extent  of  the  changes.  In  fact  the 
alterations,  though  of  the  same  character,  are  greater  in  extent  and 
occupy  a  longer  period  during  gestation  than  during  menstruation. 
These  considerations  force  us  to  the  conclusion  that  the  gravid  uterus 
is  passing  through  the  menstrual  cycle  prolonged  and  intensified.  The 
function  of  gestation  is  a  direct  modification  of  the  function  of  men- 
struation, and  the  two  are  physiologically  homologous.  The  deduction 
is  so  evident  that  I  have  been  surprised  not  to  have  yet  encountered 
it  clearly  enunciated  in  any  of  the  authors  I  have  consulted. 

That  the  decklual  cells  perform  some  very  important  function 
seems  to  me  likewise  evident  from  their  great  prominence,  but  until 


26  INTRODUCTION. 

their  history  has  been  elucidated  even  as  to  details,  we  can  hardly 
hope  to  ascertain  what  that  function  is.  We  may  surmise  that  they 
are  either  organs  of  regeneration,  or  of  nutrition  for  the  embryo,  or 
of  both  functions. 

The  cause  of  the  formation  of  the  decidua  either  in  menstruation 
or  in  gestation  is  unknown.  The  presence  of  the  impregnated  ovum 
in  the  upper  end  of  the  Fallopian  tube  seems  to  be  the  cause  of  the 
arrest  of  the  menstrual  changes  and  the  preservation  of  the  decidua 
upon  the  uterine  wall.  How  it  produces  this  effect  is  unknown,  but 
it  is  fair  to  assume  that  it  takes  place  through  the  central  nervous 
system.  Experiment  might  demonstrate  the  nervous  pathways  fol- 
lowed by  the  irritation  and  the  reflex,  and  perhaps  discover  a  trophic 
centre  in  the  cord  for  the  uterus.  That  the  impregnated  ovum,  when 
it  exerts  this  influence,  lies  in  the  upper  end  of  the  oviduct  quite 
remote  from  the  uterus  seems  certain  from  analogy  with  mammals. 
Presumably  the  ovum  undergoes  rapid  degeneration  during  its 
passage  through  the  oviduct,  and  can  be  saved  only  by  fertilization 
at  the  start.  Lowenthal,  85.1,  who  shares  the  too  frequent  misap- 
prehensions of  gynecologists  in  regard  to  the  site  of  impregnation, 
and  thinks  in  his  philosophy  that  it  is  impossible  for  a  remote  ovum 
to  exert  such  a  marked  influence  on  the  uterus,  has  advanced  the 
hypothesis  that  the  ovum  is  fertilized  in  the  uterus  and  affects  it  by 
direct  contact.  His  critic,  Wyhoff  (Centralbl.  f.  Gyncek.,  1885, 
No.  26,  401),  thinks  impregnation  may  occur  either  at  the  ovary, 
in  the  Fallopian  tube,  or  in  the  uterus !  Such  references  to  opinions 
on  this  subject,  advanced  without  proper  knowledge,  might  be 
readily  multiplied. 

But  if  the  decidua  graviditatis  is  produced  by  the  influence  of  the 
impregnated  "ovum  on  the  menstrual  membrane,  we  have  still  to  ask, 
What  causes  the  formation  of  the  decidua  menstrualis?  To  this 
no  answer  is  possible.  Pfliiger  has  advanced  the  theory,  65. 1,  that 
the  ripening  Graafian  follicle  exerts  through  the  central  nervous  sys- 
tem a  reflex  action  upon  the  uterus ;  but,  inasmuch  as  the  attempt 
to  establish  a  fixed  relation  in  time  between  the  ripening  of  the  follicle 
and  menstruation  failed  (Leopold,  83.1),  it  is  impossible  to  accept 
Pfliiger 's  theory  at  present.  That  menstruation  is  connected  with 
ovulation  appears  probable,  but  that  ovulation  has  a  constant  casual 
relation  to  the  monthly  period  is  by  no  means  demonstrated.  The 
belief  in  the  connection  is  favored  by  the  fact  that  the  operative  extir- 
pation of  both  ovaries  usually,  but  not  invariably,  causes  menstrua- 
tion to  cease.  Putnam-Jacobi  has  advanced  a  theory  in  regard  to 
the  cause  of  menstruation  (see  Amer.  Journ.  Obst.,  Apr.,  1885), 
which  is  based  upon  singular  false  homologies  between  the  ovary 
and  uterus,  and  some  physiological  assumptions  which  are,  I  think, 
not  admissible.  Other  theories,  likewise  not  tenable  in  my  judgment, 
have  been  advanced,  but  it  seems  undesirable  to  dwell  upon  specula- 
tive views. 

The  cause  of  the  formation  of  the  reflexa  is  connected  with  the 
ovum,  since  wherever  the  ovum  is  attached  the  reflexa  is  formed 
around  it  ;  how  the  ovum  after  its  attachment  exerts  its  influence,  is 
unknown.  Since  the  position  of  the  ovum  determines  that  of  the 
reflexa  it  becomes  the  more  interesting  to  put  the  question,  What 


THE    UTERUS.  27 

determines  the  site  of  attachment  of  the  ovum?  which,  unfortunately, 
is  at  present  an  unanswerable  inquiry. 

The  cause  of  delivery  is  not  ascertained,  but  has  been  much  de- 
bated. Various  suggestions  have  been  made  to  explain  why  the 
decidua  cleaves  in  two,  and  why  the  uterus  contracts  to  expel  the 
foetus.  Our  inquiry  as  to  the  cause  of  birth  may  be  resolved  into 
two  component  questions:  1,  What  is  the  stimulus  which  causes  the 
uterus  to  expel  the  foetus;  2,  What  causes  the  stimulus  to  act  at  a 
certain  period  after  conception,  i.e.,  what  determines  the  duration  of 
pregnancy?  The  second  question  I  hope  to  discuss  elsewhere. 

As  regards  the  first  question,  What  stimulus  causes  delivery?  it  is 
well  known  that  various  operative  procedures  can  excite  apparently 
by  reflex  action  contractions  of  the  pregnant  uterus  which  will  result 
in  the  expulsion  of  the  ovum.  It  is  by  taking  advantage  of  this 
possibility  that  abortions  (premature  deliveries)  are  procured.  Such 
stimulations  as  are  referred  to  may  be  caused  in  the  following 
ways:  1,  by  rupturing  the  amnion  and  allowing  the  amniotic  fluid 
to  escape  from  the  uterus ;  2,  by  the  introduction  of  foreign  bodies 
between  the  walls  of  the  ovum  and  those  of  the  uterus ;  3,  by  me- 
chanical irritation  of  various  parts,  especially  the  cervix  uteri,  the 
external  genitalia,  or  the  breasts.  With  these  facts  in  mind  the 
hypothesis  is  unavoidable  that  the  normal  contractions  of  the  uterus 
at  full  term  are  due  to  reflex  stimulation.  Various  authors  have 
accepted  this  opinion  and  endeavored  to  ascertain  the  starting-point 
of  the  stimulation.  Mauriceau  sought  it  in  the  uterus  having 
reached  the  limit  of  its  expansibility;  Naegele  in  the  irritation 
caused  by  the  embryo,  acting  like  a  foreign  body  in  the  uterus ; 
Engelmann,  at  least  partly  in  the  degeneration  of  the  decidual  cells ; 
Harse  and  others,  in  the  accumulation  of  carbonic  acid  in  the  blood 
of  the  uterus.  None  of  these  views  are  very  well  founded ;  the  two 
last  deserve,  however,  a  little  more  consideration.  The  fatty  de- 
generation is  not  adequate,  because  in  several  instances  it  has  been 
found  wanting  both  before  and  immediately  after  birth  (Sinety, 
76.1,  Meola,  84.1).  The  carbonic-acid  theory  is  presented  in  its 
most  plausible  form  by  Leopold,  and  has  been  already  stated  (p.  43) . 
To  what  is  there  said  may  be  added  that  it  is  not  shown,  1,  that 
venous  thrombi  cause  the  venous  congestion  of  the  uterus  assumed 
by.  Leopold,  and  2,  that  such  congestion  would  charge  the  uterus 
with  sufficient  carbonic  acid  to  excite  contractions  in  it.  Compare 
also  Spiegelberg's  "Lehrbuch,"  1880,  p.  120. 

We  evidently  have  to  do  with  a  progressive  maturation  of  the 
uterus— a  series  of  changes  we  cannot  explain,  but  which  is,  as  al- 
ready pointed  out,  closely  similar  to  the  series  of  changes  during 
menstruation.  Hence  it  is  probable  that  there  is  a  common  cause 
tor  the  ending  of  the  series  (the  casting  off  of  the  superficial  part  of 
the  mucosa  in  both  cases) ;  in  the  delivery  there  is  superadded  the 
contraction  of  the  uterus,  and  for  this  we  must  see  a  cause  also. 
Therefore  it  seems  to  me  that  it  is  undesirable  to  search  for  one  cause 
only  for  the  whole  process  of  birth. 

The  physiology  of  delivery  does  not  fall  within  our  scope ;  for  fur- 
ther information  the  reader  is  referred  to  Hensen's  "Physiologie  der 
Zeugung." 


CHAPTER   II. 
GENERAL   OUTLINE    OF   HUMAN    DEVELOPMENT. 

THIS  chapter  is  designed  especially  for  the  convenience  of  stu- 
dents of  medicine  and  biology.  Advanced  students  will  find  in  it 
little  of  value  to  them,  since  all  the  subjects  it  considers  are  more 
fully  treated  in  other  portions  of  the  volume. 

I.  RETROGRESSIVE  HISTORY  OF  THE  FCETUS  AND  ITS  ENVELOPES. 

Uterus  Eight  Months  Pregnant. — If  we  examine  a  pregnant 
uterus  at  any  time  during  the  sixth  to  ninth  month  of  gestation,  we 
find  essentially  the  same  relations  of  the  parts — the  most  marked 
difference  being  in  the  size  of  the  uterus,  which  increases  with  the 
duration  of  gestation,  to  correspond  to  the  growth  of  the  foetus.  A 
description  of  a  uterus  of  the  eighth  month  after  conception  will 
suffice,  therefore,  for  our  present  purpose. 

Such  a  uterus  is  a  large,  rounded  bag,  with  muscular  walls,  and 
measures  seven  or  eight  inches  in  diameter.  It  renders  the  abdomen 
very  protuberant.  Examined  externally  it  is  remarkable  especially 
for  the  numerous  large  sinus-like  blood-vessels ;  its  surface  is  smooth ; 
the  texture  of  the  walls  is  firm  to  the  touch,  but  the  walls  yield  to 
pressure,  so  that  the  position  of  the  child  can  be  felt.  As  the 
placenta  is  generally  upon  the  dorsal  side,  it  is  usual  to  open  the 
uterus  by  a  crucial  incision  upon  the  ventral  side.  The  walls  are 
about  one-half  of  an  inch  thick,  sometimes  more,  sometimes  less, 
and  as  soon  as  they  are  cut  open  we  enter  at  once  into  the  cavity  of 
the  uterus  containing  the  fcetus  and  nearly  a  pint  of  serous  liquid— 
the  liquid  is  the  amniotic  fluid.  The  fcetus  normally  lies  on  one 
side,  has  the  head  bent  forward,  the  arms  crossed  over  the  chest,  the 
thighs  drawn  against  the  abdomen,  and  the  legs  crossed ;  it  resembles 
closely  the  child  at  birth,  but  is  smaller ;  its  head  is  relatively  to  the 
size  of  the  body  larger ;  the  abdomen  is  more  protuberant,  and  the 
limbs  proportionately  smaller.  The  inner  surface  of  the  uterus  is 
smooth  and  glistening ;  if  it  is  touched  with  the  finger  it  is  found  to 
be  covered  by  a  thin  but  rather  tough  membrane,  called  the  amnion, 
which  is  only  loosely  attached.  Examination  of  the  uterine  wall, 
where  it  has  been  cut  through,  shows  that  its  thickness  is  formed 
principally  by  the  muscular  layer,  which  is  made  up  by  numerous 
laminse  of  fibres,  between  which  are  the  large  and  crowded  blood 
sinuses,  similar  to  those  distinguishable  on  the  external  surface 
of  the  uterus.  About  one-fifth  or  less  of  the  wall  inside  the  mus- 
cularis  has  a  different  texture  and  can  be  partly  peeled  off  as  two 
distinct  membranes,  the  innermost  of  which  is  the  amnion  already 
mentioned,  and  the  outer  is  the  chorion  united  with  the  decidua. 
The  amnion  and  chorion  are  appendages  of  the  embryo;  the  de- 


OUTLINE    OF    HUMAN    DEVELOPMENT. 


29 


citlua  is  the  modified  mucous  membrane  of  the  uterus.  The  inner 
portion  of  a  microscopical  section  through  the  uterine  wall  is  shown 
in  Fig.  15.  The  amnion,  am,  consists  of  two  layers,  a  cubical- 
celled  epithelium  facing  the  embryo,  and  a  connective-tissue  stratum 
facing  the  uterus.  The  chorion,  Cho,  is  likewise  two-layered,  but 


its  epithelium,  c,  is  next  the  uterus,  its  connective  tissue  next  the 
amnion ;  the  amnion  and  chorion  are  loosely  held  together  by  shreds 
and  bands  crossing  from  one  membrane  to  the  other.  The  decidua 
occupies  the  whole  space  between  the  chorion,  CTio,  and  muscularis, 
muse;  it  contains  blood-vessels,  v,  and  remnants,  gZ,  of  gland  cavi- 
ties. Let  us  return  to  the  embryo.  From  its  abdomen  there  springs 


30  INTRODUCTION. 

a  long,  whitish  cord,  known  as  the  umbilical  cord ;  it  ix  usually  about 
one- third  to  one-half  an  inch  in  diameter  and  40  cm.  long,  but  its 
dimensions  are  extremely  variable ;  it  always  shows  a  spiral  twist, 
and  contains  three  large  blood-vessels,  two  arteries,  and  one  vein, 
all  of  which  can  be  distinguished  through  the  translucent  tissue. 
The  distal  end  of  the  cord  is  attached  to  the  wall  of  the  uterus,  usu- 
ally near  the  middle  of  the  dorsal  side  of  the  organ.  It  is  easily  seen 
that  the  blood-vessels  of  the  umbilical  cord  radiate  out  from  its  end 
over  the  surface  of  the  uterus  underneath  the  amnion,  branching  as 
they  go ;  they  spread,  however,  only  over  a  circumscribed  area,  the 
placental,  where  the  wall  of  the  uterus  is  very  much  thickened.  A 
vertical  section  through  the  placental  area  shows  that  the  amnion 
and  chorion  are  widely  separated  from  the  decidua  and  muscularis 
by  a  spongy  mass  soaked  with  maternal  blood.  This  mass  consists 
of  numerous  trees  of  tissue,  which  spring  with  comparatively  thick 
stems  from  the  chorion  and  branch  again  and  again.  In  these  stems 
and  branches  are  to  be  found  the  final  ramifications  of  the  vessels  of 
the  umbilical  cord ;  the  trees  are  known  as  chorionic  or  placental 
villi.  Some  of  their  end-twigs  are  very  closely  attached  to  the  sur- 
face of  the  decidua.  In  the  centre  of  the  placental  area  the  villi 
form  a  mass  about  three-fourths  of  an  inch  thick,  but  toward  the 
edge  of  the  area  the  mass  gradually  thins  out  until  at  the  very  edge 
the  chorion  and  decidua  come  into  immediate  contact.  The  mass  of 
villi,  together  with  the  overlying  portions  of  the  chorionic  and  am- 
niotic  membranes  and  the  underlying  portion  of  the  decidua,  consti- 
tutes what  is  known  as  the  placenta.  The  decidua  of  the  placental 
area  is  called  the  decidua  serotina;  the  chorion  of  the  placenta  is 
known  as  the  chorion  frondosum.  When  birth  takes  place  the  whole 
placenta  is  expelled  after  the  delivery  of  the  child ;  the  placenta  of 
the  obstetrician  is  therefore  partly  of  foetal,  partly  of  maternal,  origin. 
Uterus  Three  Months  Pregnant. — The  uterus  measures  about 
3*  inches  in  transverse  diameter,  and  shows  well-marked  inlaid 
sinuses  on  its  external  surface.  If  it  is  opened,  as  before,  lay  a  cru- 
cial incision  on  the  anterior  side,  its  walls  will  be  found  about  half 
an  inch  or  more  in  thickness ;  it  contains  a  grayish-red  bag  (decidua 
reflexa) ,  which  nearly  fills  the  cavity  of  the  uterus  and  incloses  the 
embryo,  so  that  upon  opening  the  womb  we  do  not  encounter  the 
foetus  directly.  The  inner  bag  has  a  smooth  surface,  but  shows  a  few 
small  pores;  it  is  without  blood-vessels  and  is  attached  to  the  dorsal 
wall  of  the  uterus.  The  inner  surface  of  the  uterus  shows  a  rich  net- 
work of  blood-vessels,  many  of  which  are  large,  irregular  sinuses. 
The  walls  are  seen  to  consist  of  an  outer  muscular  layer,  and  an 
inner  decidual  layer,  which  takes  up  nearly  half  the  thickness  of  the 
wall,  and  is  known  as  the  decidua  vera.  As  compared  with  the 
eighth-month  uterus  the  proportion  of  the  layers  shows  us  that  dur- 
ing gestation  the  muscular  layer  increases  and  the  decidual  layer 
diminishes  in  thickness.  The  inner  bag  when  opened  shows  the 
large  cavity  in  which  the  embryo  lies,  floating  in  amniotic  fluid. 
The  bag  is  formed  by  three  very  distinct  membranes,  of  which  the 
outermost,  decidua  reflexa,  is  the  thickest  and  opaque ;  the  two  inner 
ones  are  thin  and  transparent ;  the  innermost  is  the  delicate  amnion ; 
the  middle  membrane  is  the  chorion  and  is  quite  distinct  from  both 


Ol'TLINE   OF   HUMAN   DEVELOPMENT.  31 

the  amnion  and  reflexa ;  with  the  latter  it  is  connected  by  a  number 
of  small  branching  villi  scattered  at  some  distance  from  one  another 
over  the  surface ;  the  villi  adhere  firmly  to  the  reflexa  by  their  tips. 
The  embryo  resembles  a  child  in  its  general  appearance;  the  length 
of  the  head  and  rump  together  is  about  eight  centimetres,  and  the 
head  is  approximately  of  equal  bulk  to  the  rump.  The  umbilical 
cord  is  5-7  mm.  in  diameter  and  usually  about  13  centimetres  long. 
From  its  distal  end  the  blood-vessels  spread  out  over  the  placental 
area,  and  around  the  edge  of  the  area  rises  the  decidua  reflexa,  which 
does  not  extend  on  to  the  placenta.  Floating  in  the  amniotic  fluid 
is  a  pear-shaped  vesicle,  the  yolk-sack,  which  is  about  8  mm.  in 
diameter;  it  has  a  fine  network  of  blood-vessels  upon  its  surf  ace,  and 
is  connected  at  its  pointed  end  with  a  long  slender  pedicle,  the  yolk- 
stalk,  which  runs  to  the  placental  end  of  the  umbilical  cord,  there 
enters  the  cord  itself,  and  runs  through  its  entire  length  to  its  attach- 
ment to  one  of  the  coils  of  the  intestine  of  the  embryo.  Over  the 
whole  of  the  placental  area  the  chorion  gives  off  large  villous  trunks, 
each  of  which  has  numerous  branches,  with  ramifications  of  the 
foetal  vessels ;  the  villi  fill  a  space  about  one  centimetre  wide  between 
the  membrane  of  the  chorion  frondosum  and  the  surface  of  the  uter- 
ine decidua  serotina,  to  which  the  tips  of  some  of  the  villi  are  at- 
tached. With  care  the  villi  may  be  separated  from  the  decidua, 
which  is  seen,  when  it  is  thus  uncovered,  to  be  cavernous ;  the  cav- 
erns are  rounded  in  form  and  may  be  followed  on  the  one  hand  until 
they  connect  with  the  blood  sinuses  of  the  uterus,  and  on  the  other 
until  they  open  into  the  intervillous  spaces,  which  therefore  receive 
a  direct  supply  of  blood  from  the  mother. 

The  principal  difference  to  be  noted  in  the  relations  of  parts  be- 
tween  the  uterus  before  and  that  after  the  fifth  month  is  in  the  pres- 
ence or  absence  of  the  decidua  reflexa  as  a  distinct  membrane.  Dur- 
ing the  fourth  month  the  reflexa  stretches  as  the  membranes  expand 
and  becomes  thinner  and  thinner  until  by  the  end  of  the  fourth 
month  it  is  as  delicate  and  transparent  as  the  chorion  and  lies  close 
against  the  wall  of  the  uterus  (decidua  vera) .  It  is  probable  that 
the  decidua  reflexa  degenerates  and  is  resorbed,  compare  p.  19. 

Uterus  Five  Weeks  Pregnant.— The  relations  are  best  shown 
by  a  median  antero-posterior  section,  Fig.  4.  The  arrangement  of 
the  uterine  parts  is  essentially  the  same  as  at  three  months.  The 
mucosa  uteri  is  changed  into  the  decidua  graviditatis.  On  the  dor- 
sal side  from  s  to  s  is  the  decidua  serotina  of  the  placental  area, 
where  the  villi  of  the  chorion  are  fastened  by  their  tips  to  the  uterus. 
From  the  edge  of  the  placental  area  on  all  sides  rises  the  decidua 
reflexa,  r  r,  which  is  much  thinner  than  the  other  parts  of  the  de- 
cidua, and  which  forms  a  closed  dome  over  the  embryo;  hence  when 
we  pass  through  the  cervix  uteri,  c,  we  enter,  not  the  cavity  contain- 
ing the  ovum,  but  the  fissure-like  space  between  reflexa,  r  r,  and 
the  vera,  c/  y,  which  includes  the  whole  of  the  modified  mucosa  of 
the  body  of  the  uterus,  except  that  part  to  which  the  ovum  is  at- 
tached and  which  produces  the  reflexa  and  serotina.  The  vera  is 
that  portion  of  the  decidua  which  is  not  in  direct  contact  with  the 
ovum.  No  stage  of  gestation  earlier  than  the  completed  formation 
of  the  reflexa  has  been  observed. 


INTRODUCTION. 


The  embryo  differs  greatly  from  the  three  months'  foetus.  Be- 
ginning with  the  envelopes  we  notice  that  the  chorion  is  beset  with 
well- developed  villi  over  its  entire  surface,  but  the  villi  over  the  pla- 
cental  area  are  larger  than  those  over  the  parts  adjacent  to  the  de- 
cidua  reflexa.  The  amnion  does  not  lie  close  to  the  chorion,  but  close 
around  the  embryo,  leaving  a  wide  space  between  the  two  mem- 
branes, which  space,  as  we  have  seen,  is  subsequently  obliterated  by 
the  expansion  of  the  amnion.  The  embryo  itself  is  very  small  and 
not  human  in  appearance,  and  its  organs  are  only  partially  differen- 
tiated. The  umbilical  cord  is  very  short ;  the  amnion  springs  from 
it  at  a  short  distance  from  the  embryo.  The  yolk-stalk  leaves  the 
cord  just  beyond  the  amnion,  is  comparatively  short,  and  ends  in  the 
pear-shaped  yolk-sack,  which  is  about  the  same  size  as  at  three 
months.  Beyond  the  point  where  the  amnion  and  yolk-stalk  part 
from  it,  the  umbilical  cord  continues  a  short  distance  with  its  blood- 
vessels, which  ramify  over  the  entire  chorion  and  penetrate  all  the 
villi  thereof.  To  produce  the  relations  found  at  three  months  the 
blood-vessels  and  villi  of  the  chorion  must  abort  except  over  the  pla- 
cental  area ;  the  umbilical  cord  must  elongate  greatly ;  the  amnion 

must  expand  until  it  touches  the 
chorion,  and  the  foetus  must  grow 
and  change. 

We  must  now  trace  back  the  his- 
tory of  the  embryo  still  farther,  in 
order  to  understand  the  relation  of 
the  embryo  to  the  embryonic  mem- 
branes and  appendages. 

Ovum  of  Three  Weeks.*— Nor- 
mal  human  ova  of  this  age  very 
rarely  indeed  reach  the  embryolo- 
gists,  but  a  few  have  been  described. 
The  chorion  forms  a  closed  vesicle 
beset  on  all  sides  with  crowded, 
clumsily-branching  villi ;  the  vesicle 
measures  about  3  cm.  in  diameter; 
the  villi  are  about  3  mm.  long,  and 
as  yet  show  no  regional  inequality  in 
their  development.  If  the  vesicle  is 
opened  the  embryo  is  found  within 
rolled  up,  the  back  being  convex ;  it 
measures  in  its  natural  attitude  about 
44  mm.  The  head  is  bent  toward 
the  right;  the  caudal  extremity  to- 
ward the  left ;  the  head  and  tail  are 
almost  in  contact,  so  that  it  is  diffi- 
cult to  observe  the  insertion  of  the 
umbilical  cord.  With  care  this  may 
be  done,  and  it  will  then  be  seen  that 
the  amnion  arises  from  the  embryo, 

and  is,  in  fact,  a  prolongation  of  the  body-wall ;  the  amnion  itself  is 
extremely  thin   and   lies   close  about  the    embryo.     The   umbilical 

*  For  figures  see  Chapter  XIII. 


FIG.  16. — Human  embryo,  4.2  mm.  long 
(His'  Lr).  After  W.  His.  Explanation 
in  text. 


OUTLINE    OF    HUMAN   DEVELOPMENT.  33 

cord*  unites  with  the  abdomen;  in  front  of  it,  i.e.,  headward,  is  a 
small  opening  through  which  the  stalk  of  the  yolk-sack  enters  the 
body  to  unite  with  the  intestine ;  it  is  from  the  edges  of  this  opening 
that  the  amnion  arises,  and  as  the  amnion  passes  around  the  umbili- 
cal cord,  it  may  be  said  that  the  cord  and  the  yolk-stalk  both  enter 
the  body  through  the  opening,  but  whereas  the  cord  is  in  contact 
with  the  amnion  the  yolk-stalk  is  not.  The  opening  may  be  called 
the  umbilical  foramen.  The  yolk-sack  is  pear-shaped,  measures 
about  3  mm.  in  diameter,  and  is  attached  by  its  pointed  end  to  a 
loop  of  the  embryonic  intestine.  The  yolk-stalk  is  developed  by  the 
subsequent  prolongation  of  the  pointed  end  of  the  sack. 

In  an  embryo  a  little  younger  the  relations  can  be  more  clearly 
recognized,  Fig.  1C.  The  embryo  is  nearly  straight,  although  both 
head  and  tail  are  bent  over  ventrally.  The  umbilical  foramen, 
from  the  edge  of  which  the  amnion 
arises,  is  very  wide  and  long ;  at  its 
tailward  edge  runs  out  the  umbilical 
cord  (Bauenstiel),  to  which  the  am- 
nion is  attached,  and  which  joins 
the  chorion  a  short  distance  from 
the  embryo.  The  neck  of  the  yolk- 
sack,  Yks,  is  also  much  wider;  if 
the  sack  is  cut  open  we  find  its  neck  to 
be  a  large  opening  into  the  cavity  of 
the  intestine ;  in  fact,  the  yolk-sack  is  / 
an  appendage  of  the  intestinal  canal,  I 
which  at  this  stage  is  very  simple,  | 
being  hardly  more  than  a  straight  * 
tube  running  lengthwise;  the  open- 
ing between  the  sack  and  intestine 
may  be  called  the  vitelline  foramen. 

The  younger  the  embryo  the  FIG.  IT.— Embryo.  2.15  mm.  long.  After 
longer  are-relatively  to  the  size  of 

the  embryo — the  foramen  umbilicale  and  the  foramen  vitellinum,  as 
is  well  shown  in  Fig.  17.  The  line  of  attachment  of  the  amnion  ex- 
tends almost  the  entire  length  of  the  embryo,  beginning  just  in 
front  of  the  heart,  and  ending  upon  the  umbilical  cord  (Bauch- 
stiel  or  allantois-stalk),  close  to  the  chorion.  The  yolk-sack  has 
also  a  long  attachment,  beginning  just  behind  the  heart,  and  extend- 
ing nearly  to  the  allantois  stalk,  which  now  appears  to  the  eye  very 
much  what  it  is  morphologically,  a  prolongation  of  the  posterior  ex- 
tremity of  the  body  of  the  embryo. 

Going  back  still  farther,  we  find  the  relations  to  be  as  represented 
by  the  accompanying  diagram,  Fig.  18.  The  embryo,  Emb,  rests 
upon  the  yolk-sack,  and  is  scarcely  longer  than  the  umbilical  fora- 
men ;  the  end  of  the  embryo  is  prolonged  posteriorly  as  the  relatively 
large  allantois-stalk,  Al,  by  which  the  embryo  is  attached  to  the 
chorion.  The  amnion  springs  from  the  sides  of  the  embryo  and  of 
its  allantois  stalk,  and  forms  a  closed  sack  over  the  embryo.  This 
stage  is  almost  the  youngest  in  the  series  of  known  human  embryos, 
and  has  been  only  imperfectly  described. 

*  At  this  stage  more  properly  to  be  called  the  Bauchstiel^  see  Chapter  XVI. 
3 


34 


INTRODUCTION. 


The  following  generalized  diagram,  Fig.  19,  of  a  young  amniote 
vertebrate  embryo  is  intended  to  render  clear  the  essential  relations 
of  the  embryo  and  its  appendages.  The  figure  represents  a  trans- 
verse section  of  the  embryo,  together  with  all  the  membranes.  The 
embryo  consists  of  an  axial  mass,  from  which  runs  out  on  each  side 
a  lamina  or  plate  of  tissue,  Som,  to  form  the  body- wall ;  this  plate 
extends  beyond  the  embryo  to  form  the  amnion.  Am;  as  the  plate 
from  one  side  joins  that  from  the  other,  the  amnion  makes  a  closed 
sack  over  the  back  of  the  embryo.  From  the  axial  mass  there  run 
out  two  other  plates,  Spl,  to  form  the  walls  of  the  intestinal  canal, 
In;  these  plates  are  likewise 
prolonged  beyond  the  body  to 
form  the  large  yolk-sack,  Yolk, 
upon  the  top  of  which  the  em- 


Spl. 


Cho. 


bom 


FIG.  18. — Diagram  of  an  embryo  of  fifteen  to 
sixteen  days. 


FIG.  19.— Generalized  diagram  of  an  amniote 
vertebrate  embryo. 


bryo  rests.  The  space  between  the  walls  of  the  intestine  and  the  body- 
walls  is  of  course  the  body-cavity,  Coe.  Where  the  body-wall, 
Som,  passes  over  into  the  amnion,  Am,  there  is  an  opening  by  which 
the  body-cavity  communicates  directly  with  the  space  between  the 
amnion  and  yolk-sack  on  one  side  and  the  chorion  on  the  other ;  this 
opening  is  the  umbilical  foramen.  Similarly  there  is  a  passage  by 
which  the  cavity  of  the  intestine,  In,  communicates  with  that  of  the 
yolk-sack,  Yolk;  this  passage  is  the  vitelline  foramen. 

For  our  conceptions  of  the  probable  history  of  the  human  ovum  up 
to  the  fourteenth  day,  we  must  rely  mainly  on  analogy,  drawn  from 
our  knowledge  of  the  development  of  other  mammals  and  of  birds 
and  reptiles.  From  these  sources  we  learn  that  the  amnion  and 
chorion  are  originally  portions  of  the  same  membrane,  which  is  an 
extension  of  the  body- wall  of  the  embryo.  In  reality  the  differen- 
tiation of  the  amnion  is  quite  a  complex  process,  as  is  shown  by  the 
detailed  history  given  in  Chapter  XV.  The  essential  steps  can  be 
made  clear,  however,  by  a  brief  description.  Fig.  20  is  a  diagram 
of  a  stage  in  the  development  of  amniota  a  little  earlier  than  that 
shown  in  Fig.  19.  Both  the  vitelline  and  umbilical  foramens  are 
much  wider  than  in  the  preceding  figure.  The  body-wall  of  the 
embryo,  Som,  passes  over  as  before  into  the  amnion,  Am,  but  the 
amnion  of  one  side  does  not  join  that  of  the  other,  but  instead  bends 


OUTLINE   OF   HUMAN   DEVELOPMENT. 


35 


SP1 


Cho 


over  and  is  continuous  with  the  chorion,  Cho.  Thus  the  amnion 
and  chorion  conjointly  form  a  fold  on  each  side  of  the  embryo ;  if 
the  two  folds  enlarge  and  arch  over  the  embryo  until  they  meet  and 
unite  by  their  edges  the  condition  illustrated  by  the  preceding  dia- 
gram, Fig.  19,  will  be  established. 
Returning  to  the  earlier  condition, 
Fig.  20,  we  may  say  that  the  ovum 
consists  of  two  closed  vesicles 
united  together  by  the  axial  mass 
of  the  embryo.  The  membrane, 
which  forms  the  outer  vesicle,  is 
subdivided  into  three  principal  re- 
gions, to  wit :  the  body-wall  of  the 
embryo,  the  amnion,  the  chorion, 
each  having  its  separate  history. 
The  membrane  which  forms  the 
inner  vesicle  is  subdivided  into 
two  principal  regions,  to  wit :  the 
wall  of  the  intestine  and  the  wall 
of  the  yolk-sack,  each  having  its 
separate  history.  It  will  be  re- 
membered that  the  posterior  end 
of  the  embryo  is  prolonged  as 
the  allantois-stalk,  by  means  of 
which  it  remains  permanently  and  directly  united  with  the  chorion. 

It  is  unnecessary,  for  our  present  purpose,  to  follow  back  the  earlier 
histor^tep  by  step.  Suffice  it  to  say  that  in  younger  stages  the 
two  vesicles  are  represented  only  by  one,  and  earlier  yet  there  is 
merely  a  cluster  of  cells. 

The  stages  of  development  preceding  this  are  not  to  be  found  in 
the  uterus,  but  in  the  Fallopian  tubes.  They  exhibit  to  us  merely  an 
agglomeration  of  a  few  cells,  the  so-called  segmented  ovum.  The 
earlier  the  stage  the  fewer  the  cells,  until  we  reach  the  condition 
when  there  are  but  few  cells,  then  two,  and  finally  one  only.  This 
cell  is  the  impregnated  ovum,  the  beginning  of  all  development, 
but  is  itself  formed  of  two  separate  parts,  very  different  in  their 
origin  and  constitution,  namely,  the  egg-cell  or  ovum  and  the  sper- 
matozoon, whose  union  is  the  act  of  impregnation — the  beginning 
of  a  new  existence. 


FIG.  20.— Generalized  diagram  of  an  amniote 
vertebrate  embryo  before  the  separation  of  the 
amnion,  Am,  and  chorion,  Cho. 


II.  PROGRESSIVE  HISTORY  OF  THE  FCETUS  AND  ITS  ENVELOPES. 

The  ovum  enters  the  upper  end  of  the  Fallopian  tube,  and  ig  there 
impregnated.*  Very  slowly  it  moves  down  the  Fallopian  tube, 
undergoing  meanwhile  the  process  of  so-called  segmentation,  by 
which  it  is  separated  into  a  gradually  increasing  number  of  cells, 
that  arrange  themselves  so  as  to  begin  the  formation  of  the  embryo 
and  its  appendages.  Probably  about  the  eighth  day  the  ovum 
reaches  the  uterus,  where  it  becomes  adherent  to  the  mucosa  upon 


*  It  is  possible  that  impregnation  may  occur  while  the  ovum  is  passing  from  the  ovary  to  the 
fimbriate  opening  of  the  Fallopian  tube. 


36  INTRODUCTION. 

the  dorsal  side  of  the  uterus  usually,  and  by  an  unknown  process  of 
agglutination.  The  decidua  reflexa  grows  up  around  it  by  a  pro- 
cess not  yet  observed.  The  amnion  is  differentiated  from,  the  cho- 
rion.  The  portion  of  the  mucosa  uteri  in  contact  with  the  ovum  is 
transformed  into  the  decidua  serotina ;  the  remaining  portion  of  the 
mucosa  becomes  the  decidua  vera.  The  allantois-stalk  unites  the 
embryo  with  the  chorion,  and  carries  the  blood-vessels  of  the  foetus 
^o  ramify  upon  the  chorion.  The  embryo  is  enclosed  by  the  amnion ; 
the  amnion  is  enclosed  by  the  villous  chorion ;  the  chorion  is  enclosed 
by  the  decidua  reflexa  and  serotina.  The  vesicle  formed  by  the  close 
adherence  of  the  chorion  to  the  reflexa  is  suspended  from  the  wall 
of  the  uterus.  The  mass  of  tissue  resulting  from  the  union  of  the 
chorion  with  the  serotina  forms  the  placenta.  The  umbilical  cord 
(allantois-stalk)  is  always  attached  to  the  placental  area,  and  later 
the  ramifications  of  the  umbilical  vessels  are  restricted  to  that 
area.  During  the  fifth  month  the  decidua  reflexa  coalesces  with  the 
decidua  vera,  and  the  space  between  them  is  of  course  obliterated. 
Finally,  we  find  that  the  amnion  enlarges,  lays  itself  against  the 
chorion,  and,  uniting  loosely  with  it,  becomes  the  innermost  constit- 
uent of  the  vesicle  enclosing  the  embryo. 


PART  I. 

THE  GENITAL  PRODUCTS 


CHAPTER   III. 
THE    HISTORY    OF    THE    GENOBLASTS   AND   THE    THEORY    OF   SEX. 

THE  term  genoblast  is  used  to  designate  the  sexual  elements.  I 
apply  it  exclusively  to  sexual  elements  proper,  and  not  to  the  acces- 
sory parts  with  which  those  elements  are  associated.  The  spermato- 
zoon is  a  genoblast;  a  spermatophore  is  not.  The  egg-cell  after 
maturation  is  a  genoblast,  but  not  before. 

I.  SPERMATOZOA. 

1.  Summary. — The  spermatozoa  of  mammals  are  filaments  con- 
sisting of  a  short,  thick  end  called  the  head,  and  a  very  long  and 
delicate  thread  called  the  tail.     The  head  varies  greatly  in  shape, 
according  to  the  species ;  in  man  it  is  broad  and  thin,  Fig.  22,  and 
is  widest  at  a  little  distance  from  the  tail.     The  head  contains  chro- 
matin,  and  may  be  colored  by  the  usual  nuclear  dyes.     The  tail 
consists  of  three  parts:  1,  the  middle-piece,  which  is  next  the  head, 
and  the  thickest  of  the  three  parts ;  it  contains  an  axial  thread,  and 
probably  always  has  a  very  fine  spiral  thread  running  round  it;  2, 
the  main-piece;  and,  3,  the  end-piece,  which  is  not  more  than  a 
line,  even  as  seen  with  very  high  magnifying  powers.     The  human 
spermatozoon  is  0.055  mm.  long — the  head  being 0.005  mm.,  the  tail 
0.050,  and  the  middle-piece  0.009. 

The  development  of  the  mammalian  spermatozoa  begins  with  a  so- 
called  parent  or  mother-cell,  which  lies  near  the  outer  wall  of  the 
seminiferous  tubule.  The  mother-cell  produces  a  number  of  daugh- 
ter-cells, which  also  multiply  by  division ;  the  daughter- cells  break 
down,  forming  a  column  of  matter  (protoplasm),  in  which  lie  their 
nuclei,  and  at  the  base  of  which  lies  the  nucleus  of  the  mother-cell; 
the  nucleus  of  the  mother-cell  and  the  column  of  matter  both  ulti- 
mately disappear,  but  exactly  how  is  not  determined ;  the  nuclei  of 
the  daughter-cells  produce  each  a  spermatozoon.  The  head  and  tail 
of  the  future  spermatozoon  become  visible  within  the  nuclear  mem- 
brane ;  the  head  is  formed  chiefly  by  the  chromatin  of  the  nucleus ; 
the  nuclear  membrane  finally  ruptures,  and  it  as  well  as  the  contents 
of  the  nucleus  which  have  not  taken  part  in  the  formation  of  the 
spermatozoon  are  lost.  Among  the  lost  parts  is  a  special  round  body 
of  small  size,  which  appears  in  the  nucleus  while  the  spermatozoon 
is  developing ;  this  body  may  be  stained  by  chloride  of  gold,  but  not 
by  haematoxylin ;  its  significance  is  unknown.  The  long  column 
holding  the  spermatozoa  together  has  usually  been  regarded  as  a  cell, 
and  is  the  supporting  cell  auct.,  or  Sertoli's  column. 

2.  Spermatozoa  are  the  essential  fertilizing  elements  secreted 
by  the  male  gland.     They  are  minute  bodies,  capable  of  active  loco- 


40  THE    GENITAL    PRODUCTS. 

motion,  and  having  a  characteristic  form  in  each  species.  In  a  few 
instances  (certain  snails,  etc.)  there  are  two  distinct  forms  of  sper- 
matozoon for  a  single  species,  but  usually  there  is  only  one  form, 
and  that  little  variable.  In  a  small  number  of  animals  the  sper- 
matozoa, as  in  the  nematods,  are  distinctly  cell-like ;  but  in  the  great 
majority  of  animals,  and,  so  far  as  I  know,  in  all  vertebrates,  they 
are  long  and  thread-like ;  hence  their  common  German  name,  Samen- 
faden,  first  proposed,  I  think,  by  Kolliker. 

The  mammalian  spermatozoa  are  long,  slender  bodies,  varying  con- 
siderably in  configuration,  but  all  presenting  at  least  the  following 
features  in  common :  One  end  is  thickened  and  is  called  the  head ; 
it  has  a  strong  affinity  for  nuclear  staining  fluids ;  this  affinity  must 
be  attributed  to  the  chromatin,  which  the  head  contains,  as  is  shown 
by  the  history  of  its  development ;  the  remainder  of  the  spermatozoon 
is  long  and  slender,  and  constitutes  the  tail ;  the  tail  consists  of— 
1,  a  middle  part  (Mittelstiick) ,  a  little  thicker  than  the  rest,  and  sit- 
uated next  to  the  head ;  the  middle  part  is  traversed  by  a  very  fine 
axial  thread,  and  ends  abruptly ;  and,  2,  a  hind-piece,  which,  accord- 
ing to  some  writers,  may  be  subdivided  naturally  into  two  segments, 
the  main-piece  (Hauptstiick)  and  end-piece. 

The  spermatozoa  of  the  various  species  differ  in  size  in  the  pro- 
portions of  the  parts,  and  often  very  strikingly  in  the  shape  and 
structure  of  the  head ;  those  of  the  opossum  are  especially  remark- 
able for  being  double ;  two  apparently  complete  spermatozoa  being 
united  to  a  common  plate  by  their  heads  (Selenka :  "  Studien  iiber 
Entwickelungsgeschichte,"  Heft  IV.,  p.  100).  Twin  spermatozoa 
have  also  been  observed  in  the  rat  by  Neumann,  75.1,  313,  Taf. 
XVII.,  Fig.  16,  b.  Compare  also  Max  von  Brunn,  84. 1,  and  Brock, 
87.5. 

The  largest  known  mammalian  spermatozoon  is  perhaps  that  of 
the  marsupial,  Phascogale;  the  spermatozoon  of  this  animal  is  0.263 
mm.  long — the  head,  however,  being  only  0.013  mm.  (Fiirst,  87.1, 
354).  The  spermatozoon  of  the  rat  is  0.144  mm.  long,  the  head 
0.009,  the  tail  0.135,  and  the  middle-piece  0.045  mm. 

La  Vallette,  71.1,  gives  a  synopsis  concerning  the  forms  of  verte- 
brate spermatozoa  nearly  as  follows :  Fish  :  The  spermatozoa  of  Am- 
phioxus  are  threads  with  round  heads.  In  Petromyzon  the  head  is 
rod-like  or  egg-shaped.  The  teleosts  generally  have  pin-like  sper- 
matozoa; but  in  the  salmonida9  (Owsjannikow)  the  head  is  pointed 
and  shaped  like  a  heart-tip.  The  spermatozoa  of  selachians  are 
much  larger,  with  the  head-end  spindle-shaped  and  often  spirally 
twisted.  Amphibia  :  The  head  is  long,  generally  pointed,  the  mid- 
dle-piece short,  and  the  tail  is  often  provided  with  an  undulatory 
membrane  (Retzius,  81.1).  Reptiles  and  birds:  The  head  is  usu- 
ally long,  often  twisted.  Mammals :  The  head  is  more  or  less  elon- 
gated ;  in  ungulates  the  head  is  flattened  and  usually  more  or  less 
egg-shaped  in  outline,  the  pointed  end  toward  the  tail.  Among 
rodents  there  is  considerable  variety  of  form.  In  the  dog  the  head 
is  pear-shaped ;  in  the  hedgehog  the  head  is  truncated  inferiorly,  and 
the  tail  is  inserted  laterally.  No  comprehensive  summary  of  the 
observed  forms  of  spermatozoa  has  been  made  since  the  publication 
of  Wagner  and  Leuckart's  article  in  "Todd's  Cyclopaedia." 


SPERMATOZOA. 


41 


B 


The  most  minutely  studied  mammalian  spermatozoon  is  that  of 
the  rat,  thanks  especial!}'  to  the  patience  of  O.  S.  Jensen,  whose 
posthumous  paper,  87.1,  furnishes  the  basis  of  the  ensuing  de- 
scription.  The  rat's  spermatozoon  measures  144 
/*;  its  head,  Fig.  21,  C,  is  a  broad  hook,  pointed 
at  one  end  and  obliquely  truncated  at  the  other ; 
from  one  corner  of  the  truncated  end  starts  the 
very  long  slender  tail,  which  is  divisible  into  the 
thicker  middle-piece  (Mittelstilck,  or  Jensen's 
Verbindungssiiick)  and  the  thinner  main-piece 
(Hauptstiick),  Fig.  21,  A,  which  terminates  in  a 
short  and  still  finer  thread  called  the  end-piece 
(End*  f  tick) .  The  appearance  of  the  spermatozoon 
varies  according  to  its  degree  of  development,  it 
not  attaining  full  maturity  until  it  has  left  the 
seminiferous  tubule.  The  changes  referred  to 
affect  principally  the  head  and  the  middle- 
piece.  The  head  is  covered,  while  the  spermato- 
zoon remains  in  the  seminiferous  tubules,  by  a 
membranous  cap,  Fig.  21,  A,  which  subsequently 
disappears.  The  middle-piece  has  a  spiral  thread 
running  round  its  outside,  Fig.  21,  B.  The  spiral 
thread  appears  soon  after  the  rupture  of  the  nu- 
clear membrane,  by  which  the  developing  sper- 
matozoon is  set  free  (c/.  infra).  The  thread  is 
at  first  indistinct  and  makes  only  a  few  turns ;  it 
rapidly  becomes  more  distinct  and  the  number  of 
turns  increases,  until  they  become  so  numerous 
that  in  a  spermatozoon  taken  from  the  vas  defer- 
ens  only  a  series  of  thick-set  cross-lines  can  be 
distinguished ;  these  lines  have  been  seen  by  sev- 
eral observers  and  variously  interpreted ;  the  spiral 
may  run  to  the  right  or  to  the  left.  The  thread 
becomes  loosened  off  by  the  action  of  glycerin  (1 
part)  and  water  (4  parts),  and  is  destroyed  in  one 
to  two  hours  by  0.(>  per  cent  salt  solution,  leaving 
then  the  axis  uncovered.  The  thread  can  be 
stained  by  chloride  of  gold,  though  the  axis  can- 
not. The  axis,  when  the  spermatozoa  are  treated 
with  acetic  acid,  often  breaks  up  into  threads  (cf. 
Ballowitz,  86.1);  it  shows  a  lighter  line  in  its 
centre.  These  observations  lead  Jensen  to  the 
conclusion  that  the  axis  is  formed  by  a  wall  of 
fibrillaB  surrounding  a  central  core  or  cavity. 
The  axis  does  not  reach  quite  to  the  head,  but  ends  with  a  little 
knob,  leaving  a  small,  perfectly  transparent  space  between  the  knob 
and  the  head,  Fig.  21,  C.  In  some  spermatozoa — e.  g.,  of  horse  and 
ox — though  npt  in  those  of  the  rat,  there  is  a  minute  opening  in  the 
head  called  the  micropoms,  and  situated  just  opposite  the  knob  of 
the  axis.  When  the  spermatozoa  are  stained  with  nuclear  dyes, 
most  of  the  head  is  colored,  but  the  tip  of  the  hook,  which  contains 
no  chromatin,  and  is  probably  formed  out  of  a  scrap  of  the  nuclear 


FIG.  21. —Structure 
of  a  rat's  spermato 
zoon.  B.  young  sper- 
matozoon, end  of  the 
middle-piece  and  be- 
ginning of  the  main- 
piece  to  show  the 
spiral  thread — greatly 
magnified  ;  A,  head, 
with  part  of  the  axial 
thread ;  C,  immature 
spermatozoon,  ante- 
rior half  only.  After 
O.  S.  Jensen. 


42 


THE    GENITAL    PRODUCTS. 


membrane,  remains  uncolored :  011  the  concave  side  of  the  tip  a  fine 
line  can  be  distinguished,  due,  apparently,  to  a  rod  of  substance. 
Sometimes  a  minute  fragment  of  the  nuclear  membrane  is  left  ad- 
herent to  the  lower  end  of  the  middle-piece;  for  the  explanation 
of  this  possibility,  compare  the  section  below  on  de- 
velopment. 

The  human  spermatozoa  are  described  by  Retzius, 
81.1,  85,  as  follows:  The  head,  seen  from  the  flat 
side,  appears  oval,  Fig.  22,  A,  with  the  front  end 
generally  tapering  a  little,  but  never  pointed;  the 
anterior  half  or  two-thirds  has  a  brighter  and  more 
transparent  part.  Seen  on  edge,  Fig.  22,  B,  the 
head  has  a  pointed  form,  with  a  posterior  thicker, 
round,  dark  part.  By  adjustment  of  the  focus  it  can 
be  ascertained  that  the  sides  near  the  point  are  de- 
pressed somewhat  like  those  of  red  blood  corpuscles. 
Retzius  could  nowise  succeed  in  demonstrating  a 
special  tip  (Spiess)  corresponding  to  that  in  the  sala- 
mander, but  Edw.  M.  Nelson  (Journ.  Quekett  Club, 
1889,  III.,  310)  has  observed  a  slender  thread  pro- 
longed from  the  head,  and  also  a  hook  at  the  end  of 
the  thread ;  these  observations  have  been  confirmed 
by  Bardeleben,  91.1.  The  latter  also  describes  ad- 
ditional details  of  the  structure  of  the  head.  The 
following  piece  (Schweigger-Seidel's  Mittelstuck)  is 
directly  united  with  the  head  by  a  -transverse  joint ; 
there  is  no  neck  in  Eimer's  sense;  the  middle-piece 
is  cylindrical  and  relatively  small — about  as  long,  or 
a  little  longer,  than  the  head;  its  surface  is  often 
granular  or  rough,  and  there  cling  to  it  a  few  shreds 
of  protoplasm,  as  has  been  described  by  several  in- 
vestigators ;  the  spiral  thread  was  long  overlooked, 
but  has  been  recognized  and  figured  by  K.  Bardele- 
ben, 91.1.  The  undulatory  membrane,  supposed  by  Gibbes,  79.1, 
and  W.  Krause,  81.4,  to  be  present,  was  perhaps  an  abnormally 
loosened  spiral  thread.  The  main-piece  of  the  tail  is  about  half  as 
thick  as  the  "  Mittelstuck, "  gradually  tapers,  and  ends  abruptly  at 
the  beginning  of  the  still  finer  and  very  short  end-piece. 

3.  Spermatogenesis. — The  seminiferous  tubules  are  cylindrical, 
i.  e. ,  in  cross-sections  they  appear  round ;  a  large  part  of  the  tubule  is 
filled  with  spermatozoa  in  various  stages  of  development.  The  outer 
boundary  is  marked  by  a  distinct  line  corresponding  to  the  tunica 
propria,  a  layer  of  endothelial  cells,  with  flat  oval  nuclei  (Neumann, 
75.1,  306) .  Next  to  the  tunica  comes  a  layer  which,  as  far  as  known, 
presents  pretty  much  the  same  appearances,  whatever  may  be  the 
stage  of  development  of  the  spermatozoa  within.  This  layer  contains 
two  kinds  of  cells:  First,  the  large  Sertoli's  columns,  as  they  may 
be  called,  after  their  discoverer.*  These  cells  are  identical  with 
Merkel's  Stutzzellen,  La  Vallette's  spermatogonien,  Swaen  and  Mas- 
quelin's  cellules  folliculaires.  Second,  small  granular  cells,  vary- 


FIG.  22.—  Human 
spermatozoa.  A, 
complete  sperma- 
tozoon ;  B,  head 
seen  from  the  side ; 
C,  extremity  of  the 
tail.  All  highly 
magnified.  After 
Retzius. 


*  First  described  by  Sertoli  in  ii.  Morgagni  (cf.  Henle's  Jahresberichte  for  1864,  p.  120). 
pare  Sertoli,  Arch.  Sci.  mediche,  ii.,  1W  OH77).' 


Com- 


SPERMATOZOA. 


ing  in  appearance  according  to  the  exact  stage  of  their  develop- 
ment. Examined  in  surface  views,  Fig.  23  (compare  also  Figs. 
:>,  »;;  and  41  of  Fiirst's  paper,  87.1),  the  large  cells  are  seen  to 
be  mostly  hexagonal  in  outline,  to  touch  one  another,  and  to  pass 
below,  f."e.  outside,  the  small  cells;  they  have  large,  clear,  oval 
nuclei  with  sharp  outlines,  and  usually  a  single  well-marked  nu- 
cleolus.  The  nuclei  lie  quite  near  the  tunica  propria,  but  in  man  lie 
farther  inward,  and  are  in  this  case  not  so  near  the  tunica  as  are  the 
small  cells.  Around  the  nucleus  there  lie  a  few  highly  refractile 
granules  which  may  be  stained  by  arsenic  acid,  and  are  probably  fat. 
The  small  cells  'lie  in  depressions  or  cups  of  the  large  cells,  Fig.  23, 
}>,  mid  when  the  small  cells  are  knocked  out — as  sometimes  happens 
in  teasing — the  partitions  between  the  cups  appear  more  distinctly 
and  create  a  network  figure,  which  formerly  misled  Von  Ebner  and 
others  into  describing  a  real  network  as  constituting  the  layer.  The 
large  cells  also  have  long  columnar  prolongations,  as  can  be  best  seen 
in  transverse  sections  of  the  tubules,  Fig.  29 ;  the  prolongations  are 
united  with  bundles  of  developing  spermatoblasts.  The  small  cells 
are  very  different ;  they  lie  over  the  outlines  of  the  large  cells  and 
between  their  centripetal  prolongations,  Fig.  29;  they  are  gran- 
ular, have  comparatively  little  protoplasm,  and  their  nuclei  are 
nearly  spherical  in  shape.  The  nuclei  vary  considerably  in  appear- 
ance, as  these  cells  multiply  by  indirect  division ;  usually  they  con- 
tain a  chromatin  network  or  a  coiled  chromatin  cord ;  sometimes 
the  network  is  concentrated  at  one  side  of  the  nucleus,  leaving  the 
other  side  comparatively  clear.  At  certain  periods  the  nuclei  are 
found  in  various  stages  of  karyokinesis.  The  cells  resulting  from 
the  division  of  the  small  cells  form  the  packing  between  the  inward 
columns  of  the  large  cells,  hence  in  cross-sections  we  get  alternating 
columns,  Fig.  29. 
The  descendants  of 
the  small  cells  pro- 
duce the  spermuto- 
hlaxts,  and  the  sper- 
matoblasts are  con- 
verted into  the  sper- 
matozoa. The  small 
cells  are  then  the 
parents  of  the  sper- 
matozoa and  may  be 
called  the  parent  - 
cells ;  a  great  variety 
of  names  have  been 
emplo3^ed  to  designate  them,  such  as  mother-cells,  spore-cells,  ger- 
minative  cells,  Samenstammzellen,  etc.  The  nomenclature  of  the 
small  cells  is  very  confused ;  those  of  them  in  process  of  indirect 
division  are  often  smaller  than  the  others  and  have  been  designated 
as  the  "growing  cells"  by  H.  H.  Brown,  85.1,  and  this  term  has 
been  used  by  other  writers  since.  The  small  cells  in  the  resting 
stage  are  called  "  Stammzellen  "  by  most  German  writers,  as  an  equiv- 
alent for  which  I  have  adopted  parent-cell. 

FORMATION  OF  THE  SPERMATOBLASTS. — The  parent-cells  divide 


FIG.  23. — Peripheral  layer  of  the  seminiferous  tubule  of  a  rat. 
Two  views  from  a  teased  preparation.     After  Neumann. 


44 


THE    GENITAL   PRODUCTS. 


and  produce  probably  three  cells,  although  the  number  has  never 
been  accurately  ascertained.  One  cell  remains  as  a  parent-cell,  and 
the  other  two  are  the  mother-cells  (Mutterzellen)  and  are  well  char- 
acterized by  their  appearance.  According  to  Biondi,  85.1,  the 
nucleus  of  the  parent-cell  remains  and  becomes  like  the  nucleus  of  the 
large  cells  (Sertoli's  or  supporting  cells).  The  mother-cells  divide 
and  their  descendants  also  divide  until  there  is  produced  a  column 
of  cells,  Fig.  24,  which  stretches  in  a  radial  line  from  the  mother- 
cell  toward  the  centre  of  the  tubule,  and  is  packed  in  between  the 
columnar  centripetal  prolongations  of  Sertoli's  cells  (cf.  Figs.  24  and 
29) .  Probably,  then,  although  investigators  are  not  agreed  in  regard 
to  this  point,  the  parent-cells  divide  in  such  a  way  that  the  cells  re- 
sulting from  the  division  are  unlike,  one  of  them  preserving  the 
character  of  the  parent-cell,  and  the  others  differing  from  it  in  hav- 
ing a  relatively  larger  nucleus  and  a  finer  chro- 
matin  network;  the  appearance  of  the  nuclei 
varies,  of  course,  according  as  they  are  in  the 
resting  or  divisional  (kinetic)  phase.  * 

The  cell  most  like  the  original  one,  and  which 
we  may  call  still  the  parent-cell,  lies  at  the  outer 
edge  of  the  tubule,  while  the  others  or  mother-cells 
lie  toward  the  centre,  Fig.  24.  The  parent-cell, 
as  already  stated,  produces  at  least  a  second  and 
perhaps  more  mother-cells,  so  that  the  column 
grows  centripetally.  The  column  also  grows  by 
multiplication  of  the  mother-cells,  but  the  cells 
thus  formed  lie  in  the  innermost  part  of  the  col- 
umn; they  are  smaller,  Fig.  24,  than  the  first 
generation  of  (mother)  cells ;  they  have  relatively 
large  nuclei,  with  the  chromatin  gathered  into 
two  or  three  spots — nucleoli.  We  thus  have  a 
column  of  cells  in  which  we  can  distinguish  three 
zones:  1,  the  outer  zone  of  the  parent-cell ;  2,  the 
middle  zone  of  the  mother-cells ;  3,  the  inner  zone 
of  the  daughter-cells.  These  zones  remain  more 
or  less  marked  for  a  considerable  period ;  for,  as 
the  cells  of  the  inner  zone  change  into  spermato- 
blasts,  those  of  the  middle  zone  change  into  second 
daughter-cells,  and  as  the  inner  spermatoblasts 
change  into  spermatozoa  the  cells  of  the  second 
zone  change  into  spermatoblasts;  the  innermost 
zone  long  continues  one  stage  ahead.  The  trizonal  arrangement  is 
very  conspicuous  in  cross-sections. 

The  division  of  the  mother  and  daughter-cells  presents  many  pe- 
culiarities, and  does  not  conform  exactly  to  Flemming's  well-known 
scheme  of  phases  for  indirect  division.  Attention  was  first  directed 
to  these  peculiarities  by  Carnoy,  in  an  important  memoir,  85.1,  and 
W.  Flemming,  87. 1,  has  since  confirmed  these  discoveries,  in  large 
part,  by  observations  on  the  salamander,  and  gives  a  plate  of  dia- 
grams which  is  instructive  as  a  facile  means  of  comparison.  La 


FIG.  24.— Column  of 
spermatocytes  from 
the  rat ;  a,  parent  cell ; 
&,  mother  cells.  After 
Binodi.  x  600  diams. 


*For  figures  of  the  karyokinetic  division  of  the  daughter-cells,  see  Fiirst,  87.  1,  Figs.  10-13. 


SPERMATOZOA. 


45 


Vallette,  Niessing,  88. 1,  p.  44,  and  others  find  that  when  the  mother- 
cells  multiply  there  is  often  a  stage  to  be  found  where  several  nuclei 
(two  to  twelve)  lie  within  one  large  cell.  The  multinucleate  giant- 
cells  are  best  found  by  teasing  the  fresh  specimen.  As  to  their  place 
in  the  spermatogenetic  history  we  possess  no  definite  knowledge. 

The  spermatoblasts  arise  from  the  nuclei  of  the  daughter- cells 
(spermatocy tes) ,  and  not  as  H.  H.  Brown,  85.1,  and  many  others 
have,  I  think,  erroneously  be- 
lieved, each  out  of  a  whole  cell. 
Biondi,  85.1,  seems  to  me  right 
in  his  statement  that  the  bodies 
of  the  cells  break  down,  or  at 
any  rate  lose  their  boundaries, 
thus  creating  a  granular  proto- 
plasmatic column  in  which  the 
nuclei  lie.  Compare  also  Nies- 
sing,  88.1.  The  protoplasm  of 
the  parent-cell  participates  in 
these  changes,  hence  its  nucleus 
comes  to  lie  at  the  base  of  the 
column.  This  nucleus  has 
meanwhile  altered  its  charac- 
ter, and  become  large,  clear, 
and  nucleolated.  Now,  these 
columns  are  the  same  as  the 
large  Sertoli's  or  supporting 
cells  above  described.  By  no 
means  all  writers  agree  with 
this  account  of  the  origin  of 
Sertoli's  cells,  but  all  other  ex- 
planations that  I  have  found 
appear  to  me  vague  and  con- 
fused, and  the  history  of  the 
changes  here  advocated  is  clear, 
and  accounts  for  the  well-estab- 
lished grouping  of  the  sperma- 
toblasts in  the  substance  of 
Sertoli's  column;  this  essential 
phase  is  explained  satisfactorily 
by  no  other  theoiy. 

The  nuclei  congregate  at  the 
inner  end  of  the  column,  and 
there  change  their  character  and 
become  recognizable  spermato- 
blasts, Figs.  '25  and  29. 

DEVELOPMENT  OF  THE  SPERMATOBLASTS  INTO  SPERMATOZOA.— 
The  nuclei  change  into  spermatozoa  as  follows:  The  chromatin  is 
at  first  unequally  distributed  throughout  the  nucleus;  it  then  in 
great  part  accumulates  at  the  end  of  the  nucleus  toward  the  outer 
wall  of  the  tubule ;  at  this  stage  the  chromatin  is  densest  near  the 
equator  of  the  nucleus,  where  the  edge  of  the  chromatin  is  sharply 
marked,  and  toward  the  outer  pole  of  the  nucleus  the  chromatin  is 


FIG.  25.— Developing  Bpennatoblaota  of  the  rat: 

a.  />.   '•.  (l.  ,-.  /,  gr,  /i,  successive  stages.      X  about 
Aft< 


750  diameters. 


ter  H.  H.  Brown. 


46 


THE    GENITAL   PRODUCTS. 


I 


B 


FIG.  26.— Developing  spenna- 
tozoa  of  a  marsupial:  Meta- 
chirus  Quica.  A,  B,  C,  differ- 
ent stages.  After  Furst. 


less  condensed  (Niessing,  88.1,  p.  40,  Taf.  I.,  Figs.  6,  7,  and   8). 
It  is  from  the  equatorial  plate  that  the  future  tail  grows  out  at  the 
start.     Particles  of  the  chromatin  are  said  to  remain  in  other  re- 
»     — ^  gions  of  the  nucleus,  and  finally  to   gather 

*B^5^  together  to  form  the  small  accessory  corpuscle 

mentioned  below.  According  to  Platner, 
89.2,  131,  132,  the  portion  of  the  nucleus 
which  forms  the  head  of  the  spermatozoon 
in  pulmonate  snails  is  homologous  with  his 
Nebenkern.  The  main  mass  of  the  chro- 
matin is  concerned  in  the  formation  of  the 
head  of  the  spermatozoon ;  it  is  at  first  quite 
round,  Fig.  25,  a  and  6,  but  soon  begins  to 
alter  its  shape,  gradually  assuming  the  form 
of  the  spermatozoon  head,  Fig.  25,  c,  d,  e,  /. 
The  tail  appears  very  early  as  a  delicate  fila- 
ment, growing  out  from  the  chromatin  and 
lying  entirely  within  the  nucleus,  Fig.  25,  a, 
but  shortly  after  is  found  to  project  beyond 
the  nuclear  membrane,  6,  and  lengthens  rap- 
idly, e,  /,  g.  The  nuclear  membrane  is  very 
distinct ;  it  elongates  into  an  oval  bag,  6,  c, 
one  end  of  which  lies  close  against  the  chro- 
matin, while  the  other  surrounds  part  of  the 
tail  and  is  wide ;  the  lengthening  continues, 
e->  f,  9i  with  accompanying  changes  of  form, 
best  indicated  by  the  figures ;  the  part  of  the  tail  within  the  nuclear 
membrane  becomes  the  middle-piece,  Fig.  20,  but  the  spiral  thread  is 
not  developed  until  later.  The  accessory  body  may  be  readily  seen 
in  the  rat ;  unlike  the  chromatin  of  the  head  it  can  be  stained  by 
chloride  of  gold :  hence,  if  it  is  formed  of  chromatin  at  all,  the  chro- 
matin must  have  undergone  alteration.  Finally,  the  nuclear  mem- 
brane ruptures,  Fig.  27,  a  portion  of  the  membrane  remains  upon 
the  head,  and  the  caudal  bag  sometimes 
endures  longer,  Fig.  25,  #,  but  at  last 
also  disappears,  except  that  in  certain 
cases  a  trace  of  it  remains  visible  as  a 
fine  cross-line  at  the  end  of  the  middle- 
piece. 

Furst  and  others  think  that  the  axis 
of  the  tail  is  formed  from  the  chromatin, 
and  that  the  sheath  of  the  axis  arises 
from  the  achromatine  substance  of  the 
nucleus  (caryoplasma) . 

After  the  rupture  of  the  nuclear 
membrane  the  young  spermatozoa  still 
develop  a  little  farther.  The  spermato- 
zoa are  ultimately  liberated,  and,  falling 
into  the  lumen  of  the  tubule,  pass  off. 

From  their  mode  of  development,  it  is  evident  that  the  sperma- 
tozoa necessarily  lie  in  bundles,  each  bundle  being  held  together  by 
a  Sertoli's  column,  Fig.  28;  at  first  they  lie  at  the  inner  end  of  the 


FIG.  27.  -  Human    spcrmutoblasts,  to 


SPERMATOZOA. 


47 


Fio.28.— Sertoli's  col- 
umn, with  a  basal  nu- 
cleolated  nucleus  and  a 
cluster  of  developing 
spermatoblasts.  After 
H.  H.  Brown. 


column,  at  a  considerable  distance  from  the  basal 
nucleus,  but  as  the  nuclei  (spermatoblasts)  length- 
en, the  heads  push  their  way  toward  the  base  of 
the  column,  Fig.  %^0.  Now  as  the  development  of 
the  daughter-cells  (spermatocytes)  is  continually 
progressing  between  Sertoli's  columns,  we  obtain 
in  sections  the  long-known,  remarkable  appear- 
ances shown  in  Fig.  21),  of  bundles  of  spermatozoa 
alternating  with  columns  of  proliferating  cells. 

4.  Historical.  —  The  seminal  animalcules 
were,  it  is  stated,  first  discovered  by  Ludwig 
Hamm,  then  a  student  at  Leyden,  in  August, 
1677.  Leeuwenhoek  claimed  the  merit  of  having 
made  the  discovery  in  November  of  the  same  year, 
and  in  1G78  Hartsoeker  published  an  account  of 
them,  professing  to  have  seen  them  as  early  as 
1  •', ;  4.  They  were  long  considered  to  be  probably 
parasites,  and  it  was  not  until  Prevost  and  Du- 
mas' researches  that  it  was  definitely  ascertained 
that  the  "  animalcules  "  were  the  essential  fertiliz- 
ing element.  Thus  Richard  Owen,  in  his  article 
on  "Entozoa"  (1836),  in  Todd's  " Cyclopsedia, " 
includes  the  spermatozoa  under  that 
head,  although  he  writes :  "  It  is 
still  undetermined  whether  they 
are  to  be  regarded  as  analogous 
to  the  moving  filaments  of  the 
pollen  of  plants  or  as  inde- 
pendent organisms  "  (Vol. 
II.,  p.  412).  But  just 
after  he  adds:  "Al- 
no  distinct 


though 

organs    of    genera- 
tion  have  been    detected,    there   is 
reason  to  suspect  that  the  sperma- 
tozoa   are    oviparous;  they   are 
also    stated    to    propagate    by 
spontaneous   fission,  the  sep- 
aration taking  place  between        sp 
the  disc  of  the  body  and 
the    caudal    appendage, 
each  of  which   develop 
the  part  required  to  form 
a  perfect  whole." 

Meanwhile  the  inves- 
tigations of  Spallanzani, 
Wagner,  Czermak,  and 
many  others  gradually 
increased  the  knowledge 
of  the  forms  of  the  sper- 
matozoa. Dujardin  was 
the  first  to  consider  the 


FIG.  29.— Part  of  a  cross-section  of  a  seminiferous  tubule  of 
a  rat.     x  about  750  diameters.     After  H.  H.  Brown. 


4S  THE   GENITAL,   PRODUCTS. 

spermatozoa  as  generated  from  the  inner  layer  of  the  seminiferous 
tubules,  and  therefore  not  as  parasites.  The  discovery  of  the  sperma- 
toblasts  or  immature  spermatozoa  by  Von  Siebold  (Miiller's  Archiv, 
1836  and  1843),  soon  confirmed  by  Kolliker  and  Reichert,  marks  an 
important  step.  Now  follows  a  series  of  publications  by  which  one 
detail  after  another  was  added  to  our  knowledge.  During  the  past 
twenty  years  there  has  been  .  rapid  progress,  which  may  be  said  to 
have  begun  with  Schweigger-Seidel's  important  memoir,  65.1,  and 
to  have  made  us  acquainted  with  the  minute  structure  of  the  sper- 
matozoa, and  their  development.  Another  line  of  investigation  was 
opened  by  O.  Herwig  (1875),  in  following  up  the  history  of  the  sper- 
matozoon within  the  ovum  after  impregnation.  For  further  histor- 
ical data,  see  Waldeyer's  address,  87.2. 

II.  OVA. 

Definition. — The  term  ovum  is  employed  in  various  senses.  It  is 
applied — 1,  to  the  cell  distinguished  as  the  ovarian  cell,  or  immature 
ovum,  out  of  which  the  female  product  or  mature  ovum  is  developed ; 
2,  to  the  mature  ovum,  or  true  female  spore;  3,  to  the  mature 
ovum  plus  the  fecundating  spermatozoon  united  with  it — that  is, 
to  the  impregnated  ovum;  4,  to  various  stages  of  development  of 
the  embryo.  In  this  article  we  consider  only  the  ovum  in  the  strict 
sense — namely,  as  the  female  sexual  product. 

Summary. — The  ovum  arises  as  a  cell,  which  matures  by  a  series 
of  changes,  of  which  the  last  and  most  striking  is  the  expulsion  of 
the  so-called  polar  globules ;  there  are  many  important  changes  which 
occur  earlier.  The  genesis  of  the  mature  ovum  may  be  conveniently 
divided  into  three  arbitrary  stages :  (1)  Differentiation  of  the  ovic 
cell ;  (2)  growth  of  the  cell  and  accumulation  of  nutritive  material  in 
it ;  (3)  maturation  proper. 

1st.  The  Origin  of  the  Primitive  Ova  (Ureier  or  Ovic  Cells).* — It 
seems  to  me,  in  the  light  of  the  recent  investigations  of  the  origin  of 
the  ova  in  vertebrates,  safe  to  assert  that  they  arise  from  cells  of 
the  mesothelium  (peritoneal  epithelium)  covering  the  genital  ridge 
of  the  embryo,  the  ridge  giving  rise  to  the  adult  ovary.  On  account 
of  its  function  the  epithelium  of  the  genital  ridge  has  been  called 
the  germinal  epithelium  (KeimepitheT) .  In  mammals,  which  alone 
will  be  here  considered,  according  to  the  best  authorities  the  ureier 
are  developed  as  follows :  Certain  cells  of  the  germinal  epithelium 
become  larger  than  the  others ;  these  cells  are  soon  carried  into  the 
interior  of  the  ovary  by  being  included  in  cord-like  ingrowths  of 
the  epithelium.  These  cords  are  the  Pfluger'schen  Schlciuche  of 
German  writers.  The  primitive  ova  exist  in  multiple  in  the  cords, 
but  each  of  them  early  becomes  surrounded  by  a  separate  envelope 
of  epithelial  cells.  A  little  later  each  ovum  separates  from  its 
neighbors  and  appears  as  a  round  cell,  with  a  clear  nucleus  and  dis- 
tinct nucleolus,  /,  closely  invested  by  a  layer  of  cells  smaller  than 
itself.  The  young  egg-cell,  together  with  its  epithelial  envelope, 
constitutes  the  so-called  primordial  follicle  (Fig.  31). 

*  For  a  full  discussion  of  this  subject  see  Chapter  XXIII. 


OVA.  49 

2d.  General  Growth  of  the  Ovum  and  Development  of  the  Yolk.— 
The  modifications  which  occur  in  growing  egg-cells  are  as  follows: 
1st.  Change  of  size ;  the  cell  enlarges,  it  being  a  rule — no  exception 
to  which  is,  I  believe,  known — that  the  mature  egg-cell  is  much 
larger  than  any  of  the  other  cells  in  the  body  of  the  parent.  \M. 
Change  of  shape ;  the  cell  usually  becomes  nearly  or  quite  spherical ; 
the  shape  of  the  egg  does  not  necessarily  remain  spherical,  but 
may  be  altered  by  external  pressure,  as  in  the  uterus  of  Arion 
("Hdbk.,"  Vol.  IV.,  p.  7,  Fig.  1815),  or  as  when  several  are  laid 
in  one  capsule  (Lumbricus,  Nephelis,  Planaria,  etc.),  or  when  com- 
pressed by  an  unyielding  shell.  An  instance  of  the  last-mentioned 
kind  has  been  described  by  Repiachoff  (Z.  f.  wiss.  Zool.,  XXX., 
Suppl.),  who  figures  the  egg  of  a  European  bryozoon  found  on  eel- 
grass  as  fusiform,  Fig.  30.  3d.  The  nucleus  becomes  larger,  spher- 
ical, and  assumes  an  eccentric  position  within  the  cell;  the  chro- 
matin  usually  gathers  into  one  nucleolus,  as  in  mammalia ;  the  nu- 
cleolus  is  large,  distinct,  highly  refringent,  easily  stained,  and  placed 
eccentrically  within  the  nucleus.  The  achromatic  substance  or  pro- 
toplasm of  the  nucleus  develops  into  a  coarse  network,  which  radi- 
;it<'s  irregularly  from  the  nucleolus  as  a  centre. 
4th.  The  cellular  network  becomes  very  distinct; 
its  interspaces  become  filled  with  ovoid,  round 
or  crystalline  solid  inclosures,  which  are  usual!}*, 
if  not  always,  mainly  of  an  albuminoid  character. 
The  inclosures  form  the  part  which  is  called  the 
deutoplasm  by  Edouard  van  Beneden  and  others.  FIG.  so.  -Egg  of  Tendra 
The  deutoplasm  is  the  same  as  the  yolk-substance  z°st£ricoia;  after  Repia- 

-,,  *  .  ...  c         i_    •!•  ciion.     iUagnineu. 

of  older  writers,  and  is  a  store  of  nutritive  mate- 
rial from  which  the  protoplasm  draws  subsequently  to  support  its 
growth.  The  term  yolk  has  no  very  exact  meaning,  for  it  is  used 
to  designate  sometimes  the  deutoplasm  alone,  sometimes  the  whole 
ovum  proper,  as  when  the  segmentation  of  the  yolk  is  spoken  of. 
5th.  In  all  vertebrates  an  ovarian  envelope,  the  zona  radiata,  is 
formed.  6th.  It  is  probable  that  a  vitelline  or  true  cell-membrane 
is  always  formed  inside  the  zona  by  the  egg-cell  before  it  reaches 
maturity. 

Primordial  Ovum.*— In  the  ovary  at  birth,  and  thereafter  up 
to  the  period  of  the  climacteric,  are  small  egg-cells,  some  of  which 
develop  from  time  to  time  into  mature  ova.  At  all  ages  these  small 
egg-cells  together  with  their  follicles  present  a  constant  appearance. 
It  is  currently  stated  in  text-books  that  there  are  some  seventy 
thousand  egg-cells  in  the  human  ovary  at  birth ;  but  upon  what  au- 
thority this  assertion  rests  I  do  not  know.  In  any  case  the  number 
is  very  large,  and  it  is  probable  that  a  good  many  of  them  never 
develop,  but  degenerate.  These  youngest  egg-cells  are  known  as  the 
primordial  ova ;  they  lie  in  a  layer  immediately  below  the  albuginea 
of  the  ovary  and  never  in  the  medullary  region,  Fig.  32.  They  are 
slightly  irregular  globules,  usually  50-60  <>.  in  diameter;  48  by  54  <*, 
54  by  58  //,  (54  by  68  /;.,  exemplify  actual  measurements.  The  proto- 
plasm is  finely  and  evenly  granular,  and  consists  of  a  uniformly 

*  In  the  ensuing  account  of  the  ovarian  ovum  up  to  its  maturation,  I  have  been  guided  chiefly 
by  Nagel's  article  88.  1 


50  THE    GENITAL   PRODUCTS. 

clear  matrix  (hyaloplasma)  and  a  fine  reticulum,  which  may  be 
brought  out  by  eosine  staining.  In  birds  His,  68.1,  has  found 
"protagon"  granules  in  the  primordial  ovum,  and  Ed.  van  Beneden 
affirms,  70. 1,  that  yolk-grains  are  present  in  the  primordial  ovum  of 
various  mammals ;  but  in  man  this  is  not  the  case.  The  protoplasm 
is  naked — that  is,  not  enclosed  in  a  cell  membrane.  The  nucleus  is 
round,  lies  in  the  centre  of  the  cell,  measures  from  29  to  32  p.  in  di- 
ameter, is  bounded  by  a  very  distinct  membrane,  and  contains  a 
round  excentrically-placed  nucleolus,  about  9  /*  in  diameter.  Be- 
tween the  nucleolus  and  the  membrane  there  is  a  loose  network  of 
fibres,  attached  to  both;  the  substance  of  the  network  is  different 
from  that  of  the  nucleolus,  as  is  shown  by  its  different  staining. 
The  network  was  first  observed  by  Flemming,  75.1,  in  Unio  and 
Anodonta,  and  has  since  been  often  observed  in  many  species ;  it  was 
first  described  in  human  ova  by  Trinchese  (Mem.  A  cad.  Sci.  Bo- 
logna, Ser.  III.,  T.  VII.) .  Some  of  the  primordial  ova  of  very  young 
children  have  no  nucleolus,  and  in  bats  all  of  them  are  at  first  with- 
out it  according  to  E.  van  Beneden.  The  position  of  the  nucleolus 
is  variable ;  it  may  lie  close  to  the  membrane  of  the  nucleus  or  nearly 
in  the  centre.  A  peculiarity  worthy  of  mention  is  that  once  in  a 
great  while  a  primordial  ovum  has  two  or  even  three  nuclei.  This 
occurs  so  very  rarely  that  it  cannot  be  considered  as  any  evidence  of 
multiplication  of  the  ova,  but  only  as  an  extremely  abnormal  vari- 
ation (see  Nagel,  88.1,  372-375).  Each  primordial  ovum  is  sur- 
rounded by  a  very  thin  epithelial  envelope,  Fig.  31, /,  with  scattered 
fusiform  nuclei  easily  distinguished  in  stained  specimens  from  the 
similarly  shaped  nuclei  of  the  neighboring  connective  tissue. 

The  shape  of  the  follicular  nuclei  has  misled  Schron,  63.1,  Foulis, 
76.1,  Klebs,  63.1,  and  others  into  maintaining  that  the  follicle  is 
derived  from  the  stroma-cells,  instead  of,  as  is  really  the  case,  from 
the  germinal  epithelium ;  Kolliker  traces  the  origin  of  the  follicular 
cells  to  the  "  Markst range  " ;  others,  as,  notably,  Harz,  83.1,  and 
Sabatier,  84.2,  derive  the  follicular  cells  from  the  ovum.  Both 
views  must,  it  seems  to  me,  be  discarded  (compare  for  details, 
Chapter  XXIII.).  The  follicle  forms  a  closed  wall  around  the  entire 
ovum,  and  not  one  with  an  opening,  as  certain  authors  have  main- 
tained. The  primary  follicles  of  mammals  were  first  described  by 
Barry,  38. 1,  under  the  name  of  ovisacs;  his  observations  were  soon 
confirmed  by  Bischoff,  42.3.  They  are  now  familiarly  known  to 
all  histologists. 

Growth  of  the  Ovum  and  Primary  Follicle. — The  follicles 
remain  for  a  long  time  without  change,  but  from  time  to  time  cer- 
tain ones  of  them  develop.  In  a  mature  ovary  we  can  find  always 
several  stages.  The  cause  of  the  development  of  the  follicles  is  un- 
known. The  primary  follicles  are  always  near  the  surface;  as  they 
grow  in  size  they  move  deeper  into  the  stroma.  The  first  step  is  the 
multiplication  of  the  cells  of  the  follicle,  Fig.  31,  A,  which  converts 
the  follicle  into  a  layer  of  cubical  cells  with  the  nuclei  at  an  even 
height.  During  this  change  in  the  follicle  the  primordial  ovum  does 
not  alter  in  size. 

The  second  step  is  the  elongation  of  the  cells  into  a  cubical  form, 
with  an  accompanying  enlargement  of  the  ovum.  The  growth  of 


OVA.  51 

the  ovum  affects  the  protoplasm,  the  nucleus,  and  the  nucleolus,  all 
of  which  increase  their  dimensions.  The  follicular  wall  steadily 
increases  in  thickness;  at  first  it  remains  single-layered,  but  the 
nuclei  take  their  places  at  various  levels;  a  little  later  it  be- 
comes several-layered,  and  then  the  formation  of  the  first  en- 
velope (zon a  pellucida)  around  the  ovum  begins.  Nagel,  88.1, 
380-382,  calls  attention  to  the  large  clear  cells  with  large  nuclei, 
which  show  a  distinct  reticulum  and  one 
or  several  chromatin  granules;  they  are 
found  in  somewhat  larger  follicles,  but 
only  up  to  the  time  when  the  yolk  granules 
begin  to  form  in  the  ovum.  Nagel  inter- 
prets these  cells  as  having  a  nutritive 
function,  and  calls  them  NahrzeUen;  he 
offers  very  little  evidence  in  favor  of  his 
view.  The  cells  in  question  measure  16- 

21  /;-,  and  are  much  smaller  than  the  prim-  '^^•^ftz^Z'' 

ordial  ova,  which  they  somewhat  resemble      FIG.  31.- Primary  follicles  from  the 
in   appearance.      These  cells  have  been    °™r?  °£a  Co°nSiveiSueeiayt? 

Seen    by   Various  authors,    e.Q.,     Call    and    /,  epithelical  follicle;  z,  beginning' 
TT,  /OM.  v  ITT-  AIT        ixr*  zona  pellucida;  nu,  nucleus  or  ger- 

Exner      (Sltzber.      VVien.      Akad.      WlSS.,     minative  vesicle.    After  W.  Nagel. 

etc.,  15  April,  1805).  It  is  more  prob- 
able that  these  cells  have  to  do  with  the  formation  of  the  liquor 
of  the  Graafian  follicle,  which  begins  while  they  are  present.  The 
cells  of  the  granulosa  multiply  by  indirect  division,  as  has  been 
shown  by  Harz  and  also  Flemming  (Arch,  mikrosk.  Anat.,  XXIV. 
376-384).  I  have  found  the  numerous  karyokinetic  figures  in  the 
follicles  of  the  rabbit's  ovary,  though  hardly  quite  as  abundant  as 
Flemming's  description  led  me  to  expect.  The  mitoses  have  not 
been  found  in  the  first  stages  of  follicular  growth.  During  the 
growth  of  the  follicle  there  is  formed,  as  was  first  described  by 
Schron,  63.1,  410,  a  network  of  blood-vessels  close  around  the  folli- 
cle ;  the  layer  of  blood-vessels  constitutes  the  so-called  tunica  vascu- 
losa  or  theca  folliculi;  the  first  vessel  is  a  simple  loop,  which 
embraces  the  young  follicle;  other  loops  approach  and  unite  with 
their  fellows  to  form  a  network. 

Development  of  the  Graafian  Follicle. — After  the  epithelium 
of  the  primary  follicle  has  become  many-layered,  there  appear  in  it 
rounded  vacuolated  spaces,  which  increase  in  size  and  finally  become 
confluent,  so  that  there  is  a  space  or  fissure  in  the  epithelium,  Fig. 
32,  4-  This  fissure  divides  the  epithelium  into  two  layers,  an  inner 
one  immediately  surrounding  the  ovum,  and  an  outer  one  next  the 
stroma  of  the  ovary.  Since  the  fissure  does  not  extend  completely 
around  the  follicle  there  is  one  place  where  the  two  layers  are  united, 
Fig.  32 ;  the  place  of  union,  though  variable  in  position,  is  always  on 
the  side  of  the  follicle  away  from  the  surface.  The  fissure  is  filled 
with  a  serous  fluid  known  as  the  liquor  folliculi.  In  man  and  most 
mammals  there  is  a  single  continuous  fissure ;  but  in  the  rabbit,  and 
perhaps  other  rodents,  there  are  often  cords  of  cells  stretching  across 
from  the  outer  to  the  inner  lamina  of  the  epithelium ;  the  cords  vary 
in  number  from  two  to  ten ;  they  were  first  described  by  Barry,  have 
been  beautifully  figured  by  Coste,  47. 1,  Lapin,  L.  I.,  Fig.  2,  and  are 


52  THE    GENITAL    PRODUCTS. 

known  as  the  retinacula.     The  development  of  the  fissure  changes 
the  primary  into  a  Graafian  follicle. 

The  Graafian  follicle  is  bounded  by  a  layer  of  epithelium  known 
as  the  membrana  granulosa,  from  its  appearance  when  examined 
in  the  fresh  state ;  it  is  surrounded  by  a  vascular  layer,  characterized 
not  only  by  its  blood-vessels,  but  also  by  the  condensation  of  the  con- 
nective tissue  composing  it.  The  follicle  lies  a  little  below  the  layer 
of  primordial  ova.  To  a  part  of  its  walls  on  the  side  away  from  the 
surface  of  the  ovary  is  attached  a  mass  of  cells  more  or  less  globular 
in  shape;  this  mass  is  known  as  the  discus  or  cumulus proligerus; 
it  encloses  the  ovum ;  the  cavity  between  the  discus  and  granulosa 
is  the  cavity  of  the  follicle,  and  contains  the  liquor  folliculi.  The 
further  history  consists  principally  in  growth  and  secondary  modifi- 
cations. The  follicular  wall  and  the  discus  increase  in  thickness ; 
there  is  added  a  very  thin  basement  membrane,  Waldeyer's  mean- 


FIG.  32.— Ovary  of  cat: 


2. — Ovary  of  cat:    1-11,  successive  stages   of   the  ova;    1,    primordial  ovum;   8, 
Graafian  follicles;  11,  corpus  luteum;  Fe,  blood-vessels;  fit,  nilus.     After  Scliron. 


9,    10, 


brana  propria,  close  around  the  outside  of  the  granulosa  and  sep- 
arating it  from  the  tunica  vasculosa ;  the  membrana  propria  is  said 
to  be  an  endothelium  derived  from  the  connective-tissue  cells  of  the 
ovary.  The  vascular  membrane  or  theca  folliculi  becomes  differen- 
tiated into  an  outer  fibrous  layer  (Henle's  tunica  flbrosa)  carrying 
the  larger  blood-vessels,  and  an  inner  less  fibrous  layer  carrying  the 
smaller  blood-vessels  (tunica  propria).  The  distinction  between 
the  membrana  propria  and  tunica  propria  should  not  be  overlooked. 
The  smallest  blood-vessels  running  around  the  follicle  from  below, 
and  minutely  subdivided  on  its  upper  surface,  converge  toward  a 
point  near  the  surface  of  the  ovary ;  this  point  is  called  the  stigma, 
contains  no  blood-vessels,  and  marks  the  spot  where  finally  the  folli- 
cle is  to  rupture  to  allow  the  ovum  to  escape.  The  stigma,  owing  to 


OVA.  53 

the  absence  of  blood-vessels,  is  yellowish-white.  In  mammals  and 
birds  it  is  elongated  and  rounded  in  outline,  but  in  lizards  is  angu- 
lar (Coste,  47. 1 ,  160) .  The  cells  of  the  granulosa  acquire,  at  least  in 
the  cow,  highly  characteristic  forms  (Lachi,  84. 1) ;  there  are,  1st, 
very  narrow  elongated  cells,  which  stretch  through  the  entire  thick- 
ness of  the  layer,  and  present,  when  isolated,  curious  irregular 
forms;  they  have  oval  nuclei,  about  which  there  is  usually  a  small 
amount  of  protoplasm ;  the  nuclei  of  these  cells  lie  in  the  half  of  the 
granulosa  next  the  cavity  of  the  follicle.  3d,  cells  with  rounded 
nuclei,  larger  cell-bodies,  and  a  few  fine  processes  of  irregular 
shapes ;  these  cells  lie  between  the  processes  of  the  others  in  the  outer 
half  of  the  membrane.  3d,  cells  that  are  probably  immigrated  leu- 
cocytes. The  cells  of  the  discus  have  not  yet  been  minutely  stud- 
ied; those  next  the  ovum  are  cylindroid,  and  radiate  around  the 
zona,  constituting  thus  the  so-called  corona  radiata  of  authors — 
compare  Fig.  34.  The  cells  of  the  outer  layer  of  the  discus  are 
more  rounded  in  form ;  it  is,  of  course,  probable  that  the  two  forms 
of  discus-cells  .resemble  the  cells  of  the  granulosa  in  actual  shape. 

Just  before  the  primary  follicle  changes  into  the  Graafian  follicle 
the  ovum,  at  least  in  man,  has  attained  its  full  diameter,  but  still 
contains  no  yolk  (deutoplasm) .  At  this  time  there  appears  a 
clear,  delicate  membrane  close  around  the  ovum,  separating  it 
from  the  cells  of  the  follicular  wall.  In  the  Graafian  follicle  this 
membrane  steadily  grows  until  it  attains  a  diameter  of  20-24  // ;  it 
is  called  the  zona  radiata  or  pellucida;  its  structure  is  described 
in  the  subsequent  section  on  the  full-grown  egg-cell. 

The  first  yolk-grains  appear  in  the  human  species  when  the  zona 
pellucida  has  attained  a  thickness  of  1  //  or  more,  and  are  situated 
always  in  the  centre  of  the  egg-cell  (Nagel,  88. 1,  385,  386) .  In  other 
mammals  they  are  said  to  appear  earlier.  The  yolk-granules  must 
be  considered  as  the  direct  products  of  the  vital  activity  of  the  egg- 
cell  itself,  and  in  my  judgment  there  is  no  sufficient  basis  for  any 
other  view.  Various  hypotheses  as  to  the  origin  of  the  yolk-grains 
have  been  advanced.  Thus  Waldeyer,  70.1,  has  maintained  that 
the  grains  are  produced  by  the  cells  of  the  follicle,  and  are  trans- 
ferred from  them  across  the  zona  into  the  ovum.  It  is  not  impossi- 
ble that  very  small  young  granules  may  arise  in  the  follicular  cells 
and  be  transmitted  along  the  fine  processes  by  which  the  cells  are 
connected  through  the  zona  radiata  with  the  ovum,  and  that  these 
granules  subsequently  grow  within  the  egg-cell,  as  Caldwell,  87.1, 
asserts  is  the  case  in  monotremes  and  marsupials.  Caldwell's  state- 
ments are  so  aphoristic  that  the  question  must  remain  unsettled 
until  more  fully  investigated.  Lindgren,  77. 1,  asserts  that  the  cells 
of  the  granulosa  immigrate  through  the  zona  to  form  the  yolk-gran- 
ules; his  observations  were  made  on  ova  which  had  already  been 
somewhat  macerated,  and  which  had  the  processes  of  the  follicular 
cells  swollen  in  consequence.  That  the  yolk-grains  are  produced  by 
the  gradual  enlargement  of  small  ones  has  been  shown  by  Sarasin's 
researches  on  reptiles,  83.1.  He  found  in  Lacerta  a  central  area  of 
small  grannies  which  gradually  enlarge;  this  area  (Herd  der  Dot- 
terbildung}  persists  even  after  the  embryo  has  appeared,  and  the  egg 
increases  in  volume  and  weight  after  the  segmentation  has  begun. 


54  THE   GENITAL   PRODUCTS. 

The  characteristics  of  the  human  yolk-grain  have  not  been  accu- 
rately investigated,  nor  have  those  of  any  of  the  higher  mammalia 
been  studied  carefully.  In  the  human  ova  the  grains  are  1  p-  or  less 
in  diameter;  highly  refringent  and  of  various  kinds.  In  a  sheep's 
ova  Bonnet,  84.1,  found  small  granules,  fat-globules  in  considerable 
abundance,  p.  178,  and  larger  granules  which  stain  with  eosine,  /.  c., 
p.  183.  Their  accumulation  continues  centrifugally,  forcing  the 
nucleus  of  the  ovum  to  an  eccentric  position ;  when  the  maximum 
of  the  vitelline  deposit  is  reached  there  is  only  a  very  thin  layer  of 
protoplasm  around  the  outside  of  the  egg-cell,  Fig.  34,  and  a  court 
of  protoplasm  around  the  nucleus.  This  disposition  is  particularly 
well  shown  in  the  ova  of  the  monotremes  and  marsupials.  See 
Caldwell,  87.1,  PI.  XXIX. ,  Fig.  5.  The  cortical  layer  is  readily  dis- 
tinguished in  fresh  ova,  but  in  hardened  specimens  is  quite  or  wholly 
indistinguishable.  In  the  ovum  of  the  placental  mammalia  the  yolk 
never  attains  a  great  development ;  but  in  most  vertebrates  the  gran- 
ules gradually  enlarge,  and  in  some  cases  they  are  quite  big.  When 
they  are  thus  developed  it  is  easy  to  see  that  they,  are  of  various 
sorts.  Thus  in  the  hen's  ovum  there  are  two  principal  kinds  of 
yolk-grains,  the  yellow  and  the  white.  The  yellow  grains  are 
spheres  of  from  25  p.  to  100  p.  in  diameter,  filled  with  numerous 
minute,  highly  ref ractile  granules ;  these  spheres  are  very  delicate, 
and  easily  destroyed  by  crushing.  When  boiled 
or  otherwise  hardened  in  situ9  they  assume  a 
polyhedral  form  from  mutual  pressure.  The 
white  grains  are  vesicles,  for  the  most  part  smaller 
(4  M  to  75  P)  than  the  spheres  of  the  yellow  yolk 
with  a  highly  refractive  body,  often  as  small  as 
1  fi  in  the  interior  of  each.  There  are  also  larger 
spheres,  each  of  which  contains  a  number  of 
spherules  similar  to  the  smaller  vesicles.  The 
yolk-plates,  of  plagiostomes,  which  consist  prin- 
cipally of  lecithin  and  nuclein,  are  not  present  in 
the  younger  ova,  but  are  present  in  great  num- 
FIG.  33. —Egg-ceil  of  bers  in  the  full-grown  ones;  they  are  oval,  barrel- 

Tegenana  domestica.    n,       ,  ,         ,      ,'.  •J_i  -i    j 

nucleus;     fc,    laminate    shaped,  or  rectangular  bodies,  with  rounded  cor- 
goik-nudeus  After Bai-   nerg   and  edgeg .   the   surface)  especially  in  the 

larger  plates,  shows  a  fine  transverse  striation, 
corresponding  to  the  laminate  structure  of  the  grain.  As  no  thorough 
comparative  investigation  of  the  yolk-granules  has  been  made,  it  is 
not  worth  while  to  enter  into  further  details. 

Besides  the  yolk-grains  there  may  also  be  present  one  or  several 
large  masses  of  nutritive  material,  such  as  the  "  oil-globules "  of 
many  teleosts,  or  the  so-called  yolk-nucleus.  The  "  oil-globules  "  are 
produced  by  the  liquefaction  of  the  yolk,  and  are  not  oily.  The 
yolk-nucleus  has  been  described  by  Balbiani  in  the  Araclmida,  The 
eggs  of  some  spiders  contain,  besides  the  nucleus,  a  second  body 
(Fig.  33,  &),  of  about  the  same  size  as  the  nucleus,  solid,  resistant,  and 
exhibiting  indications  of  a  series  of  concentric  Iamina3 ;  this  is  the 
so-called  yolk-nucleus,  and  is  probably  only  a  specialized  form  of 
deutoplasm,  and  might  be  compared,  for  instance,  to  the  four  large 
oil-globules  described  by  Spengel  in  the  eggs  of  Bonellia  viridis. 


OVA.  55 

A  yolk-nucleus  has  since  been  recorded  in  the  ova  of  various  verte- 
brates; thus  Schiitz  ('' Ueber  den  Dotterkern,"  Diss.  Inaug.,  Bonn, 
18S2)  found  in  the  ovarian  ova  of  the  pike,  in  September  and  Octo- 
ber, a  round  or  oval  bod}',  not  sharply  delimited,  clear,  and  more 
homogeneous' than  the  protoplasm,  and  which  increased  in  size  with 
the  growth  of  the  egg.  A  }Tolk-nucleus,  consisting  of  an  accumula- 
tion of  larger  and  smaller  granules,  has  also  been  observed  in  the 
frog  and  newt  (O.  Schultze,  87. 1),  but  is  apparently  wanting  in  Bufo 
and  Bombinator  (Gotte) . 

The  amount  of  yolk  varies  in  different  animals  very  greatly,  and 
determines,  apparently,  the  size  of  the  ovum.  It  has  been  observed 
that  the  process  of  segmentation  varies  according  to  the  amount  of 
yolk,  and  this  has  led  to  the  arbitrary  division  of  ova  into  meroblas- 
fa'c-and  holoblastic  (see  Segmentation  of  the  Ovum).  The  yolk  usu- 
ally, perhaps  always,  leaves  a  peripheral  layer  of  protoplasm  free. 
In  all  vertebrate  and  in  some  invertebrate  ova  this  layer  of  proto- 
plasm is  thickened,  often  considerably,  around  one  pole  of  the  ovum, 
which  is  then  distinguished  as  the  animal  pole,  the  opposite  pole 
being  called  the  vegetative.  These  are  old  terms,  which  have  come 
down  to  us  from  the  time  when  the  ectoderm,  which  is  produced 
during  segmentation,  principally  from  the  substance  of  the  animal 
pole,  was  called  the  animal  layer  and  the  entoderm  the  vegetative 
layer.  It  is  at  the  animal  pole  that  the  extrusion  of  the  polar  glob- 
ules under  normal  conditions  invariably  takes  place. 

The  Graafian  follicle  grows  very  much  more  than  the  ovum,  until 
it  becomes  a  large  cyst,  Fig.  32,  the  position  of  which  is  marked  by 
an  external  protuberance  on  the  surface  of  the  ovary.  To  the  deep 
wall  of  this  cyst  is  attached  the  discus  proligerus  with  the  ovum, 
which  is  now  nearly  full  grown.  The  stigma  is  at  the  protuberant 
point  of  the  follicle,  which  is  covered  by  very  little  ovarial  tissue,  so 
that  there  is  a  very  thin  wall  only  separating  the  cavity  of  the  folli- 
cle from  that  of  the  abdomen. 

The  degeneration  of  the  Graafian  follicles  with  the  contained  ovum 
occurs  normally  in  the  ovary ;  but  as  the  process  has  no  direct  inter- 
est for  the  embryologist,  it  will  suffice  to  refer  to  Frommann's  very 
admirable  summary  (Eulenburg's  "Real.  Encyclop.  Heilkunde,"  V., 
602-604). 

Full-grown  Ovum  before  Maturation. — The  full-grown  hu- 
man ovum  is  distinguished  among  mammalian  ova  for  the  clear 
development  and  ready  visibility  of  all  its  parts — a  peculiarity  due 
chiefly  to  the  small  amount  of  the  yolk  and  fewness  of  the  fat-gran- 
ules it  contains.  Fig.  34  represents  an  ovum  from  a  nearly  mature 
Graafian  follicle  of  a  woman  of  thirty  years ;  the  specimen  was  ob- 
tained by  ovariotomy  and  examined  and  drawn  in  the  fresh  state, 
being  kept  in  the  liquor  follicle.  This  specimen  gave  the  following 
measures-  The  diameter  of  the  whole  ovum,  including  the  zona 
radiata,  165-170  <>.\  thickness  of  the  zona,  20-24  /'-;  perivitelline  fis- 
sure, 1.3  //;  the  clear  outer  zone  of  the  yolk,  4-0  /*;  the  protoplasmic 
zone,  10-21  // ;  the  deutoplasm  zone,  82-87  // ;  the  nucleus,  25-27  /*. 
The  corona  radiata,  cor.  r.,  exhibits  the  features  already  described. 
The  zona  pellucida,  Z,  shows  a  distinct  radial  striation ;  this  is  prob- 
ably due  to  the  presence  of  minute  pore  canals  running  through  the 


5(3 


THE    GENITAL   PRODUCTS. 


,cor.r. 


FIG.  34. — Full-grown  human  ovum  before  matura 
tion :  cor.  r,  part  of  the  corona  radiata ;  Z,  zona  pellu 
cida ;  PI,  protoplasm ;  F,  yolk ;  JVw,  nucleus.  After 
W.  Nagel. 


zona,  and  which,  at  least  in  early  stages,  give  passage  to  processes  of 
the  cells  of  the  corona  radiata,  which  unite  with  the  ovum.  These 
processes  have  not  yet  been  observed  in  man  in  an  altogether  satis- 
factory manner,  and  indeed  Nagel,  88.1,  402,  expressly  denies  their 
existence,  as  well  as  that  of  the  pore  canals.  The  processes  are,  how- 
ever, readily  seen  in  the  low- 
er vertebrates,  in  the  mono- 
tremes  and  marsupials,  Cald- 
well,  87.1,  and  have  been 
observed  in  the  placental 
mammalia,  Fig.  35.  Hence 
it  seems  probable  that  they 
are  present  in  man  at  least 
while  the  ovum  is  growing, 
though  they  may  be  obliter- 
ated at  the  stage  we  are  now 
considering.  Several  observ- 
ers record  "  dumb-bell  cells"  * 
with  the  thin  portion  of  the 
cell  passing  through  the  zona, 

yom^:  :^J,;*y^cW  ^u-      and  one  knob  lyingontheoutr 

side,  the  other  on  the  inside, 
of  the  zona,  compare  H.  Vir- 
chow,  85.1.  But  apparently 
such  observations  have  been 
made  solely  on  ova  that  had 
been  somewhat  macerated,  and  therefore  the  "  dumb-bell  cells"  result 
probably  from  post-mortem  changes,  and  cannot  be  interpreted  as  by 
Lindgren,  77.1,  to  prove  the  actual  normal  passage  of  cells  of  the 
discus  proligerus  through  the  zona.  The  zona  has  no  micropyle  or 
special  open  channel  for  the  entrance  of  the  spermatozoon.  For 
additional  details,  see  the  following  section  on  the  envelopes  of  the 
ovum. 

The  ovum  proper  is  separated  by  a  narrow  fissure,  p  v,  the  peri- 
vitelline  space,  from  the  zona,  within  which  it  lies  free  and  loose,  so 
that  when  a  fresh  specimen  is  examined  the  same  side  of  the  ovum 
—that  containing  the  nucleus,  which  is  the  lightest  part — is  always 
found  uppermost. 

The-  ovum  has  no  vitelline  membrane,  according  to  Nagel,  88. 1, 
405 ;  but  in  several  mammals  such  a  membrane  has  been  described, 
appearing  as  a  thin,  delicate  line  about  the  time  the  ovum  matures, 
Fig.  35,  v.m.  The  body  of  the  ovum  may  be  divided  into  an  inner 
kernel  containing  the  yolk-granules  and  an  outer  protoplasmatic  zone, 
of  which  the  very  outermost  thin  layer  is  clear,  and  therefore  more 
or  less  differentiated  from  the  broader,  deeper  layer,  which  is  granular 
and  constitutes  most  of  the  zone.  Frommann,  89. 1,  has  shown  that 
many  of  the  granules  in  the  ova  of  the  sea-urchin  are  part  of  the 
protoplasmic  reticulum;  in  the  living  egg  they  are  incessantly 
changing  in  shape  and  in  their  connections,  even  disappearing  and 
reappearing;  the  disappearance  Frommann  terms  liquefaction,  the 
reappearance  a  new  formation.  It  seems  to  me  possible  that  the 

*  Nagelzellen,  Spundzellen,  Zwillingszellen,  or  Hantelzellen  of  German  writers. 


OVA. 


changes  seen  are  probably  in  part  effects  of  contraction  in  the  reticu- 
lum.  The  nucleus  is  nearly  spherical,  always  eccentric  in  position, 
and  has  a  nucleolus  which  in  the  fresh  specimen  shows  amoeboid 
movements  even  at  ordinary  summer  temperatures  for  several  hours 
after  removal  from  the  ovary,  Nagel,  88.1,  407.  In  hardened  speci- 
mens the  nucleus  shows  its  reticulum,  as  already  described. 

In  certain  ova  there  has  been  observed  a  special  band  of  proto- 
plasm leading  from  the  surface  of  the  ovum  to  the  egg  nucleus. 
This  is  found  in  the  ovum  of  Petromj-zon,  having  been  first  described 
and  figured  by  Calberla,  78. 1,  who,  however,  erroneously  designated 
the  nucleus  as  the  female  pronucleus,  and  interpreted  it  as  the  path- 
way performed  for  the  passage  of  the  spermatozoon — an  error  which 
Boehm  has  corrected  by  showing  that  the  true  pronucleus  is  formed 
later.  As  shown  in  the  section  on  impregnation,  p.  69,  the  path- 
way of  the  spermatozoon  can  be  traced  in  certain  amphibian  ova. 

Peculiar  names  have  been  applied  to  the  nucleus  and  nucleolus  of 
the  ovum,  and  are  still  in  general  use.  The  nucleus  was  first  dis- 
covered in  1830  by 
Purkinge  ("  Sym- 
bolse  ad  ovarium 
historiam,  "1 830)  in 
birds,  and  by  Ooste 
(1837)  in  mam- 
mals, and  became 
known  as  the  vest- 
(  nl«  germinativa, 
(Pu  r  k  utjc.'sches 
Blaschen,  or  ger- 
mmal  vesicle). 
The  nucleolus  was 
first  described  in 
1835  by  R.  Wag- 
ner, 35.1,  and  be-  FIG.  35.—  Part  of  the  ovum  of  a  mole:  En.  cells  of  corona  radia- 
Cailie  kllOWll  as  ta ;  Zt  ZODa  Pellucida '  v- m- »  vltelline  membrane ;  p.  c. ,  pore  canals. 

the  germinative  or 

Wagnerian  spot  (Wagner*  seller  Fleck).  It  was  not,  however,  until 
1839  that  Theodore  Schwann  for  the  first  time  interpreted  the  ovum 
as  a  cell ;  but  before  then  the  terms  germinal  vesicle  and  germinal 
spot  had  established  themselves,  and  since  then  they  have  remained 
in  general  use. 

The  Envelopes  of  the  Ovum. — The  eggs  of  different  classes, 
and  even  species  of  animals,  are,  as  is  well  known,  extremely  unlike 
in  appearance.  The  dissimilarity  refers  chiefly  to  size,  to  the  char- 
acter ot  the  yolk,  and  the  nature  and  number  of  membranes  or  other 
envelopes,  by  which  the  ovum  or  egg-cell  proper  is  surrounded. 
Thus  in  the  hen's  egg  the  yolk  alone  represents  the  part  correspond- 
ing to  the  egg-cell,  while  the  white  of  the  egg  and  the  egg-shell  are 
only  secondary  envelopes,  the  former  serving  to  nourish,  the  latter 
to  protect,  the  so-called  yolk,  which  is  the  essential  part,  the  true 
egg  The  various  envelopes  which  eggs  ever  have  may  be  classed 
under  tour  categories:  First,  a  very  thin  and  delicate  one,  the 
proper  membrane  ot  the  cell  itself,  and  which  ought  always  to  be 


58  THE    GENITAL    PRODUCTS. 

distinguished  as  the  vitelline  membrane;  second,  the  ovarian  enve- 
lopes, which  are  secreted  around  the  egg-cell  by  the  tissues  of  the 
ovary;  third,  the  envelopes  secreted  by  the  oviduct,  which  may 
form  a  coating  of  nutritive  material,  or  a  protective  shell,  or  both, 
as  in  the  hen's  egg,  of  which  the  nutritive  white  is  secreted  by  the 
upper  part,  the  calcareous  shell  by  the  middle  part  of  the  oviduct ; 
fourth,  coverings  secreted  by  accessory  glands,  such  as  the  slime  in 
which  the  eggs  of  snails  are  embedded,  or  the  tough  capsules  in 
which  leeches  lay  their  eggs.  By  adhering  to  this  classification  it  is 
possible  to  find  one's  way  through  the  labyrinth  of  special  descrip- 
tions. It  is  impossible  to  review  here  the  manifold  variations  in 
the  ovarian  coverings  of  animals,  and  we  shall  attempt  only  to  de- 
scribe those  of  the  higher  forms. 

All  vertebrate  ova  probably  have  two  envelopes :  first ,  a  very  thin 
inner  one,  the  vitelline  membrane  proper ;  second,  a  thicker  ovarian 
membrane,  known  as  the  zona  radiata  or  pellucida.  The  vitelline 
membrane  is  described  by  Heape,  in  the  mole,  as  a  very  thin  but 
distinct  membrane  (Fig.  35,  v.  m.),  immediately  against  the  yolk, 
separated  by  a  narrow  space  from  the  zona ,  it  is  to  be  regarded  as 
a  product  of  the  ovum  itself.  It  appears  a  short  time  before  the 
ovum  matures,  and  is  most  distinct  at  the  time  of  the  formation  of 
the  polar  globules;  its  fate  during  segmentation  has  not  been  ascer- 
tained. The  so-called  vitelline  membrane  (Dotterhaut)  of  amphibia 
is  really  the  homologue  of  the  zona  (Frommann).  Considerable 
doubt  in  regard  to  the  presence  of  this  membrane  in  vertebrates,  and 
especially  in  mammals,  has  been  expressed  by  various  writers,  but 
its  existence  seems  to  me  to  have  been  sufficiently  demonstrated.  It 
was  first  described  by  Reichert  in  1841,  and  again  by  H.  Meyer  in 
1842.  In  recent  years  it  has  been  redescribed  by  Ed.  van  Beneden, 
by  Heape,  and  others.  Balf our,  in  his  "  Embryology, "  pronounces  in 
favor  of  its  occurrence.  The  zona  radiata  (pellucida  of  C.  E.  v. 
Baer),  Fig.  35,  Z,  is  a  membrane,  usually  of 
considerable  thickness,  which  can  be  distin- 
guished around  the  ovarian  ovum  quite 
early,-  being  at  first  very  thin,  but  gradually 
increasing  in  thickness  until  it  attains  in 
man  a  diameter  of  about  20  p.  in  the  mature 
ovum.  In  the  pig  the  diameter  becomes  7 
to  9  /j. ;  in  the  sheep,  7  to  1 2  p '•',  in  the  cow,  7 
to  8  n  (Schulin) ;  in  the  mole,  8  to  11  P.,  ac- 
cording to  Heape.  In  the  mature  ovum  it 
is  a  tough,  clear,  glistening  membrane,  very 
FIG.  ae.-ovum  of  a  sea  ur-  resistant  to  acids,  and  soluble  in  alkalies  only 
After  o^iStwiT8  with  difficulty.  IMs  pierced  by  numerous 

radiating  pores,  which  produce  the  appear- 
ance to  which  the  term  zona  radiata  refers.  These  pores  were 
first  observed  by  Johannes  M  tiller  and  Remak  in  fish  eggs,* 
and  they  have  since  been  observed  in  the  ova  of  many  other  verte- 
brates, including  several  species  of  mammalia.  It  is  probable  that 
they  always  exist,  despite  the  doubts  expressed  by  Schulin,  Lind- 

*  The  homologies  o£  the  two  envelopes  around  fish  ova  are  somewhat  uncertain, see  E   L   Mark, 
9u.  j. 


OVA.  59 

gren,  Von  Sehlen,  Nagel,  and  others.  While  the  ovum  is  still  in  the 
ovary  it  is  surrounded  by  the  cells  of  the  discus  proligerus ;  these 
cells  send  processes  through  the  pores  of  the  zona  (Fig.  35).  It 
is  now  commonly  supposed  that  these  processes  are  channels  of  nutri- 
tion for  the  ovum.  The  zona  is  somewhat  granular  in  its  outer  por- 
tion, next  the  cells  of  the  corona.  Balfour  has  suggested,  not  very 
plausibly,  I  think,  that  the  granular  portion  does  not  belong  to  the 
zona,  but  represents  the  remains  of  a  hypothetical  primary  vitelline 
membrane,  within  which  the  zona  proper  arose  subsequently.  An- 
other very  hypothetical  homology  is  suggested  by  C  aid  well,  87.1, 
who  finds  two  membranes  around  the  ovarian  ovum  of  marsupials ; 
the  inner  membrane  resembles  the  zona  pellucida,  and  is  termed  by 
Caldwell  erroneously  the  vitelline  membrane ;  the  outer  membrane 
is  the  proalbumen,  which,  during  the  passage  of  the  ovum  through 
the  oviduct,  swells  up  and  becomes  the  albuminous  envelope  of  the 
egg.  Caldwell  homologizes  the  inner  clear  layer  of  the  zona  of  the 
placenta!  mammals  with  the  zona  of  marsupials,  and  the  outer  gran- 
ular layer  with  the  proalbumen.  In  Petromyzon  (Boehm,  88. 1),  as 
in  some  teleosts  (J.  Brock,  78.1),  there  are  two  ovarian  envelopes 
which  are  quite  probably  homologous  with  two  envelopes  found  in 
marsupialia,  but  that  the  zona  of  the  placentalia  represents  'two 
envelopes  united  in  one  is,  at  least,  very  uncertain.  It  seems  to 
me  that  the  zona  radiata  is  to  be  regarded  as  a  modified  intercellu- 
lar substance,  and  that  the  processes  going  through  its  pores  are  to  be 
homologized  with  the  ordinary  intercellular  protoplasmic  bridges  of 
epithelial  cells. 

Heape  thus  describes  the  pores  of  the  zona  in  the  mole:  "The 
radially  striated  appearance  of  the  zona  has  long  been  shown  to  be 
due  to  a  vast  number  of  fino  canals  passing  radially  through  it.  The 
canals,  I  find,  open  on  the  inner  side  of  the  zona  by  a  slightly  dilated 
mouth,  while  on  the  outer  side  of  the  zona  they  communicate  with 
the  exterior  by  a  considerably  wider  opening,  Fig.  35.  Into  the 
external  openings  of  these  canals  I  have  been  able  to  trace  prolonga- 
tions of  those  cells  of  the  discus  which  are  immediately  in  contact 
therewith,  Fig.  35,  and  there  appears  to  me  no  room  to  doubt  that 
the  contents  of  these  f ollicular  cells  are  thus  rendered  available  for  the 
nutriment  and  growth  of  the  ovum." 

The  term  micropyle  is  used  to  designate  a  passage  through  the 
envelopes  of  the  ovum,  which  serves  to  admit  the  spermatozoon. 
The  micropyle  is  present  in  many  invertebrate  ova,  notably  in  those 
of  insects,  arid  may  have  a  quite  complicated  structure.  In  the 
vertebrates  it  is  very  rarely  found,  having  been  thus  far  positively 
demonstrated  only  in  certain  teleost  eggs.  Calberla,  78.1,  affirmed 
that  a  micropyle  was  present  in  Petromyzon ;  but  Boehm,  after  a 
later  and  more  thorough  investigation,  88. 1,  expressly  denies  its  ex- 
istence, Kupffer  and  Benecke,  78.2,  having  previously  shown  that 
the  spermatozoa  penetrated  the  lamprey  ovum  at  several  points. 
Sundry  authors  frcm  time  to  time  have  asserted  that  a  micropyle 
was  present  in  the  mamamlian  ovum,  but  the  evidence  against  it 
seems  to  me  conclusive. 

The  corona  railiatti  is  the  name  given  to  the  envelope  of  cells  of 
the  discus  proligerus,  which  adheres  for  a  short  time  to  the  zona 


60  THE    GENITAL   PRODUCTS. 

radiata  when  the  ovum  is  discharged  from  the  Graafian  follicle. 
The  corona  may  be  represented  only  by  a  few  patches  of  cells,  or 
may  be  a  complete  envelope;  in  either  case  the  cells  are  entirely 
lost  soon  after  the  ovum  begins  its  descent  through  the  Fallopian 
tube.  The  egg  of  Lepidosteus  has  two  envelopes ;  the  outer  one  is 
homologized  by  Beard  with  the  corona  radiata,  but  E.  L.  Mark,  90. 1 , 
denies  this  homology. 

The  disappearance  of  the  zona  has  been  specially  studied  by  Tour- 
neux  et  Hermann  (C.  R.  Soc.  Biol.,  Paris,  1887,  p.  49),  who  found 
that  it  could  be  distinguished  in  rabbits'  ova  of  ninety-five  hours, 
but  not  in  those  of  one  hundred  and  sixteen  hours.  According  to 
Hensen  the  zona  in  guinea-pigs  is  ruptured,  and  the  ovum  escapes 
during  the  descent  through  the  oviduct. 

Polarity  of  the  Ovum. — The  mature  egg-cell  has  a  distinct 
axis,  the  two  poles  of  which  are  unlike  in  character,  while  around 
the  axis  there  is  a  complete  radial  symmetry  so  far  as  known.  In 
my  opinion  the  essential  difference  between  the  two  poles  is  that  the 
nucleus  is  nearer  one  than  the  other,  and  consequently  the  proto- 
plasm of  the  egg-cell  is  more  concentrated  at  one  pole  than  at  the 
other ;  for,  as  is  well  known,  the  nucleus  usually  has  an  accumula- 
tion of  protoplasm  around  it.  The  eccentric  position  of  the  nucleus 
is,  I  think,  probably  universal.  Curiously  it  is  frequently  stated 
that  the  nucleus  lies  in  simple  ova  in  the  centre,*  and  the  notion  is 
prevalent  that  the  accumulation  of  yolk  is  the  cause  of  the  eccentric 
position  in  certain  ova.  This  notion  is  not  quite  correct;  on  the  con- 
trary, we  must  assume  that  the  position  of  the  nucleus  causes  the' 
eccentricity  of  the  yolk  material.  There  is  unquestionably  a  strong 
tendency  for  nucleus  and  protoplasm  to  keep  company :  thus  we  see 
when  cells  are  connected  with  one  another  by  protoplasmatic  bridges, 
a  main  cell-body  around  each  nucleus.  Again,  within  single  cells,  the 
protoplasm  often  forms  a  court  around  the  nucleus  and  a  looser  net- 
work throughout  the  rest  of  the  cell;  in  ova  with  incomplete  segmen- 
tation each  nucleus  is  imbedded  in  its  special  accumulation  of  proto- 
plasm ;  it  appears  to  me,  accordingly,  that  the  disposition  in  the  egg- 
cell  is  only  a  special  instance  of  a  more  general  principle. 

The  eccentric  position  of  the  ovic  nucleus  is  due  to  as  yet  un- 
known causes ;  but  being  given  it  determines  the  accum'ulation  of 
yolk-grains  at  the  opposite  pole ;  it  will  be  remembered  that  in  the 
developing  egg-cell  the  nucleus  becomes  eccentric  before  the  yolk- 
grains  appear.  The  amount  of  yolk  undoubtedly  affects  the  degree 
of  the  nuclear  eccentricity.  The  nucleus  reigns  over  a  compara- 
tively small  territory,  within  which  there  is  no,  or  but  very  little, 
yolk-matter  developed ;  in  all  vertebrate  ova  the  perinuclear  proto- 
plasm touches  the  vitelline  membrane  and  marks  externally  the  site 
of  the  nuclear  or  so-called  u  animal"  pole.  In  the  rest  of  the  egg-cell 
the  yolk-grains  may  be  freely  developed,  and  as  they  increase  in 
number  and  size  there  is  a  corresponding  distention  of  the  region  of 
the  cell  which  they  occupy.  This  distention  may  go  so  far  that, 
as  in  the  birds'  ovum,  the  perinuclear  territory  is  minute  compared 
with  the  great  bulk  of  the  deutoplasmic  territory,  and  consequently 

*  For  example  O.  Heruvi^.  l**8  1,  p.  8,  says  "das  Keimbliischen  lagert  gewohnlich  in  der 
Mitte  des  Eies,"  yet  his  own  figures  correctly  represent  it  as  eccentric. 


OVA.  Gl 

the  nucleus  lies  far  away  from  the  centre  of  the  ovum.*  The  yolk- 
grains  centre  about  the  pole  opposite  the  nucleus,  which  might 
therefore  be  called  the  vitelline  or  deutoplasmic  pole,  though  it  is 
still  generally  known  by  the  inappropriate  name  of  vegetative  pole, 
which  has  come  down  to  us  from  long  ago. 

F.  M.  Balfour,  in  his  "  Comparative  Embryology,"  divided  ova  into 
three  classes,  as  follows :  1st,  alecithaly  without  any  deutoplasm ; 
2d,  telolecitlial,  with  the  deutoplasm  collected  opposite  the  animal 
pole;  3d,  centroletithal,  with  the  deutoplasm  in  the  centre  sur- 
rounded by  a  cortex  of  protoplasm.  It  is  probable  that  all  ova  are 
telolecitlial  in  the  sense  that  they  have  a  nuclear  pole,  and  that  the 
yolk-matter  is  developed  away  from  the  nuclear  pole.  The  alecithal 
ova  are  those  in  which  the  nuclear  eccentricity  is  at  a  minimum ; 
the  cent  rolecithal  ova,  which  occur  only  among  invertebrates,  are 
likely  to  prove  to  be  really  telolecithal.  All  known  vertebrate  ova  are 
telolecithal. 

The  polarity  of  the  ovum  dominates  the  process  of  the  ripening  of 
the  egg-cell,  and  has  a  very  important  influence  on  the  process  of 
segmentation  after  impregnation.  The  extent  of  this  domination 
has  been  thus  summarized  by  E.  L.  Mark,  81.1,  515:  "The  migra- 
tion of  the  germinative  vesicle  toward  a  definite  point  of  the  sur- 
face; the  radial  position  assumed  by  the  maturation  spindles;  the 
waves  of  constriction  which  precede  the  formation  of  the  polar 
globules,  and  the  inequalities  in  the  sizes  of  the  latter ;  the  union 
of  the  pronuclei  at  a  point  nearer  the  primary  than  the  secondary 
pole,  and  the  consequently  (?)  eccentric  position  of  the  first  seg- 
mentation spindle;  the  appearance  of  the  first  segmentation 
furrow  earlier  at  the  primary  than  at  the  opposite  pole;  the  for- 
mation of  pseudopodia-like  elevations,  often  most  conspicuous  at 
the  primary  pole ;  the  accumulation  of  finely  granular  protoplasm  at 
the  secondary  pole  after  the  elimination  of  the  polar  globules ;  and 
the  appearance  of  '  polar  rings  '  and  '  ring  rays  (Clepsine)  at  both 
ends  of  the  primitive  axis,  are  all  indications  of  a  polar  differentia- 
tion of  the  egg." 

The  polarity  of  the  ovum  also  evinces  itself  in  the  difference  of 
the  specific  gravity  of  the  two  poles ;  usually,  as  in  mammals,  birds, 
amphibians,  many  fish  and  invertebrates,  the  deutoplasmic  pole  is 
heavier,  and  the  ovum  always  presents  the  animal  pole  uppermost 
as  soon  as  it  is  left  free  to  turn ;  in  the  ripe  mammalian  egg  the 
yolk  has  room  to  turn  within  the  zona :  hence  when  the  fresh  ovum  is 
examined  under  the  microscope,  the  animal  pole  is  toward  the  ob- 
server and  the  eccentric  position  of  the  nucleus  cannot  be  observed. 
In  various  pelagic  teleost  ova  the  animal  pole  is  the  heavier,  and  the 
embryo  develops  accordingly  on  the  under  side  of  the  egg. 

Maturation  of  the  Ovum. — The  term  maturation  is  restricted 
by  usage  to  the  series  of  phenomena  accompanying  the  expulsion 
of  the  polar  globules  which  occurs  after  the  egg-cell  has  attained  its 
full  size,  and  just  before  or  just  after  the  separation  of  the  ovum 
from  the  ovary.  A  polar  globule  is  a  small,  nucleated  mass,  extruded 
from  a  fully-grown  egg-cell. 

When  an  ovum  is  about  to  mature  its  nucleus  moves  nearer  that 

*J   A.  Ryder  has  published  a  semi-popular  discussion  of  nuclear  displacement,  83.  1. 


62  THE    GENITAL   PRODUCTS. 

point  of  the  surface  which  may  be  regarded  as  the  centre  of  the  ani- 
mal pole,  and  there  also  occurs  a  contraction  of  the  vitellus.  The 
centrifugal  movement  of  the  nucleus  was  first  observed  by  Von 
Baer,  27. 1,  29,  in  the  hen's  egg,  and  has  since  been  seen  by  very 
numerous  observers  and  in  very  numerous  species ;  it  must,  there- 
fore, be  considered  as  an  unvariable  phenomenon.  Concerning  the 
force  which  moves  the  nucleus  we  have  no  definite  conception ;  for 
discussion  of  the  question,  see  Whitman,  87.3.  The  contraction  of 
the  yolk  is  probably  also  a  constant  phenomenon ;  it  is  apparently 
effected  by  the  expulsion  of  fluid  from  the  protoplasm,  so  that  a  clear 
space  separates  the  zona  and  yolk.  The  observations  have  not  been 
collated  yet  on  this  point,  and  it  is  impossible  to  state  whether  there 
is  a  constant  rule  as  to  the  extent  and  epoch  of  the  contraction. 

After  reaching  the  surface  the  nucleus  as  such  disappears.  This 
fact  was  known  to  Purkinje,  30. 1,  15,  the  discoverer  of  the  nucleus, 
and  has  been  shown  to  occur  in  all  eggs  which  have  been  accurately 
examined.  K.  E.  von  Baer  maintained  both  in  1827  and  subse- 
quently, 37.1,  4  and  9,  37.1,  28,  157,  297,  the  opinion  that  the  dis- 
appearance of  the  germinal  vesicle  was  connected  with  the  maturation 
of  the  ovum — a  conclusion  which  is  now  established  beyond  ques- 
tion. Reichert  in  1846,  46.1,  199,  205,  maintained  that  the  disap- 
pearance was  the  first  result  of  impregnation,  and  in  this  error  he  has 
had  sev^-al  followers  (A.  Muller,  Haeckel,  Biitschli,  and  others). 
In  birds  the  nucleus  assumes  a  very  large  size,  and  migrates  to  the 
surface  of  the  ovum,  when  it  disappears  as  shown  by  Oellacher. 
M.  Holl,  90. 1,  records  that  in  a  newly  hatched  chick  the  ova  meas- 
ured about  14  /Jt  X  9  /-/-,  while  an  ovum  nearly  ready  to  leave  the  ovary 
measured  40  X  35  mm ;  in  the  former  the  nucleus  was  about  9  />-,  in 
the  latter  315X117  />-  in  diameter.  No  polar  globules  have  yet  been 
observed  in  birds,  though  we  must  assume  that  they  are  formed. 

The  disappearance  of  the  germinal  vesicle  is  only  apparent,  not 
actual,  being  in  reality  a  metamorphosis.  It  is  probable  that  the  first 
step  is  the  discharge  of  nuclear  fluid  (Kernsaft)  into  the  surrounding 
protoplasm.  This  is  indicated  by  two  appearances — 1st,  the  shrink- 
ing of  the  nucleus,  the  outline  of  which  becomes  shrivelled ;  2d,  a 
clear  space  which  arises  around  the  nucleus.  The  shrivelling  of  the 
nucleus  has  been  observed  in  several  mammals  (Van  Beneden,  Rein, 
Bellonci,  Tafani)  in  various  vertebrates — as,  for  instance,  in  teleosts 
by  Oellacher,  72.1,  3,  in  Amphibia  by  O.  Schultze,  87.1,  and  in 
many  invertebrates,  e.g.,  Serpula  by  Schenk  (Sitzber.  Wien.  Akad. 
LXX.  Abth.  3,  291-294,  1875),  in  Hydra  by  Klemenberg,  72. 1,  42,  in 
Asterocanthion  by  Ed. van  Beneden, 76. 1 .  The  clear  perinuclear  space 
has  been  noticed  especially  in  Anura  by  Gotte,  75.1,  20-22,  and  O. 
Schultze,  87. 1,  217.  The  second  step  is  the  dissolution  of  the  mem- 
brane of  the  nucleus,  so  that  the  nuclear  contents  are  brought  into 
direct  contact  with,  and  partly  mix  with,  the  cell-plasma.  Very 
likely  this  mixture  of  nuclear  and  cell  substance  is,  as  O.  Schultze 
suggests,  87. 1,  215,  one  of  the  essential  factors  of  maturation.  The 
dissolution  of  the  nuclear  membrane  has  been  found  to  occur  in  so 
many  species  that  we  may  safely  predicate  it  of  all.  We  now  find 
the  contents  of  the  nucleus  lying  together  in  the  centre  of  the  proto- 
plasm of  the  animal  pole.  The  contents  themselves  are  altered  in 


OVA. 


63 


Fio.  37,  —Ovarian  egg    of 
Haemops :  sp ,  nuclear  spindle ; 


character,  the  most  noticeable  change  being  the  breaking  up  of  the 
chromatin  into  separate  granules ;  in  mammals  the  formation  of  the 
granules  by  the  cleavage  of  the  iiucleolus  occurs  after  the  nucleus 
has  begun  its  migration  (van  Beneden,  Bellonci,  Tafani) ;  in  Am- 
phibia the  nucleus  becomes  multinucleate  during  the  early  growth 
of  the  ovum.  The  achromatic  substance  or 
reticulum  of  the  nucleus  appears  as  threads 
often  very  difficult  to  recognize. 

The  threads  and  granules  proceed  to  group 
themselves  into  a  spindle-shaped  body,  the  so- 
called  nuclear  spindle  (Kemspindel)  which 
lies  more  or  less  nearly  in  the  radius  of  the 
ovum  and  has  one  of  its  ends  close  to  the  sur- 
face of  the  yolk,  Fig.  38,  sp.  The  achromatic 
threads  run  from  pole  to  pole  of  the  spindle ; 
the  chromatin  granules  lie  in  the  centre  of 
the  spindle  in  one  plane  and  produce  the  ap- 
pearance of  a  transverse  band  or  disc  (Strass- 
burger's  Keniplatte) ;  each  chromatin  granule 
is  associated  with  one  of  the  spindle-threads,  wig. 
Each  pointed  end  of  the  spindle  lies  just  within 

a  rounded  clear  space,  from  which,  and  not  from  the  end  of  the  spin- 
dle, radiates  threads  in  the  yolk,  whence  results  a  figure  like  a  con- 
ventional sun.  The  whole  spindle  with  the  two  suns  has  been 
named  the  <nn/>hifixtcr.  As  amphiasters  occur  in  connection  with 
ordinary  indirect  cell-division  the  distinctive  term  archiamphiaster 
has  been  proposed  for  those  concerned  in  the  production  of  the  polar 
globules.  Sometimes  as  in  Limax,  Mark,  81.1,  the  astral  rays  are 
not  straight,  but  curved  as  in  a  turbine.  In  amphibian  ova  only  a 
portion  of  the  granules  enter  into  the  formation  of  the  chromatin, 
while  the  majority  of  them  are  mingled  with  the  yolk  (O.  Schultze) ; 

it  is  possible  that  this  modification  is 
connected  with  the  large  amount  of  yolk 
and  will  be  found  in  other  vertebrate 
ova.  In  the  ova  of  mammals  (all?)  the 
chromatin  enters  into  the  "  Kernplatte." 
The  shape  of  the  spindle  varies,  as  does 
also  the  distribution  of  the  granules  of 
the  nuclear  plate,  thus :  In  the  guinea- 
pig,  the  ends  are  pointed  and  the  threads 
are  straight,  so  the  outline  of  the  spindle 
is  like  a  diamond ;  in  the  bat  the  spindle 
is  barrel-shaped  and  the  threads  are 
curved.  In  certain,  possi biy  in  all,  cases 
the  spindle,  when  first  formed,  lies  ob- 
liquely, and  subsequently  becomes  erect 
to  the  surface,  as  Whitman  observed  in  the  leech  (Clepsine,  78.2) ; 
for  further  reference,  see  O.  Schultze,  87. 1,  219-221 .  The  reason  for 
the  obliquity  and  the  following  erection  is  unknown. 

The  next  changes  may  be  followed  with  the  help  of  Fig.  39.  The 
spindle,  driven  by  an  undiscovered  power,  continues  the  centrifugal 
movement  until  it  is  partly  extruded  from  the  egg,  as  shown  in  the 


FIG  38.— Egg  of  a  leech  CXephelis), 
three-quarters  of  an  hour  after  being 
laid  •  formation  of  the  first  polar  glob- 
ule, p  g.  After  O  Hertwig. 


04  THE    GENITAL   PRODUCTS. 

figure ;  the  projecting  end  is  enclosed  in  a  distinct  mass  of  protoplasm, 
p.g.,  which  is  constricted  around  its  base.  The  fragments  of  chro- 
matin  have  each  divided  into  two,  and  one-half  of  each  fragment 
has  moved  toward  one  end,  the  other  half  toward  the  other  end  of 
the  spindle.  The  lialf-fragments  of  each  set  move  together,  hence 
there  seem  to  be  two  plates  within  the  spindle.  The  translation  of 
the  groups  of  chromatin  grains  continues  until  they  reach  the  ends 
of  the  spindle;  the  achromatic  threads  then  break  through  in  the 
middle.  Thus  the  original  nucleus,  or  at  least  part  of  it,  has  been 
divided.  There  are  now  two  masses  of  nuclear  substance — one  in  the 
ovum,  the  other  in  a  little  appendage  to  the  ovum ;  this  appendage 
is  the  first  polar  globule ;  its  nuclear  substance  does  not  develop  into 
a  complete  nucleus. 

The  remnants  of  the  egg-cell  nucleus  within  the  ovum  undergo 
further  changes.  Usually  when  the  amphiastral  (indirect  or  kinetic) 
division  of  a  nucleus  is  over,  the  separated  nuclear  masses  resume 
the  structure  of  a  normal  resting  nucleus ;  but  in  the  ovum,  as  Plat- 
ner,  89. 1,  has  especially  noted,  the  nuclear  remnants  change  directly 
into  a  second  spindle,  which  lies  as  did  the  first  within  the  protoplasm 
of  the  animal  pole,  and  likewise  gives  rise  to  an  amphiaster  (second 

archiamphiaster,  zweites  Riclitungsspin- 
del) .  The  second  spindle  even  more  clearly 
than  the  first  has  been  observed  to  occupy 
an  oblique  position,  as  in  mammals  (Bellon- 
ci,  85. 1),  or  even  parallel  with  the  surface, 
as  in  amphibians  (O.  Schultze,  87. 1)  and 
certain  Crustacea,  Weismann  and  Ischika- 
wa,  88.4.  This  spindle  produces  a  second 
polar  globule  in  similar  manner  to  the  first ; 
the  globule  is  somewhat  smaller  than  the 
first,  and  is  at  least  sometimes  connected 
both  wit£  the  first  globule  and  with  the 
nucleus;  TO,  male  pronucieus.  ovum,  bometimes  the  first  globule  divides 

into  two,  Fig.  39,  A,  and  they  may  remain 

connected  together.  The  connection  of  the  globules  with  the  yolk 
persists  for  some  time,  and  in  the  case  of  leeches  is  not  dissolved  until 
segmentation  begins. 

The  polar  globules  ultimately  disappear — how  is  not  exactly  known. 
That  they  take  no  part  in  the  further  history  of  the  ovum  may  be 
considered  established ;  for  they  break  off  and  may  often  be  seen  in 
mammals  knocking  about  within  the  zona,  while  the  ovum  is  devel- 
oping after  impregnation,  and  they  then  present  a  hyaline  appear- 
ance, as  if  slowly  degenerating. 

The  number  of  polar  globules,  as  Weismann  and  Ischikawa, 
87.2,  88.4,  first  explicitly  demonstrated,  is  two.  According  to 
these  authors,  88.4,  590,  two  polar  globules  have  been  shown  to  occur 
in  8  species  of  coelenterates,  5  of  plathelminths,  6  nemathelminths, 
1  zephyrean,  10  annelids,  5  echinoderms,  22  mollusks,  6  tunicates,  1 
bryozoon,  15  crustaceans,  G  insects,  11  vertebrates.  It  can  hardly 
be  doubted  that  two  polar  globules  are  necessary  for  the  complete 
maturation  of  the  ovum,  and  that  until  they  are  formed  impregna- 
tion cannot  take  place.  On  the  other  hand,  Blochmann  discovered 


.     OVA.  05 

that  in  a  parthenogeiietic  ovum  there  is  only  one  polar  globule 
formed,  and  Weismanu  and  Ischikawa,  88.4,  have  shown  that  this 
is  true  of  many  and  presumably  of  all  parthenogenetic  ova — that  is, 
of  ova  which  develop  without  fertilization.  For  the  theoretical 
consideration  of  the  polar  globules,  see  below. 

The  polar  globules  appear  to  have  been  seen  as  long  ago  as  1837 
by  Dumortier  in  gasteropods,  and  in  1810  by  the  elder  Van  Beneden, 
and  in  1842  in  the  rabbit  by  Bischoff.  Fr.  Muller  observed  them 
more  carefully  in  181-8,  and  detected  their  constant  relation  to  the 
planes  of  segmentation,  and  gave  them  the  current  German  name  of 
Kichtungskdrperchdn.  Robin,  in  1862,  termed  them  globules  po- 
laires,  which,  translated,  has  become  the  accepted  English  designa- 
tion. Biitschli,  76.1,  in  1 876,  first  led  the  way  toward  a  correct  con- 
ception of  the  origin  of  the  globules,  and  about  the  same  time  came 
the  independent  researches  of  O.  Hertwig,  whose  able  memoirs,  75.1, 
77.1,  77.2,  7 8.1,  have  formed  the  basis  of  all  subsequent  work. 
These  were  soon  followed  by  the  investigations  of  Fol  and  many 
others.  From  these  studies  we  possess  a  tolerable  general  concep- 
tion of  the  origin  of  the  polar  globules,  but  the  comparative  study  of 
the  details  and  variations  remains  for  the  future. 

After  the  formation  of  the  second  polar  globule  there  is  a  smaU 
group  of  chromatin  elements  and  achromatic  threads,  which,  since 
they  have  been  halved  twice,  represent  approximately  one-fourth,  not 
of  the  whole  egg  nucleus,  but  of  so  much  thereof  as  entered  into  the 
formation  of  the  first  polar  spindle.  The  nuclear  remnant  lies  close 
to  the  animal  pole  and  in  the  clear  protoplasm ;  it  is  the  so-called 
female  jtron  Helens,  the  history  of  which  varies  according  to  the 
species  of  animal.  Three  tendencies  are  known  to  affect  the  pro- 
nucleus — namely,  to  move  toward  a  central  position  in  the  ovum ;  to 
unite  with  the  male  pronucleus  as  soon  as  that  is  formed  out  of  the 
spermatozoon,  which  enters  the  ovum  to  fertilize  it,  and  to  assume 
the  character  of  a  membranate  nucleus.  As  the  time  of  the  forma- 
tion of  the  male  pronucleus  is  variable,  the  other  tendencies  being 
more  constant,  the  exact  history  of  the  female  pronucleus  may  be 
said  to  depend  principally  upon  the  appearance  of  the  male  pro- 
nucleus.  The  earlier  that  event,  the  less  does  the  female  pronucleus 
move  centripetally,  and  the  less  does  it  assume  a  nuclear  form.  In 
mammals  as  in  echinoderms,  the  female  pronucleus  acquires  a  mem- 
brane, and  lies,  when  the  spermatozoon  enters,  near  the  centre.  It 
is  very  much  smaller  than  the  egg  nucleus  (compare,  Figs.  30  and 
:>'.)),  and  is  remarkable  for  its  homogeneous  appearance  and  the  ab- 
sence of  nucleoli.  In  other  animals,  e.g.  Petromyzon,  it  is  merely 
a  cluster  of  granules.  For  further  details  as  to  the  pronuclei,  see  the 
following  section  on  impregnation. 

The  time  when  the  polar  globules  are  formed  varies,  and  accord- 
ing to  the  animal  may  be  before  or  after  the  egg-cell  leaves  the 
ovary.  In  placental  mammals  the  maturation  always  begins,  so  far 
as  known,  in  the  ovary,  and  may  be  completed  there,  or  it  may  go 
on  in  the  Fallopian  tube,  as  Tafani,  89.1,  114,  states  is  the  case  in 
white  mice.  Our  knowledge  of  the  maturation  of  the  mammalian 
ovum  is  very  imperfect,  and  rests  almost  exclusively  upon  observa- 
tions on  bats  and  rodents  (rabbits,  mice,  rats,  and  guinea-pigs),  and 


66  THE    GENITAL    PRODUCTS. 

even  on  these  the  observations  are  very  incomplete.  See  Ed.  van 
Benedeii,  80.1, Van  Beneden  et  Julin,  Rein,  83.1,  Bellonci,  85.1, 
Tafani,  89.1. 

III.    OVULATION. 

The  process  of  ovulation,  or  the  discharge  of  the  ovum  from  the 
ovary,  has  to  be  considered  from  both  the  morphological  and  physio- 
logical standpoint.  The  discharge  results  from  structural  changes  in 
the  Graafian  follicle,  and  these  changes  continue  after  the  departure 
of  the  ovum,  transforming  the  Graafian  follicle  into  a  so-called  cor- 
pus luteum.  Concerning  the  physiology  of  ovulation  we  know  al- 
most nothing  beyond  the  coincidence  in  some  species  of  mammals  of 
the  time  of  the  bursting  of  the  follicle  with  certain  periodic  changes 
in  the  uterus. 

Ovulational  Metamorphosis  of  the  Graafian  Follicle.— 
The  mature  follicle  measures  some  9  by  12  millimetres,  being  elon- 
gated in  the  same  direction  as  the  ovary,  but  its  dimensions  are 
variable.  The  granulosa  is  very  thin,  and  its  cells  show  signs  of  a 
fatty  degeneration.  It  is  probable,  I  think,  that  this  degeneration 
progresses  to  a  considerable  extent,  and  involves  the  loosening  of  the 
granulosa  cells ;  for  loose  cells,  granules,  and  fragments  are  found  in 
the  liquor  folliculi.  The  cavity  of  the  follicle  is  very  large  and  filled 
with  the  fluid,  which  seems  to  be  under  pressure,  since  it  spurts  out 
with  considerable  force  when  the  follicle  is  pricked.  It  is  to  the  pres- 
sure of  the  liquid  that  Coste  attributes  the  rupture  of  the  follicle.  Wal- 
deyer  in  Strieker's  "  Gewebelehre, "  p.  571,  describes  a  growth  of  the 
wall  of  the  follicle,  which  causes  it  to  form  a  series  of  folds  which 
protrude  into  the  follicle ;  this  ingrowth  produces  the  force  that  expels 
the  ovum.  Unfortunately,  Waldeyer  does  not  state  on  what  animal 
his  observations  were  made;  they  certainly  do  not  apply  to  the 
human  species,  for  there  is  in  man  no  considerable  growth  of  the 
follicular  wall  until  after  the  rupture.  The  stigma  becomes,  mean- 
while, very  thin,  and  finally  breaks  through.  Coste's  observations, 
47. 1,  172,  on  rabbits  eight  or  ten  hours  after  the  coitus,  showed  that 
the  rupture  is  not  abrupt  but  gradual,  the  membranes  of  the  follicle 
giving  way  first,  and  the  peritoneum  a  little  later.  When  the  stigma 
breaks,  the  liquor  folliculi,  together  with  the  ovum  surrounded  by  the 
discus  proligerus,  escapes,  and  the  ovulation,  sensu  strictu,  is  com- 
pleted. The  fate  of  the  cells  of  the  tunica  granulosa  is  uncertain, 
though  Bericldser,  84.1,  has  shown  that  in  the  pig  they  disappear 
at  the  time  of  or  soon  after  the  rupture.  I  consider  it  probable  that 
they  are  lost  in  man  at  the  time  the  ovum  escapes ;  it  may  be  that 
they  degenerate ;  it  must  be  mentioned  that  some  writers  maintain 
that  the  granulosa  persists  and  takes  part  in  the  further  metamor- 
phoses of  the  follicle.  At  the  time  of  the  rupture  there  occurs  a 
hemorrhage  of  blood  into  the  emptied  follicle,  and  this  blood  forms 
a  clot  which  fills  up  the  entire  follicle,  and  is  known  as  the  corpus 
hemorrhagicum.  The  hemorrhage  may  vary  in  amount  or  even 
be  wanting  altogether,  as  Benckiser,  84. 1,  found  in  8  cases  out  of  100 
in  the  pig.  Leopold  expressly  states,  83.1,  that  when  the  follicle  rup- 
tures at  the  menstrual  period  it  is  always  filled  and  distended  by 


OVULATION.  67 

blood  filling  it,  but  when  the  rupture  occurs  in  the  intermenstrual 
period  the  hemorrhage  is  small  or  altogether  wanting ;  the  presence 
of  blood  is  therefore  not  indispensable  to  the  formation  of  the  corpus. 
When  the  follicle  contains  no  blood  it  is  filled  with  a  whitish  coagu- 
lum  of  unknown  origin  (Coste,  47. 1, 1.  245) .  The  coagulum,  whether 
of  blood  or  not,  is  rapidly  penetrated  by  tissue  which  grows  into  it 
from  the  wall  of  the  follicle,  accompanied  by  numerous  blood-vessels ; 
the  cells  of  this  tissue  have  two  principal  forms,  His,  65.2,  186-187: 
first,  spindle-shaped  connective-tissue  cells,  which  lie  principally 
around  the  blood-vessels ;  second,  large  cells,  which  contain  granules 
of  a  pigment,  called  lutein  from  its  color ;  these  cells  are  the  lutein- 
cells,  and  are  the  characteristic  elements  of  the  metamorphosed  clot, 
to  the  margin  of  which  they  impart  a  bright  yellow  color,  whence 
the  name  corpus  luteum.  The  ingrowing  tissue  is  derived  from  the 
inner  layer  of  the  theca  folliculi.  That  the  blood-vessels  and  spindle- 
cells  have  this  origin  has  long  been  the  generally  accepted  opinion, 
and  though  the  origin  of  the  lutein-cells  is  under  dispute  it  is  prob- 
able that  they  arise  exclusively  from  the  connective-tissue  cells  of 
the  theca  interim,  which  begin  to  enlarge  even  before  the  follicle 
finally  bursts,  and  to  charge  themselves  with  lutein  granules.  Cer- 
tain writers  attribute  the  origin  of  these  cells  to  the  granulosa  either 
wholly  (Exner  and  Call)  or  in  part  (Waldeyer).  Peculiar  is  Beu- 
lin's  view  in  his  Konigsberg  dissertation,  1877,  that  they  are  derived 
from  the  membrana  propria  folliculi.  Benckiser's  observations,  84. 1 , 
prove  conclusively  that  in  the  pig  the  lutein-cells  arise  exclusively 
from  the  theca  interna.  This  view  I  accept  for  man  also,  not  only 
on  account  of  the  accuracy  of  the  observations  made  in  support  of  it 
(see  His,  65.2,  and  Frommann,  86.3),  but  also  because  specimens 
of  my  own  show  that  there  is  no  granulosa  in  the  human  corpus 
hemorrhagicum,  while  the  young  lutein-cells  can  be  easily  recog- 
nized in  the  fibrous  tunica  propria.  In  consequence  of  their  site  of 
development  the  lutein-cells  and  vessels  form  a  band  around  the 
coagulum,  and  owing  to  its  own  growth  this  yellow  band  soon  be- 
comes folded.  The  central  portion  of  the  corpus  luteum  long  re- 
mains distinguishable  as  a  separate  nucleus. 

The  exact  history  of  the  corpus  luteum  varies  according  as  ovula- 
tion  is  followed  by  pregnancy  or  not.  In  the  latter  case  the  corpus  is 
entirely  resorbed  in  a  few  weeks ;  in  the  former  it  persists  until  after 
the  birth  of  the  child.  We  distinguish  accordingly  the  corpus 
luteum  of  menstruation  from  the  corpus  luteum  of  pregnancy,  or 
corpus  luteum  verum  of  authors. 

The  corpus  luteum  of  menstruation  begins  with  a  blood-clot.  "  The 
more  recent  the  date  of  the  menstrual  flow,  the  fresher  is  the  clot  in 
the  cavity  of  a  ruptured  Graafian  follicle,  and  the  less  change  has 
taken  place  in  its  surrounding  wall.  A  few  days  later  the  wall  be- 
gins to  be  enlarged  and  thickened,  and  this  enlargement  within  a 
confined  space  causes  it  to  become  folded  upon  itself  in  short  zigzag 
reduplications,  mainly  at  the  deeper  part  of  the  follicle.  As  the  pro- 
cess goes  on  the  entire  wall  participates  in  the  hypertrophy.  Its 
convolutions  are  extended  and  multiplied,  often  in  a  very  compli- 
cated manner.  They  project  into  the  cavity  of  the  follicle,  encroach 
upon  the  central  clot,  and  become  pressed  against  each  other,  form- 


G8  THE    GENITAL   PRODUCTS. 

ing  by  their  coalescence  a  thickened,  glandular-looking  envelope. 
Previously  to  the  rupture  of  aGraafian  follicle  its  wall  is  a  uniformly 
smooth,  vascular  membrane,  not  more  than  one-fourth  of  a  milli- 
metre in  thickness.  After  the  rupture,  its  thickness  increases  to 
one-half  a  millimetre ;  but  as  the  foldings  above  described  grow  in 
number  and  in  depth  and  crowd  against  each  other  laterally,  the 
apparent  thickness  of  the  envelope  thus  formed  becomes  much 
greater,  and  may  reach  three  or  even  four  millimetres,  especially  at 
the  deepest  part  of  the  follicle.  In  this  way  there  is  produced,  dur- 
ing the  intermenstrual  period,  a  corpus  luteum,  occupying  the  sub- 
stance of  the  ovary  immediately  beneath  the  superficial  cicatrix 
which  marks  the  site  of  the  ruptured  follicle.  At  this  time  the  cen- 
tral clot  is  red  and  gelatinous,  while  the  convoluted  wall  is  of  a  light 
rosy  hue,  mixed  with  more  or  less  of  a  yellowish  tint.  Subsequently 
the  whole  structure  diminishes  in  size,  and  the  convoluted  wall  as- 
sumes a  more  decided  yellow"  (Dalton,  78.1,  p.  is). 

Leopold,  83. 1,  distinguishes  between  the  typical  and  atypical  cor- 
pora, the  former  being  those  which  start  at  the  menstrual  epoch  and 
have  a  blood-clot,  a  result  probably  of  the  ovarian  hypera?mia,  the 
latter  beginning  intermenstrually  and  having  little  or  no  blood.  He 
says,  /.  c.,  p.  399:  "The  typical  corpus  luteum  appears  on  the  first 
day  as  a  freshly  ruptured  follicle,  which  has  filled  itself  with  blood ; 
on  the  third  day  as  an  enormous  blood-cavity ;  about  the  eighth  day 
a  thin  cortex  and  a  clearer  nucleus  are  marked  in  the  clot.  From 
the  twelfth  day  on,  the  cortex  thickens  and  becomes  folded ;  by  the 
sixteenth  day  it  becomes  pale-red  or  yellowish.  Toward  the  twen- 
tieth day  the  nucleus  shrinks  markedly,  the  cortical  band  becomes 
more  and  more  yellow,  and  shoots  in  toward  the  centre  in  rays  and 
narrow  folds,  so  as  to  leave  by  the  twenty-fourth  to  thirty-fifth  day 
only  a  small,  pale  nucleus  enclosed  in  a  much-convoluted  bright- 
yellow  shell." 

The  corpus  luteum  of  pregnancy  begins  in  a  similar  manner  to  that 
of  menstruation,  but  its  growth  continues.  At  the  end  of  the  first 
month  its  wall  is  convoluted,  much  thickened,  and  of  a  brilliant  yel- 
low color;  the  central  clot  is  nearly  or  quite  decolorized  and  consti- 
tutes a  white  or  whitish  firm  central  mass,  which  in  nearly  one  case 
out  of  three  has  a  central  cavity  with  well-defined,  smooth  walls. 
Sometimes  a  few  fine  blood-vessels  penetrate  through  the  lutein 
layer.  The  external  convoluted  wall  continues  to  grow  by  encroach- 
ing upon  the  clot  or  white  nucleus  (corpus  albicans],  and  at  the 
same  time  the  brilliancy  of  the  yellow  color  diminishes.  At  term 
the  white  nucleus  takes  up  about  one-third  of  the  diameter  of  the 
corpus  and  is  still  distinctly  connected  with  the  stigma,  so  that  the 
lutein  wall  is  interrupted  at  one  point;  the  corpus  as  a  whole  is 
somewhat  smaller  than  at  from  two  to  six  months.  After  delivery, 
resorption  goes  on  rapidly  (Dalton,  78.1). 

The  brilliant  yellow  is  especially  characteristic  of  man ;  in  sheep 
the  pigment  is  pale  brown,  in  the  cow  dark  orange,  in  the  mouse 
brick-red,  in  the  rabbit  and  pig  flesh-colored.  Lutein  is  a  crystal- 
line body,  soluble  in  alcohol,  ether,  chloroform,  and  benzol,  but  of 
its  chemical  nature  we  have  no  exact  knowledge. 

Physiology  of  Ovulation. — Concerning  this  subject  and  also 


IMPREGNATION.  09 

concerning  the  functions  of  the  corpora  lutea,  we  possess  scarcely  any 
knowledge.  We  have  to  consider  only  the  relation  of  ovulation  to 
menstruation  and  coitus. 

Coste,  37.1,  454,  455,  first  showed  that  the  discharge  of  the  ova 
coincided  with  the  period  of  heat  in  various  animals.  This  was  soon 
confirmed  by  Raciborski,  44. 1,  and  since  then  by  numerous  ob- 
servers. Pouchet  ("Theorie  positive, "etc.)  attempted  to  prove  that 
this  is  also  true  of  the  human  species,  the  menstrual  period  being 
taken,  correctly,  as  the  equivalent  of  the  rut.  In  this  attempt  Pou- 
chet has  had  many  followers,  especially  among  gynecologists. 
Coste,  however,  demonstrated  long  ago,  47.1,  222,  that  the  bursting 
of  the  Graafian  follicles  may  occur  before  or  after  menstruation, 
though  it  is  most  apt  to  occur  during  the  menses.  This  conclusion 
of  Coste 's  has  been  fully  confirmed  by  Leopold,  83.1,  who  made  a 
very  careful  examination  of  twenty-five  pairs  of  ovaries  from  women 
whose  menstrual  history  was  accurately  known. 

It  was  Coste  again,  47. 1, 183-185,  who  proved  experimentally  that 
coitus  hastens  in  the  rabbit  the  rupture  of  the  Graafian  follicles. 
Unfortunately  he  gives  only  two  experiments,  and  since  then  they 
have  not  been  repeated,  so  far  as  I  am  aware,  either  upon  rabbits  or 
other  animals.  But  there  are  statements  by  many  authors,  Bary, 
Reichert,  Hensen,  88.1,  58,  Van  Beneden,  80.1,  etc.,  to  the  effect 
that  in  the  rabbit  after  coitus  during  heat  the  follicles  are  found  to 
have  burst  during  the  tenth  hour. 

IY.  IMPREGNATION. 

Impregnation  is  the  union  of  the  male  and  female  elements  to 
form  a  single  new  cell,  capable  of  initiating  by  its  own  division  a 
rapid  succession  of  generations  of  descendent  cells.  The  new  cell  is 
called  the  impregnated  or  fertilized  ovum.  The  production  of  cells 
from  it  is  called  its  segmentation.  For  the  theory  of  the  relation  of 
the  elements  to  one  another  and  to  cells,  see  the  following  section. 

In  all  multicellular  animals,  impregnation  is  effected  by  three  suc- 
cessive steps :  1 ,  the  bringing  together  of  the  male  and  female  ele- 
ments ;  2,  the  entrance  of  the  spermatozoa  into  the  ovum  and  forma- 
tion of  the  male  pronucleus ;  3,  fusion  of  the  pronuclei  to  form  the 
segmentation  nucleus.  We  proceed  to  consider  these  steps  in  their 
order. 

1.  The  Bringing  Together  of  the  Sexual  Elements.— 
This  is  effected  in  a  great  variety  of  ways,  which,  however,  fall  into 
two  groups  according  as  the  impregnation  is  effected,  a,  outside  the 
body  of  the  mother ;  or,  6,  inside.  The  simplest  manner  is  the  dis- 
charge of  the  male  and  female  elements  at  the  same  time  into  the 
water,  leaving  their  actual  contact  to  chance,  the  method  of  the  osse- 
ous fishes  for  the  most  part  and  of  many  invertebrates.  An  advance 
is  the  copulation  of  the  Anura  (frogs,  etc.) ;  the  male  embraces  the 
female,  and,  as  the  latter  discharges  the  ova,  ejects  the  sperm  upon 
them.  In  the  higher  vertebrates  the  seminal  fluid  is  transferred 
from  the  male  to  the  female  passages  during  coitus.  The  physiol- 
ogy of  this  complicated  function  does  not  fall  within  the  scope  of  this 
work. 


70  THE   GENITAL   PRODUCTS. 

For  a  long  time  it  was  not  known  how  the  semen  fertilized  the 
ova;  the  problem  was  fruitful  of  fruitless  speculation.  The  first  step 
toward  gaining  actual  knowledge  was  the  discovery  of  the  possibil- 
ity of  artificial  fecundation  by  Jacobi  in  1764.  Spallanzani  was  the 
first  to  take  advantage  of  this  and  to  show  that  fecundation  implied 
a  material  contact  of  the  semen  with  the  ova,  and  thus  to  set  aside 
De  Graaf's  notion  of  the  "aura  seminalis." 

But  not  until  fifty  years  later  did  the  memorable  experiments  of 
Prevost  and  Dumas  (Annales  des  Sciences  Naturelles,  1824)  es- 
tablish the  fact  that  the  spermatozoa  are  the  essential  factors  of  fer- 
tilization. Again,  a  little  over  fifty  years  later,  Hertwig  and  Fol 
showed  that  one  spermatozoon  suffices  to  impregnate  an  ovum. 

We  have  then  to  consider  how  the  spermatozoon,  after  the  semen 
has  been  transferred  to  the  female,  attains  the  ovum.  They  are  found 
in  mammals  after  copulation  in  the  vagina  and  even  in  the  uterus, 
but  it  is  not  clearly  ascertained  how  they  get  beyond  the  vagina.  It 
is  probable  that  they  travel  through  the  female  passages  partly  by 
the  movements  thereof,  partly  by  their  own  locomotion,  and  enter 
the  Fallopian  tubes,  though  why  or  how  is  really  unknown,  and  pass 
upward  to  meet  the  ovum.  They  are  found  in  considerable  numbers 
in  the  Fallopian  tubes.  The  ovum  meanwhile  travels  down  the  ovi- 
duct, it  probably  being  impelled  by  peristaltic  movements  of  the 
duct. 

The  meeting-point  or  site  of  impregnation  in  placental  mammals 
is  about  one-third,  perhaps  one-half,  way  down  from  the  fimbria  to 
the  uterus.  It  is  remarkably  constant  for  each  species.  Nothing 
positive  is  known  as  to  the  site  of  impregnation  in  man ;  but  there  is 
no  reason  to  suppose,  as  is  unfortunately  often  done,  that  the  site  is 
variable  or  different  from  that  in  other  mammalia. 

2.  The  Entrance  of  the  Spermatozoon  into  the  Ovum 
and  Formation  of  the  Male  Pronucleus. — With  our  present 
knowledge,  the  assumption  appears  unavoidable  that  the  ovum  exerts 
a  specific  attraction  upon  spermatozoa  of  the  same  animal  species. 
We  observe,  in  fact,  when  artificial  fecundation  is  employed,  that 
the  spermatozoa  swarm  around  the  ova  as  if  held  by  an  irresistible 
impulse.  This  phenomenon  occurs  with  every  class  of  animals,  even 
in  mammals,  whose  freshly  removed  ova  were  examined  on  a  warm 
stage  under  the  microscope  (Rein,  83. 1) .  Stassano,  83. 1,  has  main- 
tained that  the  eggs  of  echinoderms  do  exert  such  an  attraction,  and 
also  a  similar  but  less  strong  attraction  upon  the  spermatozoa  of 
allied  species.  But  since  the  brothers  Hertwig,  85. 1,  have  found  by 
their  experiments  with  sea-urchins  that  hybrid  impregnation  takes 
place  more  readily  after  the  ova  have  been  kept  awhile,  Stassano 's  view 
involves  the  further  assumption  that  the  specific  nature  of  the  attrac- 
tion fades  away  during  a  few  hours.  Very  suggestive  in  this  con- 
nection is  Pfeffer's  (" Untersuch.  Bot.  Inst."  Tubingen,  Bd.  I.,  Hft. 
3,  1884)  discovery  that  certain  chemical  substances  may  attract 
moving  spores,  etc.,  to  definite  spots.  It  is  conceivable  that  the 
ovum  may  draw  the  spermatozoa  toward  itself  by  chemical  influ- 
ence, acting  as  an  attracting  stimulus. 

There  may  be  mechanical  devices  to  facilitate  the  entrance  of  the 
spermatozoon ;  this  is,  perhaps,  generally  true  of  all  ova  with  micro- 


IMPREGNATION.  71 

pyles  serving  for  the  passage  of  the  spermatozoa.  A  careful  study 
of  such  devices  in  the  cockroach  has  been  made  by  J.  Dewitz,  85. 1, 
who  found  that  the  motions  of  the  spermatozoa  of  this  insect  are 
peculiar  and  adapted  to  increase  the  probability  of  their  passing 
through  one  of  the  micropyles  of  the  ovum.  In  ova  without  micro- 
pyles,  among  which  those  "of  mammals  are  included,  the  sperma- 
tozoa ma}',  so  far  as  we  know,  penetrate  any  part  of  the  envelopes. 

In  the  rabbit  (Rein,  83.1),  about  ten  hours  after  coitus,  the 
ovum  is  found  nearly  half-way  through  the  oviduct  and  surrounded 
by  many  spermatozoa — perhaps  a  hundred, 
more  or  less.  These  are  all,  or  nearly  all,  in 
active  motion,  for  the  most  part  pressing 
their  heads  against  the  zona  radiata.  Sev- 
eral of  them  make  their  way  through  into 
the  interior  of  the  ovum.  According  to  Hen- 
sen,  76. 1,  only  those  spermatozoa  which  en- 
ter the  zona  along  radial  lines  can  make  their 
way  through;  those  whjch  take  oblique 
courses  remain  caught  in  the  zona,  Fig.  40, 
and  may  still  be  seen  there  during  segmenta- 
tion.  As  the  ovum  at  this  time  is  already 
fully  matured,  there  is  a  space  between  the  oviduct  about  eighteen  hours 
contracted  yolks  and  the  zona.  In  this  space,  S^Sd^fe'fSSftS 
as  well  as  in  the  zona  itself,  several  sperma-  ££^JS£&£'>u*<S5y 
tozoa  may  be  observed  at  scattered  points,  in  and  within  the  zona.  After 
The  female  pronucleus  is  present,  having  c 

been  re-formed  since  the  expulsion  of  the  second  polar  globule  from 
the  ovum  while  in  the  ovary.  One  spermatozoon  gets  into  the 
yolk  proper,  and  its  entrance  apparently  prevents  the  penetration  of 
other  spermatozoa — how  is  undetermined.  The  tail  of  the  sperma- 
tozoon soon  disappears,  while  the  head  enlarges,  probably  by  the 
imbibition  of  fluid  from  the  surrounding  yolk,  and  thus  becomes  a 
nucleus-like  body — the  male  pronucleus. 

The  passage  of  the  spermatozoa  through  the  zona  was  first  discov- 
ered by  Martin  Barry  in  1843,  and  although  his  statement  was  re- 
ceived with  considerable  hesitation  by  his  contemporaries,  it  has 
since  'had  competent  confirmation  repeatedly.  VVarneck  (Bull.  Soc. 
Nati.,  Moscou,  XXIII.,  90)  is  said  to  have  been  the  first  (1850)  to 
see  the  two  pronuclei,  but  their  significance  was  not  perceived.  The 
nature  of  the  male  pronucleus  was  first  recognized  by  Oskar  Hert- 
wig.  who  traced  its  genesis  in  the  ova  of  echinoderms  from  the  sper- 
matozoon. The  fact  that  the  male  pronucleus  is  the  metamorphosed 
spermatozoon  has  since  been  confirmed  by  Selenka  ("Zool.  Stud.," 
I.);  Ed.  van  Beneden,  83.1;  Nusebaum,  84.2;  Eberth,  84.1; 
Platner,  86.1,  and  others. 

Although  a  number  of  the  spermatozoa  make  their  way  into  the 
perivitelline  space,  probably  always  one  alone  normally  enters  the 
yolk  to  there  form  a  pronucleus.  The  best  observers  are  agreed  upon 
this  point,  and  in  all  species  the  observations  upon  which  have  cov- 
ered the  whole  series  of  steps  in  the  impregnation,  there  has  been 
found  in  normal  cases  always  a  single  male  pronucleus.  Schneider's 
statements  to  the  contrary  have  been  definitely  corrected.  Bambeke, 


72  THE   GENITAL    PRODUCTS. 

76.2,  C.  Kupffer,  82.1,  and  Kupffer  und  Benecke  ("  Befruchtung 
Neunauge,"  1878),  have  observed  that  several  spermatozoa  actually 
enter  the  yolk  in  batrachians  _  and  Petromyzon.  Hertwig,  however, 
found  only  one  male  pronucleus  in  the  frog,  and  there  has  as  yet  been 
no  evidence  adduced  that  several  spermatozoa  are  concerned  in  the  final 
phases  of  impregnation.  Fol  observed  that  star-fish  eggs  are  nor- 
mally impregnated  by  one  spermatozoon ;  but  if  they  are  exposed  to 
the  action  of  carbonic  acid  they  may,  while  so  poisoned,  be  impreg- 
nated by  several  spermatozoa,  and  the  subsequent  development  in 
this  case  is  abnormal :  apparently  each  pronucleus  becomes  a  separate 
centre  of  development. 

The  manner  in  which  additional  spermatozoa  are  excluded  after 
the  first  has  entered  is  still  under  discussion.  In  cases  where  there 
is  a  single  micropyle,  which  is  used  for  entry,  it  is  possible  that  a 
portion  of  the  first  spermatozoon  may  remain  to  close  the  passage, 
or  that  in  going  through  it  sets  in  action  some  mechanism  by  which 
the  opening  is  automatically  shut.  Where  there  are  several  or 
many  micropyles,  as  in  some  insects,  or  .where  the  envelopes  may 
be  pierced  at  any  point,  as  in  mammals,  there  must  be  some  other 
device.  Fol  has  maintained  that  this  is  found  in  the  star-fish  in  the 
rapid  formation  of  a  membrane  around  the  yolk  immediately  after 
the  entrance  of  the  first  spermatozoon ;  but  Hertwig  affirms  that  this 
membrane  pre-exists.  Selenka  (Biolog.  CentralbL,  v.,  8)  describes 
the  fertilization  of  the  ovum  of  a  nemertean  worm ;  several  sperma- 
tozoa enter  within  the  vitelline  membrane ;  the  yolk  contracts  slowly. 

After  a  time  the  two  polar  glob- 
ules are    expelled,    and  before 
:^P.  they  separate  from  the  yolk  one 

spermatozoon   passes    into    the 
yolk  between  them ;  the  globules 
•:^.  /z       then  break  off  and  are  knocked 

about  by  the  spermatozoa  in  the 

perivitelline  space.     In  this  case 

!•        there  seems  to  be  a  portal  opened 

XT  .       ,  ,        f  r 

just  long  enough  tor  one  sper- 
matozoon to  enter.  As  the  phe- 
nomenon to  be  explained  is  com- 

FIG.  41.—  Anterior  pole  of  the  ovum  of  the  Petro  rnon    to  flll  nvi     it«i  r»mi«sfl-Hrvn  i<4 

myzon,  with   a   spermatozoon,   sp.,    entering   the  mO1  (TV  el,  IT 

micropyle,  mi;  p.  v.  perivitelline  space ;    z,  zona  presumably  fundamentally ideil- 

pellucida;    a.  pathway  to  female  pronucleus,  n;  7-       -t   •       «u    „  T>  1  4-1  • 

yk.,  yolk.   After  Caiberia  tical  in  all  cases.     ±>eyond  this 

surmise  our  present  knowledge 

does  not  permit  us  to  go.  The  hypothesis  may  be  suggested  that 
the  attractive  power  of  the  ovum  is  annulled  or  weakened  by  the 
formation  of  the  male  pronucleus.  This  hypothesis  was  first  sug- 
gested by  Minot  (Buck's  "  Hdbk.,"  IV.,  6),  and  has  since  been  elabo- 
rated by  Whitman,  87.3,  239-243. 

It  is  probable  that  the  tail  of  the  spermatozoon,  when  that  appen- 
dage exists,  disappears  within  the  yolk.  In  a  land-snail,  Arion, 
Platner,  86. 1,  has  traced  this  process  very  clearly.  Only  a  portion 
of  the  tail  enters  the  yolk,  but  the  part  within  acquires  the  property 
of  staining  readily,  and  so  may  easily  be  observed.  He  reports  that 
the  head  and  tail  separate ;  only  the  head  conjugates  with  the  female 


IMPREGNATION.  73 

proiiucleus,  while  the  tail  still  remains  distinct  even  after  segmen- 
tation has  been  initiated,  Fig.  44.  The  disappearance  of  the  tail 
has  been  recorded  by  most  observers.  As  Hertwig  says  (loc.  cit.y  p. 
23),  all  these  careful  observations  yield  the  assured  conclusion  that 
the  head  of  the  spermatozoon,  and  the  head  only,  becomes  the  male 
pronucleus. 

While  the  spermatozoon  is  passing  through  the  ovic  envelopes,  act- 
ive ehanges  occur  in  the  yolk.  Of  these  the  most  constant,  as  well 
as  the  most  obvious,  is  the  formation  of  a  slight  protuberance  on  the 
surface  of  the  yolk,  rising  up  toward  the  spermatozoon.  This  pro- 
tuberance may  remain,  as  in  echinoderms,  until  the  spermatozoon 
meets  it  and  by  penetrating  it  enters  the  ovum,  or  it  may  retract 
before  the  spermatozoon  passes  through  the  envelopes,  and  even  with- 
draw, as  in  Petromyzon,  Fig.  41,  so  far  from  the  advancing  sperma- 
tozoon as  to  form  into  a  depression  on  its  own  surface,  Fig.  41. 
The  protuberance  lasts  only  a  few  moments.  In  Bufo,  according  to 
Kupffer,  several  spermatozoa  enter  the  yolk  and  a  protuberance 
rises  toward  each  one,  as  if  the  yolk  were  actively  striving  to  reach 
the  male  element.  The  protuberance  always  consists  of  fine  granu- 
lar protoplasm,  which  contains  no  deutoplasm,  and  is  closely  con- 
nected with  the  nucleus.  The  size  of  the  protuberance  is  variable. 
In  Petromyzon  there  is  a  large  hummock  of  protoplasm,  which  con- 
tains the  nucleus  and  in  which  both  pronuclei  form  and  unite;  dur- 
ing these  processes  the  protoplasm  of  the  hummock  is  separated  from 
that  of  the  rest  of  the  ovum  by  a  special  membrane,  which  disap- 
pears immediately  after  the  pronuclear  copulation.  While  the  two 
pronuclei  are  meeting  the  hummock  flattens  out  and  the  protoplasm 
forming  it  travels  ceiitripetally  together  with  the  pronuclei  (Boehm, 
88. 1,  650,  051).  Whether  the  hummock  in  Petromyzon  is  homologous 
with  the  much  smaller  protuberance  in  other  ova  I  am  unable  to  say. 

The  relative  size  of  the  two  pronuclei  varies  considerably  in  differ- 
ent species,  and  is  probably  a  secondary  and  unimportant  relation. 
Each  pronucleus  when  it  first  appears  is  small  and  gradually  en- 
larges, apparently  by  the  imbibition  of  fluids  from  the  surrounding 
yolk.  Now  the  time  when  the  spermatozoon  enters  the  yolk  may 
be  either  after  or  at  some  stage  during  maturation  of  the  ovum. 
If  it  enters  early,  as  in  Limax  (Mark),  the  male  pronucleus  en- 
larges equally  with  the  female,  Fig.  42 ;  but  if  late,  as  in  the  allied 
Arion  (Platner),  then  it  appears,  Fig.  44,  considerably  smaller  than 
the  already  swollen  female  pronucleus.  O.  Hertwig,  in  his  third 
paper  on  maturation,  p.  171,  first  gave  this  explanation  and  pointed 
out  that  in  the  star-fish  ( Asterias) ,  if  the  impregnation  is  prompt, 
the  male  pronucleus  becomes  as  large  as  the  female,  but  if  impreg- 
nation is  delayed  for  four  hours  the  male  pronucleus  remains  much 
the  smaller  of  the  two.  Again  in  Hirudinea,  Fig.  42,  many  Mol- 
lusca,  Nematoidea,  etc.,  impregnation  usually  takes  place  before  the 
formation  of  the  polar  globules  is  completed,  and  the  male  pronu- 
cleus is  accordingly  as  large  as  the  female.  In  Echinus,  on  the 
other  hand,  where  the  polar  globules  are  formed  in  the  ovary,  the 
male  pronucleus  is  always  small. 

Concerning  the  path  of  the  male  pronucleus  we  possess  little  infor- 
mation. O.  Hertwig  and  Bambeke  have  found  that  in  certain  am- 


74  THE    GENITAL   PRODUCTS. 

phibian  ova  the  spermatozoon  leaves  a  trail  (Pigment-Strasse) 
apparently  by  carrying  along  with  itself  some  of  the  pigment  gran- 
ules from  the  surface  of  the  ovum.  Roux,  87.1,  has  studied  this 
path  in  the  frog's  ovum  and  finds  that  it  consists  of  one  limb,  the 

line  of  penetration  through  the  cortical 
layer  of  the  ovum,  which  is  nearly  perpen- 
dicular to  the  ovic  surface,  and  a  second 
limb  usually  forming  an  angle  with  the  first 
and  leading  directly  to  the  female  pronu- 
cleus. 

The  force  which  draws  the  pronuclei  to- 
gether is  unknown.  We  can  only  say  that, 
as  Whitman  has  thoughtfully  expounded, 
87.3,  there  is  a  relation  between  the  nu- 

FIG.  4*  .^tfxepheiis,  three  cleus  and  the  protoplasm  of  the  ovum,  such 
hours  after  laying:  m,  male,/,   that  the  nucleus  tends  to  take  a  central  posi- 

female   pronucleus;    p.   g. ,  polar     ,.  TTTI  JT  i  111  p  n 

globules    After  o.  Herfwig.        tion.      \\  hen  the  polar  globules  are  formed 

the  nucleus  becomes  repellant  and  drives 

itself  centrifugally ;  but  the  protoplasmic  attraction  remains  and 
draws  in  the  spermatozoon.  Subsequently  both  pronuclei  are  at- 
tracted toward  the  centre  and  toward  each  other,  and  the  curved 
routes  the  pronuclei  often  take  are  the  resultants  from  these  two 
attractions. 

3.  Fusion  of  the  Pronuclei. — Each  pronucleus  is  usually  found 
surrounded  by  a  space  a  little  clearer  than  the  rest  of  the  yolk.  Usu- 
ally the  yolk  around  this  clear  space  presents  a  radiating  appearance, 
which  is  known  as  the  aster,  Fig.  43;  but  this  appearance  is  not  con- 
stant, nor  is  it  known  how  it  is  caused.  Frommann,  89. 1,  395,  states 
that  in  the  egg  of  Toxopneustes  the  astral  rays  are  formed  by  very 
irregular  rows  of  angular  granules,  which  may  lie  separately,  or 
be  strung  together  by  fine  threads,  or  like  a  row  of  pearls,  and  are 
irregularly  connected  by  cross-threads.  The  great  regularity  usu- 
ally pictured  is  purety  diagrammatic.  As  the  granules  described  by 
Frommann  are  part  of  the  reticulum  of  the  ovum,  we  may  say  that 
the  astral  figure  results  from  the  arrangement  of  the  protoplasm. 
Mark,  81.1,  was  unable  to  see  it  in  Limax,  and  Rein,  83.1,  could  not 
detect  it  in  the  rabbit.  In  Arion,  as  also  in  Petromyzon,  according 
to  Boehm,  88.1,  apparently  only  the  male  pronucleus  has  an  aster, 
Fig.  44.  At  one  time  it  was  assumed  that  the  pronuclei  acted  as 
centres  of  attraction  upon  the  yolk,  and  that  the  asters  were  due  to 
their  direct  influence ;  but  since,  as  in  Arion,  Fig.  44,  the  pronucleus 
may  move  away  while  the  aster  remains  behind,  it  follows  that  the 
relations  are  more  complex  than  this  assumption  indicates,  since  the 
aster  exhibits  a  certain  independence  of  the  pronucleus.  This  is 
confirmed  by  Flemming's  observations  (loc.  c?7.,  p.  19),  that  when 
the  asters  first  appear  in  echinoderms,  the  centre  of  radiation  is  not 
the  pronucleus  itself,  but  a  clear  space  just  alongside.  Frommann, 
88.1,  396,  modifies  this  statement  by  recording  that  the  position  of 
the  centre  of  the  male  aster  varies  in  Toxopneustes  and  may  be  at 
one  side  or  the  other  of  the  male  pronucleus  or  coincident  with  it. 
Boehm,  88.1,  650,  Taf.  XXV.,  Fig.  30  c7,  notes  the  same  peculiarity  in 
the  eggs  of  Petromyzon.  These  statements  recall  the  fact  that  the 


IMPREGNATION. 


75 


asters  in  indirect  cell-division  sometimes  radiate  from  a  clear  spot  at 
the  tip  of  the  spindle.  Some  writers  have  considered  the  aster  an 
expression  of  magnetic  force  within  the  ovum  —  a  fanciful  notion 
without  any  evidence  to  support  it. 

In  the  rabbit,  Rein,  83.  1,  both  prcnuclei  lie  at  first  eccentrically, 
but  they  move  toward  each  other  and  toward  the  centre,  meeting, 
however,  before  the  central  position  is  attained.  As  they  near  one 
another,  both  pronuclei  perform  active  amoeboid  movements  ;  after 
they  meet  they  still  continue  their  amoeboid  movements  and  move 
together  to  the  centre  of  the  ovum  ;  one  of  the  pronuclei  assumes  a 
crescent  shape  and  embraces  the  other,  Fig.  45.  At  this  time  the 
yolk  displays  a  radiate  arrangement  ;  from  analogy  with  other  ani- 
mals it  must  be  assumed  that  the  two  pronuclei  fuse  into  a  single 
nucleus,  which  is  therefore  an  hermaphrodite  structure,  and  which, 
after  a  certain  period  of  repose,  itself  divides  and  so  begins  the  cleav- 
age of  the  yolk. 

The  place  where  the  pronuclei  meet  varies.  Apparently  the  female 
pronucleus  of  itself  moves  to  the  centre  or  near  the  centre  of  the  ovum  ; 
also  the  male  pronucleus  approaches  the  female  as  speedily  as  possi- 
ble. If  now  impregnation  occurs  early,  the  two  pronuclei  meet 
peripherally  ;  if  late,  they  meet  near  the  centre.  In  the  former  case 
they  move  together,  as  in  the  rabbit  (Rein),  to  a  central  position. 
The  observations  so  far  made  indicate  that  after  they  meet  the  pro- 
nuclei  both  perform  active  amoeboid  movements,  which  continue  for 
several  minutes.  Selenka  maintains  that  the  female  pronucleus 
sends  out  processes  which  embrace  the  male  pronucleus,  but  this 
has  not  been  confirmed.  Finally,  the  two  pronuclei  unite,  but  the 
process  of  union  is  very  obscure,  never  having  been  satisfactorily 
ol  (served.  Apparently  the  membranes  of  the  pronuclei,  where  the 
two  are  in  contact,  are  dissolved  away 
and  the  contents  mix.  The  best  account  ^^^ 

known  to  me  of  the  fusion  of  the  pronuclei 
is  that  given  by  Boehm  in  his  memoir, 
88.1,  on  Petromyzon.  The  outline  of 
the  female  pronucleus  is  still  diffuse  a 
quarter  of  an  hour  after  fertilization. 
The  head  of  the  spermatozoon  (male  pro- 
nucleus)  breaks  up  into  four,  more  rarely 
five,  granules.  The  female  pronucleus 
moves  centripetally,  and  acquires  a  dis- 
tinct membrane.  The  pronuclei  meet, 
the  male  granules  having  meanwhile 
multiplied  by  division.  About  this  time 
the  female  pronucleus  also  breaks  up  into 
granules.  We  then  have  a  clear  spot 
which  is  the  centre  of  an  astral  radia- 
tion, next  this  a  bunch  of  male  granules  (Boehm's  Spermatomeri- 
teri),  and  next  that  a  bunch  of  female  granules  (Boehm's  Ovomeri- 
ten),  the  whole  making  an  elongated  body  lying  at  right  angles  to 
the  radius  of  the  ovum.  Three  hours  after  fertilization  the  two 
bunches  are  fused  together  and  are  no  longer  distinguishable.  Each 
"  Merit  "  consists  of  a  body  containing  one  or  two  chromatin  specks 


FlG    ^_Ovum  of        ^  with 
two  pronuclei.    After  o. 
ound  each  pronucleus  is  shown  the 


76 


THE   GENITAL   PRODUCTS. 


(Microsomen).  In  the  Crustacea,  according  to  Weismann  and  Ischi- 
kawa,  88.4,  the  two  pronuclei,  when  they  meet,  resemble  ordinary 
membranate  nuclei ;  where  they  come  in  contact  with  one  another  the 
membranes  dissolve  away  and  the  contents  of  the  pronuclei  mingle. 
In  Ascaris  the  process  is  more  complicated.  "We  may  say,  there- 


FIG.  44.— Two  ova  of  a  land-snail,  Arion.  After  Plainer.  The  ova  are  irregular  in  shape, 
as  at  this  stage  they  are  still  in  utero,  and  mutually  compressed.  A  shows  the  segmentation 
nucleus,  n,  just  formed;  the  two  large  "Karyosomen  "  in  it  are  derived  from  the  male  pro- 
nucleus;  the  male  aster  still  remains,  as.  B  shows  the  commencing  change  of  the  segmentation 
nucleus  into  the  first  spindle.  In  both  ova  the  tail,  sp.  of  the  spermatozoon  is  distinguishable. 

fore,  that  the  fusion  of  the  pronuclei  is  the  essential  phenomenon, 
and  the  method  of  the  fusion  is  secondary  in  importance. 

Another  point  deserving  mention  is  the  rotation  of  the  copulated 
nuclei.  See.Frommann's  article  on  "  Befruchtung  "  in  "  Eulenberg 
Cyclopedia,"  p.  568. 

Now  since  the  head  of  the  spermatozoon  is  developed  chiefly  out 
of  the  chromatin  of  the  nucleus  of  a  spermatoblast,  it  follows  that 
impregnation  is  essentially  the  addition  of  chromatin  to  the  nucleus 
(female  pronucleus)  of  the  mature  ovum.  After  the  union  of  the 
pronuclei  follows  a  period  of  repose,  during  which  the  yolk  enlarges 

until  it  again  fills  or  nearly  fills  the  space 
within  the  zona  radiata  ;  a  little  room  is 
left,  which  is  chiefly  occupied  by  the  polar 
globules.  The  significance  of  the  contrac- 
tion of  the  mature  and  the  expansion  of 
,  |p:x  the  impregnated  yolk  is  unknown.  In 
certain  cases  the  parts  of  the  segmentation 
nucleus  which  are  derived  from  the  male 
pronucleus  remain  distinguishable.  This 
is  notably,  according  to  Plainer,  the  case 
writh  Arion.  The  segmentation  nucleus 
contains  a  number  of  nucleolus-like  bodies 
(Karyosomen  of  Plainer,  Fig.  44,  A,  >/), 
with  a  distinct  round  outline,  and  a  few 
granules  of  chromatin.  These  bodies  are 
of  two  kinds,  Fig.  44 ;  the  smaller  and 
more  numerous  are  produced  by  the  female  pronucleus,  while  the  two 
larger  ones  arise  from  the  division  of  the  head  of  the  spermatozoon. 


FIG.  45. — Ovum  of  a  rabbit  sev- 
enteen hours  after  coitus  with  the 
pronuclei  about  to  con  jugate*  }></. 
polar  globules :  .  ;<m,u  pellucida. 
After  Rein. 


THEORY    OF   SEX.  77 

In  the  later  stage,  when  the  nucleus  is  changing  into  the  first  seg- 
mentation spindle,  Fig.  44,  B,  the  two  large  male  •'  Karyosomen" 
are  still  distinct,  and  have  each  their  chromatin  gathered  in  little 
particles  around  the  periphery.  Edouard  van  Beneden,  83.1,  goes 
even  further,  stating  that  in  Ascaris  the  chromatin  from  the  two 
pronuclei  can  be  distinguished  in  the  nuclei  of  segmentation,  and 
that  when  it  divides,  both  the  male  and  female  chromatin  loops 
divide  also,  so  that  the  resulting  nuclei  are  truly  hermaphroditic. 

V.  THEORY  OF  SEX. 

Sex  is  a  term  employed  in  two  meanings,  which  are  often  confused 
but  which  it  is  indispensable  to  distinguish  accurately.  Originally 
sex  was  applied  to  the  organism  as  a  whole  in  recognition  of  the 
differentiation  of  the  reproductive  functions.  Secondarily,  sex,  to- 
gether with  the  adjectives  male  and  female,  has  been  applied  to  the 
essential  reproductive  elements,  spermatozoon  and  ovum,  which  it  is 
the  function  of  the  respective  sexual  organisms  (or  organs)  to  pro- 
duce. According  to  a  strict  biological  definition,  sexuality  is  the 
characteristic  of  the  male  and  female  reproductive  elements,  and  sex 
of  the  individuals,  in  which  those  elements  arise.  A  man  has  sex, 
a  spermatozoon  sexuality.  Sexuality  is  primitive  and  essential,  and 
sex  is  dependent  upon  it.  We  have  to  consider,  1st,  the  nature  of 
sexuality ;  ^d,  the  origin  of  sexuality ;  3d,  the  nature  of  sex. 

Nature  of  Sexuality. — The  essential  facts  of  sexual  reproduc- 
tion are:  That  two  bodies,  partaking  more  or  less  plainly  of  the 
character  of  cells,  fuse  together  into  a  single  body,  which  seems  in 
every  known  respect  to  be  homologous  with  a  uninucleate  cell,  and 
which  undergoes  a  series  of  divisions  (segmentation  of  the  ovum) 
resulting  in  the  production  of  an  increasing  number  of  new  cells. 
In  all  the  higher  animals  (and  plants)  the  two  bodies  are  obviously 
different.  In  all  metazoa  one  body,  the  ovum,  contains  a  store  of 
nutritive  material  and  has  a  special  envelope  of  its  own;  the  other, 
the  spermatozoon,  is  small  and  provided  with  means  of  active  loco- 
motion ;  the  details  of  their  fusion,  which  is  known  as  the  fertiliza- 
tion or  impregnation  of  the  ovum,  have  been  described. 

The  only  hypothesis,  as  to  the  nature  and  mutual  relations  of  the 
ovum  and  spermatozoon  which  rests,  such  is  still  my  opinion,  on 
much  basis  of  fact  is  Minot's  ''Theory  of  the  Genoblasts,"  17.47. 
This  hypothesis  is  based  upon  three  categories  of  facts :  1st,  Sexual 
reproduction  is  effected  by  the  union  of  a  male  and  female  element, 
which  produces  a  cell;  this  cell  is,  therefore,  hermaphroditic,  or, 
perhaps,  one  should  say,  asexual  or  neuter,  since  it  is  neither  male 
nor  female.  2d.  When  the  cell,  which  gives  rise  to  the  female  ele- 
ment matures  into  an  ovum,  it  undergoes  a  remarkable  process  of 
unequal  division,  known  as  the  extrusion  of  the  polar  globules.  In 
other  words,  the  cell  divides  into  three  bodies — ft,  two  polar  globules; 
ft,  a  single  female  element.  In  some  cases  the  polar  globules  sub- 
divide further.  3d.  When  a  cell  divides  into  the  male  elements 
there  remains  one  cell  which  does  not  form  a  spermatozoon.  In 
mammals  it  is  probable  that  the  parent-cell  divides  into  three  cells, 
one  of  which,  6,  remains  to  form  the  base  of  a  Sertoli's  column,  and 


78  THE    GENITAL    PRODUCTS. 

two  of  which,  «,  subdivide  further  to  produce  the  spermatoblasts  and 
ultimately  the  spermatozoa.  Unfortunately  our  knowledge  of  the 
development  of  the  spermatozoa  is  extremely  unsatisfactory,  no  two 
authors  agreeing,  so  that  extreme  caution  is  necessary.  There  are, 
however,  reasons  for  thinking  that  the  statements  just  made  in  re- 
gard to  mammalian  spermatogenesis  correctly  specify  the  essential 
steps,  and  it  is  probable  that  the  essential  steps  are  the  same  through- 
out the  animal  kingdom.  Assuming  then  that  the  view  of  sperma- 
togenesis here  adopted  is  correct,  our  further  deductions  from  the 
premises  are  almost  self-evident.  In  the  cells  proper  both  sexes  are 
potentially  present ;  to  produce  sexual  elements  the  cell  divides  into 
its  sexual  parts,  the  genoblasts ;  in  the  case  of  the  egg-cell  the  male 
polar-globules  are  cast  off,  leaving  the  female  ovum  (oospore  of  Bal- 
four) ;  in  the  case  of  the  sperm-cell  the  male  spermatoblasts,  which 
by  the  hypothesis  of  Minot  are  homologous  with  the  polar  globules, 
multiply  considerably,  and  their  descendants  give  rise  by  further  spe- 
cialization (in  mammals  of  their  nuclei)  to  the  male  elements,  while 
the  parent-cell,  or  homologue  of  the  oospore,  atrophies.  In  both 
cases  the  sexual  cell  separates  into  a  single  female  element  or  thely- 
blast,  and  probably  two  male  elements  or  arsenoblasts,  which  are 
capable  of  multiplication  by  division ;  but  in  one  case  it  is  the  thely- 
blast,  in  the  other  the  arsenoblast,  which  is  preserved,  differentiated 
further,  and  utilized.  To  make  a  complete  cell  there  must  be  a 
union  of  the  male  and  female,  and  this  is  accomplished  by  "  impreg- 
nation of  the  ovunio" 

Minot's  hypothesis  is  strictly  morphological  and  offers  us  no  in- 
sight at  present  into  the  physiological  aspects  of  sexuality.  It  has 
been  adopted  by  Balfour,  and  Ed.  van  Beneden,  neither  of  whom 
cite  Minot.  Since  the  theory  of  the  genoblasts  was  first  advanced 
in  1877,  it  has  been  confirmed  by  important  discoveries,  especially 
by  the  series  of  investigations  which  have  proven  that  polar  globules, 
as  stated  in  the  section  on  the  maturation  of  the  ovum,  occur  in  all 
classes  of  the  animal  kingdom,  and,  secondly,  through  the  investi- 
gations on  the  relation  of  the  polar  globules  to  parthenogenesis.  The 
general,  may  we  not  say  the  universal,  occurrence  of  the  formation 
of  the  polar  globules  as  a  necessary  step  in  rendering  the  ovum  capa- 
ble of  impregnation,  is,  of  course,  a  very  important  confirmation  of 
the  theory,  since  the  theory  assumes  that  the  production  of  the  polar 
globules  is  the  essential  step  in  converting  the  egg-cell  into  an  oospore 
or  thelyblast. 

Minot,  in  his  original  article,  briefly  indicated  the  application  of 
his  theory  to  parthenogenesis,  and  the  question  was  more  and  ably 
discussed  by  Balfour  in  his  "Comparative  Embryology,"  I.,  63-64. 
In  his  article  of  1883,  47,  367,  Minot  is  more  explicit.  He  says :  "  If 
one  assumes  that  the  ovum  becomes  female  by  the  removal  of  the  polar 
globules,  then  it  must  remain  asexual  so  long  as  no  globules  are 
formed.  If  one  further  assumes  that  no  polar  globules  are  formed  in 
ova,  which  develop  parthenogenetically,  then  these  ova  would  remain 
simple  cells,  and  their  reproductive  process  would  depend  on  ordi- 
nary cell-division.  If  the  globules  are  developed  then  impregna- 
tion is  an  unavoidable  preliminary  of  further  development."  In 
other  words,  parthenogenesis  is  only  an  extreme  case  of  asexual  repro- 


THEORY   OF   SEX.  70 

duction  and  in  nowise  the  development  of  a  female  element  (oospore 
or  thely blast)  without  impregnation.  The  correctness  of  this  view 
has  since  become  extremely  probable  through  the  observations  of 
Blochmann,  87. 1,  88. 1,  of  Weismann,  and  of  Weismann  and  Ischi- 
kawa,  88.2,  4,  who  find  that  in  parthenogenetic  ova  there  is  only 
one  polar  globule  formed,  while  in  eggs  requiring1  fertilization  there 
are  two.  Now  by  Minot's  theory  the  cells  must  be  hermaphroditic 
in  order  to  develop,  and  the  egg-cell  becomes  a  thelyblast  by  the 
ejection  of  two  polar  globules;  if,  therefore,  one  polar  globule  is 
removed  and  the  other  not,  the  egg-cell  retains  part  of  its  male  con- 
stituent. The  significance  of  the  two  polar  globules  has  already 
been  discussed,  p.  Go;  Weismann's  interpretation  is  considered  in 
the  following  section  on  Heredity. 

To  my  theory  of  the  genoblasts,  I  feel  justified  in  making  an 
essential  addition — namely,  that  sexuality  is  a  relation  of  substances 
or  forces,  and  not  dependent  on  any  special  substance.  The  chief 
evidence  in  favor  of  this  assumption  is  the  fact  that  in  all  male  ele- 
ments the  proportion  of  protoplasm  to  the  nucleus  is  small,  while  in 
female  elements  (thely blasts)  it  is  small;  and,  moreover,  to  produce 
spermatozoa  there  is  an  excessive  growth  of  the  nuclei,  while  to  pro- 
duce ova  there  is  an  excessive  growth  of  the  protoplasm.  It  is 
remarkable,  as  Minot  has  demonstrated  (Address,  Proc.  A.  A.  A.  S., 
l.v.K)),  that  a  relative  increase  of  protoplasm  is  the  anatomical  char- 
acteristic of  senescence.  The  ovum  resembles  an  old  cell,  the  sper- 
matozoon a  young  cell,  and  these  resemblances  cannot  be  considered 
fortuitous. 

There  is  no  material  basis  of  sexuality  in  the  sense  that  there 
is  any  visible  male  or  female  substance  known  to  the  biologist, 
nor  is  it  probable  that  a  male  or  female  substance  exists.  The  func- 
tions of  life,  according  to  our  present  conceptions,  are  not  each  con- 
nected with  particular  chemical  compounds  or  with  particular  visible 
constituents  of  the  cells,  but  rather  depend  upon  the  complex  inter- 
relations of  numerous  different  substances, which  enter  into  the  com- 
position of  the  cell.  There  are  certain  functions  which  are  connected 
more  intimately  with  one  part  than  with  another — as,  for  instance, 
contractility  with  the  protoplasm,  heredity  with  the  nucleus;  but 
even  in  these  cases  we  cannot  say  that  the  functions  in  question 
could  go  on  without  the  interplay  of  the  other  portions  of  the  cells. 
The  genoblasts  contain  nuclear  substance,  protoplasm,  and  enchy- 
lema,  and  we  can  ascertain  the  sex  of  a  genoblast  only  by  observing 
its  history,  not  by  any  direct  test.  It  is  probable  that  male  or  female 
sexuality  is  an  intracellular  relation  of  parts,  some  modification  of 
the  interplay  of  forces  within  the  cells,  and  for  the  present  this  view 
must  hold  against  the  opposite  view  that  there  is  a  male  matter  and 
a  female  matter. 

Several  interpretations  of  the  polar  globules  have  been  advocated, 
which  are  incompatible  with  Minot's  theory.  The  first  of  these  is 
that  of  Whitman,*  who,  in  his  first  article  on  the  development  of 
Clepsine,  78.2,  p.  48,  maintains  that  a  series  of  cell  generations  is 
produced  by  a  series  of  divisions,  and  the  separation  of  the  polar 

*  Compare  also  O.  Hertwig.  90.  1. 


80  THE    GENITAL   PRODUCTS. 

globules  is  merely  the  last  of  these  divisions.  Inasmuch  as  this 
view  overlooks  the  fact  that  polar  globules  are  part  of  the  process  of 
maturation  and  that  no  ovum  can  be  impregnated  until  they  aiu 
formed,  and  the  further  fact  that  the  products  of  division  (globules 
and  oospore)  are  extremely  unlike,  while  in  ordinary  divisions  the 
two  daughter-cells  have  close  resemblance  to  one  another — inasmuch 
as  these  fundamental  facts  are  overlooked,  it  seems  to  me  that  Whit- 
man's explanation  cannot  be  adopted. 

Allied  to  Whitman's  view  is  that  of  Butschli,  84.1,  who,  starting 
from  the  idea  of  a  sexual  colony,  such  as  is  found  in  certain  unicellu- 
lar animals  (Flagellata) ,  considers  that  the  tendency  to  form  such 
colonies  is  preserved  in  the  metazoa,  and  shows  itself  in  the  bundle 
of  spermatoblasts  and  in  a  more  rudimentary  form  in  the  egg-cell, 
forming  a  colony  with  its  two  polar  globules.  The  essential  objec- 
tion to  this  view  is  that  it  overlooks  the  fact  that  the  divisions  of 
cells  to  produce  the  sexual  products  are  divisions  into  unlike  bodies, 
while  in  the  sexual  colonies  of  the  Flagellata  the  divisions  are,  so  far 
as  known  at  present,  into  like  cells. 

O.  Hertwig's  criticisms,  90.1,  against  Minot  are  based  on  the 
study  of  the  differentiation  of  the  sexual  elements  of  Ascaris.  He 
overlooks  the  fact  that  the  theory  of  Minot  depends  on  the  origin  of 
the  sexual  elements,  not  on  their  differentiation;  yet  nothing  is 
known  as  to  the  origin  of  the  genoblasts  in  Ascaris. 

Besides  the  theory  of  sex  already  discussed,  there  are  three  others 
which  must  be  noticed.  The  first  of  these  has  been  advanced  by 
Sabatier,  and  defended  by  him  in  a  series  of  articles,  several  of 
which  have  been  reprinted,  making  in  their  reprinted  form  the  fifth 
volume  of  Sabatier 's  "  Travaux."  *  Sabatier  considers  that  the  cells 
are  neuter  or  hermaphroditic,  agreeing  in  this  respect  with  Minot, 
and  that  the  casting  off  of  the  male  portion  converts  the  cell  into  a 
female  element,  and  vice  versa,  but  he  goes  farther  than  Minot  in 
attempting  to  specify  which  parts  of  the  cell  are  male  and  which  are 
female.  He  directs  attention  first  to  the  fact  that  in  certain  inver- 
tebrates there  is  a  central  mass  (Bloomfield's  blast  oph  or  e) ,  to  which 
there  are  attached  spermatoblasts  or  spermatozoa.  He  endeavors  to 
prove  that  this  is  the  primitive  method  of  spermatogenesis,  and  con- 
cludes that  the  male  element  is  peripheral,  and  the  product  of  a  cen- 
trifugal action.  He  directs  attention,  second,  to  the  various  products 
that  are  thrown  off  from  the  cell,  which  ultimately  forms  the  ovum. 
Summarizing  his  conclusions  in  regard  to  the  egg  (1.  c.  V.,  202-203) 
he  says :  "  If  we  recapitulate  now  the  various  groups  of  globules 
which  are  eliminated  from 'the  ovule,  commencing  at  the  asexual 
cell-stage  of  its  life  up  to  the  moment  when  it  attains  the  complete 
dignity  and  signification  of  a,n  egg,  we  see  that  there  may  be : 

"  1st.  Globules  precoces  on  du  debut,  which  become  usually  the 
elements  of  the  follicle  and  give,  so  to  speak,  the  first  impulse  to  the 
march  of  the  cell  toward  sexuality. 

"  2d.  Globules  tardifs,  which  are  at  times  formed  well  before  the 
epoch  of  maturity,  but  are  eliminated  at  a  late  period,  and  sometimes 
very  near  the  maturity.  They  are  all  formed,  as  are  the  globules 

*  "Travaux  du  Laboratoirede  Zoolo^ie  de  la  FacultS  des  Sciences  de  Montpellier  et  de  la  Sta- 
tion Zoologique  de  Cette."  Ire  Ser. ,  5me  vol 


THEORY   OF   SEX.  81 

,  by  simple  differentiation  in  the  midst  of  the  protoplasm 
and  without  karyokinetic  phenomena. 

"  3d.  Globules,  which  are  contemporary  with  the  period  of  com- 
plete maturity,  and  of  which  the  elimination  accentuates  in  the  egg 
a  very  pronounced  attraction  for  a  male  element  coming  from  another 
cell,  or  even  from  another  organism.  These  are  the  globules  de 
maturation  parfciite.  Most  of  these  globules  result  from  phenom- 
ena of  cellular  division,  and  form  the  polar  globules  properly 
so-called." 

From  this  quotation  it  will  be  clear  that  Sabatier  classes  together 
the  follicular  cells  surrounding  the  ovum,  the  non-cellular  masses 
excreted  from  the  egg-cell  during  its  development,  and  the  polar 
globules.  All  of  these  are — so  he  maintains — thrown  off  from  the 
central  ovum,  hence  he  concludes  that  the  female  element  is  central 
and  the  product  of  a  centripetal  action.  In  brief  the  male  element 
represents  a  centrifugal  force,  the  female  element  a  centripetal  force. 

A.  Prenant  has  adopted  a  theory  which  is  apparently  a  modifica- 
tion of  Sabatier's,  but  until  his  memoir  is  published  (Journ.  de 
rAnat.  et  PhysioL,  1892)  discussion  of  his  theory  must  be  deferred. 

I  am  unable  to  accept  Sabatier's  theory  for  many  reasons,  of  which 
the  following  may  be  mentioned:  1st.  It  cannot  be  shown  that  the 
differentiation  of  the  spermatozoa  does  occur  typically  at  the  periph- 
ery ;  on  the  contrary,  in  the  great  majority  of  cases,  it  is  distinctly 
polar,  since  it  takes  place  at  the  inner  end  of  an  epithelial  cell.  2d. 
It  is  impossible  to  maintain  a  homology  between  cells  and  masses 
which  are  not  nucleated  at  any  period  of  their  history,  and  Sabatier's 
views  as  to  the  maturation  of  the  ovum  oblige  us  to  draw  such  an 
homology.  3d.  Sabatier,  to  establish  the  centrifugal  removals, 
which  produce  the  ovum,  relies  largely  upon  the  history  of  the  glob- 
ules tardifs,  which,  therefore,  must  by  his  hypothesis  be  male.  He 
bases  his  defence  largely  on  observations  on  the  "  testa-cells  "  of 
Ascidians,  which  he  considers  to  belong  under  the  head  of  globules 
tardifs;  but  these  observations  have  been  called  in  question  by  Fol 
(Recueil  Suises  Zool.,  No.  1),*  so  that  there  is  doubt  as  to  one  of 
Sabatier's  chief  foundations.  Now  some  of  these  globules — sup- 
posed to  be  male — contain  no  nuclear  substance,  yet  all  the  sexual 
elements,  which  we  know  positively  to  be  such,  do  contain  nuclear 
substance. 

Balbiani's  theory,  79. 1,  is  the  exact  inverse  of  the  two  previously 
mentioned ;  for  him  every  sexual  element  is  the  product  of  the  copu- 
lation of  two  elements :  1st,  the  epithelial  cells  of  the  follicle,  which 
are  male;  2d,  the  Urei,  which  is  always  female.  Balbiani  has 
not  observed  any  such  copulation,  nor  has  he  any  valid  indirect  evi- 
dence of  it  to  bring  forward;  on  the  contrary,  he  disregards  in 
several  respects  what  others  consider  elementary  principles  of  his- 
tology. 

Nussbaum's  theory  appears  to  me  valuable  and  suggestive.  It 
was  first  advanced,  so  far  as  I  know,  in  1880,  though  similar  con- 
ceptions are  to  be  found  in  earlier  writers.  Nussbaum,  80. 1,  starts 
with  the  conjugation  of  two  similar  unicellular  individuals,  as 
occurs  in  certain  protozoa;  the  two  individuals  fuse,  and  after  fusion 

*  For  Sabatier's  answer  see  same  Recueil,  No.  a. 


THE   GENITAL    PRODUCTS. 

divide  into  successive  generations  of  cells.  He  next  points  out  that 
in  the  higher  animals  all  the  sexual  differences  are  secondary  not 
only  in  the  so-called  "secondary  sexual  characteristics,"  but  also  in 
the  sexual  organs  themselves.  He  then  goes  on  to  emphasize  the 
presence  of  the  sexual  cells  (Ureier  of  German  authors,  Hamann's 
Urkeimzellen)  in  the  embryo,  and  maintains  that  as  these  both  give 
rise  to  the  sexual  products  the  ovum  and  spermatozoon  are  strictly 
homologous  cells.  He  writes,  p.  106  :  "  There  come  together  during 
impregnation  accordingly  not  two  heterogeneous  elements  which 
complement  one  another  and  together  form  a  whole,  but  rather 
there  come  together  two  homologous  cells,  of  which  one  to  facilitate 
conjugation  is  transformed  into  a  more  movable  body ;  the  other  is 
laden  with  nutritive  material,  and  is  furnished  wTith  protective  de- 
vices." And  again,  p.  113:  "The  differentiation  of  sex  is  not  the 
transmission  of  two  originally  united  functions  to  the  differing 
descendants  of  a  common  original  Anlage;  it  is  rather  the  variation 
of  homologous  cells  for  the  better  achievement  of  their  conjugation." 
The  sexual  elements,  according  to  Nussbaum,  are  cells  which  are 
set  apart  for  reproductive  functions  from  the  rest  of  the  cells  of  the 
body,  and  there  is  no  primary  difference  between  male  and  female. 
He  does  not  consider  in  any  way  the  significance  of  the  polar  glob- 
ules or  Sertoli's  columns,  and  therefore  does  not  argue  directly 
against  Minot's  theory.  His  generalization  that  separate  cells  alike 
in  character  are  set  apart  early  in  embryonic  development  to  form 
both  the  male  and  female  elements  is  a  very  important  one,  and  has 
been  adopted  by  embryologists.  Weismann  accepts  it  and  applies  it 
to  his  theory  of  heredity,  and  it  has  received  a  valuable  confirmation 
in  Hamann's  paper,  87. 1.  But  this  generalization  leaves  the  ques- 
tion of  the  final  differentiation  of  the  Ureier  into  sexual  elements 
untouched,  and  is  not  necessarily  in  any  way  in  conflict  with  the 
conception  of  that  differentiation  advocated  by  Minot. 

It  seems  to  me,  therefore,  that  although  Minot's  hypothesis  cannot 
be  proven  at  present,  yet  there  is  no  other  hypothesis  of  sex  having 
nearly  as  strong  evidence  in  its  favor. 

Origin  and  Objects  of  Sexuality. — THE  ORIGIN  of  sexuality 
is  involved  in  much  obscurity.  In  the  lowest  unicellular  organisms 
there  is  certainly  no  clear  sexual  differentiation,  and  some  biologists 
assert  that  there  is  nothing  comparable  to  sexual  reproduction,  but 
the  observations  are  far  too  imperfect  at  present  to  justify  any  such 
assertion.  The  question  involved  is,  whether  sexuality  is  coexten- 
sive with  life  or  not ;  in  the  latter  case  it  is  the  result  of  evolution 
from  asexual  organisms,  and  is  a  secondary  and  not  a  primary  or 
essential  characteristic  of  life.  The  problem  is,  therefore,  a  funda- 
mental one,  but  we  cannot  hope  for  its  solution  until  our  knowledge 
of  the  lowest  organisms  is  greatly  extended. 

The  precursor  of  the  sexual  process  is  undoubtedly  to  be  found  in 
the  conjugation  of  two  similar  cells,  which  fuse  into  a  single  organ- 
ism, as  occurs  in  certain  cryptogamous  plants  and  among  the  pro- 
tozoa, notably  the  rhizopods.  In  the  next  stage  the  cells  which  fuse 
together  are  obviously  different,  as  in  the  Flagellata.  If  now  we 
pass  to  the  colonies  of  the  Flagellata  we  find  that  certain  cells  only 
act  as  conjugators,  and  thus  we  approach  the  disposition  of  the  mul- 


THEORY   OF   SEX.  83 

ticellular  animals,  Metazoa,  which  have  bodies  composed  of  cells, 
certain  of  which  produce  the  sexual  elements,  and  these  elements 
conjugate.  In  conjugation  and  impregnation  alike  the  process  is  the 
fusion  of  a  nucleated  protoplasm  with  another  nucleated  protoplasm 
of  different  origin.  In  plants  also,  as  we  ascend  from  the  lower  to 
the  higher  forms,  we  find  the  differences  between  the  conjugating 
bodies  to  increase:  thus  in  zygopliytes  the  conjugating  cells  are 
alike,  in  phanerograms  the  pollen  and  ovicell  are  unlike.  The  ques- 
tion arises  whether  the  conjugation  of  the  like  or  of  the  unlike  pro- 
toplasms (or,  in  other  words,  of  similar  cells,  or  of  genoblasts)  gives 
the  clew  to  what  is  essential.  Is  the  dissimilarity  of  the  conjugating 
bodies  essential?  If  Minot's  theory  of  the  genoblasts  is  correct  it 
is  probable  that  the  dissimilarity  is  essential,  in  which  case  it  is 
conceivable  that  when  similar  cells  conjugate  each  cell  contains  both 
male  and  female,  and  the  male  of  one  saturates  the  female  of  the 
other,  and  vice  versa.  On  the  other  hand,  the  whole  tendency  of 
evolution  is  from  the  simpler  to  the  complex,  and,  a  priori,  it  is 
more  plausible  to  consider  that  complete  sexuality  is  a  differentiation 
of  a  simpler  process  rather  than  the  mere  separation  of  what  was 
united  in  one  cell.  The  last-mentioned  conception  is  undoubtedly 
the  one  which  would  appeal  to  most  biologists  at  the  present  time. 
Yet  we  see  that  the  functions  which  exist  in  a  cell  do  undergo  sep- 
aration, so  that  they  become  excessively  predominant  in  certain 
cells ;  for  instance,  the  nervous  functions  have  been  thus  selected  out 
for  the  superfluous  endowment  of  certain  cells,  and  it  appears  to  me 
perfectly  conceivable  that  male  and  female  may  be  united  in  a 
unicellular  organism  just  as  completely  as  assimilative  and  nervous 
functions,  and  as  these  latter  are  differentiated,  so,  too,  are  the 
former. 

The  above  considerations,  and  others  which  might  be  given,  were 
it  worth  while  to  lengthen  the  discussion  of  so  obscure  a  subject, 
lead  me  to  the  hypothesis  that  sexuality  is  coextensive  with  life; 
that  in  protozoa  *  the  male  and  female  are  united  in  each  of  the 
conjugating  cells,  and  impregnation  is  double;  and,  finally, 
that  in  the  metazoa  the  male  and  female  of  the  cells  separate 
to  form  genoblasts  or  true  sexual  elements,  and  impregnation 
is  single.  It  need  hardly  be  pointed  out  that  this  hypothesis  is 
purely  tentative,  and  may  have  to  be  rejected  altogether  when  we 
have  sufficient  knowledge  to  decide  as  to  its  validity. 

THE  OBJECT  OF  SEXUALITY  is,  likewise,  known  only  by  hypothe- 
sis. Three  views  are  to  be  considered :  its  purpose  is,  1st,  rejuvena- 
tion; 2d,  to  produce  variability;  3d,  to  check  variability.  1st.  The 
theory  that  the  purpose  of  sex  is  to  produce  a  young  organism  is 
very  old,  and  is  based  on  every-day  observation ;  it  involves,  as  its 
corollary,  that  organisms  become  old,  and  thereby  incapable  of 
maintaining  their  own  existence.  That  sexual  reproduction  does 
produce  a  young  organism  is  the  universal  law ;  it  is  also  true  that 
every  young  organism  does  possess  certain  morphological  and 
physiological  characteristics  by  which  it  may  be  distinguished 
from  an  old  organism.  When  sexual  reproduction  occurs  life  pro- 

*  Very  possibly  this  is  not  true  for  all  protozoa  for  there  may  be  protozoa  with  true  geno- 
blasts. 


84  THE   GENITAL   PRODUCTS. 

ceeds  in  cycles ;  the  sexual  conjugation  produces  a  single  cell,  which 
divides  again  and  again,  until  at  last  the  process  cannot  proceed 
further ;  then  a  renewed  conjugation  follows  and  a  new  cycle  of  cell- 
generations  ensues ;  in  the  higher  animals  the  cells  remain  together 
as  they  multiply ;  in  the  protozoa  the  cells  each  lead  a  separate  life, 
but  in  both  the  cell-cycle  is  dominant ;  the  body  of  a  metazoon  is 
comparable  to  the  set  of  individual  unicellular  protozoa  resulting 
from  one  sexual  act.  In  one  case  the  cells  of  a  cycle  remain  together, 
in  the  other  they  separate.  So  far,  then,  as  it  is  known  to  occur, 
the  sexual  process  is  a  rejuvenating  one ;  but  this  does  not  prove  that 
all  living  organisms  require  sexual  rejuvenation  from  time  to  time, 
nor  does  it  prove  that  there  is  no  other  means  of  rejuvenation.  It 
may  be  that  all  cells  as  they  divide  asexually  lose  their  growth-power, 
so  that  there  comes  a  time  when  there  must  be  a  rejuvenation  or 
restoration  of  the  growth-power,  but  it  is  improbable  that  sexual 
reproduction  is  the  only  means  to  effect  the  necessary  restoration  of 
vitality.  2d.  That  the  object  of  sex  is  to  increase  variability  and 
so  afford  a  wider  scope  for  natural  selection  has  been  maintained  by 
Weismann.  At  first  sight  the  notion  of  the  mingling  of  two  hered- 
itary strains  of  different  character  producing  variety  in  the  offspring 
seems  very  plausible ;  but  the  notion  does  not  bear  examination,  for 
it  renders  the  commencement  of  variability  impossible,  and  fails  to 
account  for  the  divergence  in  the  offspring  of  the  same  parents. 
3d.  The  view  that  sexual  reproduction  checks  variability  has  been 
advanced  by  Hatschek,  87.1,  38G,  who  points  out  that  the  mingling 
of  hereditary  strains  tends  to  restore  the  specific  norm,  since  in  the 
long  run  the  variations  counterbalance  one  another.  Galton  has 
shown  that  in  human  stature  the  tendency  of  heredity  is  to  restore 
the  normal  height,  and  the  same  is  presumably  true  of  other  char- 
acteristics. I  am  strongly  inclined  to  accept  Hatschek's  theory,  and 
to  maintain  with  him  that  one  result  of  sexual  reproduction  is  to 
correct  variations  and  so  preserve  the  specific  type. 

Nature  of  Sex. — Sex,  as  we  encounter  it  in  the  human  species, 
is  the  result  of  a  long  evolution  affecting  a  large  number  of  organs — 
perhaps  all  of  the  organs — so  as  to  result  in  characteristic  differences 
between  the  male  and  female ;  but  the  essential  difference  is  in  the 
relation  of  the  two  sexes  to  the  production  of  the  genoblasts ;  the 
male  produces  the  spermatozoa,  the  female  the  ova,  and  in  this  lies 
the  whole  essence  of  the  sexual  differentiation;  all  other  distinctive 
morphological  and  physiological  traits  of  men  and  women  are  second- 
ary. Thus  the  structure  and  functions  of  the  genital  ducts,  of  the 
uterus,  mammary  glands,  etc.,  though  eminently  characteristic  of 
the  sexes,  in  man  are  not  from  a  biological  point  of  view  funda- 
mental. 

As  we  ascend  the  animal  scale  there  is  an  increasing  divergence 
between  the  sexes,  owing  to  the  increasing  adaptation  to  the  repro- 
ductive functions.  It  is  generally  believed  that  the  primitive  con- 
dition is  hermaphroditic,  and  that  the  female  is  an  individual  in 
which  the  power  of  producing  male  elements  is  lost,  and  a  male  an 
individual  in  which  the  power  of  producing  female  elements  is  lost. 
In  a  certain  sense  this  conception  appears  true,  for  in  the  embryo 
there  is  an  indifferent  stage  in  which  the  sexual  glands  are  already 


HEREDITY.  So 

differentiated,  but  in  which  the  future  sex  is  unrecognizable;  sub- 
sequently by  unknown  factors  the  sexual  gland  is  converted  into  an 
ovary  or  a  testis.  In  some  cases,  as  in  certain  teleosts  and  in  the 
snails,  the  sexual  glands  develop  both  ova  and  spermatozoa.  These 
facts  suggest  that  the  primitive  sexual  gland  is  potentially  herma- 
phroditic. It  is  to  be  remembered,  however,  that  if  hermaphroditism 
\\vre  the  primitive  form  we  should  expect  to  find  the  lowest  metazoa 
hermaphroditic ;  but  this  is  no*  the  case  either  with  all  Coelenterata 
or  all  sponges,  although  it  k  the  case  in  some  higher  classes  of  the 
animal  kingdom — as,  for  instance,  the  trematode  worms  and  pul- 
monate  gasteropods.  These  and  other  considerations  have  led  me 
to  the  hypothesis  that  primitively  each  individual  animal  is  sexually 
indifferent  when  young,  and  becomes  either  male  or  female  when 
adult;  by  a  secondary  modification  in  certain  forms  the  individual 
becomes  both  male  and  female.  This  is  contrary  to  the  prevalent 
opinion  that  the  hermaphroditic  condition  is  the  primitive  one. 

VI.  HEREDITY. 

In  regard  to  the  process  of  hereditary  transmission  there  are  two 
theories,  each  of  which  appears  in  several  modifications '  1st,  the 
theory  of  pangenesis;  2d,  the  theory  of  germinal  continuity,  The 
latter  does,  the  former  does  net,  appear  to  me  to  conform  to  our 
present  knowledge. 

Pangenesis. — The  theory  of  pangenesis  was  first  formulated  by 
Darwin,  thourh  it  had  been  crudely  foreshadowed  by  Buffon,  Bon- 
iiet,  and  Herbert  Spencer.  The  following  quotation  from  Darwin's 
"Animals  and  Plants  under  Domestication"  (Amer.  edit.,  1868, 
II.,  448,  449)  gives  his  statement  of  his  theory:  "I  have  now 
enumerated  the  chief  facts  which  every  one  would  desire  to  connect 
by  some  intelligible  bond.  This  can  be  done,  as  it  seems  to  me,  if 
we  make  the  following  assumptions :  if  the  first  and  chief  one  be  not 
rejected,  the  others,  from  being  supported  by  various  physiological 
considerations,  will  not  appear  very  improbable.  It  is  almost  uni- 
versally admitted  that  cells  or  the  units  of  the  body  propagate  them- 
selves by  self-division  or  proliferation,  retaining  the  same  nature 
and  ultimately  becoming  converted  into  the  various  tissues  and  sub- 
stances of  the  bod}^.  But  besides  this  means  of  increase  I  assume 
that  cells,  before  their  conversion  into  completely  passive  or  "  form- 
material,"  throw  off  minute  granules  or  atoms,  which  circulate 
freely  throughout  the  system,  and  when  supplied  with  proper  nu- 
triment multiply  by  self -division,  subsequently  becoming  developed 
into  cells,  like  those  from  which  they  were  derived.  These  granules, 
for  the  sake  of  distinctness,  may  be  called  cell-gemmules,  or,  as  the 
cellular  theory  is  not  fully  established,  simply  gemmules.  They  are 
supposed  to  be  transmitted  from  the  parents  to  the  offspring,  and 
are  generally  developed  in  the  generation  which  immediately  suc- 
ceeds, but  are  often  transmitted  in  a  dormant  state  during  many 
generations  and  are  then  developed.  Their  development  is  supposed 
to  depend  on  their  union  with  other  partially  developed  cells  or  gem- 
mules,  which  precede  them  in  the  regular  course  of  growth.  Why  I 
use  the  term  union  will  be  seen  when  we  discuss  the  direct  action  of 


86  THE   GENITAL   PRODUCTS. 

pollen  on  the  tissues  of  the  mother-plant.  Gemmules  are  supposed 
to  be  thrown  off  by  every  cell  or  unit,  not  only  during  the  adult  state, 
but  during  all  stages  of  development.  Lastly,  I  assume  that  the 
gemmules  in  their  dormant  state  have  a  mutual  affinity  for  each 
other,  leading  to  their  aggregation  either  into  buds  or  into  the  sexual 
elements.  Hence,  speaking  strictly,  it  is  not  the  reproductive  ele- 
ments nor  the  buds  which  generate  new  organisms,  but  the  cells 
themselves  throughout  the  body.  These  assumptions  constitute  the 
provisional  hypothesis  which  I  have  called  Pangenesis. " 

This  hypothesis  is  the  suggestion  of  a  masterly  mind,  and,  as  a 
succinct  and  comprehensive  expression  of  the  facts  of  heredity,  must 
always  command  admiration.  But  the  real  worth  and  real  signifi- 
cance of  the  hypothesis  have  not  been  grasped  by  those  who  have 
tried  to  better  it ;  its  value  is  not  in  explaining,  for  it  does  not  do 
that,  but  in  expressing  heredity  in  hypothetical  terms,  which  are 
at  once  suggestive  and  comprehensible.  Haeckel,  in  an  amusing 
pamphlet,*  which  no  competent  critic  can  assign  the  slightest  value 
to,  asserted  that  the  gemmules  are  rhythmical  vibrations,  but  he 
gives  no  reasons  to  justify  his  opinion.  Elsberg  has  also  written  on 
the  subject  in  the  Proc.  Amer.  Assoc,  Adv.  Sci.,  XXV,,  178,  and  cites 
there  earlier  writings  of  his  own.  f 

Brooks'  modification,  76 . 1  ,of  the  theory  of  pangenesis  well  deserves 
consideration,  although  the  subsequent  progress  of  biology  does  not 
lead  me  to  think  it  felicitous;  but  we  can  now  recognize  it  as  a  step 
toward  Nussbaum's  valuable  theory  of  germinal  continuity,  and  also 
toward  Weismann's  conception  that  sexual  reproduction  has  for  its 
object  the  maintenance  of  variability,  Brooks'  theory  is  advocated 
in  his  book  on  "  Heredity"  (Baltimore,  1879) ;  he  states  it  succinctly 
as  follows  •  J  "  This  paper  proposes  a  modification  of  Darwin's  hy- 
pothesis of  the  same  name  (pangenesis) ,  removing  most  of  its  diffi- 
culties, but  retaining  all  that  is  valuable.  According  to  the  hy- 
pothesis in  its  modified  form,  characteristics  which  are  constitutional 
and  already  hereditary  are  transmitted  by  the  female  organism  by 
means  of  the  ovum ;  while  new  variations  are  transmitted  by  gem- 
mules,  which  are  thrown  off  by  the  varying  phsyiological  units  of 
the  body,  gathered  up  by  the  testicle  and  transmitted  to  the  next 
generation  by  impregnation."  If  this  theory  was  tenable,  there 
should  be — to  mention  a  single  objection — little  variation  in  individ- 
uals produced  by  parthenogenesis ,  and  they  ought  always  to  be  fe- 
males, whereas  they  are  sometimes  males.  There  remains  not  a  new 
theory  of  pangenesis,  but  the  valuable  suggestion  that  the  maternal 
influence  causes  less  variability  than  the  paternal.  I  am,  however, 
strongly  disinclined  to  anticipate  the  confirmation  of  this  suggestion, 
especially  because  the  males  are  not  more  variable  than  the  females, 
as  we  should  expect.  I  have  some  extensive  statistics  which  show 

*E.  Haeckel  "Peregenesis  der  Plastidule,"  Berlin.  1876  For  some  criticisms  which  consid- 
ering the  character  of  this  pamphlet,  are  very  gentle,  see  Ray  Lankester  in  Nature.  July  13th. 
1876,  xiv  235-238 

+  The  perusal  of  Elsberg's  article  has  not  enabled  me  to  recognize  anything  novel  except 
the  substitution  of  the  term  plastidule  for  gemmule.  used  by  Darwin  and  speculations  as  to  the 
composition  of  plastidules  as  if  he  were  groping  after  the  conception  of  the  unicella  of  Nageli, 
with  which  he  was  apparently  unacquainted 

I  Proc  Amer  Assoc  Sc.  Buffalo  1876  p  177  abstract  of  a  paper  read  before  the  section  of 
natural  history 


HEREDITY.  87 

that  in  mammals,  at  least,  there  are  no  essential  differences  between 
the  sexes  in  variability.  Even  if  Brooks'  thesis  should  be  established 
it  would  prove  only  that  the  inheritance  from  the  mother  is  stronger 
than  from  the  father,  and  there  would  lack  reasons  for  his  abstruse 
hypothesis. 

The  theory  of  pangenesis  is  to  be  resigned,  not  so  much  on  account 
of  the  direct  arguments  against  it,  as  on  account  of  the  accumulation 
of  evidence  in  favor  of  the  theory  of  germinal  continuity. 

Germinal  Continuity. — There  are  various  theories  to  be  consid- 
ered under  this  head ;  but  they  all  have  in  common  the  conception 
that  there  is  a  formative  force  in  organisms — that  the  force  depends 
upon  a  special  material  substratum,  and  that  some  of  the  supply  of 
that  substratum  is  given  by  the  parent  to  the  sexual  elements  it 
produces. 

The  first  important  step  toward  the  substitution  of  a  new  theory 
vice  pangenesis  was  taken,  so  far  as  I  am  aware,  by  Moritz  Nuss- 
baum,  whose  memoirs,  80.1,  84.2,  on  the  differentiation  of  sex  de- 
serve great  attention.  August  Weismann  *  has  adopted  Nuss- 
baum's  conception  and  defended  it  with  insistent  energy,  adding  also 
several  modifications.  Nussbaum  pointed  out  that  there  is  note- 
worthy evidence  in  the  development  of  various  animals  tending  to 
show  that  the  germinal  cells,  from  which  the  sexual  products  arise, 
are  separated  off  very  early  from  the  other  cells  of  the  embryo  and 
undergo  very  little  alteration.  Hence  he  concluded  that  some  of 
the  germ  substance  is  directly  abstracted  from  the  developing  ovum 
and  preserved  without  essential  alteration  to  become,  by  giving  rise 
to  the  sexual  elements,  the  germ  substance  of  another  generation. 
\Veismann  insists  upon  the  corollary  that  the  whole  nature  of  the 
animal  or  plant  depends  upon  its  germinal  substance  (Keiinplasmd) , 
and  that  the  reason  why  the  offspring  is  like  the  parent  is  that  in 
every  genoblast  some  of  the  germinal  matter  is  preserved  unchanged. 
He  calls  this  view  the  theory  of  the  continuity  of  the  germ-plasma. 
He  follows  Nussbaum  also  in  emphasizing  the  fact  that  this  theory  is 
inconsistent  with  the  theory  of  pangenesis  and  with  the  theory  that 
parental  characteristics  acquired  through  the  influence  of  external 
causes  ar"e  transmissible  to  the  offspring.  On  these  two  points  Weis- 
mann's  second  and  third  papers  are  especially  important.  Nussbaum 
and  Weismann  lay  great  stress  upon  the  separation  of  the  cells  of  the 
embryo  into  two  kinds:  1,  the  germ -cells,  which  are  converted  into 
the  sexual  elements;  2,  the  somatic- cells,  which  constitute  the  body 
of  the  organism.  The  germ-cells  descend  directly  from  the  ovum, 
according  to  Weismann,  who  has  carried  his  speculations  to  a  great 
extreme,  and  undergo  little  alteration,  so  that  they  have  (in  suspen- 
sion) the  power  to  produce  a  whole  organism,  which  the  somatic-cells 
do  not  have.  It  is  impossible  to  agree  to  this  extraordinary  view. 


* -\\eismann  s  first  paper  was  read  before  the  University  of  Freibure  as  a  Prorectoratsrede 
and  was  published  in  pamphlet  form  at  Jena  in  1883.  83.1.     A  second  paper  waS  read  before  the 
ierinan  Naturforscherversannnhm*  in  1885.  and  appeared  in  the  Ta^latt  of  that  \^"ation 
it  was  subsequently   amplified  and   republished.  85  1.     A  third    paper   86  3 :    was   Ml?  vise 
dressedto  the  Xaturforscherversammlung  in  1886.  and    published  ^  Jena   the  same  year      A 
not.ice  of  this  last  is  given  by  Kollmann.  Biol.  Cbl.,  v..  673  and  70.5.    At  the  same  meefinJ  of  the 
Naturforscher.  R.  Virchow  also  delivered  an  address  fsee  Virchow's  \reh     ciii      FIoV  4W !   and 
abstract  in   Biol.    Cbl..  vi.    97.    129.1151.,    in    which  he  attacked  Weismann      To  kollniann  w 
\  irchow  Weismann  has  replied  in  Biolog.  Centralbl     vi    38 


88  THE    GENITAL    PRODUCTS. 

Minot,  70,  has  expressly  emphasized  the  fact  that  the  formative 
force  is  certainly  a  diffused  one,  as  is  amply  proven  by  the  processes 
of  regeneration,  by  the  phenomenon  of  duplication  of  parts,  and  by 
asexual  reproduction,  since  in  all  these  cases  the  formation  of  a  part 
or  the  whole  of  the  organism  proceeds  without  the  participation  of 
the  sexual  elements.  Kolliker,  also,  85. 1,  44-40,  clearly  demonstrates 
that  a  sharp  division  between  germ-cells  and  somatic-cells  cannot  be 
maintained.  The  same  position  has  been  adopted  by  Whitman, 
87.3,  and,  of  course,  by  many  others.  It  is  to  be  further  remem- 
bered that  the  cells  for  the  different  organs  of  the  body  are  all  set 
apart  very  early  indeed,  and  in  the  case  of  vertebrates  the  germ- 
cells  are  among  the  very  latest  to  become  distinguishable ;  thus  the 
nerve-cells,  muscle-cells,  notochord-cells,  etc.,  etc.,  all  can  be  seen 
to  precede  the  germ-cells  in  their  differentiation.  Weismann's 
assumption  that  the  germ-cells  are  set  apart  specially  early  is  simply 
false;* all  the  organs  have  their  cells  set  apart  early,  and  that  too 
while  they  are  in  the  embryonic  condition ;  and  it  is  not  true  that  the 
germ-cells  differ  essentially  as  to  their  mode  of  origin  or  differen- 
tiation from  the  so-called  somatic-cells.  The  early  divergence  of  the 
cells  according  to  the  organs  or  parts  they  are  destined  for  was 
pointed  out  explicitly  by  W.  His  many  years  ago,  74.1,  18.  19. 
Weismann's  error  consists  in  attributing  a  peculiar  significance  to 
a  fact  by  connecting  it  only  with  the  development  of  the  sexual  ele- 
ments, whereas  it  is  a  fact  common  to  all  parts  of  the  body.  All, 
therefore,  of  Weismann's  further  speculations  as  to  the  difference 
between  germ  plasma  and  "  histogenes  plasma"  are  without  foun- 
dation. 

Nageli  was  probably  the  first  to  reach  the  definite  conception  of 
a  material  basis  of  heredity,  to  which  basis  he  gave  the  name  of 
idioplasma.  This  idioplasma  is  essentially  identical,  it  seems  to 
me,  with  Weismann's  Keimplasma.  Nageli 's  views  are  presented 
very  fully  in  a  large,  abstruse,  and  little-studied  volume,  of 
which  a  useful  abstract  has  been  given  by  Kollmann  (Biol.  CbL, 
IV.,  488,  517).  Nageli  is  led  to  the  theory  that  there  are  in  every 
living  cell  two  substances,  one  of  which,  the  idioplasma,  alone  car- 
ries on  the  function  of  hereditary  transmission,  while  the  other,  the 
nutritive  plasma  (Nahrplasma)  is  the  seat  of  the  remaining  func- 
tions. In  other  words,  Nageli  put  forward  in  a  definite  form  the 
theory  of  germinal  continuity,  for  he  assumes  the  formative  force  to 
reside  in  a  specific  material  substratum,  which  reproduces  and  per- 
petuates itself,  occurs  throughout  the  organism,  and,  therefore,  in 
the  genital  products  also.  The  argument  in  support  of  this  theory  is 
very  able,  and  well  deserves  the  cordial  praise  which  Kolliker  and 
others  have  bestowed  upon  it. 

Nageli  did  not  specify  what  constituent  of  the  cell  corresponds  to 
his  idioplasma.  O.  Hertwig,  85. 1,  was  the  first  to  indicate  the  nu- 
cleus as  the  organ  of  heredity,  and  this  view  has  been  ably  defended 
by  Kolliker,  85. 1,  Strassburger,  and  others.  This  notion  rests  upon 
the  consideration  of — 1st,  various  facts  which  suggest  that  the  nucleus 
has  special  control  over  the  organization  of  the  cell ;  2d,  the  prob- 
ability that  impregnation  consists  essentially  in  the  fusion  of  the 
pronuclei;  3d,  the  development  of  the  spermatozoon  from  the  nu- 


HEREDITY.  89 

cleus.  That  the  nucleus  presides  over  the  cells  is  naturally  suggested 
by  the  phenomena  of  cell-division,  especially  indirect  division 
(karyokinesis,  mitosis) ,  for  during  the  process  the  nucleus  leads  the 
way,  and  its  visible  alteration  precedes  that  of  the  protoplasm ;  the 
astral  rays  both  during  kar}Tokinesis  and  those  around  the  pronuclei 
during  impregnation  may  be  interpreted  as  results  of  nuclear  control. 
The  opposite  conception  that  the  protoplasm  leads  has  not  lacked  de- 
fenders (see  Auerbach,  Biitschli,  76.1,  Nussbaum,  86.1,  504,  and 
Whitman,  88. 1) .  I  may  point  out  that  in  interpreting  the  observa- 
tions bearing  upon  this  discussion,  we  must  not  forget  that  the  nu- 
cleus and  protoplasm  are  interdependent,  neither  being  able  to  main- 
tain its  existence  permanently  without  the  other.  "The  fact,"  says 
Minot,  85,  125,  "that  the  visible  alteration  of  the  protoplasm  in  a 
certain  rare  case  comes  before  that  of  the  nucleus  shows  that  the 
protoplasm  probably  has  an  active  role  in  cell-division ;  but  since 
even  then  its  arrangement  depends  on  the  position  of  the  nucleus, 
the  evidence  of  the  superiority  of  nuclear  control  is,  I  think,  not 
affected."  On  the  other  hand,  there  are  many  observations,  which 
may  be  interpreted  as  proofs,  that  the  nuclei  have  a  regulating  power 
over  the  cells,  especially  as  regards  their  division  and  organization. 
A  few  of  these  may  be  instanced:  1st.  After  a  cell  is  formed,  its 
nucleus  enlarges  first,  and  the  cell-body  follows  it  in  growth.  2d. 
Kolliker,  in  his  paper,  85. 1,  on  heredity  (p.  29  ff.),  discusses  the  re- 
lation of  nuclei  to  growth  very  fully  and  ably.  The  great  extent  of 
his  learning  has  enabled  him  to  present  the  manifold  aspects  of  the 
question  more  thoroughly  than  any  other  writer.  His  argumentation 
seems  to  me  so  satisfactory  that  it  does  not  require  the  weight  of  his 
great  authority  to  establish  the  conclusion  that  without  nuclei  there 
is  no  growth.  Of  this  the  most  faith-compelling  evidence  is  offered 
by  the  important  experiments  jf  Nussbaum  and  Gruber,*  who  found 
that  when  unicellular  animals  are  artificially  divided,  the  fragments 
containing  nuclei  continue  to  grow,  while  pieces  without  nuclei  die 
off.  3d.  The  large  unicellular  Thallophytes,  such  as  Caulerpa  and 
Codium,  become  multinuclear  before  they  attain  their  adult  size. 
Further  illustrations  are  given  by  Kolliker  (/.  c.,  pp.  11),  20).  4th. 
Perhaps  the  most  striking  demonstration  of  the  importance  of  the 
nucleus  is  afforded  by  the  experimental  alteration  of  the  plane  of 
division  of  the  ovum.  Pfluger,  83.6,  showed  that  the  plane  of  the 
first  division  of  the  ovum  is  altered  by  tilting  the  ovum  before  the 
division  begins,  and  keeping  it  in  the  same  position  during  division ; 
normally  the  plane  passes  through  the  white  pole,  but  when  the  ovum 
is  fastened  in  an  oblique  position,  the  plane  is  not  in  the  axis  of  the 
ovum,  but  in  the  line  of  gravity.  Born,f  84.3,  has  continued  these 
remarkable  experiments,  and  discovered  that  the  nucleus  changes  its 
position  when  the  ovum  is  kept  tilted,  and  that  the  site  of  the  nucleus 
determines  the  plane  of  division  of  the  ovum.  The  second  and  third 
points  (the  importance  of  the  pronuclei  and  the  nuclear  origin  of  the 

*  Science,  vol.  vi. ,  p.  4.  See  also  Nussbaum's  later  paper  in  the  Archiv  fur  mikroskop.  Anat. , 
xxvi  .  p  4&5.  Nussbaum  also  cites  Fr.  Schmitz's  experiments  on  the  artificial  division  of 
plants  Schmitz's  paper  I  have  not  seen;  it  was  published  in  1879,  in  the  Festschrift  der  natur- 
forschenden  gesellschaft  zu  Halle. 

t  I  have  not  seen  the  original.  There  is  an  abstract  in  Hofmann  und  Schwalbe's  Jahresbe- 
richt  for  1884  p  444 


90  THE    GENITAL   PRODUCTS. 

spermatozoon)  have  been  sufficiently  elucidated  in  previous  divisions 
of  this  chapter.  Now,  it  is  obvious,  since  qualities  may  be  inherited 
from  the  father,  that  the  nucleus  alone  can  furnish  the  means  of 
transmission  from  parent  to  offspring;  and,  since  it  can  accomplish 
this  on  the  paternal  side,  it  is  probable  that  it  can  do  as  much  on  the 
maternal  side — an  assumption  against  which  no  evidence  has  been 
brought  forward ;  hence  the  hypothesis  that  the  nucleus  is  the  organ 
of  hereditary  transmission.  For  criticism  of  this  view  see  J. 
Frenzel,  86.5,  p.  89,  whose  arguments  have  been  controverted  by 
Minot,  85,  127. 

We  may  go  one  step  farther :  Since  the  chromatin  is  the  character- 
istic of  the  nucleus,  and  since  spermatozoa  in  some  cases  consist 
almost  exclusively  of  chromatin,  it  is  probable,  as  maintained  by 
Minot,  85, 127,  that  chromatin  is  the  essential  factor  in  the  func- 
tion of  heredity.  It  is  my  conviction  that  the  hypothesis  of  pan- 
genesis,  both  in  its  original  form  and  in  all  its  subsequent  modifica- 
tions, has  been  definitely  set  aside.  In  its  place  we  have  the  theory 
that  the  nature  of  germ,  i.  e.,  of.  the  impregnated  ovum,  is  the  same 
over  and  over  again,  not  because  there  is  ini  each  case  a  similar 
collocation  of  gemmules  or  plastidules,  but  because  the  chromatin 
perpetuates  itself  so  that  the  same  kind  of  chromatin  is  found  in 
the  one  generation  as  in  the  generations  preceding  it  and  following 
it.  The  child  is  like  the  parents  because  its  organization  is  reg- 
ulated by  not  merely  similar,  but  by  some  of  the  same,  chromatin 
as  that  of  the  parents.  Perhaps  instead  of  "  chromatin  "  we  ought 
to  say,  in  order  to  avoid  an  unjustifiable  explicitness,  "nuclear 
substance." 

The  validity  of  this  hypothesis  remains  for  the  future  to  decide. 
There  is  one  general  objection  to  it — that  of  connecting  a  special 
function  with  a  special  substance,  which  is  against  the  general  con- 
ception of  vital  functions  as  the  resultants  of  interlocking  activities 
extending  throughout  each  cell.  Compare  the  remarks  a  propos  of 
the  theory  of  sex,  ante,  p.  70.  The  objection  is,  to  my  mind,  a 
real  and  very  serious  one. 


PART  II. 

THE    GERM-LAYERS. 


CHAPTER   IV. 
SEGMENTATION     FORMATION    OF    THE   DIADERM. 

THERE  follows  after  impregnation  a  short  pause,  and  then  the 
ovum  begins  its  process  of  repeated  division,  which  is  known  as  the 
"  segmentation  of  the  ovum,"  the  term  having  been  introduced  before 
it  was  known  that  each  "  segment"  is  a  cell.  The  division  or  cleav- 
age (Furchung)  of  ova  was  described  by  Prevost  and  Dumas,  1824, 
and  again  by  Rusconi,  36.1.  By  usage  the  term  segmentation  is 
restricted  to  the  production  of  cells  up  to  the  period  of  development 
when  the  two  primitive  germ-layers  are  clearly  differentiated  and 
the  first  trace  of  organs  is  beginning  to  appear. 

Segmentation  Nucleus. — The  impregnated  ovum  has  a  single 
nucleus,  which  is  known  as  the  segmentation  nucleus,  and  which  is 
formed,  as  stated  in  Chapter  III.,  by  the  union  of  the  male  and  fe- 
male pronuclei.*  It  is  the  parent  of  all  the  nuclei  subsequently 
found  in  the  organism,  and  participates  actively  in  the  process  of 
segmentation.  It  is  very  much  smaller  than  the  nucleus  of  the  egg- 
cell  before  maturation ;  it  is  usually  membranate  and  has  numerous 
fine  granules  of  chromatin,  microsoina,  derived  from  the  pronuclei; 
in  some  cases  the  microsoma  from  the  male  pronucleus  are  distin- 
guishable from  those  of  female  pronucleus  (see  under  Impregnation, 
ante,  p.  70).  In  the  rabbit  the  nucleus  when  first  formed  has  in- 
distinct contours,  irregular  shape,  and  a  homogeneous  appearance 
(Ed.  van  Beneden,  75. 1,  699) ;  it  soon  enlarges,  becomes  regular,  and 
acquires  a  distinct  centrally  situated  nucleolus  (Bischoff,  42.1,  50, 
Coste,  47. 1,  Lapin,  PI.  II.,  Fig.  4),  presumably  by  the  gathering  to- 
gether of  the  microsoma. 

The  position  of  the  nucleus  is  always  eccentric,  f  so  far  as  known, 
and  aproximately,  if  not  exactly,  the  same  as  that  of  the  egg-cell  nu- 
cleus before  maturation.  Accordingly,  the  degree  of  eccentricity 
varies  as  the  amount  of  yolk  or  deutoplasm,  being  least  in  alecithal 
and  greatest  in  telolecithal  ova.  In  brief,  it  may  be  said  the  nucleus 
tends  to  take  the  most  central  position  possible  with  regard  to  the 
protoplasm  of  the  ovum.  The  vitelline  granules  are  not  to  be  re- 
garded as  protoplasm,  hence  their  accumulation  may  produce  a  one- 
sided distension,  without,  however,  in  the  least  disturbing  the  uni- 
form radial  distribution  of  the  protoplasm.  The  nucleus  is  sur- 
rounded by  protoplasm  with  few  or  no  yolk-grains ;  in  telolecithal 

*lSd.  van  Beneden  in  his  first  paper  on  Ascaris.  83.1,  affirmed  that  there  was  no  real  union 
of  the  pronuclei  in  the  impregnated  ova  of  that  species;  but  Carnoy,  86.  1,  shows  that  Van 
Beneden's  observations  were  incomplete,  and  Zacharjas  has  stated,  87.1.  that  they  are  so  de- 
fective as  to  be  fundamentally  erroneous  in  regard  to  important  phases,  and  he  points  out  that 
in  reality  the  eggs  of  Ascaris  offer  another  proof  of  the  actual  union  of  the  pronuclei.  The  im- 
pregnation in  this  nematod  has  since  formed  the  subject  of  numerous  articles;  see  Van  Beneden 
and  Neyt  87.1,  Carnoy  87.1.  Boveri  88.1,  O.  Hertwig  90.1,  etc. 

t  It  is  often  stated  that  the  nucleus  'ies  exactly  in  the  centre,  but  1  have  been  unable  to  find  a 
single  observation  to  justify  the  statement. 


94  THE    GERM-LAYERS. 

ova  the  perinuclear  accumulation  is  the  court  of  protoplasm  at  the 
animal  pole. 

Period  of  Repose. — After  the  segmentation-nucleus  is  formed 
there  occurs  a  pause,  which  lasts,  according  to  observations  on  several 
invertebrates,  from  half  to  three-quarters  of  an  hour.  It  is  probable 
that  a  similar  pause  ensues  in  the  mammalian  ovum,  but  there  are 
as  yet  no  observations  to  show  whether  it  occurs  or  not.  During 
this  period  the  yolk  expands  slightly,  unless,  indeed,  the  expansion 
observed  is  due  to  the  influence  of  hardening  agents,  *  and  the  mono- 
centric  radiation,  which  is  present  when  the  nuclei  copulate,  grad- 
ually fades  out,  and  is  replaced  by  a  dicentric  radiation,  which  marks 
the  end  of  the  period  of  repose  and  the  commencement  of  the  first 
division  of  the  ovum. 

Karyokinesis  of  the  Ovum. — For  convenience  I  interpolate  a 
sketch  of  the  process  of  cell-division  as  encountered  in  the  ovum, 
based  on  O.  Hertwig,  88.1,  37,  and  C.  Kabl,  84.1.  My  sketch  is 
by  no  means  complete. 

It  is  probable  that  the  resting  nucleus  has  one  pole  at  which  the 
connection  between  the  reticulum  of  the  nucleus  and  the  surrounding 

grotoplasm  is  more  intimate  than  elsewhere,  as  suggested  by  Rabl, 
9.1.  This  pole  is  marked  by  a  clearer  spot  outside  the  nucleus, 
close  against  it,  and  much  smaller  than  it.  This  clear  spot  becomes 
the  centre  of  a  radiating  arrangement  of  the  protoplasm.  It  was, 
I  believe,  first  observed  by  Flemming  in  the  eggs  of  Echinoderms, 
has  been  seen  in  Ascaris  megalocephala  by  Van  Beneden  and  Neyt, 
87. 1,  and  by  Boveri,  88. 1,  in  Siredon  by  Kolliker,  89. 1,  and  in  other 
cases.  It  is  now  designated  as  the  sphere  of  attraction,  f  and  is  seen, 
at  least  in  certain  phases,  to  contain  a  separate  central  body  (cen- 
trosoma  of  Boveri) .  It  is  not  improbable  that  the  "  sphere  of  at- 
traction" is  identical  with  the  Nebenkern  of  recent  German  writers. 
In  a  number  of  instances  a  small  part  of  the  nucleus  is  seen  to 
separate  off  and  to  lie  as  a  distinct  body,  Nebenkern,  alongside  the 
nucleus ;  this  body  has  a  colorable  portion,  which  is  comparable  to 
the  "centrosoma."  •  For  an  account  of  the  scattered  observations  on 
the  Nebenkern,  together  with  the  relation  of  these  bodies  to  Gaule's 
so-called  cytozoa,  see  G.  Platner,  86.3.  For  additional  observations 
see  Prenant,  88.1,  and  Platner,  89.2.  The  sphere  of  attraction  di- 
vides, as  does  also  its  central  body,  and  its  two  parts  move  to  op- 
posite sides  of  the  nucleus.  There  thus  appear  two  opposite  accu- 
mulations of  clear  protoplasm,  from  each  of  which  as  a  centre  astral 
rays  or  radiating  lines  are  formed  in  the  cell-body.  Meanwhile 
within  the  nucleus  changes  go  on ;  the  threads  of  the  intranuclear 
network  radiate  out  from  the  pole,  where* the  sphere  of  attraction  lies 
before  its  division,  and  the  chromatic  substance  forms  a  number  of 
distinct  grains.  When  the  sphere  of  attraction  divides  and  its 
halves  go  asunder  the  nuclear  substance  preserves  its  radiating  re- 
lation to  each  sphere,  and  as  the  membrane  of  the  nucleus  disappears 
during  these  changes  the  final  result  is  the  transformation  of  the  nu- 


*  Van  Beneden  states  that  arsenic  acid  produces  an  artificial  expansion  of  the  ovum  within 
the  zona. 

t  The  history  and  significance  of  the  spheres  of  attraction,  as  here  presented,  cannot  by  any 
means  be  regarded  as  final.  The  observations  are  few,  and  until  recently  the  exact  history  of 
the  spheres  of  attraction  has  received  no  attention  from  investigators 


SEGMENTATION:    FORMATION    OF    THE    DIADERM.  95 

cleus  into  a  spindle-shaped  body,  the  points  of  which  rest  just  within 
the  clear  centre  of  each  astral  system,  so  that  the  spindle  stretch*- 
from  one  protoplasmic  mass  to  the  other.  The  spindle  consists  of 
fine  threads  extending  from  pole  to  pole  and  having  almost  no  affin- 
ity for  the  dyes  of  the  histologist — a  peculiarity  which  causes  them 
to  be  known  as  the  achromatic  threads.  These  threads  are  probably 
always  compounded  of  a  considerable  number  of  exceedingly  fine 
fibrillaB  (see  Rabl,  89.1,21,22).  The  colorable  substance  forms  a 
number  of  separate  grains,  each  of  which  is  united  with  one  of  the 
achromatic  threads,  and  all  of  which  lie  at  the  same  level  in  the 
centre  of  the  spindle ;  when  the  spindle  is  seen  from  the  side,  the 
chromatine  grains  appear  to  constitute  a  central  band  or  disc  (Strass- 
burger's  Kernplatte),  but  when  the  spindle  is  seen  endwise  the  sep- 
arate grains  are  at  once  recognized.  The  shape  of  the  grains  is 
variable ;  some  authors  without  sufficient  observational  proof  have 
advanced  the  opinion  that  the  grains  are  always  V-shaped.  The 
spindle,  together  with  the  polar  accumulations  of  protoplasm  and  the 
two  accompanying  radiations,  constitute  a  so-called  amjthnixtcr. 

The  domain  of  the  radiation  extends,  the  two  protoplasmatic  cen- 
tres move  farther  apart,  the  nuclear  spindle  elongates  correspond- 
ingly, and  the  chromatiii  grains  of  the  Kernplatte  divide.  Flem- 
miiig  maintains  that  the  division  is  always  lengthwise  of  the 
V-shaped  grain,  but  this  has  been  controverted  by  Carnoy.  How 
the  division  occurs  in  the  mammalian  ovum  is  unknown.  By  the 
division,  however  it  is  effected,  the  number  of  chromatiii  grains  is 
doubled;  they  form  two  sets:  one  set  moves  toward  one  pole,  the 
other  toward  the  other  pole;  the  grains  of  each  set  keep  at  the 
same  level  as  they  move  until  they  reach  the  end  of  the  spindle, 
where  they  appear  as  a  polar  disc  (Carnoy 's  couronne  polaire). 
Next  the  achromatic  threads  of  the  spindle  break  through  and  are 
apparently  drawn  in  toward  each  polar  crown.  There  are  now  two 
nuclear  masses,  each  near,  but  not  at,  the  centre  of  a  radiation,  and 
each  consisting  of  chromatin  and  achromatic  substance.  Each  mass 
develops  into  a  complete  membranate  nucleus,  but  the  steps  of  this 
process  have  yet  to  be  followed  in  detail  in  the  vertebrate  ovum. 

The  signs  of  division  of  the  protoplasm  usually  become  visible 
about  the  time  the  polar  crowns  are  formed,  but  when  the  ovum 
contains  much  deutoplasm  the  division  may  be  retarded.  In  the 
plane  which  passes  through  the  equator  of  the  nuclear  spindle  there 
appears  a  furrow  on  the  surface  of  the  ovum,  which  gradually 
spreads  and  deepens  until  it  is  a  complete  fissure  around  the  cell ;  it 
cuts  in  deeper  until  at  last  only  a  thin  stalk  connects  the  two  halves 
of  the  cell,  and  thereupon  the  stalk  breaks  and  the  cell  is  divided. 
There  next  ensues  a  pause,  during  which  the  astral  rays  of  the  proto- 
plasm disappear  in  the  daughter-cells,  and  the  daughter-nuclei 
assume  each  the  form  of  an  ordinary  resting  membranate  nucleus. 

The  external  appearances  of  segmentation  in  the  living  ovum 
vary,  of  course,  especially  according  to  the  amount  and  distribution 
of  the  yolk-material.  The  appearances  in  holoblastic  ova  with  very 
little  yolk  are  well  exemplified  by  Limax  campestris.  Mark's  de- 
scription, 81.1,  is,  nearly  in  his  own  words,  as  follows:  In  Limax, 
after  impregnation,  the  region  of  the  segmentation  nucleus  remains 


96  THE    GERM-LAYERS. 

more  clear,  but  all  that  can  be  distinguished  is  a  more  or  less 
circular,  ill-defined  area,  which  is  less  opaque  than  the  surrounding 
portions  of  the  vitellus.  After  a  few  moments  this  area  grows  less 
distinct.  It  finally  appears  elongated.  Very  soon  this  lengthening 
results  in  two  light  spots,  which  are  inconspicuous  at  first,  but 
which  increase  in  size  and  distinctness,  and  presently  become  oval. 
If  the  outline  of  the  egg  be  carefully  watched,  it  is  now  seen  to 
lengthen  gradually  in  a  direction  corresponding  to  the  line  which 
joins  the  spots.  As  the  latter  enlarge  the  lengthening  of  the  ovum 
increases,  though  not  very  conspicuously.  Soon  a  slight  flattening 
of  the  surface  appears  just  under  the  polar  globules ;  the  flattening 
changes  to  a  depression,  Fig.  46,  which  grows  deeper  and  be- 
comes angular.  A  little  later  the  furrow  is  seen  to  have  extended 
around  on  the  sides  of  the  yolk  as  a  shallow 
depression,  reaching  something  more  than  half- 
way toward  the  vegetable  or  inferior  pole,  and 
in  four  or  five  minutes  after  its  appearance  the 
depression  extends  completely  around  the  yolk. 
This  annular  constriction  now  deepens  on  all 
sides,  but  most  rapidly  at  the  animal  pole ;  as 
it  deepens  it  becomes  narrower,  almost  a  fis- 
sure. By  the  further  deepening  of  the  constric- 
tion on  all  sides  there  are  formed  two  equal 
i^  d££g°tto  flS  masses  connected  by  only  a  slender  thread  of 
cleavage.  The  envelopes  protoplasm,  situated  nearer  the  vegetative  than 
Lanark  x^eo  diams  •  ]  the  animal  pole,  and  which  soon  becomes  more 
attenuated  and  finally  parts.  The  first  cleavage 

is  now  accomplished.  Both  segments  undergo  changes  of  form; 
they  approach  and  flatten  out  against  each  other,  and  after  a  certain 
time  themselves  divide . 

Primitive  Type  of  Segmentation. — In  the  lower  animals 
there  is  not  found  that  excessive  amount  of  deutoplasm  in  the  ovum 
which  is  so  characteristic  of  the  vertebrates,  and  in  their  ova  we 
have  what  is  undoubtedly  the  earlier  and  more  primitive  type  of 
segmentation.  In  these  cases  the  cleavage  extends,  as  in  the  egg  of 
Limax  (see  above),  through  the  whole  of  the  dividing-cell.  The  two 
cells  first  produced  are  almost  if  not  quite  alike,  and  each  of  them 
produces  two  cells  which  are  also  very  similar  to  one  another ;  then 
comes  a  division  of  the  four  cells  into  eight,  four  of  which  resemble 
one  another  and  differ  from  the  remaining  cells  which  are  also 
similar  among  themselves.  Four  of  the  cells  are  derived  chiefly 
from  the  substance  of  the  animal  pole  of  the  ovum  and  are  very 
protoplasmatic ;  and  the  other  four  cells  are  constituted  out  of  the 
substance  of  the  vegetable  pole  and  accordingly  contain  most  of  the 
deutoplasm  of  the  ovum  The  eight  cells  form  an  irregular  spheroid, 
in  the  centre  of  which  there  is  a  space  between  the  cells ;  this  space 
is  known  as  the  segmentation  cavity. 

The  four  cells  of  the  animal  pole  progress  in  their  divisions  more 
rapidly  than  the  four  of  the  vegetable  pole;  but  the  latter,  when  the 
yolk  matter  is  at  a  minimum,  as,  for  instance,  in  echinoderms,  do  not 
lag  much  From  their  unequal  rates  of  division  the  two  sets  of  cells 
come  to  differ  more  and  more  in  size,  those  of  the  animal  pole  being 


SEGMENTATION  :    FORMATION    OF   THE   DIADERM.  97 

much  the  smaller.     The  divisions  of  the  cells  take  place  so  that  the 
ceils  form  a  continuous  layer  of  epithelium,  one  cell  thick,  stretching 
around  the  enlarged  central  segmentation  cavity,   Figs.  47  and  60; 
the  epithelium  consists  of  a  larger  area  of  the 
small  cells  of  the  animal  pole  and  a  small  area         ^SSSSi^\  IT 
of  the  large  cells  of  the  vegetable  pole.     This 
stage  of  segmentation  is  known  as  the  hlitxjtila 
staue,  the  small  cells  are  destined  to  form  the 
of   the   embryo;    the  large  cells  the 
,  the   central  space  is  the  fsciinn'nf<(- 
cdrift/:  the  line  along  which  the  two  parts 
of  the  epithelium  (ectoderm  and  entoderm)  join 
is  known  as  the  cctcntal  Inn* 

Vertebrate  Type  of  Segmentation.  - 
In  the  vertebrates  we  find  that  segmentation 
also  results  in  two  epitheha,  an  ectoderm  and 
entoderm,  joined  at  their  edges,  and  surround- 
ing  a  segmentation  cavity,  but  the  resemblance 
to  the  typical  blastula  is  marked  by  changes 
in  both  ectoderm  and  entoderm;  the  vertebrate  ectoderm  when 
first  fully  differentiated  consists  of  several  layers  of  cells,  and  not 
merely  of  a  single  layer  of  cells,  as  in  the  primitive  type  of  seg- 
mentation ,  the  entoderm  contains  a  very  large  amount  of  nutritive 
material  (deutoplasm) ,  and  is  represented  either  by  a  large  mass  of 
large  cells  (marsipobranchs,  ganoids,  amphibians)  or  a  mass  of  pro- 
toplasm, not  divided  into  cells  or  but  partially  divided  into  cells,  and 
containing  an  enormous  quantity  of  deutoplasm  (sauropsidans  and 
monotremes) .  In  the  higher  mammals  there  are  further  modifica- 
tions, described  below. 

The  more  primitive  form  among  vertebrates  is,  I  think,  presum- 
ably that  in  which  the  entoderm  consists  of  separate  cells;  for  this 
mode  of  segmentation  is  the  one  which  most  resembles  that  of  inver- 
tebrates, and  it  occurs  in  the  lowest  vertebrates,  and  in  ova  which 
are  not  excessively  charged  with  yolk. 

In  the  primitive  form  of  vertebrate  segmentation,  which  is 
preserved  in  the  marsipobranchs,  ganoids,  and  amphibia,  there  is  a 
well-marked  difference  between  the  cells  of  the  two  poles.  The  fol- 
lowing account  refers  especially  to  the  frog's  egg  and  is  an  adapta- 
tion of  Balfour's  summary  ("Comp.  Embryol.,  "I. .  78,  79).  The  first 
formed  furrow  is  vertical ,  it  commences  in  the  upper  half  of  the 
ovum,  which  corresponds  to  the  animal  pole,  and  is  characterized  by 
the  black  pigment — the  lower  or  vegetable  pole  being  whitish.  The 
first  furrow  extends  rapidly  through  the  upper,  then  more  slowly 
through  the  lower  half  of  the  ovum,  so  that  the  divergence  in  the 
two  polar  rates  of  development  is  indicated  already.  As  soon  as 
the  furrow  has  cleft  the  egg  into  halves,  a  second  vertical  furrow  ap- 
pears at  right  angles  to  the  first  and  behaves  in  the  same  way,  Fig.  48. 
The  next  furrow  is  at  right  angles  to  both  its  predecessors,  and  there- 
fore parallel  to  the  equator  of  the  egg;  but  it  is  much  nearer  the 
animal  flnm  the  rw/etative  pole,  It  extends  rapidly  around  the 
egg  and  divides  each  of  the  four  previous  segments  into  two  parts : 
larger  trith  a  great  deal  of  yolk  and  the  other  smaller  with 
1 


98  THE    GERM-LAYERS. 

very  little  yolk.  The  eight  segments  or  cells  have  a  small  segmen- 
tation cavity  in  the  centre  between  them.  This  cavity  increases  in 
size  in  subsequent  stages,  its  roof  being  formed  by  the  small  cells 
further  divided,  and  its  floor  by  the  large  cells  also  multiplied  by 
division,  though  to  a  less  extent  than  the  small  cells.  All  the  devel- 
opmental processes  progress  more  rapidly  at  the  animal  pole.  After 
the  equatorial  furrow  there  follow  two  vertical  or  meridional  fur- 
rows, which  begin  at  the  animal  pole  and  divide  each  of  its  four  cells 
into  two,  making  eight  small  cells.  After  a  short  period  these 
furrows  extend  to  the  lower  pole  and  divide  each  of  the  large  cells 
into  two,  Fig.  48,  4-  The  so-called  meridional  cleavages  after  the 
first  and  second  are  not  true  meridional  cleavages,  since  they  do 
not  pass  through  the  folds  of  the  ovum,  but  through  the  poles  of  the 
cells  (blastomeres) ,  which  they  divide  (see  Rauber,  Morph.  Jalirb., 
VIII.,  287).  A  pause  now  ensues,  after  which  the  eight  upper  cells 
become  divided  by  a  furrow  parallel  to  the  equator,  and  somewhat 
later  a  similar  furrow  divides  the  eight  lower  segments.  Each  of 
the  small  cells  is  now  again  divided  by  a  vertical  furrow,  which  later 
divides  also  the  corresponding  large  cell.  The  segmentation  cavity 


FIG    48. — Segmentation  of  the  egg  of  the  common  Frog 

is,  therefore,  now  bounded  by  32  small  and  32  large  cells.  After 
this  the  upper  cells  (ectoderm)  g'ain  more  and  more  in  number  beyond 
the  lower  cells  (entoderm).  After  the  64  segments  are  formed  two 
equatorial  furrows  appear  in  the  upper  pole  before  a  fresh  furrow 
arises  in  the  lower,  making  128  ectodermal  cells  against  only  '-\'l 
entodermal.  The  regularity  of  the  cleavage  cannot  be  followed 
further,  but  the  upper  pole  continues  to  undergo  a  more  rapid  seg- 
mentation than  the  lower.  At  the  close  of  segmentation  the  egg 
forms  a  sphere  containing  an  eccentric  segmentation  cavity,  Fig. 
49,  s.  c. ,  composed  of  two  unequal  parts,  an  upper  arch  of  several 
layers  of  cells,  Bl,  the  primitive  blastoderm  of  Minot  or  ectoderm, 
and  a  lower  mass,  Yolk,  of  large  cells  rich  in  protoplasm.  At 
the  edge  of  the  mass  of  large  cells,  kiv,  there  is  a  gradual  passage  in 
size  to  the  cells  of  the  blastoderm,  and  it  appears  that  the  small 
cells  receive  additions  at  the  expense  of  the  large  ones ;  this  zone 
corresponds  to  the  so-called  germinal  wall  of  large  vertebrate  ova, 
and  also  to  what  we  have  defined  as  the  ectental  line. 

The  secondary  type  of  vertebrate  segmentation  differs  from  the 
primary  principally  in  the  retarded  development  of  the  entoderm,  due 
apparently  to  the  increase  of  the  yolk-matter.  The  yolk-granules 


SEGMENTATION:  FORMATION  OF  THE  DIADERM. 


99 


Bl 


are,  as  already  mentioned,  found  to  be  situated  not  quite  exclusively, 
though  almost  so,  in  those  parts  of  the  ovum  out  of  which  the  ento- 
dermal  cells  are  formed.  Hence,  when  there  is  a  great  deal  of  yolk 
the  anlage  of  the  entoderm  becomes  bulky,  and  when  it  segments 
the  entodermal  cells  it  pro- 
duces are  correspondingly 
big,  as  we  have  seen  is  the 
case  in  amphibian  ova.  (  hi 
the  other  hand,  when  the 
amount  of  yolk  is  small,  as 
in  the  primitive  type  of  seg- 
mentation, e.g.  echinoderms, 
the  entodermal  cells  are  small. 
In  the  reverse  case,  when  the 
,  amount  of  yolk  is  exceedingly 
g;reat,  as  in  selachians,  rep- 
tiles, and  birds,  the  yolk  may 
not  divide  into  cells  as  f;i>t 
as  the  nuclei  multiply,  so  that 
it  seems  that  the  presence  of 
the  deutoplasm,  though  it 
does  not  affect  the  nuclear 

/-lit^ic.i^T,a 
divisions 


'Fl°-  40  —  Section  of  the  segmented  ovum  of  Axolotl  : 
blastoderm:     «.   c.,    segmentation   cavity;    Yolk, 

kw  (Keimwall)   &erminal  wal1- 


, 

impedes  very  much  the  di-  V'fter1!ienoncierm 
vision  of  the  protoplasm,  and 
c<  msequently  in  these  ova  we  find,  at  certain  stages  of  development, 
a  multinucleate  yolk.  The  impediment  is  not  encountered  by  the 
protoplasm  of  the  animal  pole,  hence  we  see  the  animal  pole  seg- 
menting while  the  yolk  does  not  ;  in  this  case  the  segmentation  ap- 
pears confined  to  one  portion  of  the  ovum,  and,  accordingly,  such 
ova  are  termed  meroblaxtic  in  contradistinction  to  the  holoblastic 
ova,  in  which  the  first  cleavage  furrow  divides  the  whole  ovum  ;  but 
the  difference,  it  must  be  expressly  remembered,  is  one  of  degree,  not 
of  kind. 

The  best  known  example  of  a  vertebrate  meroblastic  ovum  is 
undoubtedly  the  hen's  egg.  The  so-called  yolk,  or  "yellow,"  is  the 
ovum  ;  the  white  and  the  shell  are  both  adventitious  envelopes  added 
b}T  the  oviduct  as  the  ovum  passes  down  after  leaving  the  ovary. 
The  segmentation  begins  while  the  ovum  is  passing  through  the 
lower  part  of  the  oviduct,  and  shortly  before  the  formation  of  the 
shell  commences.  If  an  ovum  from  the  upper  part  of  the  oviduct 
be  examined  it  is  found  to  be  surrounded  with  more  or  less  white 
(albumen)  .  Its  animal  pole  is  represented  by  a  whitish  disc  from 
2.5-3.5  mm.  in  diameter,  and  0.30-0.35  mm.  in  thickness;  this  disc 
is  known  by  many  names  :  Formative  yolk,  germinal  disc,  cicatri- 
cula  (Narbe,  Hahnentritt,  Keimscheibe,  stratum  s.  discus  pro- 
ligerus).  The  animal  pole  consists  chiefly  of  protoplasm,  and  is 
peculiar  only  in  its  small  size  compared  with  the  whole  ovum  ;  it 
contains,  when  the  ovum  leaves  the  ovary,  the  egg-cell  nucleus  ;  the 
ovum  then  matures,  impregnation  occurs,  and  finally  segmentation 
begins.  Viewing  the  ovum  from  above  we  see  the  first  furrow 
appear  as  a  groove  running  across  the  germinal  disc,  though  not  for 


100 


THE    GERM-LAYERS. 


its  whole  width,  and  dividing  it  into  halves ;  this  furrow  is  developed 
in  accompaniment  with  the  division  of  the  segmentation  nucleus. 
The  primary  furrow  is  succeeded  by  a  second  furrow  nearly  at  right 
angles  to  the  first ;  the  surface  of  the  germinal  disc  is  cut  up  into  four 
segments  or  quadrants,  Fig.  50,  A,  which  are  not,  however,  sepa- 
rated from  the  underlying  substance.  The  number  of  radiating 
furrows  increases  from  four  to  seven  or  nine,  when  there  arises  a 
series  of  irregular  cross-furrows,  by  which  the  central  portion  of  each 
segment  is  cut  off  from  the  peripheral  portion,  giving  rise  to  the 
appearance  illustrated  by  Fig.  50,  C ;  there  are  now  a  number  of  small 


FIG.  50.— Four  stages  of  the  segmentation  of  the  Hen's  ovum.     After  Coste.    Only  the  germinal 
disc  seen  from  above  and  part  of  the  surrounding  yellow  yolk  are  represented. 

central  segments  surrounded  by  larger  wedge-shaped  external  seg- 
ments. Division  of  the  segments  now  proceeds  rapidly  by  means  of 
furrows  running  in  various  directions.  Not  only  are  the  small  cen- 
tral segments  divided  into  still  smaller  ones,  D,  but  their  num- 
ber is  increased  also  by  the  addition  of  cells  cleft  off  from  the  central 
ends  of  the  large  peripheral  segments,  which  are  themselves  sub- 
divided by  additional  radiating  furrows.  Sections  of  the  hard- 
ened germinal  disc  show  that  segmentation  is  not  confined  to 


SEGMENTATION:    FORMATION   OF   THE   DIADERM.  101 

the  surface,  but  extends  through  the  protoplasmatic  mass  of  the 
animal  pole,  there  being  deep-seated  cleavages  in  planes  parallel  to 
the  surface,  of  the  ovum.  According  to  Duval,  84. 1,  when  the  first 
few  small  central  cells  are  separated  off,  there  is  a  small  space 
between  them  and  the  underlying  egg-substance  (see  Figs.  2,  3,  4, 
5,  and  G  of  his  PI.  I.),  and  this  space  he  calls  the  segmentation 
cavity ;  but  in  this  I  think  he  is  in  error,  for  the  cells  formed  below 
this  space  are  incorporated  in  the  ectoderm  or  primitive  blastoderm ; 
the  cells  referred  to  are  those  marked  in  in  Fig.  8  of  Duval's  PL  I. 
The  true  segmentation-cavity,  as  we  have  seen,  is  bounded  on  one 
side  by  ectoderm,  on  the  other  side  by  entoderm.  This  fundamental 
characteristic  Duval  has  entirely  overlooked.  From  the  processes 
described  there  results  a  disc  of  cells,  which  receives  peripheral  addi- 
tions ;  the  border  from  which  these  additions  come  is  known  as  the 
xct/niciitiuf/  zone.  The  whole  mass  of  cells  derived  from  the  germi- 
nal disc  represents  the  ectoderm,  and  the  segmenting  zone  may  be 
homologized  with  the  cells  around  the  edge  of  the  primitive  blasto- 
derm of  the  frog,  Fig.  40,  kw.  A  section  through  the  segmented 
germinal  disc  shows  the  following  relations :  The  blastoderm  is  a 
disc  of  cells;  its  upper  layer  is  epithelioid ;  its  lower  layers  consist 
of  rounded  cells  more  or  less  irregularly  disposed;  at  its  edge  it 
merges  into  the  yolk,  which  continues  to  produce  cells;  between  the 
blastoderm  and  the  yolk  is  a  fissure,  the  segmentation  cavity ;  the 
yolk  under  the  fissure  contains  a  few  nuclei,  which  have  each  a  little 
protoplasm  about  them,  but  do  not  form  parts  of  discrete  cells. 

In  reptiles  the  process  of  segmentation  is  very  similar  to  that  in 
birds.  Our  knowledge  is  based  principally  upon  observations  upon 
the  eggs  of  the  European  lizards  (Lacerta  agilis  and  viridis) ,  which 
have  been  studied  by  Kupffer  and  Benecke,78.3,  Balfour,  79. 1,  Sara- 
sin,  83.1,  Weldon,  83.1,  and  Hofmann  (Archives  neerlandaises^ 
XVI.,  1881).  Hofmann  gives  a  resume  in  Brown's  "Thierreich," 
VL,Abth.  III.,  pp.  1877-1881.  The  process  is  very  irregular,  forsmall 
cells  are  budded  off  singly  and  in  scattered  clusters  from  the  larger 
segments.  As  Strahl,  87.1,  290,  has  pointed  out,  the  blastoderm 
receives  direct  accretions  from  the  underlying  yolk,  cells  being  sepa- 
rated off  by  horizontal  cleavages.  At  the  close  of  segmentation  the 
germinal  disc  is  converted  into  a  membrane  consisting  of  several 
layers  of  cells  and  parted  from  the  underlying  yolk  by  a  thin  space, 
the  segmentation  cavity ;  at  its  edge  this  membrane,  the  primitive 
blastoderm,  is  united  with  the  yolk,  it  being  immediately  surrounded 
by  a  segmenting  zone,  from  which  it  receives  accretions.  The  layer 
of  the  yolk  immediately  under  the  segmentation  cavity  contains  scat- 
tered nuclei,  lying  singly  or  in  clusters;  each  nucleus  is  surrounded 
by  protoplasm ;  the  nuclei  are  not  all  alike ;  some  are  very  large, 
round  with  very  distinct  nuclear  threads ;  other  are  small  and  often 
bizarre  in  shape ;  probably  the  latter  are  budded  off  from  the  former. 

In  Elasmobranchs  the  germinal  disc  is  thicker,  and  consequently 
the  mass  of  cells  resulting  from  its  segmentation  cuts  in  quite  deeply 
into  the  yolk  (Balfour,  "  Comp.  Embryol,"  I.,  Fig.  4G ;  Riickert,  85. 1, 
2fe).  Kastschenko,  88.2,  has  shown  that  before  the  germinal  disc  is 
segmented  into  cells  there  are  nuclei  scattered  through  it,  and  he 
has  rendered  it  probable,  88. 1,  that  these  nuclei  come  from  the  seg- 


102 


THE    GERM-LAYERS. 


mentation  nucleus.  It  is  possible  that  in  other  meroblastic  verte- 
brates proliferation  of  the  nuclei  precedes  the  cleavage  of  the  germinal 
disc  into  discrete  cells.  As  segmentation  progresses,  the  cells  spread 
out  into  a  layer  which  shows  the  same  essential  relations  as  have 
been  described  in  birds  and  reptiles.  There  is  the  several-layered 
primitive  blastoderm,  with  its  edges  connected  with  the  yolk  and 
itself  overlying  the  segmentation  cavity,  the  lower  floor  of  which  is 
formed  by  the  multinucleate  yolk,  the  representative  of  the  cellular 
yolk-mass  of  the  frog,  Fig.  49,  Yolk.  The  nuclei  are  confined  to 
the  layer  immediately  under  the  segmentation  cavity,  and  this  layer 
corresponds  to  the  sub-germinal  plate  in  teleost  ova.  Of  the  yolk- 
nuclei  some  are  large,  others  are  small  as  in  reptiles ;  they  are  the 
Parablastkerne  of  His,  the  Merocytenkerne  of  Riickert. 

In  bony  fishes  also  we  find  the  same  type,  but  modified  somewhat. 
The  process  of  segmentation  has  been  very  carefully  studied  by  C.  O. 
Whitman,  84. 1,  to  whom  I  am  indebted  for  the  accompanying  semi- 
diagrammatic  figure  of  the  segmented  ovum  of  a  flounder.  The 
ovum  is  surrounded  by  a  vitelline  membrane,  z,  from  which  it  has 
slightly  withdrawn,  notably  at  the  upper  pole,  where  lies  the  thick 
cap  of  cells  constituting  the  blastoderm,  Bl;  in  the  stage  represented 
the  outer  layer  of  cells  is  just  beginning  to  assume  an  epithelioid 
character ;  underneath  the  blastoderm  is  the  well-marked  segmenta- 
tion cavity,  s.  c.;  everywhere  at  the  edge  of  the  blastoderm  lies  the 
segmenting  zone,  k  w,  a  ring  of  granular  protoplasm  with  rapidly- 
dividing  nuclei;  the  cells  re- 


-BL 


suiting  from  these  divisions 
are  added  to  the  edge  of  the 
blastoderm,  which  thus  en- 
larges peripherally.  The  pro- 
toplasm of  the  segmenting 
zone  is  prolonged  inward, 
forming  the  floor  of  the  seg- 
mentation cavity;  this  sheet 
of  protoplasm,  s.g.,  is  known 
as  the  sub-germinal  plate. 
The  segmenting  zone  is,  of 
course,  the  homologue  of  the 
similar  zone  in  amniote  ova, 
or  the  so-called  germinal  wall, 
but  it  is  quite  sharply  defined 
against  the  yolk,  and  therein 
differs  from  the  wall  in  the 
chick,  because  in  the  latter  the 
germinal  wall  merges  gradu- 
ally into  the  yolk.  The  process 
of  segmentation  differs  from 
that  in  elasmobranchs  and  sauropsida  in  that  the  cleavage  of  the 
germinal  disc  is  strikingly  regular,  and  further  in  that  the  whole 
width  and  thickness  of  the  germinal  disc  is  involved  in  the  segmen- 
tation from  the  very  start.  The  segmentation  in  teleosts  is  further 
interesting  as  affording  proof  that  all  the  nuclei,  as  shown  by  Whit- 
man's investigations,  arise  from  the  segmentation  nucleus. 


51.— Ovum  of  a  Flouuuer  in  transverse  verti- 
cal section ;  semi-diagrammatic  figure  by  Dr.  C.  O. 
Whitman,  z,  vitelline  membrane  (or  zona?) ;  kiv., 
segmenting  zon^  (Keimwall) ;  Bl,  blastoderm  or 
primitive  ectoderm ;  s.c. ,  segmentation  cavity ;  s.g. , 
subgerminal  plate;  gl,  oil  globule  of  yolk. 


SEGMENTATION:    FORMATION   OF   THE    DIADEKM.  103 

To  summarize :  In  vertebrate  ova  with  a  large  yolk,  which  does 
not  divide  into  cells  until  segmentation  is  considerably  advanced,  the 
substance  of  the  animal  pole  segments  completely,  and  produces 
several  layers  of  cells  (the  uppermost  becoming  epithelioid)  which 
are  the  ectoderm  or  primitive  blastoderm ;  the  edge  of  the  blastoderm 
touches  the  yolk,  and  is  surrounded  by  a  nucleated  zone  in  which  the 
production  of  cells  is  continuing ;  underneath  the  blastoderm  is  the 
fissure-like  segmentation  cavity ;  the  floor  of  this  cavity  is  formed  by 
the  unsegmentated  yolk  (entoderm)  which  is  furnished  with  scattered 
nuclei  in  the  layer  immediately  underneath  the  yolk ;  the  yolk  nuclei, 
at  least  in  selachians  and  reptiles,  are  of  two  kinds,  very  large  ones 
and  smaller  ones,  which  arise  probably  from  the  large  nuclei ;  the 
uninucleated  layer  may  be  termed  the  sub-germinal  plate. 

Modified  Segmentation  of  Placental  Mammals. — The  low- 
est mammals  resemble  the  reptiles  in  many  respects.  Among  other 
reptilian  characteristics  of  the  mono- 
tremes  we  find  ova  of  large  size  and 
rich  in  deutoplasm.  That  these  ova 
segment  in  similar  manner  to  those 
of  reptiles  and  during  their  passage 
through  the  oviduct  was  first  ascertained 
by  direct  observation  by  Caldwell  in 
1*S4,  87.1. 

In  marsupials  and  the  placental  mam- 
malia the  amount  of  yolk-substance  is 
greatly  reduced,  and  the  ovum  is  of 
small  size.  It  is,  therefore,  holoblastic, 
that  is  to  say,  the  cleavage  planes  cut 
through  the  entire  cell,  as  in  the  prim- 
itive type  of  segmentation;  but  the 
arrangement  of  the  cells  at  the  dose  of 

Segmentation    appears   to  be  a  direct  in-     the  polar  globules;  numerous  sperma- 

heritance  from  the  reptilian   ancestors   *ozoa  11,  m  and  within  the  zona  peiiu- 
of  the  mammals. 

The  segmentation  of  the  mammalian  ovum  was  first  clearly  recog- 
nized by  Bischoff,  though  it  had  been  previously  seen  and  misinter- 
preted by  Barry,  38.1,  39.1,  40.1;  very  beautiful  figures  of  seg- 
mentation in  the  rabbit  have  been  given  by  Coste,  47.1.  More 
recently  observations  have  been  published  by  Hensen  on  the  rab- 
bit, 76.1,  Van  Beneden  on  the  rabbit,  76.1,  80.1,  Kupffer  on  ro- 
dents, 8.23,  Selenka  on  rodents,  82.1,  83.1,  84. 1,  and  opossums, 
86. 1,  Van  Beneden  and  Julin  on  bats,  80. 1,  Tafani  on  white  mice, 
89. 1.  The  ovum  when  discharged  from  the  ovary  is  surrounded  by 
the  corona radiata  (cf.  ante,  p.  59),  which  is  lost  when  impregnation 
takes  place.  Segmentation  begins  when  the  ovum  is  one-half  to 
two- thirds  of  the  way  through  the  oviduct.  The  ovum  spends  about 
seventy  hours  in  the  oviduct  in  the  rabbit  and  about  eight  days  in  the 
dog.  The  first  cleavage  plane  passes  through  the  axis  of  the  ovum, 
which  is  marked  by  the  polar  globules.  When  first  formed  the  two 
segmentation  spheres  are  oval  and  entirely  separated  from  one 
another,  but  subsequently  they  flatten  against  one  another  and  be- 
come appressed — a  remarkable  phenomenon,  of  which  we  possess 


104 


THE    GERM-LAYERS. 


no  explanation  whatever.     The  second  cleavage  plane  is  also  meri- 
dional. 

The  ovum  next  divides  into  eight  and  then  into  twelve  segments, 
of  which  four  are  larger  than  the  rest. 

The  succeeding  cleavages  have  never  been  followed  accurately ;  but 
from  Heape's  observations  on  the  mole,  86. 1, 106,  we  know  that  the 
divisions  progress  with  great  irregularity,  and  it  is  probable  that  the 
commonly  assumed  regularity  of  mammalian  segmentation  does  not 
exist  in  nature.  After  a  time  (in  the  rabbit  about  seventy  hours)  there 
is  reached  the  stage  termed  Metagastrula  by  Van  Beneden,  80. 1 , 
153-160,  in  accordance  with  his  view  of  the  homologies  of  this  stage. 
The  metagastrula  consists  of  a  single  layer  of  cuboidal  hyaline  cells 
lying  close  against  the  zona  pellucida,  Fig.  53,  en;  the  space  within 
this  layer  contains  an  inner  mass  of  cells,  im,  which  are  rounded  or 
polygonal  and  densely  granular.  At  one  point  the  outer  layer  is 
interrupted  and  the  space  is  filled  by  one  of  the  granular  segments 
of  the  inner  mass,  Fig.  53.  The  nuclei  of  all  the  cells  are  some- 
what nodulated  and  have  sev- 
eral highly  ref  ractile  granules 
each.  The  granules  in  the 
bodies  of  the  cells  of  the  outer 
layer  are  somewhat  concen- 
trated around  the  nucleus, 
leaving  the  cortices  of  the 
cells  clear.  Van  Beneden, 
76.1,  28,  29,  has  observed 
that  sometimes  (21  ova  out  of 
29)  the  first  two  segmentation 
spheres  are  of  unequal  size 
in  the  rabbit,  and  similar 
variability  occurs  in  the  mole, 
Heape,  86.1,  165;  Tafani, 
on  the  other  hand,  expressly 
denies  its  occurrence  in  white 
mice.  It  is,  I  think,  very 
improbable  that  this  differ- 
ence, which  sometimes  occurs  and  sometimes  does  not,  has  any  fun- 
damental significance.  Van  Beneden,  however,  has  maintained  that 
the  small  cell  gives  rise  in  the  rabbit  to  the  inner  mass  of  cells  (see 
blow) ,  which  he  terms  the  entoderm,  but  which  must,  it  seems  to 
me,  be  homologized  with  the  ectoderm,  as  explained  below.  That 
Van  Beneden  is  in  error  as  to  the  genetic  relation  of  the  small  cell 
to  the  inner  mass  has  been  demonstrated  by  Heape,  86.1,  166. 

The  second  cleavage  plane  is  probably  also  meridional,  and  is  cer- 
tainly at  right  angles  to  the  first,  so  that  four  similar  cells  are  pro- 
duced as  in  the  primitive  type  of  segmentation,*  Fig.  54.  These 
four  cells  are  also  rounded  at  first  and  probably  become  fitted  against 
one  another  so  as  to  produce  the  disposition  observed  by  Tafani, 
79.1,  116,  in  mice  ova  at  this  stage.  Tafani  describes  each  cell  as 
having  the  form  of  a  three-sided  pyramid  with  the  apex  at  the  cen- 

*  The  distinction  here  made  between  "primitive  type  of  segmentation"  and  "primitive  type 
of  vertebrate  segmentation  "  should  be  borne  in  mind  by  the  reader 


FIG.  53.— Rabbit's  ovum  of  about  seventy  hours. 
After  E.  van  Beneden.  z,  zona  pellucida;  EC,  ento- 
derm; i.m.  inner  mass  of  granular  cells. 


>K<- MENTATION:    FORMATION   OF   THE   DIADERM. 


105 


tre  of  the  ovum  and  a  convex  base  forming  part  of  the  external  sur- 
face of  the  yolk.  That  the  two  first  cleavage  planes  are  meridional 
is  rendered  probable  by  the  arrangement  in  the  four-cell  stage 
observed  by  Selenka  in  the  Virginian  opossum,  Fig.  55. 

During  all  these  early  stages  the  cells  (segmentation  spheres)  are 
naked,  i.e.,  without  any  mem- 
brane; the  nuclei,  when  not  in 
karyokinetic  stages,  are  large, 
clear,  and  vesicular;  the  yolk- 
granules  are  small,  highly  re- 
fractile,  and  more  or  less  nearly 
spherical ;  they  show  a  marked 
tendency  to  lie  in  the  cell  half- 
way between  the  nucleus  and  the 
edge  of  the  cell,  or  when  the 
cells  are  large  around  the  nu- 
cleus and  at  a  little  distance 
from  it. 

It  is  at  about  this  stage  that 
the  ovum  passes  from  the  Fal- 
lopian tube  into  the  uterus, 
where  it  dilates  into  what  is 

known      as     the      blastodermic        FIG.  54.-Ovum  of  a  Bat,  Vespertilio  murina, 

vesicle.  This  dilatation  is  due  3£  indTjSE* ntati°a  spheres'  AfterVanBen 
principally  to  the  multiplication 

and  flattening  out  of  the  cells  of  the  outer  layer  and,  of  course,  in- 
volves the  expansion  and  consequent  thinning  of  the  zona  pellucida, 
compare  Figs.  5G  and  58.  The  inner  mass  meanwhile  remains  pas- 
sively attached  to  one  point  on  the  circumference  of  the  vesicle, 
Fig.  56,  i.  m.  By  this  process  the  thin  fissure  between  the  inner 
mass  and  the  outer  layer  becomes  a  considerable  space,  Fig.  59,  s.  c. , 
the  cavity  of  the  blastoderm  or  segmentation  cavity  (blastococle) . 


X300 


FIG.    55.— Ovum   of    Virginian    Opossum, 
with  four  segments.     After  Selenka. 


FIG.  56. —Young  blastodermic  vesicle  of  a 
Mole,  z,  Zona  pellucida;  i.m, inner  mass  of 
cells;  s.z.,  sub-zonal  layer  of  cells.  After 
W.  Heape. 


The  blastodermic  vesicle  continues  to  expand,  and  in  the  rabbit 
and  mole  there  is  a  corresponding  enlargement  of  the  tubular  uterus 
at  the  point  where  the  vesicle  is  lodged.  "  It  is  clearly  impossible 
for  the  delicate-walled  ovum  to  expand  in  the  form  of  a  vesicle,  and 


106 


THE    GERM-LAYERS. 


distend  the  uterine  walls  by  virtue  of  the  growth  of  its  cells ;  it 
must  be,  therefore,  concluded  that  it  obtains  some  support.  This 
support  is  rendered  from  within.  The  vesicle  contains  a  transparent 

fluid,  the  nature  of  which  I 
am  only  sufficiently  conver- 
sant with  to  say  that  after 
treatment  with  alcohol  a 
white  precipitate  is  present 
in  the  vesicle.  It  is  equally 
evident  that  this  fluid  can 
only  have  been  obtained  from 
the  uterus,  and  that  it  is 
present  within  the  vesicle  at 
a  very  considerably  greater 
pressure,  than  in  the  uterus 
itself.  Such  a  condition  is 
caused  by  means  of  the  cells 
of  the  wall  of  the  vesicle; 
they  secrete  the  fluid  within 
the  vesicle,  this  function 
being  performed  against  a 
pressure  which  is  greater  on 
their  inner  than  on  their  outer 
side,  exactly  as  the  cells  of 
the  salivary  glands  are  known 
to  act.  The  uterine  fluid  is 
secreted  by  glands  present  in 
great  numbers  in  the  uterine 
tissue,  and  is  poured  through 
their  open  mouths  into  the 

FIG.  57.— Sections  through  the   inner  mass  of  the  £  JA  mi 

blastodermic  vesicle  of  the  Mole  at  three  successive    Cavity  OI   tne  Utei'US.        Inere 

fnnSSmassZonapellucida:*    '  subzonal>yer :  <  ™ -   is  every  probability   it   has 

nutritive  qualities,  since  it  is 

thence  taken  up  into  the  cavity  of  the  embryonic  vesicle,  which 
eventually  functions  as  a  yolk-sac,  in  the  walls  of  which  embryonic 
blood-vessels  ramify  "  (Heape) . 

The  inner  mass,  Fig.  56,  i.  m.,  does  not  at  first  grow  much  and  re- 
tains its  rounded  form,  becoming,  at  least  in  the  mole,  nearly  globu- 
lar, Fig.  57,  A.  The  inner  mass  subsequently  flattens  out,  becoming 
lens-shaped,  thinner,  and  of  larger  area,  Fig.  57,  B.  It  continues 
spreading  laterally  and  separates  into  three  distinct  layers.  The  ovum 
now  consists  of  a  very  thin  zona  pellucida,  Fig.  58,  z,  close  against 
which  is  a  single  layer  of  thin  epithelial  cells,  En;  at  one  pole  this 
layer  is  interrupted  by  a  lens-shaped  mass,  i.  m.,  formed  by  three 
layers  of  cells.  These  three  layers  were  first  clearly  described  by  E. 
van  Beneden,  76.1,  and  have  been  since  figured  by  him,  80. 1 ;  Van 
Beneden  identified  these  three  layers  with  the  three  permanent  germ- 
layers  which  do  not  arise  until  later.  Rauber,  however,  showed  that 
both  the  outer  layers  enter  into  the  formation  of  the  ectoderm,  while 
the  inner  layer  is  concerned  in  the  production  of  the  permanent  ento- 
derm;  the  outermost  layer  Rauber  terms  the  Deckschicht.  Lieber- 
kiihn,  79. 1,  and  others  have  since  then  confirmed  Rauber's  results. 


SEGMENTATION:    FORMATION   OF   THE    DIADERM. 


107 


Homoloyies  of  the  Mammalian  Blastodermic  Vesicle. — We 
have  so  little  accurate  information  concerning  the  details  of  the 
formation  of  the  blastodermic  vesicle  that  any  interpretation  must 
be  tentative.  I  still  consider,  however,  the  view  which  I  brought 
forward  in  1885,  uHdbk,"  I.,  5*28,  as  the  most  satisfactory,  and  pre- 
ferable to  the  similar  explanation  advanced  independently  and  simul- 
taneously by  Haddon,  85.1,  and  reproduced  by  him  briefly  in  his 
"Practical  Embryology,"  47,  48.  F.  Keibel,  87.1,  advocated 
similar  interpretations  two  years  later,  but  without  quoting  Minot 
or  Haddon.  I  regard  the  subzonal  epithelium  as  the  entoderm  and 
the  inner  mass  of  cells  as  the  primitive  blastoderm  or  ectoderm;  by 
so  doing  the  parts  can  be  readily  and  exactly  homologized  with  the 
parts  in  the  frog's  ovum,  as  will  be  evident  at  once  if  the  diagram, 
Fig.  59,  of  the  mammalian  vesicle  be  compared  with  the  section  of 
a  segmented  amphibian  ovum,  Fig.  40.  The  primitive  blastoderm 
Bl,  or  ectoderm,  consists  of  several  layers  of  cells  rich  in  protoplasm ; 
below  it  is  the  large  segmentation  cavity,  s.  c.,  relatively  much 
larger  in  the  mammalian  than  in  the  amphibian  ovum.  At  its 
edge  the  primitive  blastoderm  joins  the  entoderm  Yolk,  which  in 
amphibia  is  a  large  mass,  in  mammals  only  a  single  layer  of  cells. 
Now,  we  know 
that  the  ancestors 
of  the  higher 
mammalia  had 
ova  with  a  large 
amount  of  deuto- 
plasm,  which  in 
the  course  of  evo- 
lution has  been 
lost,  so  that  in  the 
ova  of  the  placen 
talia  there  is  ver; 
little  yolk-mate 
rial ;  we  know 
further  that  the 
readiness  of  cel- 
lular divisions 
depends  en  the 
amount  of  yolk, 
hence,  when  the 
yolk  is  lost,  we 
should  expect  to 
find  the  e  n  t  o  ~ 

FIG   58  —Ovum  of  a  Rabbit,  ninety-four  hours  after  coitus.     After 
We    have    Seen,   IS     van  Beneden      En.  subzonal  epithelium  (entoderm)  :  Z,  zona  pelluci- 

derived  from  the   c 

vegetative  substance  of  the  ovum,  to  be  represented  by  relatively 
small  cells,  if  we  imagine  the  number  of  entodermic  cells  in  the 
frog's  ovum,  Fig.  40,  Yolk,  reduced,  their  connection  with  the  prim- 
itive blastoderm  and  their  character  as  a  continuous  layer  being 
preserved,  we  obtain  at  once  the  characteristic  arrangement  of  the 
mammalian  blastodermic  vesicle,  Fig.  50.  The  homology  here  es- 


108 


THE    GERM-LAYERS. 


tablished  is  further  confirmed  by  the  coarse  network  of  protoplasm 
in  the  cells  of  the  outer  layer  of  the  vesicle  (Ed.  van  Beneden,  80. 1), 
suggesting  at  once  the  meshes  which  have  been  emptied  of  their 
deutoplasm.  Adam  Sedgwick,  86.1,  has  shown  that  in  the  ova  of 
Peripatus  capensis  the  yolk-matter  has  been  lost,  though  abundant 
in  other  species  of  the  same  genus,  and  the  coarseness  of  the  proto- 
plasmic network  is  preserved  as  evidence  of  the  granules  formerly 
present.  This  observation  serves  to  confirm  the  view  I  have  sug- 
gested as  to  the  significance  of  the  wide-meshed  reticulum  of  the 
cells  of  .the  mammalian  subzonal  layer,  Fig.  59,  Yolk. 

The  disposition  of  the  animal  pole  in  the  ovum  before  segmenta- 
tion also  conforms  to  the  homologies  here  advocated.  It  will  be 
remembered,  ante,  p.  55,  that  the  protoplasm  of  the  animal  pole 
extends  far  into  the  ovum  and  is  enveloped  by  a  cup  (deutoplasm 
zone)  of  the  substance  of  the  vegetable  pole.  Hence,  when  the 
animal  pole  forms  cells,  they  lie  as  an  inner  mass,  Fig.  56,  i.m. 
If  Minot's  view  be  adopted,  then  the  ectoderm  lies  within  the 

entoderm  at  a  certain  stage  of 
development,  for  the  one  cell 
which  retains,  as  shown  in  Fig. 
53,  the  connection  of  the  ecto- 
derm with  the  exterior  is  sub- 
sequently overgrown  by  the 
outer  layer  of  cells  (Van  Bene- 
den,  Heape).  There  is,  then,  a 
complete  inversion  of  the  germ- 
layers  in  all  (?)  placental  mam- 
malia. In  most  cases  the  inver- 
sion is  temporary;  the  inner 
mass  as  described  above  flattens 
out,  and  probably  flattens  out 
inside  the  outer  epithelial  layer ; 
if  this  is  the  case  then  the  ex- 
ternal layer  of  the  lens -shaped 
mass,  Fig.  57,  B  and  C,  is  real- 
ly entoderm;  this  layer  is 
Rauber's  Deckschicht,  which, 
as  already  stated,  usually  disappears,  leaving  the  true  inner  mass  or 
permanent  ectoderm  to  form  part  of  the  surface  of  the  blastodermic 
vesicle,  so  that  with  the  exception  of  the  reduction  in  the  dimension 
of  the  entoderm  the  relations  are  the  same  as  in  other  vertebrate  ova. 
The  inner  layer  of  the  flattened  inner  mass  gives  rise  to  the 
entoderm,  and  this  at  first  sight  appears  to  be  conclusive  evidence 
against  the  homology  here  draAvn  between  the  inner  mass  and  the 
primitive  ectoderm  of  other  vertebrates.  The  same  thing  was 
formerly  supposed  to  occur  in  the  blastoderm  of  other  vertebrates,  but 
it  is  now  known  that  the  entoderm  is  added  from  another  source  to 
the  under  side  of  the  primitive  blastoderm  or  ectoderm,  and  though 
we  possess  no  exact  information  whatever  as  to  the  origin  of  the 
entodermic  cells  under  the  primitive  blastoderm  of  the  mammalia, 
there  is  no  reason  to  assume  that  they  arise  in  a  manner  fundamen- 
tally different  from  that  typical  of  other  vertebrates.  We  may, 


FIG.  59.— Diagram  of  a  segmented  mammalian 
ovum :  Z,  zona  pellucida ;  Bl.  primitive  blasto- 
derm; s.c. ,  segmentation  cavity;  Yolk,  layers  of 
cell  representing  the  remnant  of  segmented  yolk. 


SI;<;M  I:\TATIOX:  FORMATION  OF  THE  DIADERM.  109 

therefore,  dismiss  this  objection.  The  origin  of  the  entodermic 
cavity  and  its  lining  is  described  in  the  next  chapter. 

Planes  of  Division  During  Segmentation.— The  plane  of 
the  first  division  determines  those  of  the  subsequent  divisions,  and 
also  perhaps  the  axes  of  the  embryo  ;*  it  is  itself  determined  by  the 
position  of  the  long  axis  of  the  first  amphiaster  or  nuclear  spindle  to 
which  it  is  at  right  angles.  It,  therefore,  is  a  matter  of  great  Interest 
to  ascertain  what  factors  determine  the  position  of  the  first  spindle, 
or,  in  other  words,  the  axis  of  elongation  of  the  segmentation 
nucleus.  So  far  as  at  present  known,  there  are  two  factors:  1, 
relation  to  the  axis  of  the  ovum ;  2d,  position  of  the  path  taken  by 
male  pronucleus  to  approach  the  female  pronucleus.  The  axis  of  the 
ovum  is  fixed  before  impregnation ;  it  passes  through  the  centre  of 
the  animal  and  that  of  the  vegetable  pole.  Usually  the  nuclear 
spindle  which  leads  to  the  formation  of  the  polar  globule  has  its  long 
axis  coincident  with  that  of  the  ovum,  hence  the  point  of  exit  of  the 
polar  globule  marks  one  end  of  the  ovetic  axis.  The  first  amplii- 
uxtcr  or  xjtimUr  /.s  (i/tr<tf/x  at  right  angles  to  the  ovic  axis.  This, 
however,  leaves  the  meridian  plane  undetermined.  Roux,  87.1, 
from  a  series  of  interesting  experiments  on  frogs'  ova,  concludes  that 
the  plane  is  fixed  by  the  path  of  the  spermatozoon.  So  far  as  I  know 
this  idea  was  first  suggested  by  Selenka  in  1878,  in  his  paper  on 
"The  Development  of  Toxopneusters  Variegatus;"  compare,  also, 
Mark,  81.1,  p.  500.  In  the  frog's  egg  the  path  of  the  male  pro- 
nucleus  is  marked  by  a  line  of  pigment,  as  was  first  described  by 
Van  Bambecke,  70. 1,  Go,  and  has  been  well  figured  by  O.  Hertwig, 
77.2,  PI.  V.,  Fig.  48.  The  pigment  renders  it  easy  to  ascertain  the 
position  of  the  male  road  even  after  the  first  cleavage  of  the  ovum. 
This  Roux  has  done  in  sectioned  ova,  and  from  experiments  and 
observations  reaches  this  result:  The  long  axis  of  the  first  segmen-. 
tattoii- N/)in(1/c  //V*  ///  a  plane,  which  passes  through  the  axis  of 
the  onuii  <d/<!  tlic  jxith.  of  the  male  pronucleus.  If  Roux's  conclu- 
sion is  confirmed,  it  will  become  of  fundamental  importance.  Yet 
there  must  be  other  factors  which  can  at  least  replace  the  male  pro- 
nucleus  in  this  special  role,  since  the  development  of  parthenogenetic 
ova,  in  which  there  is  no  male  pronucleus  at  all,  is  equally  determinate. 
It  is  probable  that  the  distribution  of  the  protoplasm  is  the  real  cause 
determining  the  position  of  the  nucleus;  thus  in  oval  eggs  the 
spindle  lies  in  the  direction  of  the  long  axis ;  it  is  quite  probable  that 
if  the  male  pronucleus  has  the  effect  ascribed  to  it  by  Rotlx,  it  pro- 
duces it  indirectly  by  altering  the  distribution  of  the  protoplasm 
within  the  ovum ;  that  such  alteration  takes  place  is  indicated  by 
the  occurrence  of  the  male  aster. 

That  the  first  cleavage  plane  is  determined  by  relations  existing  in 
the  unimpregnated  ovum,  has  been  suggested  by  O.  Schultze  in 
consequence  of  his  finding  the  germinal  vesicle  lying  eccentrically 
in  the  eggs  of  the  brown  frog.  Schultze  suggests  that  the  first  plane 
passes  through  the  ovic  axis  and  the  eccentric  nucleus.  Roux  (Biol. 

*  In  certain  cases,  notably  in  birds  as  described  above,  the  segmentation  is  irregular;  and  it  is 
therefore  not  known  yet  whether  the  scheme  of  arrangement  of  the  cleavage  planes  here  given 
can  be  applied  to  all  ova  or  not.  We  may  say,  however,  that  the  scheme  is  the  primitive  one, 
from  which  any  modifications  arose  phylogenetically.  The  best  discussion  is  by  A.  Agassiz  and 
Whitman,  84.1.  34-41. 


110 


THE    GERM-LAYERS. 


C6/.,  VII.,  420),  maintains  that  this  suggestion  is  set  aside  by  his 
own  observations  cited  above.  For  further  discussion  see  Schultze's 
short  note,  87.2,  andRoux's  rejoinder,  88.1.  I  think  the  question 
whether  the  first  cleavage  plane  is  determined  by  the  ovum's  struc- 
ture or  not  is  still  an  open  one. 

As  already  stated  in  the  primitive  segmentation,  both  invertebrate 
and  vertebrate,  the  second  cleavage  plane  is  at  right  angles  to  the 
first  and  also  meridional,  while  the  third  plane  is  at  right  angles 
to  both  the  first  and  therefore  equatorial.  In  meroblastic  vertebrate 
ova  this  regularity  is  entirely  lost. 

Relation  of  the  Segmentation  Planes  to  the  Embryonic  Axis.— 
It  has  been  assumed  by  some  writers  that  the  first  cleavage  plane 
coincided  with  the  future  median  plane  of  the  embryo.  This  con- 
ception is  rendered  extremely  improbable  by  the  fact  that  the  seg- 
ments of  the  ovum  have  been  observed  to  migrate  in  various  cases 
so  as  to  destroy  the  symmetrical  grouping.  Miss  Clapp's  observa- 
tions, 91. 1,  499,  on  the  toad-fish  show  that  the  median  plane  of  the 
embryo  may  form  almost  any  angle  with  the  first  cleavage  plane. 

Differentiation  of  the  Ectoderm  and  Entoderm. — As 
already  pointed  out,  the  essential  feature  of  segmentation  is  the 

unlikeness  of  the  cells  produced;  the 
manifold  variations  in  the  process  of  seg- 
mentation depend  chiefly  on  the  amount 
of  yolk. 

Minot  in  1877,  17,  first  established 
the  generalization  that  in  all  animals 
the  ovum  undergoes  a  total  segmenta- 
tion during  which  the  cells  of  the  ecto- 
derm divide  faster  and  become  smaller 
than  the  cells  of  the  entoderm;  com- 
pare Fig.  60.  There  are,  however,  a 
small,  and  I  think  diminishing,  number 
of  cases,  where  the  process  of  segmenta- 
tion is  imperfectly  understood,  and  which 
cannot  yet  be  shown  to  conform  to  this 
generalization.  "  All  the  known  varia- 
tions in  the  process  of  segmentation  de- 
pend merely  upon :  1st,  the  degree  of  dif- 
ference in  size  between  the  two  sets  of  cells ;  2d,  the  time  when  the 
difference*  appears ;  3d,  the  mode  of  development,  whether  polar  or 
by  delamination,*  either  of  which  may  or  may  not  be  accompanied 
by  axial  infolding.  In  Gasteropods,  Planarians,  "  Calcispongia3, 
Gephyrea,  Annelida,  fish,  birds,  and  Arthropods,  the  difference  is 
great  and  appears  early.  In  Echinoderms,  most  Ccelenterates,  some 
sponges,  in  Nematods,  Amphibians,  etc.,  it  is  less  marked  and  ap- 
pears later." 

In  most  cases  the  entodermic  cells  are  very  decidedly  larger  and 
less  numerous  than  those  of  the  ectoderm.  This  distinction  is 
obviously  necessary  on  account  of  the  mutual  relations  of  the  two 
primitive  layers.  The  ectoderm  has  to  grow  around  the  entoderm, 
which  it  can  do  only  by  acquiring  a  greater  superficial  extension ; 

*  It  does  not  occur  among  vertebrates. 


FIG.  60.— Ovum  of  Amphioxus  lan- 
ceolatus  during  segmentation-stage, 
with  88  cells,  x  280  diams.  After 
B.  Hatschek.  One  pole  is  occupied 
by  large  entoclermal,  the  other  by 
smaller  ectodermal  cells. 


SEGMENTATION:   FORMATION    OF   THE   DIADERM.  Ill 

this  the  ectoderm  accomplishes  by  dividing  very  quickly  at  first 
into  small  cells.  After  the  entoderm  is  fully  enveloped  it  may  then 
continue  to  grow  until  its  superficies  is  much  greater  than  that  of 
the  outer  layer,  within  which,  however,  it  still  finds  room  by  form- 
ing numerous  folds ;  thus  is  gradually  reached  the  condition  in  the 
higher  adult  animals  where  the  intestine  sometimes  has  an  enor- 
mous surface,  but  is  nevertheless  contained  in  body- walls  -covered  by 
ectoderm  presenting  much  less  surface.  It  is,  therefore,  only  during 
the  early  stages  of  segmentation  that  we  find  the  entoderm  expand- 
ing more  slowly  than  the  ectoderm. 

The  terms  holoblastic  and  meroblastic  are  applied  to  ova  accord- 
ing to  their  manner  of  segmentation.  The  first  is  employed  for  those 
ova  in  which  there  is  either  very  little  or  only  a  moderate  amount  of 
yolk,  so  that  the  whole  of  the  ovum  splits  up  into  distinct  masses 
(cells)  which  enter  into  the  composition  of  the  embryo.  The  second 
designates  ova  with  a  very  large  amount  of  yolk,  so  that  while  the 
protoplasm,  from  which  the  ectoderm  arises,  divides  rapidly  into 
distinct  cells,  the  entodermal  portion  merely  develops  nuclei  at  first, 
with  the  result  that  while  one  portion  of  the  egg  is  "  segmenting" 
another  portion  (the  entodermal)  remains  unsegmented,  so  far  as  the 
external  appearances  are  concerned.  Eggs,  then,  with  much  yolk, 
undergo  the  so-called  partial  segmentation;  hence  the  adjective 
meroblastic. 

Whatever  the  exact  mode  of  segmentation  there  results  always  the 
same  type  of  organization,  to  which  Minot  has  applied  the  term 
t/itiifcnn;  it  is  characterized  by  consisting  of  two  plates  of  cells, 
differing  in  character,  joined  at  their  edge  (ectental  line) ,  and  sur- 
rounding a  central  segmentation  cavity ;  the  two  plates  or  lamina 
are  the  two  primitive  germ-layers,  the  ectoderm  and  entoderm. 
The  earliest  form  of  the  diaderm  is  that  known  as  the  blastula,  as 
Haeckel  has  felicitously  named  the  first  larval  form  of  the  lower 
animals.  In  the  blastula  we  have  a  simple  epithelial  vesicle,  the 
cavity  of  which  is  the  large  segmentation  cavity,  Fig.  47 ;  the 
epithelial  layer  is  one  cell  thick  and  divided  into  two  regions ;  one 
composed  of  smaller  cells  is  the  ectoderm,  EC,  and  the  other  of  larger 
cells  is  the  entoderm.  This  stage  occurs  with  sundry  modifications 
in  a  great  many  invertebrates.  These  modifications  are  due  princi- 
pally to  the  increase  in  size  of  the  entodermic  cells,  which,  as  already 
pointed  out,  results  from  the  increase  of  the  yolk-matter  in  the 
ovum.  Thus  in  many  mollusks  the  entodermic  cells  are  very  large 
and  at  first  few  in  number.  By  a  still  further  modification  the 
cellular  yolk  is  replaced  by  a  mass  rich  in  deutoplasm,  but  not 
divided  into  cells,  while  at  the  same  time  the  segmentation  cavity 
is  reduced  by  the  invasion  of  the  yolk-mass.  In  vertebrates  we  have 
the  additional  modification  that  the  cells  are  several  layers  deep  in 
the  ectoderm  and  primitively  in  the  entoderm  also;  compare  the 
section  of  the  axolotl's  ovum,  Fig.  49;  in  certain  forms,  as  we  have 
seen,  the  entoderm  is  not  divided  into  discrete  cells,  but  remains 
one  mass;  this  is  the  case  in  Elasmobranchs  and  the  amniota,  but  in 
the  highest  amniota  (Placentalia)  the  yolk  is  lost  and  the  ento- 
derm is  again  represented  by  a  single  layer  of  cells,  Fig.  59. 

It  seems  to  me  evident  that  the  first  step  of  development  in  the 


112  THE    GERM-LAYERS. 

segmenting  ovum  is  the  differentiation  of  the  two  germ-layers, 
ectoderm  and  entoderm,  resulting  in  the  diaderm  stage.  Diaderm 
is  a  term  preferable  to  blastula,  because  the  latter  is  applicable  strictly 
only  to  a  special  larval  form,  while  the  former  is  a  general  term 
which  refers  to  the  essential  differentiation  at  this  stage.  It  is 
important  to  remark  that  the  two  layers  are  distinct  in  the  diaderm 
or  blastula  stage ;  it  is  often  erroneously  affirmed  that  the  blastula 
consists  of  a  uniform  layer  of  cells,  part  of  which  subsequently 
becomes  the  entoderm. 

The  segmentation  cavity  comprises  the  whole  space  between  the 
entoderm  and  ectoderm ;  it  is  very  early  invaded  by  cells  produced 
from  the  two  primitive  germ-layers.  These  cells  are  in  vertebrates 
of  many  kinds  and  enter  the  segmentation  cavity  at  various  periods. 
It  is  customary  to  group  the  cells  which  enter  early  into  this  cavity 
under  the  common  name  of  mesoderm^  and  to  consider  them  as 
a  third  and  distinct  germ-layer.  For  convenience  we  may  adopt 
this  custom,  for  to  a  certain  extent  the  mesoderm  of  authors  is  a 
separate  germ-layer,  but  it  by  no  means  includes  all  the  tissues 
which  occupy  ^he  space  between  the  two  primitive  germ-layers.  As 
the  space  between  the  entoderm  and  ectoderm  is  always  homologous 
with  itself,  it  follows  that  the  entire  room  between  the  epithelium 
(entoderm)  of  the  digestive  tract  and  its  appendages  on  the  one  side 
and  the  epidermis  on  the  other  is  homologous  with  the  segmentation 
cavity. 

The  mesoderm  of  authors  comprises  three  tissues:  1,  free  wander- 
ing cells  (mesamoeboids] ;  2,  embryonic  connective  tissue  or  cells 
connected  together  by  processes  (mesenchyma)  ;  3,  epithelium,  which 
forms  two  or  more  separate  sacs.  The  origin  of  the  mesoderm  and 
the  relations  of  the  three  tissues  it  contains  are  discussed  in  the  next 
chapter. 

The  Gastrula  Theory. — In  invertebrates  with  holoblastic  ova 
the  blastula  passes  into  a  stage  known  as  the  gastrula.  Gastrula 
is,  properly  speaking,  a  new  name  for  a  larval  form  called  planula 
by  older  writers ;  but  the  term  is  now  generally  employed  to  desig- 
nate an  ideal  embryonic  stage,  supposed  to  be  common  to  all  multi- 
cellular  animals. 

The  blastula  changes  into  a  gastrula  by  a  process  of  invagination. 
The  entodermal  area  of  the  blastula  flattens  out,  the  ectoderm  mean- 
while expanding  by  multiplication  of  its  cells ;  after  flattening,  the 
entoderm  turns  inward,  forming  at  first  a  shallow  cup,  then  a  pit 
which  has  an  opening  or  mouth,  the  rim  of  which  is  the  ectental 
line.  The  larva  is  now  a  double  sac,  and  has  an  external  wall  or 
ectoderm  and  an  internal  wall  or  entoderm ;  the  entodermic  cavity 
is  entirely  distinct  from  the  segmentation  cavity.  The  process  of 
gastrulation  is  here  described  as  it  occurs  among  the  lower  inver- 
tebrates. 

Typical  gastrulse  are  the  free-swimming  Iarva3  of  many  marine 
invertebrates ;  we  may  take  as  an  example  that  of  a  sea-urchin,  Fig. 
61.  The  larva  is  round;  at  one  pole  it  has  an  opening,  m,  the 
gastrula  mouth  leading  into  an  internal  cavity ;  as  this  is  a  free- 
swimming  larva  it  is  provided  with  long  cilia  for  organs  of  locomo- 
tion ;  the  cilia  in  many  gastrulas  are  distributed  over  limited  areas 


SEGMENTATION:   FORMATION   OF   THE   DIADERM. 


113 


FIG.  61.— Section  of  a  gastrulaof  Toxopneustes 
lividus;  after  Selenka.  ec,  ectoderm;  en,  ento- 
derm;  mes,  mesoderm;  m,  mouth. 


or  they  may  be  wanting  altogether.  The  larva  consists  of  a  double 
sac,  a  larger  outer  one  of  small  epithelial  cells,  ec,  the  ectoderm, 
and  a  much  smaller  inner  sac  composed  of  larger  entodermal  epi- 
thelial cells,  en;  at  the  mouth, 
///,  of  the  inner  sac  the  two 
layers  are  continuous  with  one 
another;  in  the  space  between 
the  two  sacs,  which  corresponds 
to  the  segmentation  cavity,  are 
a  few  scattered  cells,  the  first 
members  of  the  mesoderm,  mes. 

The  entodermal  sac  of  the 
gastrula  is  known  as  the  arch- 
fiiteron;  other  terms  are  also 
in  use,  e.  g.,  mid-gut,  coelente- 
rou,  urdarm,  etc.  The  opening 
is  known  as  the  <j<txtrn  la  mouth 
(archistome,  urmund,  etc.). 
The  ccelenterates  preserve  the 
gastrula  organization  through- 
out life,  but  in  all  higher  classes 
the  archenteron  gives  rise  not  only  to  the  permanent  digestive  tract, 
but  also  to  many  appendages  and  derivatives  thereof ;  and,  moreover, 
the  gastrula  mouth  closes  over,  and  in  vertebrates  the  true  mouth  is 
an  entirely  new  formation,  which  arises  without  any  connection 
whatever,  so  far  as  known,  with  the  gastrula  mouth.  By  gastrula- 
tion  the  ectental  line  becomes  the  rim  of  the  gastrula  mouth. 

A  line  passing  through  the  centre  of  the  mouth  and  the  opposite 
pole  of  the  gastrula  is  the  so-called  axis.  Now,  if  the  mouth  be 
elongated,  there  would  at  once  be  a  new  longitudinal  axis  marked 
out,  and  the  gastrula  would  become  bilaterally  symmetrical.  If, 
further,  the  mouth  is  pulled  out  into  a  slit,  and  in  the  process  of 
evolution  the  lips  come  together  and  unite  in  their  middle  part,  the 
animal  would  still  have  the  two  ends  of  the  original  mouth  left 
open,  and  would  so  acquire  two  apertures  to  its  archenteron — one 
anterior  to  serve  as  mouth,  and  one  posterior  to  serve  as  anus.  This 
hypothesis  of  the  conversion  of  a  gastrula  into  a  bilaterally  symmet- 
rical animal  by  the  elongation  of  the  mouth  and  concrescence  of  the 
lips  or  ectental  line,  was  first  suggested,  so  far  as  I  am  aware,  by  Rabl, 
76.1.  A  very  perfect  exemplification  of  the  process  is  afforded  by 
the  developing  ova  of  Peripatus capensis,  as  shown  by  Balfour,  83.1, 
and  Sedgwick,  85.2,  PL  XXXII.,  Figs.  23-26.  There  are,  how- 
ever, serious  difficulties  in  applying  the  theory  to  bilateral  inver- 
tebrates ;  I  am  strongly  inclined  to  think  that  further  research  will 
obviate  these  difficulties. 

In  certain  vertebrates  and  annelids  the  concrescence  of  the  ectental 
line  has  been  clearly  demonstrated,  but  the  process  is  rendered  by 
secondary  modifications  much  more  complex  than  that  described  in 
the  preceding  paragraph — the  detailed  account  of  it  forms  the  sub- 
ject of  the  next  chapter. 

The  gastrula  theory  is  that  all  metazoa  have  a  common  inherited 
stage  of  development,  which  follows  immediately  after  the  diaderm; 
8 


114 


THE   GERM-LAYERS. 


this  stage  is  characterized  by  there  being  an  outer  ectodermal  sac 
with  a  perforation  to  the  edge  of  which  is  attached  the  entoderm, 
which  forms  a  closed  inner  sac,  the  archenteron.  The  embryology 
of  coelenterates  teaches  us  that  the  gastrula  is  a  secondary  type,  and 
thus  the  interesting  problem  of  the  origin  of  the  gastrula  is  to  be 
solved  by  the  invertebrate  embryologist  (see  J.  P.  McMurrich, 
91.1,310.) 

The  term  gastrula  was  introduced  by  Haeckel,  and  is  now  univer- 
sally used  by  embryologists.  The  discovery  of  the  importance  of  the 
gastrula  is  due  to  the  brilliant  researches  of  Kowalewski  on  various 
invertebrates,  including  Amphioxus,  then  supposed  to  be  a  verte- 
brate. Haeckel  then  seized  upon  the  idea  of  the  gastrula  and  wrote 
an  essay,  74.2,  (compare  also  75.1),  upon  it,  which  from  its  brilliant 
style  attracted  notice,  and  did  much  to  direct  attention  to  the  impor- 
tant discovery  of  Kowalewski.  Although  Haeckel  indulged  his 
fantasy  unduly  and  was  misled  into  speculations  which  are  now 
unheeded  and  almost  forgotten,  he  did  great  good  by  starting  the 
interest  of  zoologists  in  the  right  direction.  By  a  remarkable  coinci- 
dence, Lankester  published  an  essay,  73.1,  of  very  similar  purport 
to  Haeckel' s,  at  about  the  same  time. 

The  gastrula,  like  the  diaderm,  varies  greatly,  the  chief  modifica- 
tions depending  on  the  amount  of  yolk  present ;  this  is  illustrated  by 

the  accompanying  diagrams,  Fig. 
62 ;  the  mesoderm  is  intentionally 
omitted;  A  corresponds  to  such  a 
larva  as  Fig.  61;  the  difference 
in  size  between  the  two  sets  of 
cells  is  slight  but  evident.  In  B, 
the  difference  is  more  marked, 
and  fairly  represents  a  gastrula 
of  Amphioxus.  In  C,  the  differ- 
ence is  very  great  and  corresponds 
to  that  observed  in  certain  gaste- 
ropod  larvae.  In  D,  the  inner 
set  is  no  longer  separated  into 
distinct  cells,  although  there  are 
a  number  of  nuclei,  each  of  which  marks  the  centre  of  a  future  cell ; 
in  such  instances  we  must  regard  the  whole  inner  portion  as  not  yet 
transformed  into  a  definite  entodermic  ce/Mayer.  This  figure  is  par- 
ticularly instructive,  because  it  shows  that  what  we  call  the  yolk  is 
not  something  distinct  from  the  germ,  but  really  belongs  to  the  inner 
layer  of  the  embryo.  E  shows  a  similar  egg,  in  which  the  outer  set 
of  cells  has  not  yet  grown  around  the  yolk.  F  shows  the  same  egg 
not  in  section,  but  seen  from  the  outer  surface  in  order  to  exhibit 
the  cap  of  small  cells  (blastoderm)  resting  upon  the  yolk. 


FIG.  62.  —Diagrams  of  the  Principal  Modiflca 
tions  of  the  Gastrula  (see  text).  A— E.  repre 
sents  sections. 


CHAPTER  V. 

CONCRESCENCE:  ORIGIN    OF    THE   PRIMITIVE   STREAK  AND  ARCH- 

ENTERON. 

THIS  chapter  was  published  in  a  preliminary  form  in  the  American 
Naturalist,  June- August,  1890.     Since  then  the  researches  of  Van 

Beneclen,    88.3,    011   the  rabbit,  and   of  L.  Will,  B? 

89.1,   90.1,   92.1,  on   reptiles  have   cleared   up  ||s 

many  obscure  points.     The  chief  gain,  as  Prenant  £jj 

has  shown  in  his  "  Embryologie, "  is  the  knowledge  *y 
that  probably  in  all,  certainly  in  many  vertebrates 

(excluding  Amphioxus) ,  the  entodermal  canal  arises  o £ 

by  the  fusion  of  two  cavities;  one  of  these  is  the  s?g 

long-known  notochordal  or  blastoporic  canal,  which  £  a 

communicates   with  the  exterior   by  an   opening  §:?" 

(blastopore)  at  its  posterior  end ;  the  other  cavity  £  | 

is  formed  in  the  yolk  immediately  underneath  the  ,|! 

notochordal  canal  and  is  completely  closed.     Very  ~  2, 
early  the  partition  between  the  two  cavities  disap- 

pearr  and  they  fuse,  making  the  definite  cavity  of  || 

the  entodermal  canal.     This  primitive  canal,  from  o^ 
which  the  pharynx,  lungs,  and   digestive  organs 

are  differentiated,  is  known  as  the  archenteron.  ^® 
The  relations  with  which  we  are  now  concerned  are 

illustrated  by  Fig.  63.  J  ? 


I.  THE  LAW  OF  CONCRESCENCE. 

Yolk  Cavity.  —  Concerning  the  formation  of 
the  yolk-cavity  we  possess  very  imperfect  knowl- 
edge.  Undoubtedly  a  patient  search  might  collate 
many  facts  from  the  literature  of  the  early  stages, 
but  until  such  a  collation  shall  be  made  and  sup- 
plemented by  further  observations,  no  positive  his- 
tory  of  the  yolk  cavity  can  be  given.  We  can  say 
that,  when  the  notochordal  canal  begins  to  form, 
there  is  already  a  large  cavity  under  the  germ  and 
entirely  surrounded  by  entodermal  material.  In 
elasmobranchs  and  Sauropsida  the  floor  of  the  cav- 
ity  is  the  yolk  itself,  while  the  roof  is  formed  by 
cellular  material  ;  the  cavity  expands  over  a  con- 
siderable  area,  but  is  flattened;  it  is  completely 
separated  from  the  segmentation  cavity  ;  it  is  des- 
ignated often  by  the  name  of  sub-germinal  cavity  , 
but  unfortunately  the  same  name  is  also  applied  to 
the  morphologically  different  segmentation  cavity. 


2    !WJ 


In  Amphibia 


the  yolk  cavity  lias  been  recognized  by  O.  Schultze;  it  is  not  large. 


116 


THE   GERM-LAYERS. 


In  mammals  the  yolk-cavity,  as  soon  as  the  entodermic  layer  is 
fully  developed  —  see  below  —  comprises  the  so-called  cavity  of  the  two- 
layered  blastodermic  vesicle  ;  owing  to  the  reduction  of  the  yolk,  it 
is  bounded  wholly  by  a  layer  of  cells,  not  partly  by  a  mass  of  yolk, 
as  in  meroblastic  ova,  and  is  very  large  in  proportion  to  the  ovum. 

Concrescence.  —  The  passage  from  the  stage  of  segmentation  to 
the  first  embryonic  stage  is  effected  in  vertebrates  by  means  of  cer- 
tain migrations  of  embryonic  material  from  lateral  positions  to 
median  positions,  and  subsequent  union  in  the  middle  line.  This 
process  of  union  is  known  as  concrescence.  It  consists  in  the  grow- 
ing together  of  the  two  halves  of  the  ectental  line  to  form  the  struct- 
ural axis  of  the  future  embryo.  The  process  is  somewhat  complex, 
and  needs  therefore  to  be  described  in  detail,  the  more  so  as  it  has 
still  to  be  followed  in  mammals. 

The  accompanying  diagram  may  assist  to  render  clear  the  process 
of  concrescence,  Fig.  64.  It  is  intended  to  illustrate  the  spreading 

of  the  ectoderm  (germi- 
nal disc,  blastoderm, 
auct.)  over  the  yolk  and 
the  simultaneous  for- 
mation of  the  primitive 
axis.  The  whole  ovum 
is  represented  as  seen  in 
projection;  the  propor- 
tions are  such  as  have 
been  suggested  by  the 
ova  of  flounders  and 
frogs.  Three  successive 
stages  of  the  expanding 
blastoderm  are  repre- 
sented ;  the  first  position 
of  the  embryonic  rim 
(ectental  line)  corre- 
sponds to  the  dotted 
line  a"  a";  the  concres- 
cence reaches  only  to  the 

Pomt    marked    1  . 

lateral         margins,       o 

v  •    -i 
which    are  to   COnCreSCe 

lofov  cfill  fnrm  r»arf  nf 
later'  ,STUJ 

the  edge  of  the  blasto- 
derm. At  the  next  stage  the  ectoderm  has  grown  very  much  and 
has  moved  its  edge  to  a'  a',  while  the  margins,  £,  have  coalesced  so 
that  the  primitive  axis  extends  to  2.  The  extension  continues,  bring- 
ing the  ectental  line  to  a  a  a  S,  and  carrying  the  primitive  axis 
back  to  3  ;  behind  the  primitive  streak  a  small  area,  Yk,  of  the  yolk 
is  still  uncovered,  and  is  homologous  with,  first  ,  the  anus  of  Rusconi 
in  amphibian  ova,  and,  second  (in  my  belief),  with  the  so-called 
primitive  streak  of  the  amniota.  The  portion  of  the  ectental  line 
bounding  this  area  differs  from  that  which  is  immediately  concerned 
in  the  formation  of  the  primitive  streak,  8;  although  it  now  lies  be- 
hind the  primitive  streak,  it  was  previously  in  front  of  it,  when  the 


FIG.  64.—  Diagram  illustrating  the  growth  of  the  blastoderm 
and  concrescence  of  its  rim  to  form  the  primitive  axis, 
pr.  s.  JV.  neural  or  medullary  groove;  nr.  neural  ridges; 
%  blastoderm;  «,  part  of  the  .blastodermic  rim,  termed  the 
Sichel  in  German  ;  pr.  s,  primitive  axis;  bl",  blastopore;  Yk, 
uncovered  yolk.  Compare  also  the  text. 


THE    LAW    OF    CONCRESCENCE.  117 

blastoderm  covered  only  the  minor  portion  of  the  ovum,  see  S"  a"a" . 
Ultimately  the  yolk  is  entirely  covered  by  the  blastoderm,  thus  fixing 
the  length  of  the  primitive  streak.  It  is  essential  to  notice  that  the 
blastodermic  rim  (ectental  line)  divides  into  two  portions,  one,  s, 
which  forms  the  primitive  streak,  and  another,  a  a" ,  which  over- 
grows the  ovum  and  at  last  closes  over  the  yolk  behind  the  completed 
primitive  axis. 

Historical  Xnte. — The  earliest  observations  on  concrescence  to 
form  the  embryonic  axis  are,  so  far  as  known  to  me,  those  of  Rathke 
on  leeches.*  Nine  years  later  Kowalewski  (Mem.  Acad.  Sci.,  St. 
Petersburg,  7me  Ser.,  XVI.,  1871)  recorded  its  occurrence  among 
insects.  Its  recognition  as  a  vertebrate  mode  of  development  we 
owe  to  the  brilliant  investigations  of  W.  His;  in  his  first  paper, 

76. 1,  lie  describes  very  accurately  and  clearly  the  process  of  concres- 
cence in  the  salmon;  in  his  second  paper,   77. 1,  he  describes  con- 
crescence in  the  sharks,  and  in  his  third  and  fourth  papers,  77.2, 

91.2,  he  discusses  again  the  general  bearing  of  his  results.    Semper, 
in  his  great  work  on  the  relationship  of  annelids  and  vertebrates, 

76.3,  271,  was  the  first  to  make  a  direct  comparison  of  the  pro- 
cesses of  concrescence   in  annelids,  insects,  and  vertebrates.     Un- 
fortunately   Balfour  entirely  failed  to   grasp  the  new  conception, 
and    by  expressing    himself  very  decidedly  against    it,     "Comp. 
Embryol.,"    II.,  306-308,  led  man}'  embryologists  to  discredit  the 
discovery.     Whitman,  78.2,  91-94,    has  ably  defended   the   com- 

rison  made  by  Semper  (see  above) ;  Rauber,  76.2,  Kollman, 
5.1,  Ryder,  85.5,  9,  and  others  have  added  to  our  knowledge 
of  the  phenomenon.  Duval's  researches  on  the  chick,  84.1, 
demonstrate  concrescence  there  also,  though  the  author  appears 
unacquainted  with  the  results  of  his  predecessors.  Minot  in  the 
article  "Foetus,"  in  Buck's  "Handbook,"  III.,  172,  173,  accepts 
concrescence  as  the  typical  mode  of  vertebrate  development. 

Concrescence  in  Bony  Fishes. — At  the  close  of  segmentation 
the  germinal  disc  forms  a  cap  of  cells  on  the  yolk.  The  disc  (primi- 
tive blastoderm)  spreads  over  the  yolk  gradually ;  when  it  begins  to 
spread  its  edge  is  already  thickened ;  this  thickened  edge  corresponds 
to  the  ectental  line ;  the  thickening  is  known  as  the  Randwulst; 
it  is  also  called  the  blastodermic  rim,  which  term  Ryder  and  others 
have  used.  When  the  blastoderm  has  spread,  so  as  to  cover  perhaps 
a  sixth  or  less  of  the  surface,  one  point  of  the  rim  ceases  f  to  move ; 
consequently,  as  the  expansion  continues  the  edge  of  the  disc  bends  in 
behind  this  point  on  each  side,  until  two  parts  of  the  blastodermic 
rim  meet  as  they  come  from  opposite  sides,  and  then  grow  together. 
This  is  illustrated  by  the  accompanying  diagram,  Fig.  65;  F  is 
the  outline  of  the  yolk ;  bl  is  the  outline  of  the  blastoderm ;  a,  the 
fixed  point ;  the  expansion  of  the  blastoderm  has  brought  the  parts 
1  1  together  and  they  have  united;  the  parts  2  2  are  about  to 
meet  and  unite ;  then  3  3  will  meet ;  i  4  and  so  on,  until  the  two 
halves  of  the  ectental  line  are  brought  together  along  their  entire 
length ;  their  junction  marks  the  axis  of  the  future  embryo,  and  pro- 
duces a  longitudinal  band  of  thicker  tissue,  which  has  long  been 

*  Rathke  and  Leuckart.  "Beitriige  zur  Entwickelungsgeschichte  der  Hirudineen;"  Leipzig,  1862. 
t  Or  perhaps  merely  moves  more  slowly. 


118 


THE    GERM-LAYERS. 


FIG.  C5. — Diagram  of  concrescence  in  a 
Teleostean  egg:  Y,  outline  of  yolk:  W, 
outline  of  blastoderm,  1  1,  lateral  parts 
already  concresced;  2  2,  lateral  parts 
about  to  concresce;  3,  4,  parts  to  con- 
cresce  later. 


known  to  embryologists,  and  may  be  named  the  primitive  axis. 
The  fixed  point  of  the  blastodermic  rim  marks  the  head-end  of  the 
embryo;  the  parts  of  the  ectental  line  which  grow  together  next 
behind  the  fixed  point  develop  into  the  head,  those  a  little  farther 

back  into  the  neck,  and  those  farthest 
back  into  the  rump  and  tail.  The 
parts  of  the  circular  rim  most  remote 
from  the  fixed  point,  «,  of  course 
concresce  last.  The  destiny  of  each 
portion  of  the  ectental  line  is  fixed 
before  concrescence  occurs.  In  fact 
in  certain  cases  the  differentiation 
of  the  tissues  advances  to  a  consider- 
able degree  in  the  "  Randwulst "  be- 
fore concrescence.  This  is  strikingly 
the  case  in  Elacate,  in  the  ova  of 
which  the  myotomes  (or  segmental 
divisions  of  the  mesoderm)  appear 
in  the  embryonic  rim  before  its  con- 
crescence (Ryder,  85.9) ;  compare 
also  Ryder's  observations  on  Belone, 
81.2.  The  development  of  the  tele- 
ostean  germ -layers  is  not  yet  fully 
worked  out.  For  the  best  history  of  the  entoderm  and  mesoderm, 
as  well  also  for  references  to  conflicting  authorities,  see  M.  Kowalew- 
ski,  86.1,2,  who,  however,  pays  no  heed  to  the  law  of  concrescence. 
That  concrescence  occurs  in  teleosts  essentially  as  here  described, 
seems  to  me  evident  from  the  figures  given  by  W.  His,  76.1,  C. 
Kupffer,  84.1,  Coste,  47.1,  and  others.  Nevertheless  the  concres- 
cence is  denied  by  Henneguy,  88.1,  H.  V.  Wilson,  91.1,  260,  and 
others,  but  the  arguments  I  have  found  against  concrescence  have 
not  appeared  to  me  valid. 

In  the  primitive  axis  is  a  mass  of  cells  below  the  ectoderm ;  this 
mass  subsequently  divides  into  mesoderm  and  entoderm.  The 
entodermal  cells  form  at  first  and  for  a  considerable  period  a  solid 
cord  (cf.  Balfour,  "Comp.  Embryol.,"  II.,  75)  in  which,  however,  a 
lumen  appears  later ;  this  lumen  I  will  tentatively  homologize  with 
the  cavity  of  the  notochordal  canal  of  amniota. 

Concrescence  in  Elasmobranchs. — Our  knowledge  rests 
mainly  on  the  researches  of  His,  77.1,  and  his  follower,  Kollmann, 
85.1.  Fig.  66,  A,  is  a  generalized  diagram  of  an  elasmobranch 
ovum,  representing  the  ectodermal  disc,  J5/,  as  seen  from  above  rest- 
ing upon  the  yolk,  which  is  not  represented  in  the  figure.  The  first 
change  noticeable  in  the  disc  after  the  close  of  segmentation  is  a 
groove  running  completely  around  its  margin  between  it  and  the 
yolk ;  as  the  disc  grows  and  expands  the  groove  is  no  longer  present 
along  the  front  edge,  a  a,  of  the  blastoderm,  but  only  on  the  sides 
and  behind.  Abou  the  same  time  there  usually  appears  a  distinct 
notch,  n,  which  marks  the  fixed  point  of  the  margin  and  the  pos- 
terior end  of  the  disc.  If  now  a  section  be  made  across  the  line, 
X  F,  the  relations  will  be  found  to  be  essentially  as  represented  in 
the  diagram,  Fig.  66,  B;  the  disc  rests  on  the  yolk,  Vi,  which  con- 


THE   LAW   OF   CONCRESCENCE. 


119 


tains  numerous  nuclei ;  between  the  yolk  and  the  ectoderm,  EC,  is 
the  segmentation  cavity,  sc;  the  groove  is  bounded  above  by  a  layer 
of  cells,  En,  which  are  larger  than  those  of  the  ectoderm,  and  have 
been  produced  by  the  yolk,  Vi;  sometimes  there  are  cells  lying  in 
the  segmentation  cavity  at  this  stage,  the  formation  of  the  mesoderm 
having  already  begun.  The  essential  point  to  note  in  this  stage,  is, 
as  Kollmann  has  shown,  the 
division  of  the  margin  of  the 
ectodermal  disc  into  two  parts, 
one,  a  a,  resting  directly  on 
the  yolk,  the  other,  S,  directly 
continuous  with  a  layer  of  en- 
todermal cells,  B,  En,  forming 
a  little  groove  under  the  mar- 
gin of  the  disc.  The  two  por- 
tions of  the  ectental  margin 
have  entirely  distinct  func- 
tions, as  already  stated;  the 
anterior,  a  a,  is  destined  to 
grow  over  and  cover  the  yolk 
by  the  extra-embryonic  portion 
of  the  ectoderm ;  the  posterior, 
S,  is  destined  to  form  the 
primitive  axis  of  the  embryo. 
Fig.  67  is  similar  to  Fig. 
66,  but  represents  a  more  ad- 
vanced stage.  The  ectodermal 
disc,  Bl,  is  much  enlarged,  and 
its  anterior  grooveless  mar- 
gin, a  a  a,  is  relatively  much  F;a.  66. -Diagram  of  an  Elasmobranch _  Blastoderm 

more  increased  than  the  poste-  *?  4SSSS  -he  form/atiKin  ?f  J*e  mareinal  groove. 

t$      At  A,   surface-view:     £1,    blastoderm;    a   a,   anterior 

nor    grooved    margin,     o;    the  grooveless  margin;  S(Sichel),  marginal  groove ;  n, 

nor.4^  nf  +1^  r^±/-.h    T?i/v    flfl    -w  marginal  notch;  X  Y,   line  of  section.    B,  section 

Ceiltie  01  the  notcn,  X  Ig.  00,  n,  aiong  the  line  .Y  Fof  A.    EC,  ectoderm;  En,  ento- 

has     remained      nearly    if    not  t^.rm;1J'1  extental I  line;  «.c.,  segmentation  cavity; 

quite  stationary,  Fig.  67,  pr.  s, 

while  the  margin,  s  s,  of  either  side  has  been  growing  toward  its 
fellow  in  the  manner  indicated  by  the  arrows,  and  as  they  meet  the 
two  side-margins  grow  together  in  the  median  line,  making  a  longi- 
tudinal structure.  The  manner  and  results  of  the  concrescence  of 
the  margins  from  the  two  sides  to  form  an  axial  structure  become 
clearer  in  section,  Fig.  07,  B.  The  margin  at  the  side,  m,  still 
shows  the  same  relations  as  in  Fig.  66,  B;  in  the  median  line, 
however,  the  margins  have  met  and  intimately  united,  "so  that 
what  were  originally  two  grooves  have  completely  united  to  form  a 
single  canal,  Ent,  bounded  above  by  entodermal  cells,  below  by  the 
entodermal  yolk,  Vi.  This  canal  is  the  primitive  entodermal 
cavity.  Whether  it  represents,  when  first  developed,  merely  the 
notochordal  canal  of  the  amniota  or  the  fused  notochordal  canal  and 
yolk  cavity,  we  are  unable  to  determine  at  present.  A  moment's 
consideration  renders  it  evident  that  the  canal  must  be  open  pos- 
teriorly ;  this  opening  is  the  blastopore,  bl.  There  are  some  further 
details  to  be  mentioned :  where  the  ectental  margins  have  united  in 


120 


THE    GERM-LAYERS. 


the  median  line  there  appears  a  lateral  outgrowth,  mes,  which  is  the 
beginning  of  the  mesoderm ;  in  some  cases  this  mesodermic  tissue 
appears  before  the  margins  concresce ;  when  viewed  from  the  sur- 
face the  mesoderm  can  be  seen  through  the  ectoderm,  as  was  ob- 
served long  ago;  it  is  this  faint  appearance  which  early  writers 

call  in  anamniota  the 
primitive  streak,  it  be- 
ing the  foreshadowing 
of  coming  organiza- 
tion. Fig.  G7,  A,  also 
shows  in  front  of  the 
primitive  axis  the  first 
trace,  JV,  of  the  central 
nervous  system,  which 
we  shall  describe  later. 
The  blastoderm  is  seen 
also  to  be  divided  al- 
ready into  two  parts, 
the  lighter  area  pellu- 
cida,  A.  p.,  and  the 
darker  area  opaca, 
"A.  o.;  the  latter  also 
shows  the  first  blood- 
islands.  For  further 
descriptions  of  these 
areas,  see  Chap.  XIII. 
From  their  observa- 
tions, His,  Kollmann, 
and  others  have  in- 
ferred that  at  the  an- 
terior ectental  margin, 
a  a  a,  there  are  pro- 
cells,  which  grow 


Fio.  67.-Diagram  of  a  Vertebrate  Blastoderm  a  little  more 
advanced  than  Fig.  66:  A,  surface-view.  B,  section  along  the 
line,  X  Y.  ^blastoderm;  a  a  a,  anterior  margin;  s  s,  pos- 
terior  margin  (Sichei;);  ^.o.,  area  opaca;  ^4.p.,  area  pellucida; 


111 


j          ,,,j 
toward 


n.  r.  ,  neural  ridges  ;  N,  neural  or  medullary  groove  ;  pr.  s.  ,  prim-    ori  A    pon  «*t,i  tn  t,P   Dfl  T't  of 

itive  streak;  6Z,blastopore;  EC.,  ectoderm;  m,  ectental  margin;    ainQ    C 

En:  entodermic  cells  ;  Fi,  yolk  :  mes,  mesoderm  ;  s.  c.  .  segmen-    the  mesoderm   and  are 

especially  concerned  in 

forming  the  first  blood,  which  is  produced  always  in  the  extra- 
embryonic  area.  This  mesoderm  of  peripheral  origin  His  has  named 
parablast  —  a  term  which,  unfortunately,  has  been  employed  differ- 
ently by  some  subsequent  writers.  The  ectoderm,  entoderm,  and 
axial  mesoderm  are  grouped  by  His  under  the  collective  name  of 
arcliiblast.  This  view  of  the  double  origin  of  the  mesoderm,  al- 
though it  has  been  adopted  in  a  modified  form  by  the  brothers  Hert- 
wig,  I  am  unable  to  accept.  The  question  is  discussed  in  Chapter  VI. 
Concrescence  in  Marsipobranchs,  Ganoids,  and  Amphib- 
ians. —  As  not  only  the  constitution  of  the  ovum,  but  also  'its  early 
development,  is  very  similar  in  the  three  classes  named,  we  may  con- 
sider them  collectively  in  the  present  connection.  The  condition  of 
the  ovum  at  the  close  of  segmentation  has  already  been  described, 
p.  99,  and  figured,  Fig.  49.  The  ectental  line  is  not  sharply  defined, 


THE   LAW    OF   CONCRESCENCE. 


121 


nor  does  there  appear  any  groove  around  the  edge  of  the  blastoderm 
as  in  meroblastic  ova.  The  small-celled  ectoderm  spreads  over  the 
yolk ;  while  it  is  doing  this  a  short  notochordal  canal  appears  at  the 
hind  edge  of  the  blastoderm  with  a  small  opening  to  the  exterior, 
known  as  the  blastopore,  Fig.  68,  bl.  The  first  indication  of  the 
canal  in  the  frog  is  easily  recognized,  being  the  appearance  of  a 
curved  area  of  pigmentation  of  semilunar  outline  amid  the  yolk-cells 
at  the  posterior  pole ;  the  convexity  of  the  area  is  directed  toward 
the  segmentation  cavity ;  the  centre  of  the  concavity  corresponds  to 
the  dorsal  lip  of  the  blastopore  (Robinson  and  Assheton,  91.1,  403). 
The  canal  runs  forward  toward  the  segmentation  cavity,  Fig.  us, 
s.c.;  above  and  in  front  of  the  blastopore  the  cells  have  multiplied 
and  accumulated  to  form  the  beginning  of  the  primitive  axis,  Pr.  In 
the  lamprey  there  is  at  this  stage  no  such  axial  accumulation  of 
cells ;  according  to  Shipley  the  ectoderm  consists  of  a  single  laj-er  of 
cells,  and  the  notochordal  canal  is  bounded  on  its  dorsal  side  by  a 
single  layer  of  cells  also,  between  which  and  the  overlying  blasto- 
derm there  are  no  cells ;  the  gathering  of  cells  corresponding  to  the 

primitive  axis  does  not  arise  un- 
til later.    The  canal,  according  to 


FIG.  68.— Ovum  of  Axolotl.  After  Bellonci. 
Longitudinal  section  to  show  the  commencing 
formation  of  the  primitive  axis,  Pr.  bl.  blas- 
topore ;  Bl,  blastoderm ;  s.  c. ,  segmentation 
cavity. 


Yin.  60.  — Ovum  of  Petromyzon  in  longitudi- 
nal section.  After  Balfour.  me,  mesoderm  of 
primitive  axis;  6Z,  blastopore;  Ffc,  yolk  at  the 
anus  of  Rusconi ;  a/,  notochordal  canal ;  s.  c. , 
segmentation  cavity. 


O.  Schultze,  ultimately  fuses  with  the  yolk  cavity  to  form  the  definite 
archenteron ;  it  is  sometimes  designated  as  the  blastoporic  invagina- 
tion.  The  canal  in  the  same  measure  as  the  blastoderm  spreads  over 
the  yolk-grooves  at  it3  hinder  end  away  from  the  segmentation  cavity, 
Fig.  69,  s.c.,  just  as  in  elasmobranchs.  A  stage  is  soon  reached  in 
which  nearly  the  entire  length  of  the  archenteron  is  formed  and 
nearly  the  whole  yolk  is  covered.  There  is  still  a  blastopore  which 
leads  into  the  cavity,  and  which  has  moved  gradually  backward 
from  its  original  position.  Behind  the  blastopore  lies  the  uncovered 
yolk,  FA;,  which  in  the  frog's  ovum  is  very  conspicuous,  because  its 
whitish  color  contrasts  with  the  dark  color  of  the  heavily  pigmented 
ectoderm  around  it ;  this  area  of  exposed  yolk  is  the  so-called  anus 
of  Rusconi.  When  the  canal  has  completed  its  full  length  the 


122 


THE    GERM-LAYERS. 


..EC. 


following  disposition  of  the  parts  is  found,  Fig.  70 :  The  archen- 
teron  is  bounded  below  by  the  large  mass  of  yolk- cells,  Vi,  and 
above  by  the  epithelium,  Ent,  of  the  entoderm;  its  posterior  end 
curves  up  to  open  at  the  blastopore,  Bl,  passing  through  a  mass  of 
cells,  which  constitute  the  end  of  the  primitive  streak;  this 
portion  of  the  archenteron  is  sometimes  called  the  blastoporic  canal. 
There  is  further  a  short  prolongation,  Al,  of  the  cavity  below  the 
blastopore.  This  diverticulum  has  been  homologized  with  the  allan- 

tois,  (see  Chapter  XII.).  It  is 
also  very  probably  homologous 
with  the  more  nearly  spheri- 
cal diverticulum  found  in  a 
similar  position  in  teleosts, 
and  now  known  as  Kupffer's 
vesicle,  from  having  been  es- 
pecially studied  by  C.  Kupf- 
fer,  66.1,  475,  68.1,  who 
has  interpreted  it  as  the 
teleostean  allantois.  Com- 
pare D.  Schwarz,  89.1,  197, 
Taf.  XIII.,  Figs.  35,  37,  etc. 
Around  the  blastopore  is  a 
mass  of  cells  (primitive  axis) 

FIG.  70. — Longitudinal  section  of  the  ovum  of  a 
Sturgeon  after  the  formation  of  the  entodermic  cavity : 
EC,  ectoderm;  Mes,  mesoderm;  Ent,  entoderm;  Bl, 
blastopore;  Al,  diverticulum  of  the  archenteron;  Fi, 
yolk.  After  Salensky. 


--Afc 


STL 


continuous  on  the  one  side 
with  the  ectoderm,  on  the 
other  with  the  epithelial  en- 
toderm lining  the  archente- 
ron, and,  thirdly,  with  a  sheet  of  cells,  Mes,  between  the  ectoderm, 
EC,  and  entoderm,  Ent. 

The  developmental  phases  just  outlined  seem  to  me  to  afford  suf- 
ficient evidence  of  concrescence.  Owing  to  the  gradual  transition 
between  the  ectoderm  (blastoderm)  and  the  entoderm  (yolk-cells) 
there  is  no  sharp  ectental  line,  as  in  some  other  types.  Moreover, 
there  is  no  differentiation  of  the  tissues  at  the  blastodermic  rim,  but 
only  after  the  cells  are  united  in  the  axis ;  hence  we  cannot  distin- 
guish parts  at  the  periphery  of  the  blastoderm  and  follow  their  union 
in  the  primitive  streak  as  we  can  in  certain  sharks  and  bony  fishes. 
Nevertheless,  we  find  all  the  essential  features  of  concrescence ;  the 
notochordal  canal  and  the  primitive  axis  begin  at  the  edge  of  the 
blastoderm  and  grow  at  their  posterior  end  away  from  the  segmenta- 
tion cavity,  and  at  the  same  rate  the  blastoderm  overspreads  the  yolk. 
Concrescence  in  Sauropsida.— The  early  stages  in  Reptilia 
have  long  been  obscure.  Clarke  (Agassiz'  "Contributions,"  II.),  in 
his  paper  on  the  embryology  of  the  turtle,  mistook  the  commencement 
of  the  notochordal  canal  for  the  commencement  of  the  amniotic 
fold.  Weldon,  83. 1,  Kupffer,  82. 1,84. 1,  Strahl,  80. 1,  2,  3,  82. 1, 
83.1,  Hoffmann  (Bronn's  "Thierreich,"  VI.,  Abth.  iii.,  1892-1897), 
and  others  partly  traced  out  the  history  of  the  canal.  Will's 
observations,  90. 1,  on  the  development  of  the  gecko  gave  the  key  to 
the  history  of  the  canal  in  the  reptiles.  In  tho  gecko  there  is  formed 
a  notochordal  canal,  which  is  at  first  very  short,  but  gradually 
lengthens  out,  apparently  chiefly  by  growth  at  its  hind  or  blastoporic 


THE   LAW   OF   CONCRESCENCE. 


123 


end,  Fig.  63,  nch.  c.  The  end  of  the  canal,  when  the  germinal  area 
is  examined  in  surface-views,  is  characterized  by  a  transverse  figure 
or  sichel,  which  is  well  known  in  reptilian  embryos  of  all  orders,  and 
which  presumably  represents  the  portions  of  the  Randwulst  which 
are  to  concresce  and  thereby  lengthen  the  primitive  axis  and  the 
notochordal  canal  inclosed  by  the  cells  of  the  axis.  Underneath  the 
notochordal  canal  is  a  layer  of  entodermal  cells,  Ent,  which  form  the 
roof  of  the  yolk  cavity ;  the  figure  does  not  show  the  inferior  or 
lateral  boundaries  of  the  yolk  cavity.  In  a  little  later  stage,  the 
tissue  between  the  canal  and  the  yolk  cavity  disappears  and  the  two 
lumina  fuse. 

In  other  reptiles  the  development  is  similar,  though  obscured  by 
the  peculiarity  that  the  anterior  part  of  the  notochordal  canal  opens 
into  the  yolk  cavity  before  the  posterior  part  is  formed.  In  such 
cases  there  is  only  a  short  section  of  the  canal  to  be  observed  with 
complete  boundaries  at  any  one  stage.  In  reptiles  then  concres- 
cence can  only  be  inferred  from  the  presence  of  the  "  sichel"  and  the 


FIG.  71 . —Formation  of  the  blastoporic  canal  in  Lacerta  muralis.  After  Weldon.  B,  C, 
longitudinal  sections  of  two  successive  stages  of  the  blastoderm,  which  in  each  case  has  been  re- 
moved from  the  yolk :  the  space  under  the  entoderm.  En,  is  the  archenteric  cavity.  D,  transverse 
section  of  the  posterior  part  of  the  blastopore  a  little  younger  than  C.  EC,  ectoderm;  En,  ento- 
derm ;  Pr,  primitive  streak ;  II.  blastopore :  Ch,  notochord ;  Hies,  mesoderm. 

growth  backward  of  the  primitive  axis.  Fig.  71  illustrates  the 
formation  of  the  canal  in  Lacerta,  as  described  and  figured  by  W. 
F.  R.  Weldon,  83.1.  En  is  the  entoderm  forming  the  roof  of  the 
yolk  cavity.  B  shows  the  notochordal  canal,  &/,  just  beginning  to 
form.  C  is  a  stage  considerably  more  advanced;  the  anterior  part 
of  the  canal  has  fused  with  the  yolk  cavity,  and  the  dorsal  wall  of 


124  THE   GERM-LAYERS. 

the  canal  has  produced  the  notochord,  nch;  only  a  short  posterior 
end,  6Z,  remains  as  a  canal.  D  is  a  transverse  action  through  the 
blast  opore. 

The  process  of  concrescence  in  birds  was  partly  indicated  by  Roller's 
investigations,  79.1,  82.1,  and  has  been  carefully  elucidated  by 
Duval,  84. 1.  The  resemblance  to  concrescence  as  known  in  elasmo- 
branchs  is  very  striking.  Around  the  edge  of  the  blastoderm  ap- 
pears very  early  a  small  groove ;  as  the  blastoderm  expands  the  front 
portion  loses  the  groove ;  one  point,  the  centre  of  the  grooved  margin, 
ceases  to  move,  or  at  least  moves  much  more  slowly  than  the  remain- 
der of  the  blastodermic  rim ;  as  the  expansion  continues  the  edges 
of  the  two  halves  of  the  groove  coalesce  gradually  behind  the  fixed 
point,  thus  producing  the  entodermal  canal  in  the  same  manner  as 
in  the  sharks ;  cells  accumulate  at  the  same  time  and  make  behind 
the  blastopore  the  so-called  primitive  streak.  There  is  some  uncer- 
tainty in  Duval's  account,  as,  unfortunately,  at  the  time  he  wrote 
the  existence  of  a  yolk  cavity  contributing  to  the  formation  of  the 
archenteron  had  not  been  recognized. 

In  birds  (hen's  ova)  there  is  a  further  peculiarity,  which  is,  I 
think,  probably  to  be  found  in  all  amniota,  namely :  that  portion  of 

the  edge  of  the  ectoderm  which  does 
not  share  in  concrescence  and  which 
corresponds  to  the  edge  of  the  anus 
of  Rusconi  closes  over  the  yolk  be- 
hind the  primitive  streak,  so  that  the 
portion  of  the  yolk  which  is  left  un- 
covered is  remote  from  the  embryonic 
««»  (or  primitive  streak)  As  a  rare 

ing  on  the  yolk  from  a  longitudinal  sec-    anomaly,  S66   Whitman,   OO.  1.     a  line 

Du?ai°f  the  blastoderm  in  situ-    After  is  visible  running   in  the  ectoderm 

from  the  hind  end  of  the  primitive 

streak  to  the  edge  of  the  uncovered  yolk ;  this  line  is  to  be  interpreted 
as  evidence  of  the  growing  together  of  the  ectoderm,  behind  the  streak 
proper.  The  ectoderm,  as  it  spreads  over  the  yolk,  receives  no  accre- 
tions from  it,  but  accomplishes'  its  expansion  by  proliferation  of  its 
own  cells.  Thus  the  uncovered  yolk  is  bounded  by  the  free  edge  of 
the  ectoderm,  Fig.  72.  The  area  of  uncovered  yolk,  which  may  be 
called  the  yolk  blastopore,*  is  not  homologous  with  the  anus  of 
Rusconi,  from  which  it  differs  in  position,  being  remote  from  instead 
of  close  (as  is  the  anus  of  Rusconi)  to  the  blastopore,  for  it  is  situated 
nearly  opposite  the  embryonic  area.  In  birds,  according  to  Duval, 
8*1.2,  the  yolk  blastopore  (Dotternabel)  is  never  closed  by  ectoderm, 
but  remains  covered  by  the  vitelline  membrane  only,  until  the 
mesoderm  spreads  over  it.  The  growing  edge  of  the  ectoderm  is 
somewhat  thickened ;  it  finally  is  reflected  around  the  edge  of  the 
yolk  blastopore,  forming,  as  it  were,  a  funnel,  at  the  bottom  of  which 
is  the  yolk  (see  Duval,  I.e.). 

Concrescence  in  Mammals. — As  shown  below  in  the  detailed 
history  of  the  mammalian  blastodermic  vesicle,  there  is  a  fixed  point 
(Hensen's  knot)  at  which  the  formation, of  the  primitive  axis  and 
notochordal  canal  begins,  and  from  which  they  lengthen  out  back- 

*  Duval  applies  to  it  the  name  of  ombilic,  ombilical. 


THE    LAW    OF    CONCRESCENCE.  125 

ward  as  they  would  do  if  formed  by  concrescence.  The  main  cavity 
of  two-layered  vesicle  is  the  yolk  cavity,  and  with  it  the  notochordal 
canal  subsequent!}"  fuses,  cf.  infra.  The  position  and  history  of  the 
ectental  line  being  absolutely  unknown  in  mammalia,  it  is  of  course 
impossible  to  form  any  definite  notions  as  to  the  process  of  concres- 
cence in  them. 

Concrescence  :  Summary. — The  evidence  that  concrescence  is 
the  typical  means  of  forming  the  primitive  streak  in  vertebrates  is :  1, 
detailed  and  conclusive  observations  upon  elasmobranchs,  teleosts, 
and  birds ;  2,  exact  and  extensive  observations  on  marsipobranchs, 
ganoids,  and  amphibians,  which  concord  with  the  theory  of  con- 
crescence ;  3,  a  great  probability  of  its  occurrence  in  reptiles,  owing 
to  the  similarity  of  their  development  with  that  of  birds ;  4,  a  prob- 
ability of  its  occurrence  in  mammals,  because  of  the  resemblance  in 
the  growth  and  structure  of  the  primitive  axis  to  that  in  other  verte- 
brates. The  theory  seems  to  me  inevitable  that  the  vertebrate 
prim  it  ire,  axis  is  formed  by  the  f/roirin(/  together  in  the  axial 
line  of  the  future  embryo  of  the  two  halves  of  the  ectental  line. 

The  development  of  the  primitive  axis  may  be  described  in  general 
terms  as  follows:  At  the  close  of  segmentation  the  edge  of  the 
primitive  blastoderm  separates  into  two  parts ;  one  part  (the  anterior) , 
as  the  blastoderm,  expands,  spreads  over  the  yolk,  gradually  covering 
it  with  ectoderm ;  the  other  part  (the  posterior)  forms  the  primitive 
axis ;  it  has  in  its  centre  one  fixed  point ;  consequently,  when  the 
blastoderm  expands  the  two  halves  of  the  posterior  part  of  the 
ectental  line  are  brought  together  and  gradually  unite  (concresce) 
along  a  line  running  from  the  fixed  point  backward  (radially  as  re- 
gards the  blastoderm).  Consequently,  the  segmentation  cavity, 
which  is  underneath  the  primitive  blastoderm,  lies  in  front  of  the 
developing  axis.  While  this  goes  on  cells  grow  out  from  the  con- 
crescing  part  of  the  ectental  line  into  the  space  between  the  ectoderm 
and  entoderm  (or  yolk) ;  underneath  the  line  of  junction  a  cavity  is 
formed  lined  by  entoderm ;  this  cavity  is  the  notochordal  canal ;  it 
lengthens  backward  as  concrescence  progresses ;  it  has,  whatever  its 
length,  a  small  entrance,  the  blastopore,  at  its  hind  end ;  the  blasto- 
pore  is  ultimately  obliterated.  The  cells  which  grow  out  from  the 
ectental  line  constitute  the  first  anlage  of  the  middle  germinal  layer 
or  mesoderm,  and  shining  through  the  ectoderm  they  produce  the 
appearance  of  a  whitish  line,  which  has  led  to  the  name  of  primitive 
axis.  The  characteristics  of  the  mesoderm  are  described  in  the 
next  section.  Along  the  line  of  junction  there  often  appears  a  slight 
furrow  in  the  ectoderm,  which  is  known  as  the  primitive  groove. 

Significance  of  Concrescence. — It  will  at  once  be  evident  that 
if  the  process  of  concrescence  went  on  without  the  actual  meeting  of 
the  two  portions  of  the  ectental  line  the  result  would  be  to  leave  the 
archenteron  open  along  its  entire  length ;  the  borders  of  the  opening 
would  be  the  ectental  line ;  and  this  line,  as  we  have  seen,  corre- 
sponds to  the  lips  of  the  gastrula  mouth ;  consequent!}",  we  should 
have  a  gastrula  with  an  elongated  mouth.  This  condition  is  illus- 
trated by  the  accompanying  diagram,  Fig.  73.  It  agrees  in  all  re- 
spects with  the  gastrula  type ;  its  most  noteworthy  peculiarities  are 
two :  first,  the  enormous  mass  of  yolk  accumulated  in  the  aboral 


126 


THE   GERM-LAYERS. 


portion  of  the  entoderm ;  second,  the  elongation  of  the  gastrula  or 
archenteric  cavity  in  a  direction  at  right  angles  to  the  gastrula  axis, 
xy.  If  now  the  lips  of  gastrula,  Fig.  64,  .s,  meet  and  unite  we 
should  obtain  at  once  the  vertebrate  type.  According  to  W.  His' 
discovery,  this  is  precisely  what  takes  place — only  the  lips  are  brought 
together  first  at  one  end,  where  they  .at  once  unite,  while  behind  they 
are  widely  separated ;  but  gradually  they  are  brought  together  and 
unite  throughout  their  entire  length. 

Concrescence  is,  then,  a  modified  method  of  uniting  the  lips  of  a 
greatly  elongated  gastrula  mouth.  Why  this  modification  is  estab- 
lished we  cannot  say  with  certainty,  though  we  may  surmise  with 
confidence  that  it  is  consequent  upon  the  great  accumulation  of  yolk 
in  vertebrate  ova. 

The  view  here  adopted  enables  us  to  speak  positively  as  to  the 
point  where  we  are  to  look  in  vertebrates  for  the  homologue  of  the 
invertebrate  mouth.  In  annelids  concrescence  is  very  well  marked, 
whenever  the  ova  contains  much  yolk ;  now  in  leeches  and  earth- 
worms the  ectental  line  does  not  concresce  along  the  entire  axial  line, 


FIG.  73.  —Diagrammatic  cross-section  of  a 
vertebrate  ovum,  in  which  concrescence  is 
supposed  to  have  been  arrested ;  xy,  median 
plane;  Ach,  archenteron;  Ent,  entoderm; 
Ect,  ectoderm. 


FIG.  74.— Dog-fish  embryo,  nearly  in  Balfour's 
stage  C.  HI,  position  of  invertebrate  mouth ;  J?, 
rim  of  the  germinative  area. 


but,  on  the  contrary,  as  shown  by  Kleinenberg  and  Whitman,  the 
foremost  part  of  the  germ  bands  (gastrula  lips)  do  not  unite,  but  leave 
a  small  opening ;  when  the  permanent  mouth  is  formed  this  opening 
is  carried  in  and  serves  as  the  passage  between  the  mouth  cavity 
(Vorderdarm,  stomodseum)  and  the  archenteric  cavity.  The  fore- 
most part  of  the  line  of  concrescence  lies,  according  to  His'  observa- 
tions, on  fishes  just  where  the  optic  outgrowths  arise,  Fig.  74,  m; 
hence  we  have  to  search  between  the  origins  of  the  optic  nerves  for 
traces  of  the  in-vertebrate  mouths.  Further  reference  to  this  question 
is  made  later  in  connection  with  the  development  of  the  nervous 
system. 

The  Notochordal  Canal. — The  existence  of  this  canal  was,  so 
far  as  I  am  aware,  first  satisfactorily  recognized  by  Lieberkuhn,  82. 1 , 
84.1,  who  discovered  that  in  mammals  it  produces  the  notochord, 


THE    LAW   OF   CONCRESCENCE. 


127 


and  by  losing  its  lower  walls  fuses  with  the  yolk  cavity.  The  canal 
is  a  narrow  tube  which  runs  forward  in  the  tissue  of  the  primitive 
axis  (Kolliker's  head  process) ;  it  ends  blindly  in  front,  but  its 
posterior  end  communicates  with  the  exterior  by  a  funnel-shaped 
opening  (the  blastopore)  through  the  ectoderm.  Immediately  behind 
the  blastopore  lies  the  accumulation  of  cells,  termed  the  primitive 
streak  in  amniota,  the  anus  of  Rusconi  in  amphibians.  The  canal 
is  lined  by  epithelium,  which  is  thickened  on  the  dorsal  side  to  form 
the  anlage  of  the  notochord.  At  the  sides  the  epithelium  merges  into 
cells  belonging  to  the  mesoderm. 

The  manner  in  which  the  canal  is  formed  by  concrescence  is  ex- 
plained in  the  preceding  pages,  and  the  manner  in  which  it  fuses  with 
the  yolk  cavity  is  described  in  the  following  section.  For  additional 
details  and  references  see  the  history  of  the  notochord  in  Chapter  VIII. 

Fusion  of  the  NotochordarCanal  and  Yolk  Cavity.— The 
fusion  of  these  two  cavities  has  been  carefully  studied  in  mammals 
and  reptiles.  The  fusion  in  amphibi- 
ans is  briefly  mentioned  by  O.  Schulze, 
88.1.  In  the  gecko  (L.  Will,  90.1) 
and  in  mammals  (Lieberkuhn,  82.1, 
84. 1 ,  Van  Beneden,  88. 3,  and  others) , 
the  canal  becomes  quite  long,  and  then 
acquires  a  series  of  irregular  openings, 
Fig.  75,  nch,  on  its  ventral  side  into 
the  very  large  yolk  cavity,  which  at 
this  stage  underlies  the  whole  germinal 
area.  The  anlage  of  the  notochord  is 
already  differentiated  on  the  dorsal 
side  of  the  canal.  The  ventral  open-, 
ings  increase  both  in  number  and  size 
until  the  entire  canal  has  fused  with 
the  yolk  cavity  except  at  the  hind  end, 
where  it  persists  for  a  while  as  the  so- 
called  blastoporic  canal.  The  fusion 
occurs  in  guinea-pigs  the  fourteenth  to 
fifteenth  day,  in  rabbits  the  eighth  day. 

In  lizards  (Strahl,  Kupffer)  and  tur- 
tles (Will)  the  fusion  occurs  in  a  simi- 
lar manner,  but  sooner,  so  that  the  anterior  portion  of  the  canal  has 
fused  with  the  yolk  cavity  before  the  posterior  portion  of  the  canal  is 
completed. 

The  union  of  the  two  cavities  produces  the  definitive  archenteron, 
which  is  a  spacious  cavity  lined  by  entoderm,  having  the  anlage  of 
the  notochord  in  its  median  dorsal  line  and  opening  to  the  exterior 
by  the  blastopore,  which  is  situated  at  the  caudal  end  of  the  primitive 
axis  and  the  head  ward  end  of  the  primitive  streak. 

Blastopore. — The  blastopore  is  the  small  opening  which  leads 
into  the  notochordal  canal,  or,  after  the  canal  has  fused  with  the 
yolk  cavity,  leads  into  the  archenteron.  It  is  situated  at  the  hind 
end  of  the  primitive  axis  (head-process),  and  marks  the  anterior 
boundary  of  the  anus  of  Rusconi  in  amphibia,  or  of  the  primitive 
streak,  properly  so-called,  of  amniota,  Fig.  71,  B. 


FIG.  75. — Germinal  area  of  a  Guinea- 
pig  at  thirteen  days  and  twenty  hours, 
seen  from  the  under-side.  After  Lieber- 
kuhn. ao,  area  opaca;  ap,  area  pellu- 
cida ;  nch ,  anlage  of  the  notochord  as  a 
canal,  with  several  irregular  openings 
on  the  entodermic  side,  x  24  diameters. 


128 


THE   GERM-LAYERS. 


While  the  concrescence  of  the  ectental  line  is  going  on  the  blasto- 
pore changes  its  position,  being  always  at  the  end  of  the  notochordal 
canal.  When  the  canal  fuses  with  the  yolk  cavity  the  end  of  the 
canal  persists  for  a  time  as  a  passage  at  the  end  of  the  primitive  axis, 
and  this  passage  is  sometimes  designated  as  the  blastoporic  canal, 
see  Figs.  70  and  71.  The  opening  is  finally  obliterated. 

The  blastopore  is  not  homologous  with  the  gastrula  mouth,  but  is 
merely  a  small  portion  thereof ;   in  front  of  it  the  gastrula  mouth  is 
closed  by  concrescence ;  while  concrescence  is  going  on  there  will  be 
a  part  of  the  gastrula  mouth  open  behind  the  blastopore ;  when  con- 
crescence is  completed  the  blastopore  is  at  the  end  of  the  elongated 
fastrula  mouth,  the  lips  of  which  are  united  throughout  the  remain- 
er  of  their  length.     The  blastopore  is  not  a  fixed  point,  being  merely 
the  opening  of  the  notochordal  canal,  and  as  by  concrescence  the 
canal  is  elongated,    in  precisely  the  same  measure  the  blastopore 
travels  backward. 

The  Meroblastic  Embryo. — Considerations  of  practical  con- 
venience have  led  to  the  custom  of  distinguishing  in  the  development 

of  meroblastic  ova  the  embryonic 
from  the  extra-embryonic  por- 
tions. The  distinction  is  in  real- 
ity entirely  arbitrary,  for  the 
whole  of  the  ovum  is  included, 
morphologically  speaking,  within 
the  body  of  the  embryo.  Custom 
has  led  to  designating  the  two 
parts  as  the  embryo  and  the  yolk ; 
but  the  student  should  be  careful 
not  to  allow  himself  to  be  misled 
by  these  terms.  In  the  laboratory 
it  is  a  general  practice  to  remove 
the  so-called  "  embryo  "  from  the 
yolk,  and  in  doing  this  the  arch- 
enteric  cavity  loses  its  inferior 
wall,  to  wit :  the  entodermic  yolk. 
Let  the  relations  be  represented 
by  the  accompanying  diagram, 
Fig.  76,  the  embryo  being  drawn 
very  much  too  large  in  proportion 
to  the  yolk,  for  the  sake  of  clear- 
ness. Suppose  the  layers  to  be 
cut  through  on  the  lines  x  x;  we  could  then  remove  the  embryonic 
portion.  This  is  what  is  actually  done  in  practice.  It  is  very  im- 
portant to  understand  clearly  that  the  yolk  is  part  of  the  embryo,  and 
that  our  sections  usually  represent  only  a  torso. 


FIG.  76. —Diagram  showing  the  relations  of  a 
vertebrate  ovum  with  an  embryo  in  cross-sec- 
tion and  a  large  yolk.  EC,  ectoderm;  JV,  neural 
groove;  mes,  mesoderm;  s  c  ,  segmentation  cav- 
ity ;  Ent ,  archenteric  cavity ;  a  a,  ectodermal 
rim,  where  the  ectoderm  is  growing  over  the 
yolk. 


II.  THE  PRIMITIVE  Axis  AND  STREAK. 

The  term  primitive  axis  is  a  new  one,  which  it  has  seemed  neces- 
sary to  introduce  to  avoid  confusion.  It  is  nearly  synonymous  with 
the  term  head-process  (Kolliker's  Kopffortsatz) .  It  is  applied  in  all 
vertebrates  to  the  median  band  of,  cells  which  runs  forward  from  the 


THE   PRIMITIVE   AXIS   AND    STREAK.  129 

blastopore ;  the  central  cells  of  the  band  are  entodermal*  and  form 
the  epithelial  wall  of  the  notochordal  canal ;  the  lateral  cells  of  the 
band  contribute  to  the  production  of  the  mesoderm.  At  the  blast- 
opore the  primitive  axis  merges  into  the  primitive  streak,  sensu 
gtrictu,  and  on  that  account  has  been  interpreted  and  described  by 
many  authors  as  the  anterior  prolongation  of  the  primitive  streak. 
After  the  ventral  wall  of  the  notochordal  canal  has  disappeared  and 
the  canal  has  fused  with  the  yolk  cavity,  the  entire  tissues  of  the 
primitive  axis  lie  on  the  dorsal  side  of  the  archenteron. 

The  term  primitive  streak  may  be  conveniently  and  properly  re- 
stricted hereafter  to  the  accumulation  of  cells  lying  immediately 
behind  the  blastopore.  In  amphibia  this  accumulation  is  known  as 
the  anus  of  Rusconi ;  it  belongs  to  the  entoderm  (and  later  to  the 
mesoderm  also),  and  is  very  conspicuous  owing  to  the  absence  of 
pigment  in  its  cells.  In  amniota  the  corresponding  accumulation 
comprises  the  cells  in  the  region  around  the  primitive  groove,  as 
described  in  detail  below ;  in  amniota  the  accumulation  has  the  yolk 
cavity  (later  archenteron)  extending  under  it,  Fig.  71,  A  B,  pr, 
and  it  is  therefore  not  directly  continuous  with  the  yolk  proper,  as 
in  amphibians. 

The  conceptions  of  the  axis  and  streak  above  presented  appear  to 
me  necessary  consequences  of  our  present  knowledge,  but  until  they 
are  accepted  by  other  embryologists,  the  reader  must  view  them  aa 
largely  my  personal  opinions,  and  must  remember  that  morpholo- 
gists  are  not  yet  agreed  as  to  the  nature  of  the  primitive  streak. 

The  Primitive  Axis. — As  above  defined,  the  primitive  axis 
is  the  median  band  of  cells  resulting  from  concrescence  and  overly- 
ing the  definitive  archenteron. 

It  is  advisable  to  begin  with  the  consideration  of  the  arrangement 
as  we  find  it  in  eggs  of  marsipobranchs,  ganoids,  and  amphibians, 
since  these  eggs  are  probably  more  primitive  in  their  mode  of  de- 
velopment than  those  of  other  vertebrates.  The  points  of  most  im- 
portance in  my  judgment  are  illustrated  in  Fig.  77,  A  and  B.  In 
A  we  have  a  section  through  the  middle  portion  of  a  young  primi- 
tive axis  of  an  axolotl,  the  axis  still  requiring  considerable  additions 
at  its  hinder  end  before  attaining  its  full  length ;  the  archenteric 
cavity,  Ae,  is  a  large  space  bounded  above  by  an  epithelium,  En, 
and  below  by  the  large  mass  of  yolk  cells,  Yk;  the  two-layered 
ectoderm,  EC,  everywhere  bounds  the  section ;  above  the  arcbenteron 
and  below  the  ectoderm  lies  the  accumulation  of  cells  constituting 
the  primitive  axis,  Pr;  the  lateral  prolongations,  Mes,  of  the  axis 
represent  the  commencing  mesodermic  outgrowths;  whether  the 
mesoderm  grows  out  from  the  primitive  axis  and  subsequently  ex- 
pands solely  by  its  own  proliferation,  or  whether  it  receives  at  its 
periphery  accretions  from  the  yolk  cells  is  uncertain.  I  am  inclined 
to  think  that  the  mesoderm  does  not  receive  additions  from  the  yolk. 
In  B  we  have  a  similar  section,  but  of  an  older  stage,  and  through 
the  hind  end  of  the  nearly  full-grown  axis ;  the  general  arrangement 
is  the  same  as  in  A ;  we  note  the  following  differences :  the  archen- 
teric cavity  is  a  mere  slit,  Ae;  the  primitive  axis,  Pr,  is  very  thick 

*  Prenant  ("Embryologie  ")  regards  them  as  ectodermal,  following  O.  Hertwig's  suggestion; 
the  terminology  in  this  case  is  largely  a  question  of  previous  definition. 


130 


THE    GERM-LAYERS. 


and  composed  of  numerous  small  cells,  and  its  lateral  mesodermic 
expansion,  Mes,  extends  farther  around  the  ovum.  In  both  sections 
we  see  that  the  cells  of  the  primitive  axis  are  not  marked  off  from 


En 


Mes 


B 


FIG.  77.  — Sections  of  Axolotl  eggs ;  A,  frontal  section  somewhat  anterior  to  the  blastopore,  from 
an  egg  in  which  the  archenteron  was  partly  formed,  but  the  anus  of  Rusconi  not  delimited.  B, 
frontal  section  of  an  older  ovum,  with  well-marked  but  large  anus  of  Rusconi ;  the  section  passes 
just  in  front  of  the  blastopore.  EC,  ectoderm ;  En,  entoderm ;  Mes,  mesoderrn ;  Ae,  archenteric 
cavity;  Ffc,  yolk ;  pr,  primitive  axis.  After  Bellonci. 

those  of  the  adjoining  entoderm.  In  a  longitudinal  section,  as  is 
illustrated  by  that  of  a  sturgeon,  Fig.  70,  the  mesoderm  of  the  primi- 
tive axis  is  seen  to  extend  far  forward  from  the  blastopore,  Bl.  The 
disposition  of  the  parts  and  the  appearance  of  the  cells  vary  in  the 
three  groups  we  are  considering,  but  for  our  purpose  it  is  unnecessary 
to  describe  these  secondary  differences.  The  points  essential  to  note 
are  that  the  primitive  axis  produces  chiefly  mesoderm,  which  is 
accumulated  along  the  axial  line,  and  is  thickest  around  the  blast- 
opore, where  it  joins  the  primitive  streak,  and  which  spreads  lat- 
erally between  the  ectoderm  and  entoderm ;  in  the  axial  region  the 
mesoderm  is  not  separated  from  the  entoderm. 

In  elasmobranchs  the  differentiations  of  the  axial  tissues  begins  in 
the  embryonic  rim  before  concrescence  takes  place,  so  that  while  the 
type  affords  peculiarly  conclusive  evidence  of  concrescence,  it  is  less 
convenient  for  the  study  of  the  primitive  axis,  since  the  hind  end  of 
the  primitive  axis  is,  'as  it  were,  divided,  being  continued  as  the 
embryonic  rim,  right  and  left.  The  degree  of  differentiation  in  the 
rim  varies  extremely :  in  Pristiurus  the  mesoderm  grows  out ;  in 
Scyllium  the  mesoderm  grows  out  and  the  differentiation  of  the 
notochord  begins;  in  Torpedo  (Ruckert,  87.1,  101)  the  myotomes 
appear  in  the  embryonic  rim  before  concrescence,  as  in  Elacate 
among  teleosts.  The  relations  are  further  complicated  by  the  ad- 
vance in  development  of  the  axial  structures  while  concrescence  is 
going  on,  so  that,  as  for  instance  in  Pristiurus,  Rabl,  89.2,  116-129, 
the  axial  notochord  may  be  differentiated,  while  the  mesoderm  is 
still  developing  in  the  embryonic  rim.  The  precocious  changes  in 
the  embryonic  rim  demand  especial  attention  when  the  origin  of  the 
mesoderm  is  discussed  (of.  Chapter  VI.).  The  ectoderm,  as  soon  as  it 
becomes  one-layered  (secondary  blastoderm,  see  Chapter  IV.),  con- 


THE    PRIMITIVE    AXIS    AND    STREAK. 


131 


s  of  high-cylinder  cells.  As  development  progresses  the  ectoderm 
thins  out  except  on  either  side  of  the  axial  line.  The  mesoderm 
arises  from  the  entoderm  close  to  the  ectental  line  and  is  there  quite 
thick,  but  as  it  stretches  away  it  thins  out.  Now  if  it  be  remembered 
that  the  ectental  line  becomes  the  axial  line,  when  concrescence  oc- 
curs, it  is  evident  that  this  mesodermic  thickening  of  the  entoderm 
is  in  reality  an  axial  thickening,  and  when  concrescence  takas  place 
it  fuses  with  the  corresponding  thickening  of  the  opposite  side  and 
constitutes  an  actual  axial  thickening  or  true  primitive  streak ;  but 
in  elasmobranchs,  as  soon  as  the  anterior  axial  structures  have  con- 
cresced,  we  find  by  precocious  development  that  the  notochord  and 
medullary  groove  appear;  now,  as  shown  in  Chapter  VII.,  the  ap- 
pearance of  these  structures  causes  the  division  of  the  axial  mesoderm 
into  completely  separated  right  and  left  portions.  It  is  only  by  keep- 
ing the  process  of  concrescence  and  the  precocious  development  of 
the  parts  constantly  in  mind  that  we  can  understand  the  development 
in  elasmobranchs  or  compare  it  rightly  with  that  of  other  types. 
From  what  has  been  said  it  is  clear  that  a  section  of  the  blastoder- 
mic  rim  from  which  the  mesoderm  was  just  growing  out  would  cor- 
respond to  half  a  section  of,  say,  a  bird's  ovum,  though  the  primitive 
axis,  and  upon  comparison  it  will  be  found  that  all  the  essential  re- 
lations are  identical. 

The  primitive  axis  and  streak  of  birds  have  been  much  in- 
vestigated and  discussed,  and  may  be  conveniently  treated  together. 
I  follow  in  the  main  Duval,  78.1,  84.1,  many  of  whose  statements 
are  confirmed  by  Zumstein, 
87.1.  Other  important  au- 
thorities to  be  consulted  are  Kol- 
liker  in  both  his  text- books ;  His, 
68.1,  77.2,  82.1,  etc.;  Roller, 
82.1;  Disse,  78.1,  79.1;  Wal- 
deyer,  69.1,  83.1;  M.  Braun, 
82.3;  Gasser,  77.1,  79.1; 
Rauber,  76.2;  C.  Rabl,  89.2, 
et  al 

The  following  description  ap- 
plies to  the  hen's  egg.  When 
the  egg  is  laid  the  centre  of  the 
segmented  blastodisc  presents  a 
circular  area  of  lighter  color; 
during  the  first  few  hours  of  in- 
cubation this  area  pellucida, 
as  it  is  called,  becomes  more  dis- 
tinct; as  the  area  pellucida 
expands,  the  primitive  streak 
appears  in  it  eccentrically  be- 
tween the  eighth  and  twelfth 
hour.  By  the  sixteenth  hour 
the  primitive  streak  has  its 
full  length.  The  rate  of  development  is  extremely  variable,  au- 
tumn eggs  developing  more  slowly  than  spring  eggs ;  the  eggs  vary- 
also  individually,  and  are,  moreover,  much  influenced  by  the  tern- 


0.0. 


FIG.  78.— Area  pellucida  of  a  Hen's  egg,  with 
completed  primitive  furrow.  After  Duval.  a.o. , 
areaopaca;  c£,  anterior  crescent;  a.p. ,  area  pellu- 
Cida;j^,  primitive  groove.  X  20  diams. 


132 


THE    GERM-LAYERS. 


perature  of  their  incubation.  For  a  fuller  discussion  of  these  varia- 
tions see  His,  68.1,  56-63.  Seen  from  the  surface  the  area  pel- 
lucida  with  completed  streak  presents  the  following  features,  Fig. 
78.  The  area  pellucida,  a.  p.,  is  considerably  elongated  and  some- 
what pear-shaped,  being  widest  at  the  anterior  end  of  the  primitive 
groove,  pt . ;  this  groove  is  well  marked  as  a  narrow  and  shallow 
furrow,  which  begins  some  distance  from  the  anterior  edge  of  the 
area  and  ends  just  before  reaching  the  posterior  edge  of  the  area ; 
the  front  end  of  the  furrow  usually  bends  slightly  to  the  left,  but  not 
invariably,  as  Koller  and  Rabl  have  maintained,  for  it  sometimes 
bends  to  the  right  or  is  quite  straight ;  a  line  of  granules  is  sometimes 
noticeable  above  the  primitive  groove;  they  were  seen  by  Dursy,  Z.c., 
and  are  called  by  Duval,  78.1,  15,  the  filament  epiaxial — compare 
Gasser,  79.1.  The  portion  of  the  area  pellucida  immediately  around 
the  primitive  groove  appears  slightly  darker  than  the  rest.  The 
anterior  portion  of  the  pellucida  is  further  distinguished  by  the 
anterior  crescent,  c£,  the  "vordere  Aussenfalte"  of  His,  68.1,  and 
other  German  writers.  The  anterior  crescent  is  a  temporary  ap- 
pearance, due,  according  to  Duval,  to  a  series  of  folds  of  the  entoderm, 
which  form  a  curving  row  of  shallow  pockets,  that,  shining  through, 
mark  out  the  crescent.  The  crescent  disappears  a  little  later,  and 
there  arises  nearly,  if  not  quite,  in  its  place  a  different  fold,  the 
amniotic.  The  similarity  of  position  has  led  to  the  anterior  cres- 
cent's being  identified  by  some  authors  with  the  true  amniotic 
fold. 

Longitudinal  and  transverse  sections  are  very  instructive.     We 


FIG.  79.  —Longitudinal  section  of  the  region  of  the  primitive  streak  of  a  Hen's  ovum  incubated 
six  hours.  After  Duval.  D,  general  view  magnified  about  40  diameters.  A,  B,  C,  details  of  D, 
with  higher  magnification.  EC,  ectoderm;  mes,  mesoderm;  En,  entoderm;  6Z,  Duval's  "blasto- 
pore ; "  kw,  germinal  wall  (Keimwall) ;  Ach,  archenteric  cavity ;  sg.  c,  segmentation  cavity. 

begin  with  the  examination  of  a  longitudinal  section  of  a  somewhat 
younger  stage.  Later  the  ectoderm  closes  behind  the  primitive 
streak,  as  already  stated  and  spreads  backward  over  the  yolk.  The 
section  shows  that  the  yolk  is  not  divided  into  cells,  although  nuclei 
are  scattered  through  it ;  the  nuclei  are  represented  as  black  dots  in 


THE    PRIMITIVE    AXIS    AND    STREAK. 


133 


A,  B,  and  C.  The  cavity  of  the  archenteron,  Ach*  is  enlarged  by 
the  formation  of  a  deep  pit  in  the  yolk,  while  the  posterior  half  of 
the  cavity  remains  a  narrow  fissure  between  the  cellular  entoderm, 
En,  and  the  yolk;  the  archenteron  communicates,  according  to 
Duval,  with  the  exterior  by  an  openin'g,  bl,  which  he  calls  the  blas- 
topore ;  as  this  supposed  opening  is  apparently  at  the  posterior  ex- 
tremity of  the  primitive  streak  it  cannot  be  the  true  blastopore.  The 
entoderm  is  a  loosely  put  together  stratum  of  cells,  which  passes  over 
anteriorly  into  a  ridge  of  the  yolk  in  which  cells  are  being  produced 
around  the  already  accumulated  nuclei ;  this  ridge,  kiv,  is  the  ger- 
minal wall.  Posteriorly  the  cell  layers  are  much  thicker,  A;  the 
ectoderm,  EC,  is  clearly  differentiated  from  the  underlying  cells, 
which  are  all  more  or  less  alike;  though  they  represent  both  the 
entoderm  and  mesoderm.  From  this  connection  and  from  the  fact 
that  the  connection  between  the  ectoderm  and  mesoderm  which  is  so 
well  known  to  exist  after  the  primitive  streak  has  attained  its  full 
length,  Duval  concludes  that  the  mesoderm  arises  primitively  from 
the  entoderm.  Transverse  sections  afford  additional  information. 


k34:;;; 


FIG.  80.  — Transverse  sections  of  a  germinative  area,  with  half-formed  primitive  streak,  of  a 
Hen's  egg.  After  Duval.  A,  through  the  anterior  region  of  the  area  pellucida.  B,  through 
the  primitive  streak.  C,  part  of  A  enlarged.  EC,  ectoderm;  mes,  mesoderm;  Ent,  entoderm; 
bl,  blastopore;  for,  germinal  wall  (Keimwall);  Ach,  archenteric  cavity;  sg.  c,  segmentation 
cavity. 

The  accompanying  Fig.  80  represents  cross-sections  of  a  germinal 
area,  the  primitive  streak  of  which  had  attained  about  one-half  its 
full  length.  The  first  section,  Fig.  80,  A,  passes  through  the 
anterior  region  of  the  area  pellucida,  and  therefore  in  front  of  the 
primitive  groove ;  it  shows  the  large  cavern,  Ach,  of  the  archenteron 
(or  yolk  cavity?)  hollowed  out  in  the  yolk;  the  entoderm,  C,  Ent, 
above  the  cavity  is  a  thin  layer  of  cells,  connected  laterally  with  a 
projecting  shelf  of  yolk  kw  (the  bourrelet  entodermo-vitellin  of 
Duval) ,  which  is  rich  in  nuclei ;  it  subsequently  expands  and  acquires 
a  more  cellular  character ;  this  shelf  is  the  commencement,  therefore, 
of  the  Keimwall  of  German  writers.  Immediately  above  the  ento- 
derm, and  intimately  connected  with  it,  are  a  few  cells,  which  be- 
long to  the  mesoderm,  C,  mes;  the  ectoderm  is  quite  thick,  C,  EC, 

*  As  previously  stated,  Duval  was  unacquainted  with  the  existence  of  yolk  cavity;  it  is  prob- 
able that  the  cavity  here  termed  archenteric  is  really  the  yolk  cavity. 


..... 
^llltofe, 

EC 


134  THE    GERM-LAYERS. 

and  consists  of  high  columnar  cells ;  toward  its  periphery  the  ectoderm 
thins  out,  and  its  edge  rests  upon  the  yolk,  with  which  it  has  no  con- 
nection. In  the  region  of  the  primitive  streak,  Fig.  80,  B,  there  are 
important  differences  in  the  germ  layers  to  note.  The  entodermic 
cavity,  Ach,  is  very  much  smaller ;  the  mesoderm  is  much  thicker 
and  in  the  axial  region  fuses  with  both  the  outer  layer  of  cells  *  and 
the  entoderm,  thus  forming  the  Achsenstrang  (axial  cord)  of  Ger- 
man writers ;  the  mesoderm  also  spreads  out  over  the  yolk  far  beyond 
the  archenteric  cavity,  and  about  one- third  of  the  way  from  the  axial 
line  to  the  distal  edge  of  the  ectoderm :  the  ectoderm  merges  in  the 
median  line  with  the  mesoderm,  and  presents  externally  a  small 
notch,  B,  pr,  corresponding  to  the  primitive  groove. 

Whether  at  the  stage  from  which  Fig.  80  is  taken  the  formation 
of  the  primitive  axis  (head-process)  has  fairly  begun  is  uncertain. 
In  slightly  older  stages  the  "  head -process"  is  present  (Kolliker, 
"Grundriss,"  2te  Aufl.,  36).  During  these  changes  the  archenteron 
(yolk  cavity?)  expands  rapidly,  the  entoderm  becomes  very  thin  in  the 
area  pellucida,  and  passes  more  and  more  abruptly,  as  development 
progresses,  into  the  so-called  germinal  wall  of  the  area  opaca ;  finally 
the  ectoderm  becomes  thinner  peripherally,  so  that  the  axial  thicker 
part  is  gradually  marked  off  more  and  more  abruptly.  Sections  of  a 
stage  with  the  primitive  groove  at  its  maximum — a  stage  which  is 
usually  found  toward  the  end  of  the  first  day  of  incubation — show 
these  changes  clearly.  A  cross-section  through  the  area  opaca  in 
front  of  the  area  pellucida  shows  the  thin  ectoderm,  the  thick  cellular 
entoderm  overlying  the  archenteric  cavity  and  charged  with  yolk 
granules ;  the  entodermic  nuclei  are  very  variable  in  form  and  ir- 
regular in  distribution ;  the  cell  boundaries  are  indistinct.  There  is 
no  mesoderm.  A  cross-section  near  the  front  of  the  area  pellucida 
likewise  shows  only  ectoderm  and  entoderm ;  the  former  is  a  high 
cylinder  epithelium  over  the  area  pellucida  and  thins  out  toward  the 
opaca  on  each  side ;  the  latter  is  a  thin  layer  over  the  area  pellucida 
and  passes  quickly  but  not  abruptly  into  the  very  thick  yolk-bearing 
entoderm  (or  Keimwall)  of  the  area  opaca.  Sections  a  short  dis- 
tance in  front  of  the  primitive  groove  show  that  the  head-process 
(Kopffortsatz)  is  a  forward  prolongation  of  the  primitive  streak,  and 
consists  of  an  axial  accumulation  of  mesodermic  cells  fused  with 
the  entoderm,  and  having  broad  extensions  sideways  to  form  the 
mesoderm  between  the  outer  and  inner  germ-layers;  the  lateral 
portions  of  the  mesoderm  have  no  connection  with  the  other  germ 
layers,  and  at  its  distal  edge  the  mesoderm  thins  out  and  rests  upon 
the  entoderm  of  the  opaca,  but  without  being  connected  with  it ;  I 
cannot  find  any  satisfactory  evidence  that  it  receives  any  additions 
from  the  opaca  entoderm,  as  many  authors  have  maintained.  The 
ectoderm  in  the  region  of  the  "  Kopffortsatz"  resembles  that  further 
forward,  but  it  very  soon  shows  a  faint  median  furrow,  the  so-called 
dorsal  groove  (Riickenrinne) ,  which  is  the  commencement  of  the 
medullary  groove  (see  Chapter  VII.) .  In  the  anterior  half  of  the 
primitive  streak  the  relations  are  different  from  those  in  the  "  head- 
process."  The  outer  layer  shows  the  primitive  groove,  Fig.  81,  prg, 

*  This  outer  layer  is  usually  termed  ectoderm,  but  I  hold  that  it  is  not  ectoderm,  but  the  homo- 
logue  of  the  outer  layer  of  yolk  cells  in  the  amphibian  anus  of  Rusconi. 


THE   MAMMALIAN   BLASTODERMIC    VESICLE.  135 

and  is  fused  with  the  axial  cord  (Achsenstrang)  of  the  mesoderm ; 
laterally  the  outer  layer  passes  into  the  true  ectoderm,  EC.  In  the 
posterior  region  of  the  primitive  groove  the  connection  of  the  meso- 
derm with  the  inner  germ-layer  is  dissolved.  Behind  the  primitive 
groove  the  mesoderm  extends,  but  lies  free  between  the  ectoderm  and 


FTG.  81.— Transverse  section  of  the  anterior  region  of  a  fully-developed  primitive  streak  of  a 
Hen's  ovum:  EC.  ectoderm :  //»<*.  mesoderm;  Ent,  eatoderm;Pr.gr,  primitive  groove.  The 
large  black  dots  represent  yolk  grains. 

entoderm.  To  recapitulate :  there  is  a  long  axial  mesodermic  thick- 
ening, which  has  the  primitive  groove  over  its  posterior  two-thirds ; 
the  thickening  in  front  of  the  groove  is  united  with  the  entoderm, 
and  constitutes  the  primitive  axis ;  tlie  thickening  under  the  front 
half  of  the  groove  is  united  with  the  entoderm ;  in  the  median  line  its 
external  surface  is  freely  exposed,  and  laterally  it  merges  into  the 
ectoderm ;  the  thickening  under  the  hind  half  of  the  groove  is  not 
united  with  the  entoderm. 

III.  THE  MAMMALIAN  BLASTODERMIC  VESICLE. 

In  all  placental  mammalia,  owing  presumably  to  the  absence  of  the 
large  amount  of  yolk  present  in  the  ova  of  other  amniota,  the  early 
development  is  modified,  and  the  germinal  area  instead  of  resting  on 
a  mass  of  yolk  rests  upon  a  vesicle.  When  the  vesicle  is  fully  de- 
veloped its  main  cavity  is  lined  by  entodermal  cells,  and  must  be,  in 
my  opinion,  homologized  with  the  yolk  cavity  of  other  vertebrates, 
for  it  fuses  with  the  notochordal  canal  to  develop  the  definitive 
archenteron. 

We  may  conveniently  distinguish  four  stages  of  the  vesicle, 
which  are  described  below  in  order :  1 ,  with  one  layer  constituting 
the  vesicle,  except  over  the  germinative  area;  2,  with  two  layers; 
3,  with  primitive  streak;  4,  with  "head-process,"  or  primitive  axis. 

1.  Vesicles  with  One  Complete  Layer. — After  the  close  of 
segmentation  we  find  that  the  inner  mass  becomes  flattened  out,  and 
in  the  reigon  it  occupies  we  can  distinguish  three  layers  of  cells,  as 
previously  described :  first,  counting  from  the  outside,  the  thin  layer 
of  cells  known  as  Rauber's  "  Deckschicht ;"  second,  a  middle  layer  of 
cylindrical  cells,  which  becomes  the  ectoderm;  third,  an  inmost 
layer  of  thin  flattened  cells,  which  belong  to  the  entoderm;  the 
Deckschicht  continues  round  the  whole  vesicle  as  a  single  layer; 
the  other  layers  do  not  so  continue,  compare  Figs.  57  and  58.  The 
next  step  in  development  is  the  formation  of  a  second  layer,  which 
spreads  out  in  all  directions  from  the  region  of  the  inner  mass ;  hence 
as  far  as  the  new  layer  reaches  the  blastodermic  vesicle  becomes  two- 


136 


THE   GERM-LAYERS. 


layered.  Meanwhile  the  Deckschicht  disappears,  leaving  two  layers 
in  the  region  of  the  inner  mass ;  it  is  to  be  remarked  that  the  Deck- 
schicht is  retained  in  certain  rodents,  undergoing  special  modification, 
as  described  in  the  section  on  inversion  of  the  germ-layers. 

Rabbit's  Vesicle  at  Six  Days. — The  following  is  a  summary  of 
Ed.  van  Beneden's  description,  80.1,  185-200.  The  vesicle  meas- 
ured 3.2  mm.  in  diameter;  it  was  nearly  spherical;  the  wall  of  one 
hemisphere  consisted  of  one  layer  of  cells;  the  other  hemisphere 
had  two  layers  of  cells,  and  besides  in  its  central  portion  a  third 
layer  intervening  between  the  other  two.  The  area  with  three  layers 
Van  Beneden  designates  as  the  tache  embryonnaire;  it  showed  no 
trace  of  the  primitive  streak ;  it  was  oval  in  outline  and  had  one 
point,  which  the  author  identifies  as  Hensen's  knot,  where  the  layers 
adhere  together  closely.  Transverse  sections  show  that  the  outer- 
most layer  of  cells  is  a  low  cylinder  epithelium,  which,  at  the  edge 
of  the  area,  passes  into  a  thin  epithelium,  quite  abruptly ;  it  cor- 
responds to  Rauber's  Deckschicht,  and  has  been  said  by  him  to 
flatten  out  and  disappear,  leaving  the  cells  underneath  as  the  per- 
manent outer  layer  of  the  embryonic  vesicle.  The  cells  of  the  inner- 
most layer  are  thin  and  wide ;  they  are  called  the  hypoblast  (ento- 
derm)  by  Van  Beneden ;  the  cells  themselves  have  round  nuclei, 
around  each  of  which  is  accumulated  a  court  of  granular  proto- 
plasm ;  the  adjacent  courts  are  connected  by  a  coarse  mesh  work  of 
protoplasmatic  threads ;  treatment  with  nitrate  of  silver  brings  out 
the  cell  boundaries  and  divides  the  reticulum  into  polygonal  areas. 
The  cells  of  the  present  outermost  layer  have  distinct  boundaries  and 
contain  granules  and  long  bacilliform  bodies,  which  Van  Beneden 
saw  also  in  the  fresh  specimens  and  found  to  be  constant  appearances. 
Similar  bodies  are  found  in  the  germinal  vesicles  of  sheep,  and  are 
held  by  Bonnet,  84. 1,  to  be  derived  from  the  uterine  milk;  the  rab- 
bit is  not  known  to  have  uterine 
milk.  The  histological  peculiari- 
ties of  these  two  layers  remain 
about  the  same  from  the  fifth  to 
the  eighth  day.  The  middle  layer 
consists  of  rounded  cells  with  nu- 
merous granules ;  seen  from  the 
surface  their  diameter  is  greater 
than  that  of  the  cells  outside  them, 
but  much  less  than  that  of  the 
cells  underlying  them.  While 
we  know  that  the  middle  layers 
are  ectodermal,  it  is  uncertain 
whether  the  inner  layer  is  ento- 
dermal  or  not. 

Blastocyst  of  the  Rabbit  of 
Seven  Days. — The  development 
is  exceedingly  variable,  so  that 
exact  times  cannot  be  given.  The 
general  appearance  is  illustrated  by  Fig.  82,  from  Kolliker.  The 
vesicle  figured  was  4.4  mm.  in  length;  the  envelopes  of  the  ovum  are 
not  shown,  though  they  were  still  present ;  at  the  upper  pole  is  the 


FIG.  82.  — Blastodermic  vesicle  of  a  Rabbit  of 
seven  days :  agr,  area  germinativa,  or  embryonic 
shield ;  ye,  line,  above  which  the  vesicle  is  two- 
layered.  From  Kolliker. 


THE   MAMMALIAN  BLASTODERMIC   VESICLE.  137 

small  embryonic  shield,  corresponding  in  position  to  the  region  of  the 
inner  massj  it  is  marked  out  by  the  greater  thickness  of  the  walls  of 
the  vesicle ;  the  developing  second  layer  extends  over  more  than  half 
the  vesicle,  reaching  to  the  line  ge. 

2.  Blastodermic  Vesicle  with  Two  Layers.— Of  this  stage 
we  have  several  descriptions ;  for  the  rabbit  by  Kolliker  ("  Grundriss," 
p.  91);  Hensen,  76.1;  C.  Rabl,  89.2,  141;  as  well  as  the  older 
accounts  by  Bischoff,  42.1,  and  Coste,  47.1,  and  the  brief  mention 
by  Heape  in  Foster  and  Balfour's  "  Embryology,"  2d  edition,  316- 
320;  for  the  mole  by  Heape,  83.1;  for  the  dog  by  Bischoff,  45.1; 
for  the  cat  by  Schafer,  76.1;  for  the  sheep  by  Bonnet,  84.1;  and 
for  several  rodents,  as  indicated  in  the  section  on  inversion  of  the  germ- 
layers,  p.  141. 

The  two-layered  stage  is  found  in  the  rabbit  about  seven,  in  the 
sheep  about  thirteen,  days  after  coitus.  The  dimensions  for  the 
sheep  are  about  4  mm.  for  the  greatest  diameter  and  2.3  mm.  for  the 
lesser  diameter. 

The  two  layers  form  each  a  closed  sack ;  the  embryonic  shield  is 
well  marked  as  a  round  spot,  less  translucent  than  the  walls  else- 
where. The  outer  layer  has  a  distinctly  epithelial  character ;  in  the 
region  of  the  shield  its  cells  are 'columnar  with  spherical  nuclei;  in 
the  rabbit  the  cells  are  low  and  the  nuclei  lie  nearly  at  one  level ;  in 
the  sheep  the  cells  are  taller  and  the  nuclei  are  at  various  levels;  in 
the  mole  (for  a  good  figure  see  Heape,  83. 1,  PL  XXI.,  Fig.  49),  and 
in  various  rodents  there  are  several  layers  apparently,  but  perhaps 
in  them  also  the  epithelium  is  columnar,  as  it  certainly  is  later.  At 
the  edge  of  the  shield  there  is  an  abrupt  change  to  a  very  thin  layer, 
with  widely  expanded  cells ;  consequently,  in  the  region  of  the  shield 
the  nuclei  are  close  set,  while  outside  the  shield  they  are  wider  apart. 
The  change  at  the  edge  of  the  shield  is  at  first  less  abrupt,  but  at  the 
present  stage  is  very  marked.  A  similar  difference  exists  in  the  inner 
layer,  although  its  cells  are  very  much  thinned  out  everywhere,  yet 
the  layer  is  slightly  thicker  in  the  region  of  the  shield ;  the  nuclei  of 
the  inner  layer  are 
somewhat  flattened, 
and  they  are  larger 
and  farther  apart  than 
the  nuclei  of  the  outer 
layer  —  a  difference 
which  is  very  obvious 
in  surface  views,  both 

during     this    and     the       FIG.  83.  -Transverse  section  of  the  embryonic  shield  of  the 
•riQvf  •PrkllrvmriTur    c-forr^a      blastodermic  vesicle  of  a  Sheep,  thirteen  days  preenant.    After 
text  lOilOWing   Stages.    Bormet.     Oi  outer  layer  of  vesicle;  6,  inner  layer  of  vesicle. 

The  inner  layer  has  an 

epithelial  character  in  the  region  of  the  shield,  but  farther  away  the 
cells  move  apart,  and  being  connected  by  processes  resemble  embry- 
onic connective  tissue  (Bonnet,  84.1,  192;  Hensen,  76.1,  Figs.  15 
and  11,  B  on  Taf.  VII. ;  E.  van  Beneden,  80.1).  The  relations  are 
illustrated  by  the  accompanying  Fig.  83,  representing  the  shield  in 
the  sheep  at  thirteen  days  and  of  a  vesicle  measuring  4  mm.  by  2 
mm. ;  at  the  left  of  the  figure  the  layers  are  accidentally  folded. 
The  next  changes  which  occur  are  principally  those  of  growth 


138 


THE    GERM-LAYERS. 


til 


both  of  the  vesicle  as  a  whole  and  of  the  embryonic  shield,  which  also 
begins  to  arch  up ;  the  vesicle  and  shield  both  become  oval ;  usually 
the  oval  shield  lies  lengthwise,  but  in  the  deer,  as  shown  by  Bischoff, 
it  lies  transversely  of  the  vesicle.  The  size  of  the  shield  is  quite 
nearly  uniform  among  the  placental  mammals  in  which  it  has  been 
studied,  but  the  size  of  the  vesicle  varies  extremely ;  especially  note- 
worthy is  the  excessively  rapid  elongation  in  ungulates  (pig,  sheep, 
goat,  and  deer) ;  in  the  sheep,  for  example,  it  trebles  or  sextuples  its 
length  in  less  than  a  single  day  after  the  shield  appears.  The  next 
step  is  the  appearance  of  a  middle  layer,  at  least  in  sheep  (Bonnet, 
84.1,  192-196,  89.1,  42),  which  shows  in  the  fresh  specimen  as  a 
slight  turbidity,  Fig.  84,  mes,  of  the  vesicular  walls  just  outside  the 
edge  of  the  shield,  while  in  the  region  of  the  shield  there  is  no  middle 
layer  whatever.  Sections  show  that  the  new  layer  consists  of  loosely 
scattered  cells  connected  by  anastomosing  processes ;  it  is  everywhere 

absolutely  distinct  from  the  outer 
layer,  but  merges  at  many  points 
with  the  inner  layer ;  from  this  con- 
nection Bonnet  concludes  that  the 
middle  layer  is  derived  from  the 
inner  layer  by  what  must  be  called 
a  process  of  delamination.  So  far 
as  known  to  me  nothing  analogous 
to  this  middle  layer  has  yet  been 
observed  in  other  mammals.  The 
next  important  step,  again  accord- 
ing to  Bonnet,  81.1,  195,  is  the  ap- 
pearance of  Hensen's  knot,  which 
takes  place  while  the  peripheral 
middle  layer  is  developing.  The 
knot,  Fig.  84,  kn,  is  at  first  a  small 
thickening  on  the  under-side  of  the 
outer  layer;  it  is  situated  on  the 
middle  line  of  the  shield  a  little 
nearer  one  end  than  the  other ;  it  is 
distinctly  separated  from  the  inner 
layer,  but  is  connected  with  the 
cells  of  the  middle  layer,  which  have 
now  developed  themselves  in  the 
middle  region  of  the  shield  also. 
Bonnet  mantains  that  the  knot  gives  off  cells  which  contribute  to  the 
formation  of  the  middle  layer.  The  knot  marks  the  front  end  of  the 
future  primitive  streak,  and  is  the  beginning  of  the  primitive  axis. 

The  appearances  in  a  sheep's  ovum  at  this  stage  are  illustrated  by 
Fig.  84  of  a  vesicle  of  twelve  to  thirteen  days  from  a  sheep ;  the  vesi- 
cle measured  55  mm.  in  length  by  about  1.5  in  breadth,  but  the 
length  of  the  vesicle  is  extremely  variable  at  this  stage :  the  specimen 
had  been  stained  to  bring  out  the  small,  close-set  nuclei  of  the  outer 
layer  and  the  larger,  more  widely  set  nuclei  of  the  inner  layer.  The 
upward  arching  embryonic  shield,  Sh,  shows  Hensen's  knot,  kn; 
around  the  edge  of  the  shield,  Sh,  the  middle  layer  makes  an  irregu- 
lar shadow,  mes. 


tifres 


FIG.  84.—  Central  portion  of  a  Sheep 'sblas- 
todermic  vesicle  or  twelve  to  thirteen  days. 
Sh,  shield;  fcn,  Hensen's  knot;  pr,  trace  of 
primitive  streak ;  mes,  "Mesoblasthof."  Af- 
ter Bonnet.  X  34  diams. 


THE    MAMMALIAN    BLASTODERMIC    VESICLE. 


139 


A  condition  of  the  blastoderm ic  vesicle  similar  to  that  described  is 
figured  by  Coste  for  the  rabbit,  47. 1 ,  by  Bischoff  for  the  rabbit,  42. 1 , 
Taf.  IX.,  fig.  42,  c,  for  the  dog,  45.1,  Taf.  III.,  fig.  28,  B;  and  the 
gradual  extension  of  the  second  layer  is  recorded  for  the  mole  by 
Heape,  83. 1.  Since  it  is  known  to  occur  in  rodents,  carnivora,  and 
insectivora,  it  is  probably  true  of  all  placental  mammals  that  the  one- 
layered  vesicle  becomes  two-layered  by  the  outgrowth  of  cells  from 
ihe  "inner  mass"  found  at  the  close  of  segmentation;  this  is  the 
first  step  of  development  after  segmentation. 

KAUBER'S  DECKSCHICHT  has  evidently  great  importance.  It  was 
first  described  by  him  in  the  rabbit,  75.2;  and  was  also  discovered 
by  E.  van  Beneden,  76. 1,  who,  however,  made  the  error  of  consid- 
ering it  as  the  permanent  ectoderm,  and  the  true  ectoderm  below  it 
as  the  mesoderm ;  this  error  has  been  amply  corrected  by  Kolliker 
and  is  now  admitted  by  Van  Beneden  (see  Van  Beneden  and  Julin, 
84.1).  Its  disappearance  in  the  rabbit  has  also  been  studied  by 
Lieberkuhn,  79.1.  Balfour  ("Comp.  Embryol.,"  II.,  219)  from  in- 
vestigations on  the  rabbit  by  himself  and  Heape,  concluded  that 
the  cells  of  the  Desckchicht  disappear  by  being  incorporated  in  the 
true  ectodermal  layer  becoming  at  the  same  time  columnar;  this 
view  is  verified  by  Lieberkuhn,  82.1,  400,  401.  As  already  stated 
the  rodent  modification  of  the  Deckschicht  is  discussed  below,  p.  141. 
In  the  rabbit  the  Desckchicht  disappears  before  the  second  layer  of 
cells  grows  completely  round  the  vesicle. 

3.  Blastodermic  Vesicles  with.  Primitive   Streak. — The 
knot  of  Hensen  marks  the  front  end  of  the  primitive  streak,  which 
lengthens  backward;  during  the  same  period 
the  vesicle  as  a  whole  enlarges ;  in  ruminants 
the  enlargement  is  enormous  and  very  rapid.* 
The  primitive  streak  always  lies  in  the  long 
axis  of  the  shield.     The  formation  of  the  prim- 
itive axis  begins  with  the  union  of  Hensen 's 
knot  with  the  inner  layer,  so  that  at  the  knot 
all  three  layers  are  actually  united — the  condi- 
tion   originally  discovered  by  Hensen,  76.1, 
268.     The  union  of  the  knot  with  the  inner 
layer  spreads  backward  until  it  reaches  the  edge 
of  the   shield,  thus   generating  the  primitive 
streak.     Next   follows   the  elongation    of    the 
streak  and  shield,  the  latter  becoming  pointed 
at  its  hinder  end.    We  thus  have  a  pear-shaped 
shield  with  the  primitive  streak  running  for- 
ward from  its  pointed  end ;  the  anterior  end  of 
the  primitive  streak  is  somewhat  enlarged  and  0fFa*Rabbit?  oTumCof  five 
the  posterior  end  is  considerably  thickened ;  the  starSktoans^ 
three   layers   are   united   along  the    primitive  of  middle  layer,  mf  m\  Af- 
streak.     Fig.  85  represents  the  embryonic  shield  ter  K611iker' 
of  a  rabbit  embryo;  the  shield  measured  1.34mm.  in  length  and 
0.85   mm.  in  width;    the  primitive  streak  is   a  broad   band,  corre- 
sponding to  the  axial  thickening,  and  extends  about  two-thirds  of 

*  Bonnet  states  that  in  the  sheep  the  blastodermic  vesicle  must  elongate  during  this  period  at 
the  rate  of  one  centimetre  an  hour. 


140  THE    GERM-LAYERS. 

the  length  of  the  shield;  the  mesoderm,  m'9  m" ,  occupies  a  circu- 
lar area  around  the  hind  end  of  the  streak ;  for  a  similar  stage  in 
the  opossum  see  Selenka,  86.1,  Taf.  XVIII.,  Fig.  6;  in  the  mole, 
Heape,  83.1,  PL  XXVIII.,  Fig.  12;  in  the  sheep,  Bonnet,  84.1, 
Taf.  X.,  Fig.  39,  40.  Cross-sections  show  the  union  of  the  three 
layers  in  the  axis ;  the  greater  width  of  the  streak  in  front  (to  this 
wide  anterior  end  of  the  streak  the  term  Hensen's  knot  continues 
to  be  applied) ;  and  show  also  the  increasing  thickness  of  the  streak 
posteriorly.  The  primitive  groove,  which  is  a  shallow  depression  of 
the  outer  layer,  appears  first  over  Hensen's  knot,  and  thence  extends 
gradually  backward  along  the  median  line  of  the  primitive  streak. 


FIG.  86.  — Section  of  the  primitive  streak  of  the  Mole :  p.  gr,  primitive  groove ;  EC,  ectoderm ; 
mes,  mesoderm ;  En,  entoderm ;  Pr,  primitive  streak.  (In  sections  nearer  the  end  of  the  groove, 
Pr  does  not  appear,  and  the  inner  layer  is  distinct,  though  not  separated  axially  from  the  mid- 
dle layer. )  After  Heape 

A  transverse  section  through  about  the  middle  of  the  streak  at  this 
stage  in  the  mole  is  represented  in  Fig.  86,  and  may  be  considered 
thoroughly  typical. 

4.  Blastodermic  Vesicles  with  Primitive  Streak  and 
Head -Process. — In  the  stage  we  are  now  considering  the  axial 
thickening  becomes  subdivided  into  two  parts,  an  anterior  known  as 
the  head-process  (Kopffortsatz) ,  and  the  true  primitive  streak.  The 
two  are  distinguished  by  the  fact  that  the  axial  thickening  in  the 
region  of  the  process  is  separated  from  the  outer  layer  but  fused 
with  the  inner  layer,  while  in  the  region  of  the  streak  it  includes,  as 
in  birds,  the  outer  layer.  Except  at  its  anterior  end,  the  axial  thick- 
ening is  not  connected  with  the  inner  layer.  Hence  cross-sections 
may  give  us  three  different  appearances  according  to  the  level  at 
which  they  are  taken. 

The  head-process  was  first  distinguished,  so  far  as  I  am  aware,  by 
Kolliker  ("  Entw.-Ges.,"  1879,p.  271),  also  83. 1.  Lieberkiihn,  82. 1, 
first  showed  that  in  it  appears  a  small  longitudinal  canal,  the  walls 
of  which  form  the  notochord.  Heape,  83.1,  discovered  that  the 
hinder  end  of  this  canal  opens  exteriorly  in  the  mole,  and  Bonnet, 
84.1,  made  the  same  observation  on  sheep.  Strahl  describes  the 
"process"  in  the  rabbit  incidentally  in  his  paper  on  the  cloaca,  86.2; 
additional  information  is  given  by  Bonnet,  89.1,  concerning  the 
sheep,  and  by  C.  Rabl,  89.2,  concerning  the  rabbit.  Especially 
valuable  is  Fr.  Carius'  dissertation,  88.1.  In  the  guinea-pig,  ac- 
cording to  Carius,  after  the  formation  of  the  primitive  streak  the 
middle  layer  grows  out  in  all  directions  and  lies  free  between  the 
inner  and  outer  layers.  In  front  of  the  primitive  streak  the  out- 
growth takes  place  in  three  divisions — one  median,  two  lateral.  The 
median  outgrowth  is  the  head-process  proper,  and  it  becomes  later 


THE    MAMMALIAN   BLASTODERMIC   VESICLE.  141 

united  with  the  inner  layer,  but  at  first  lies  entirely  free  (embryo  of 
thirteen  to  fourteen  days) .  The  first  indications  of  the  formation 
of  a  canal  is  an  alteration  of  form  in  the  cells,  which  elongate  in 
directions  at  right  angles  to  the  axis  of  the  head-process,  so  that  their 
oval  nuclei  are  radially  placed ;  the  change  begins  posteriorly  and 
progresses  forward ;  while  it  is  going  on  the  anterior  extremity  of  the 
head-process  fuses  with  the  inner  layer.  The  radial  cells  move  apart 
so  that  there  arises  a  longitudinal  canal ;  subsequently  the  canal  loses 
its  inferior  wall,  so  that  it  becomes  continuous  as  a  cavity  with  the 
cavity  of  the  vesicle  formed  by  the  inner  layer;  compare  ante,  p.  127. 
In  the  rabbit  the  head-process  is  also  free  at  first,  but  very  early 
unites  with  the  inner  layer,  in  which  condition  it  was  found  by 
Carius,  18-19,  at  seven  and  a  half  days.*  In  the  rabbit  Hensen's 
knot  presents  at  this  stage  a  small  depression  (the  front  end  of  the 
primitive  groove  into  which  a  small  plug  of  tissue  projects  up  from 
the  underlying  axial  thickening  (Carius'  Fig.  7);  Van  Beneden 
homologizes  this  with  the  anus  of  Rusconi.  The  relations  of  the 
head-process  in  the  sheep  are  very  much  as  in  the  rabbit,  Bonnet, 
89.1,  65-67;  the  cells  of  the  middle  layer  are  at  first  free,  as  they 
grow  forward  to  form  the  process,  but  subsequently  are  found  united 
with  the  inner  layer. 

The  head-process  (cf.  Lieberkiihn) ,  84. 1,  probably  always  grows— 
as  is  certainly  the  case  in  the  guinea-pig — at  its  hinder  end  and  at 
the  expense  of  the  primitive  streak ;  it  is,  I  think,  in  this  manner 
that  the  often-noticed  shortening  and  final  disappearance  of  the 
streak  is  effected .  This  mode  of  growth  concords  with  the  concres- 
cence theory. 

Homologies  of  the  Mammalian  Blastocyst. — The  homologies 
with  corresponding  stages  of  other  vertebrates  are  uncertain.  It 
seems  clear  that  the  main  cavity  of  the  two-layered  vesicle  corresponds 
to  the  yolk  cavity  and  that  the  head-process  is  identical  with  the 
primitive  axis.  But  the  homologies  during  the  stages  of  transition 
from  the  segmented  ovum  to  the  two-layered  vesicle  are  uncertain, 
and  must  remain  so  until  we  understand  the  genesis,  first  of  the  yolk 
cavity,  second  of  the  primitive  axis.  Nor  can  the  development  be 
clear  to  us  until  the  growth  of  the  primitive  axis  by  concrescence  is 
elucidated. 

Inversion  of  the  Germ-Layers  in  Rodents. — In  many  but 
not  in  all  rodents  the  outer  layer,  Rauber's  Deckschicht,  of  the 
embryonic  shield  undergoes  a  remarkable  hypertrophy  immediately 
after  the  close  of  segmentation  proper ;  the  Deckschicht,  together  with 
the  ectoderm  underlying  it,  becomes  a  plug  which  pushes  in  the  other 
layers,  thereby  profoundly  altering  the  topography  of  the  ovum.  In 
the  mole,  Heape,  83.1,  the  hypertrophy  is  not  very  great  and  the 
plug  disappears  soon,  so  that  there  is  no  great  change ;  in  guinea- 
pigs,  mice,  and  Arvicola,  the  plug  becomes  very  large  and  remains 
for  a  long  time.  The  plug  is  very  long  and  the  ovum  elongates  with 
it,  changing  into  an  almost  cylindrical  vesicle  (Selenka's  Keimcyl- 

*C.  Rabl,  89.2,  143-145,  states  expressly  that  in  the  rabbit  the  axial  thickening  is  not  con- 
nected with  the  inner  layer  either  under  the  head-process  or  under  the  primitive  streak.  He  dif- 
fers from  other  investigators  in  this  so  much  that  I  think  his  preparations  were  probably  defec- 
tive; indeed,  his  own  figures  suggest  at  once  that  the  inner  layer  has  been  artificially  separated 
from  the  overlying  one. 


142 


THE   GERM-LAYERS. 


inder) .  The  plug  becomes  hollow,  and  the  cells  corresponding  to  the 
Deckschicht  become  separated  from  those  which  are  to  form  the 
ectoderm  of  the  embryo.  Three  modifications  of  the  hollowing  out  of 
the  plug  and  of  the  separation  of  its  two  parts  are  known.  The 
changes  referred  to  are  very  clearly  illustrated  by  Selenka,  84.1, 
Taf.  XVI.,  in  a  series  of  comparative  diagrammatic  figures.  In  the 
simplest  case,  Fig.  87,  the  plug  acquires  a  single  cavity,  a;  the  cells 
around  the  upper  end,  Tr,  correspond  to  the  Deckschicht  and  serve 
partly  to  attach  the  ovum  to  the  uterine  walls ;  the  cells,  EC,  around 
the  lower  end  of  the  cavity  become  the  embryonic  ectoderm ;  all  the 
cells  around  the  cavity  a  are  homologous  with  the  outer  layer  of  the 
embryonic  shield  of  other  mammals.  The  cavity  c  of  the  vesicle  is 
very  much  reduced;  the  inner  side  of  the  shield,  i.e.  of  the  plug,  is 
lined  by  an  inner  layer,  En,  which  gives  rise  to  the  entoderm.  The 
outer  layer  of  the  vesicle  is  very  thin;  it  unites 
very  closely  with  the  walls  of  the  uterus,  and  later 
disappears.  Hence,  when  the  uterus  is  opened, 
only  the  hollow  plug  and  its  covering  of  entoderm 
can  be  removed ;  as  it  makes  a  two- walled  vesicle 
it  was  considered  to  represent  by  itself  the  two- 
layered  stage  of  the  blastodermic  vesicle.  Thus 
it  came  that  Bischoff  believed  that  in  various  ro- 
dents the  ectoderm  lies  inside,  the  entoderm  out- 
side. Bischoff 's  observations,  52. 1,  70. 1,  which 
c  were  confirmed  by  Rei chert,  62.1,  are  correct; 
but  the  inversion  of  the  layers  is  apparent,  not 
real.  The  actual  homologies  were  not  discovered 
En  until  the  improvements  in  microscopical  technique 
enabled  Selenka,  83.1,  84.1,  and  Kupffer,  82.3, 
to  make  sections  of  uteri  with  the  ova  in  situ,  and 
in  their  sections  to  follow  the  history  of  the  outer 
layer.  Their  results  have  been  in  the  main  con- 
firmed by  Fraser,  83.1,  and  extended  to  another 
derm;  oi,  outer  layer;  a,  species  by  Biehringer,  88. 1,  91.1. 
foderL^^fter^eSiifa.  In  Mus  decumanus  the  ectodermal  cells  early 
become  a  separate  spherical  mass,  thus  dividing 
the  plug  into  two  parts ;  a  cavity  appears  in  each  part ;  these  two 
cavities  soon  become  confluent,  and  the  inner  layer  of  cells  having 
meanwhile  developed,  the  relations  become  essentially  identical  with 
those  in  Mus  sylvaticus,  Fig.  87.  In  Mus  musculus  the  development 
is  similar,  but  there  is  the  additional  peculiarity  that  the  Deckschicht 
is  regularly  invaginated  at  first  so  as  to  form  a  small  pit,  into  which 
uterine  tissue  grows.  In  Arvicola  this  invagination  is  more  marked 
and  lasts  longer,  but  in  both  cases  it  is  early  obliterated. 

Arvicola  represents  the  second  modification  mentioned  above ;  it 
has  not  only  the  invagination  to  distinguish  it,  but  also  the  very 
early  formation  of  the  cavity  of  the  plug  as  a  fissure  between  the 
Deckschicht  and  the  true  ectodermal  cells. 

The  guinea-pig  offers  the  third  modification  and,  is  characterized 
by  the  early  complete  separation  of  the  plug  into  its  two  parts ;  the 
Deckschicht  remains  at  one  end  of  the  ovum  and  forms  the  Trager ; 
it  acquires  an  independent  cavity  of  its  own ;  the  ectodermal  portion 


FIG.  87. — Blastodermic 
vesicle  of  Mus  sylvati- 


THE    MAMMALIAN   BLASTODERMIC    VESICLE.  143 

of  the  plug  forms  a  solid  spherical  mass  which  is  transported  to  the 
opposite  pole  of  the  ovum ;  it  subsequently  becomes  hollowed  out,  pre- 
senting a  space,  which,  as  the  later  development  shows,  is  the  amniotic 
cavity.  The  inner  layer  passes  from  the  edge  of  the  Trager  around 
the  sphere  of  ectoderm ;  if  the  two  parts  of  the  plug  were  connected 
the  relations  of  the  inner  layer  would  be  the  same  as  in  Mus  sylvati- 
cus,  Fig.  87. 

The  subsequent  development  of  the  rodents  with  inverted  layers  is 
modified  in  various  secondary  features,  which  it  will  be  unnecessary 
for  us  to  study.  In  all  typical  respects  the  embryonic  development 
agrees  with  that  of  other  mammals  even  as  to  details. 

Duval,  90.2,  has  shown  that  in  the  rabbit  the  outer  layer  of  the 
blastodermic  vesicle  degenerates  and  disappears,  though  at  a  much 
later  stage  than  in  the  species  just  considered.  Hence  there  is  in 
the  rabbit  also  potentially  an  inversion  of  the  germ-layers. 

Graf  Spee,  89.1,  170,  suggests,  and  I  think  with  considerable 
reason,  that  the  earliest  development  of  the  human  ovum  takes  place 
by  inversion  of  the  layers.  If  this  hypothesis  is  correct,  it  explains 
many  of  the  remarkable  peculiarities  of  the  youngest  human  ova 
known  at  the  present  time, 


CHAPTER   VI. 
THE  MESODERM  AND  THE  CCELOM.* 

THE  morphology  of  the  mesoderm  is  one  of  the  most  vexed  ques- 
tions of  the  day.  Scarcely  an  embryologist  can  be  found  who  has 
not  published  opinions  on  this  question  considerably  at  variance  with 
the  opinions  of  others.  It  has  been  maintained  that  the  mesoderm 
arises  from  the  ectoderm ;  that  it  arises  from  the  entoderm,  or  from 
both ;  from  neither,  but  from  two  special  segmentation  spheres ;  that 
it  has  a  double  origin,  part  coming  from  the  blastoderm,  part  from 
the  yolk,  and  even  that  there  is  no  mesoderm. 

We  now  know  positively  that  in  all  vertebrates  there  is  a  distinct 
and  unmistakable  mesoderm,  which  spreads  out  from  the  primitive 
streak  in  all  directions  and  has  distinctive  histological  character- 
istics. 

Two  large  and  complex  cavities  appear  in  this  mesoderm,  one  on 
each  side  of  the  median  axial  line.  The  mesodermic  cells  which 
bound  these  two  cavities  assume  an  epithelial  arrangement,  and  are 
designated  as  the  mesothelium;  the  cavities  constitute  the  ccelom  or 
primitive  body  cavity ;  the  mesothelium  at  various  points  throws  off 
cells  which  compose  the  mesenchyma  (embryonic  connective  tissue). 
We  have,  accordingly,  three  distinct  phases  to  study,  viz. :  1,  the  ori- 
gin of  the  mesoderm ;  2,  formation  of  the  ccelom  and  mesothelium ; 
3,  the  origin  of  the  mesenchyma.  Finally,  we  must  review  the 
principal  theories  in  regard  to  the  morphological  significance  of  the 
mesoderm. 

I.  ORIGIN  OF  THE  MESODERM. 

Mesoderm  of  Elasmobranchs. — In  the  cartilaginous  fishes  the 
mesoderm  arises  from  the  entoderm  close  to  the  ectental  line.  The 
observations  of  Balfour  in  his  monograph,  78.3  (see  also  his  works, 
I.,  246-268),  established  the  fact  that  the  mesoderm  appears  after 
the  two  primary  layers  and  is  connected  with  the  entoderm.  This 
fact  has  since  been  abundantly  confirmed,  see  Kollmann,  85.2, 
Swaen,  87.1,  Kiickert,  85.1,  87.1,  Rabl,  89.2,  D.  Schwarz, 
89. 1,  et  al.  These  later  observations,  particularly  those  of  Riickert 
and  Rabl,  have  settled  the  exact  point,  or  rather  area,  of  entoderm 
which  is  mesoblastogenic.  Unfortunately  Rabl  overlooked  the 
phenomena  of  concrescence,  and  consequently  reached  conclusions  as 
to  the  development  of  the  mesoderm  which  I  feel  no  hesitation  in 
pronouncing  erroneous.  The  mesoderm  is  differentiated  along  the 
embryonic  rim  before  concrescence  takes  place;  hence,  when  con- 
crescence is  partly  completed,  there  is  an  axial  stretch  of  mesoderm, 

*  This  chapter  has  already  been  published  iti  the  American  Naturalist,  Oct. ,  1890,  but  as  here 
reprinted  has  been  extensively  altered. 


ORIGIN   OF   THE   MESODERM.  145 

and  from  the  hind  end  of  this  the  mesoderm  stretches  out  toward 
each  side  along  the  embryonic  rim  in  connection  with  the  entoderm, 
as  has  been  described  in  Chapter  V.  We  can  distinguish  the  axial 
mesoderm  from  the  lateral  mesoderm ;  but  later  on,  when  concrescence 
has  progressed  farther,  there  is  no  lateral  mesoderm,  for  it  has  be- 
come axial.  Rabl,  however,  failed  to  study  the  later  stages,  and 
so  came  to  consider  that  this  temporary  condition  of  the  mesoderm 
signified  a  double  origin ;  accordingly,  he  distinguishes  between  the 
"gastral"  (axial)  and  "  peristomial"  (lateral)  mesoderm,  and  makes 
the  unsuccessful  attempt  to  show  that  the  "  gastral"  and  "  peristomial" 
mesoderms  are  of  essentially  different  origin  in  all  vertebrates.  Had 
Rabl  understood  concrescence  he  would  certainly  have  not  fallen  into 
these  errors.  There  is  no  positive  evidence  that  there  is  an  evag- 
ination  of  the  entoderm  as  the  Hertwigs  maintain  can  be  shown  in 
the  amphibians — see  below.  On  the  contrary,  the  cells  grow  forth 
from  the  entoderm  so  as  to  constitute  a  sheet  between  the  primary 
germ-layers.  Soon  the  connection  with  the  entoderm  is  permanently 
severed. 

The  fact  that  the  mesoderm  appears  first  in  the  embryonic  rim 
renders  it  easy  to  make  sure  of  its  springing  from  the  entoderm. 
Later,  when  concrescence  moves  the  rim  into  the  axial  line,  all  three 
germ-layers  are  united  in  the  primitive  axis,  and  it  becomes  more 
difficult  to  decide  which  of  the  layers  the  mesoderm  is  specially  con- 
nected with.  To  conclude :  in  elasmobranchs  the  mesoderm  arises 
over  a  limited  area  of  the  entoderm  near  the  ectental  line ;  it  sepa- 
rates from  the  entoderm  apparently  by  a  process  of  delamination,  but 
the  exact  means  of  separation  have  yet  to  be  investigated ;  it  remains 
for  a  while  connected  with  the  entoderm  along  the  embryonic  axis ; 
after  its  separation  from  the  entoderm  the  mesoderm  expands  by 
proliferation  of  its  own  cells  and  receives  no  accretions  from  the 
yolk,  so  far  as  at  present  known. 

Mesoderm  of  Teleosts. — So  far  as  the  published  accounts  go 
the  middle  layer  of  bony  fishes  arises  as  maintained  by  Balfour, 
"  Comp.  Embryol.,"  II.,  74,  from  the  entoderm.  Such  appears  to  be 
the  significance  of  Ryder's  observations,  84.1,  41,  of  A.  Goette's, 
73.1,  E.  Zielger's,  Agassiz  and  Whitman's,  84.1,  and  of  others. 
For  a  good  description,  together  with  citations  of  conflicting  au- 
thorities, see  M.  Kowalewski,  86.1,  469-474.  Apparently  the 
blastodermic  rim  is  turned  under,  and  the  turned-under  portion  yields 
the  entoderm,  and  is  intimately  connected  with  the  sheet  of  mesoder- 
mal  cells,  very  much  as  in  sharks ;  the  mesoderm  is  several  layers 
thick  and  stretches  in  under  the  ectodermal  blastoderm,  gradually 
thinning  out ;  the  cells  of  the  middle  layer  are  at  first  closely  com- 
pacted. 

Mesoderm  of  Amphibia. — Here  it  seems  also  clearly  estab- 
lished that  the  mesoderm  arises  from  the  entoderm,  principally  along 
and  alongside  the  median  line,  as  a  sheet  of  cells  with  no  cavity 
(ccelom),  included  between  them;  along  the  centre  of  the  primitive 
axis  and  at  the  blastoporic  margin  the  connection  between  the 
mesoderm  and  entoderui  is  both  evident  and  intimate ;  see  Bellonci, 
84.1,  Tav.  II.,  for  figures  showing  this  point  in  the  axolotl,  and 
O.  Schultze,  88.1,  for  similar  figures  of  Rana  fusca.  These  facts 
10 


146 


THE   GERM-LAYERS. 


have  been  recorded  by  so  many  observers  that  there  can  be  little 
doubt  or  none  of  their  entire  accuracy ;  see  the  description  and  cuts, 
ante,  p.  130.  It  may  be  considered  as  still  uncertain  whether  the  sheet 
of  mesoderm  receives  accretions  at  its  distal  edge  from  the  yolk  cells 
(entodermic)  upon  which  it  rests.  There  usually  is  no  sharp  limit 
between  the  two,  and  therefore  we  must  consider  it  probable  that  at 
first  the  mesoderm  receives  additions  from  the  yolk ;  later  on  it  is 
found  divided  from  the  vitelline  cells,  and  after  it  has  split  off  it 
probably  grows  independently.  The  growth  of  the  mesoderm  at  first 
from  the  yolk  has  been  found  in  Petromyzon  by  A.  E.  Shipley, 
88.1,177,  178  (of  "  Studies") ,  although  in  later  stages  the  mesoderm  is 
severed  from  the  yolk. 

In  later  stages  the  mesoderm  is  wanting  in  the  median  line,  and 
thus  constitutes  two  masses  or  two  lateral  sheets.  This  bilateral 
division  is  effected  by  the  development  of  the  medullary  groove  and 
notochord,  as  described  in  Chapter  VIII.  The  mesodermic  connec- 
tion with  the  entoderm  is  retained,  but  is  double  owing  to  the  divis- 
ion. Along  the  median  dorsal  line  of  the  archenteron  runs  the  strip 
of  entoderm  which  forms  the  notochord ;  on  each  side  of  this  strip 
runs  the  line  of  connection  between  entoderm  and  mesoderm.  The 
study  of  this  secondary  condition  has  led  many  authors  into  the 
error  of  ascribing  a  double  origin  to  the  amphibian  mesoderm,  and 
inferentially  to  the  vertebrate  mesoderm  in  general.  This  brings  us 
to  the  consideration  of  O.  Hertwig's  views,  which  form  one  of  the 
chief  supports  of  the  "  Coelomtheorie"  of  the  brothers  Her  twig. 
For  further  discussion  of  this  theory  see  p.  155. 

O.  Hertwig,  82.1,  83.1,  studied  stages  in  which  the  notochord 
had  appeared,  and  at  this  time,  as  O.  Schultze,  88.1,  has  shown, 

the  primitive  relations  of  the  lay- 
ers no  longer  exist,  but  Hertwig 
regarded  the  secondary  arrange- 
ments in  question  as  primary.  He 
found  no  mesoderm  in  the  axial 
line  above  the  notochord;  at  the 
edge  of  the  notochord,  where  it 
joins  the  undifferentiated  epithe- 
lial entoderm  of  the  archenteron, 
there  is  on  each  side  a  groove 
which  in  cross-sections  appears  as 
a  notch,  Fig.  88;  the  notch  is  of 
variable  depth,  is  sometimes  ab- 
sent, and  is  a  temporary  feature. 
In  the  neighborhood  of  the  furrow 
alongside  the  notochord,  the  meso- 

Fm.  88.— Axolotl  embryo;  transverse  section    derm  IS   Still   intimately  Connected 


with  the  entoderm.  These  rela- 
tions are  believed  by  Hertwig  to 
indicate  that  the  mesoderm  arises 
as  two  masses,  which  is  not  the  case,  and  that  each  mass  is  really  a 
diverticulum  of  the  archenteron,  the  furrow  being  the  mouth  of  the 
diverticular  cavity.  Hertwig's  figures,  82.1,  Taf.  XIII.,  XIV., 
offer  the  plainest  representations  of  the  mesoderm  in  Triton  as  paired 


vnes, 


Ent 


of  an  early  stage:  EC,  ectoderm;  mes,  meso- 
derm ;  Md.  medullary  groove ;  Ch,  notochord ; 
Ent,  entoderm;  Ffc,  yolk;  Ach,  archenteron. 
After  Bellonci. 


ORIGIN    OF   THE    MESODERM.  147 

diverticula ;  but  these  figures  *  are  evidently  diagrammatic,  and  they 
must  be  termed  inaccurate,  I  think,  in  the  very  respects  which  are 
essential  to  Hertwig's  theory.  This  appears  from  the  investigations 
of  Goette,  75.1,  Bellonci,  84.1,  Bambeke,  68.1,  O.  Schultze,  88.1, 
and  others.  Compare  also  K.  Lambert,  83.1.  The  reader  may 
compare,  for  instance,  Hertwig's  Fig.  10,  I.e.,  Taf.  XIII.,  with 
Bellonci's  Fig.  11,  I.e.,  Tav.  III.  O.  Schultze's  detailed  criticism, 
I.e.,  344-340,  of  Hertwig's  account  seems  to  me  entirely  justified,  and 
I  accordingly  accept  it  as  a  complete  disproof.  This  criticism  shows 
that  Hertwig's  conception  is  based  upon  insufficient  and  erroneous 
observations ;  insufficient  because  he  did  not  investigate  the  early 
condition  of  the  mesoderm,  and  failed  to  recognize  the  fugitive  and 
unessential  character  of  the  parachordal  grooves ;  erroneous  because 
the  cavity  in  the  mesoderm  does  not  really  communicate  with  that  of 
the  archenteron.  There  are  other  errors  which  Schultze  points  out 
and  which  are  important.  Robinson  and  Assheton,  91.1,  495,  have 
also  failed  to  verify  Hertwig's  statements. 

We  find  in  Amphibia  at  a  certain  stage  the  axial  (Rabl's  gast  rales) 
and  lateral  (RabPs  peristomales)  mesoderm.  The  former  is  in  the 
region  of  the  completed  concrescence,  the  latter  round  the  edge  of  the 
anus  of  Rusconi.  The  former  is  connected  with  theentoderm  alone, 
the  latter  with  the  ectoderm  also,  since  the  entoderm  is  connected 
with  the  ectoderm  around  the  unconcresced  blastoporic  rim.  The 
connection  with  the  ectoderm  renders  it  possible  that  the  middle 
layer  receives  cell  3  from  the  outer  layer,  but  there  is  no  direct  proof 
of  this.  When  the  concrescence  is  completed  the  mesoderm  is  said 
to  sever,  in  the  posterior  axial  region,  its  connection  with  the  ento- 
derm, but  to  retain  awhile  its  connection  with  the  outer  germ  layer. 
The  same  phenomenon  recurs  in  the  amniota.  It  cannot,  however, 
be  taken  to  signify  that  the  middle  layer  originates  from  the  ecto- 
derm, since  at  an  earlier  stage  it  is  clearly  entodermal. 

Mesoderm  of  Sauropsida. — We  may  consider  reptiles  and 
birds  together,  since  the  early  history  of  the  middle  layer  is  very 
similar  in  the  two  classes. 

In  reptiles,  so  far  as  our  present  unsatisfactory  knowledge  en- 
ables us  to  judge,  the  mesoderm  arises  by  delamination  from  the 
entoderm,  but  remains  connected  with  it  along  the  axial  line ;  in 
front  (i.e.,  in  the  head-process)  it  is  connected  with  the  entoderm 
only,  but  posteriorly  it  is  fused  with  the  tissue  of  the  primitive 
streak,  and  thereby  is  indirectly  connected  with  the  ectoderm.  After 
its  delamination  the  mesoderm  expands  independently  of  the  other 
germ-layers  except,  perhaps,  along  the  axis.  That  the  relations  are 
like  those  in  birds  has  been  shown  clearly  by  Strahl,  83. 1,  and  also 
by  Weldon,  83.1,  whose  Figure  1  is  reproduced,  ante,  Fig.  71. 
The  intimate  connection  of  the  mesoderm  with  the  entoderm  at  the 
blastodermic  rim  before  concrescence  is  sufficiently  established  by 
Kollmann,  84.3,  403-406,  though  his  conception  that  this  part  of 
the  mesoderm  is  a  separate  structure,  which  he  terms  akroblast, 
renders  it  difficult  to  follow  certain  parts  of  his  description.  C.  K. 
Hofmann  may  also  be  cited,  though  his  account  (Bronn's  "Thier- 

*Some  of  them  are  reproduced  in  Hertwig's  "Lehrbuoh  der  Entwickelungsgeschichte  " 
6tes  Capitel. 


148  THE   GERM-LAYERS. 

reich,  Reptilien,"  p.  1881)  is  of  doubtful  accuracy  in  several  respects. 
L.  Will,  89. 1,  1127,  finds  that  in  the  gecko  the  mesoderm  is  united 
with  the  entoderm  in  the  head-process,  but  omits  to  describe  its  exact 
connection  with  the  primitive  streak ;  the  stages  showing  the  origin 
of  the  mesoderm  he  does  not  mention.  The  processes  involved  will 
undoubtedly  be  understood  as  soon  as  the  concrescence  of  the  axis 
has  been  worked  out — a  fundamental  question,  which  as  yet  not  a 
single  investigator  has  heeded. 

In  birds  the  exclusively  entodermic  origin  of  the  mesoderm  is,  in 
my  opinion,  conclusively  demonstrated  by  the  researches  of  Duval, 
84.1,  104-117;  the  entoderm  gradually  thickens  by  migrations  of 
its  cells  over  a  considerable  axial  area;  the  upper  stratum  of  this 
thickened  area  separates  off  as  the  mesoderm  except  that  in  the  axial 
line  it  retains  its  connection  with  the  entoderm ;  when  concrescence 
takes  place  the  two  layers  form  the  primitive  axis.  In  the  region  of 
the  primitive  streak  there  is  a  single  large  mass  of  cells,  Fig.  71, 
_Pr,  which  is  continuous  with  all  three  germ-layers.  Now  if  the 
homology  maintained  in  the  previous  chapter  be  correct  between  the 
primitive  streak  and  the  anus  of  Rusconi,  then  the  cells  of  the  streak 
are  also  entodermal,  and  the  middle  germ-layer  is  connected  in  both 
axial  regions  directly  only  with  the  entoderm.  After  the  mesoderm 
has  separated  from  the  entoderm  except  in  the  median  line  it  may 
continue  to  receive  accretions  from  the  entoderm  in  the  median  line, 
but,  as  far  as  known,  makes  no  peripheral  additions  except  from  its 
own  growth.  So  far  as  heretofore  observed  the  mesoderm  receives 
no  cells  from  the  ectoderm. 

Mesoderm  of  Mammals. — In  this  class,  according  to  the  best 
recent  investigations,  the  mesoderm  appears  to  have  a  distinctly 
twofold  origin.  According  to  Bonnet,  84.1,  196,  part  of  the  meso- 
derm grows  out  from  Hensen's  knot  at  a  time  when  the  knot  is  a 
thickening  of  the  outer  layer  and  has  not  yet  acquired  any  connection 
with  the  inner  layer ;  another  portion  is  produced  peripherally,  Fig. 
84,  mes,  by  delamination  from  the  inner  layer;  the  two  anlages 
grow  toward  one  another  and  unite  into  one  continuous  mesoderm, 
in  which  all  trace  of  the  primitive  double  origin  is  obliterated. 
Kolliker  has  recorded  the  outgrowth  of  the  mesoderm  from  Hensen's 
knot  in  the  rabbit,  and  his  statement  has  been  confirmed  by  Fr. 
Carius,  88. 1,  17.  In  later  stages  we  find  the  relations  of  the  layers 
similar  to  those  in  Sauropsida,  there  being  a  head-process  with  the 
mesoderm  connected  axially  with  the  inner  layer,  and  a  primitive 
streak,  with  which  the  mesoderm  fuses ;  the  inner  layer  of  the  blas- 
todermic  vesicle  is  connected  with  the  front  part  of  the  streak.  This 
stage  is  quite  well  known,  cf.  Heape,  83.1,  on  the  mole,  Bonnet  on 
the  sheep,  84.1,  Kolliker  on  the  rabbit  ("  Grundriss  ") ,  Selenka  on 
the  opossum,  86.1,  Lieberkuhn,  82.1,  and  others,  especially  the 
very  careful  descriptions  of  the  rabbit's  layers  by  C.  Rabl,  89. 2. 

At  present  it  seems  to  me  impossible  to  offer  any  satisfactory  inter- 
pretation of  the  observed  double  origin  of  the  mammalian  mesoderm. 
The  relations  of  the  mesoderm  to  the  primitive  axis  (head-process) 
and  primitive  streak  are  identical  with  those  in  birds  and  reptiles. 

The  Vertebrate  Type  of  Origin  of  the  Mesoderm.— The 
preceding  paragraphs  show  that  in  all  classes  of  vertebrates  the 


ORIGIN   OF   THE    MESODERM.  149 

origin  of  the  mesoderm  is  essentially  the  same,  except  that  in  some 
mammals  it  begins  in  two  regions  of  the  entoderm  almost  simultane- 
ously. The  relations  in  the  mammals  we  do  not  understand.  In  the 
non-mammalian  vertebrates  the  mesoderm  first  appears  as  a  thicken- 
ing of  the  entoderm  over  a  not  inconsiderable  area  around  the  concres- 
cing  blastodermic  rim,  and  it  becomes  separated  from  the  entoderm 
by  the  gradual  parting  of  the  upper  cells  to  form  the  true  mesoderm 
from  the  lower  cells  or  permanent  entoderm ;  this  delamination  does 
not  take  place  next  the  blastodermic  rim  (or — after  concrescence — in 
the  axial  line) ;  hence  in  the  region  of  the  primitive  axis  the  three 
layers  may  be  connected  for  a  time;  further,  as  the  tissue  of  the 
primitive  streak  is  at  first  connected  with  the  ectoderm,  the  mesoderm 
is  thereby  indirectly  continuous  with  the  outer  germ-layer  during  very 
early  stages.  It  is  important  to  note  that  the  mesoderm  arises  over 
a  considerable  area  during  the  same  period ;  that  its  formation  may 
be  more  or  less  advanced  before  concrescence  of  the  rim ;  and  that 
after  concrescence  it  stretches  across  the  axis  of  the  embryo  between 
the  ectoderm  and  entoderm,  thus  becoming  a  continuous  sheet  or 
layer.  This  fact  that  the  mesoderm  is  a  single  anlage  needs  to  be 
specially  emphasized.  So  far  as  known  to  me  there  is  not  a  single 
vertebrate  which  has  been  shown  to  lack  this  stage,  but  on  the  con- 
trary its  occurrence  is  established  for  all  classes  and  by  so  many 
observers  that  we  may  well  assert  that  there  are  few  facts  in  embry- 
ology better  established.  Later  the  mesoderm  becomes  divided 
in  the  axial  line,  *  and  consideration  of  this  secondary  condition  has 
led  to  several  theories  of  the  mesoderm,  which  would  hardly  have 
been  brought  forward  had  their  authors  not  neglected  to  take  into 
account  the  earlier  condition  of  the  middle  layer.  Some  of  these 
theories  are  discussed  below. 

After  its  delamination  the  mesoderm  is  a  distinct  layer  and  grows 
independently,  receiving  no  accretions  from  the  other  layers  except 
in  the  axial  line,  where  it  receives  cells  from  the  entoderm  and  in  the 
region  of  the  primitive  streak.  The  edge  of  the  expanding  sheet 
of  mesoderm  is  free,  as  has  been  pointed  out  in  the  previous  chapter, 
resting  upon  the  yolk  but  not  fused  with  it.  It  is  therefore,  it  seems 
to  me,  impossible  to  admit  that  there  is  a  peripheral  ingrowth  of 
tissues  arising  from  the  yolk  and  entering  the  mesoderm  to  form  the 
blood,  etc.  Compare  below,  Theories  of  the  Mesoderm,  p.  153. 

The  primitive  mesodermic  cells  are  embryonic  in  character; 
that  is,  they  have  a  large,  usually  nucleolated,  nucleus,  and  very 
little  protoplasm  (Minot,  125).  They  are  connected  together  by  fine 
threads,  and  may  lie  some  distance  apart,  then  presenting  an  obvious 
resemblance  to  the  mesenchyma  of  later  stages.  The  cells  become 
more  closely  compacted  as  development  progresses,  and  when  the 
coelom  appears  they  take  on  a  distinctly  epithelial  arrangement  to 
make  the  mesothelium.  The  cells  frequently  contain  yolk  grains — 
in  the  case  of  Amphibia  numerous  and  large  ones.  In  birds  the  yolk 
grains  are  few,  but  are  easily  observed,  Fig.  81 ;  in  mammals  they 
are  almost  entirely  absent. 

*Mitsukuri.  91.1,  has  attempted  to  deny  the  views  I  have  advanced,  because  in  turtles  the 
mesoderm  is  divided,  as  shown  by  his  own  observations.  He  has  overlooked  the  fact  that  his 
observations  refer  to  the  secondary  stage  only  when  the  medullary  groove  and  notochord  are 
present,  and  that  they  have  no  bearing  on  the  question  of  the  earlier  and  primitive  condition. 


150 


THE    GERM-LAYERS. 


Expansion  of  the  Mesoderm. — After  the  mesoderm  is  once 
formed  as  a  distinct  layer  without  connection  with  the  primitive 
layers  except  in  the  axial  line,  it  expands  independently — that  is,  by 
the  proliferation  of  its  own  cells.  During  its  early  expansion  the 
mesoderm  assumes  in  all  amniota  a  definite  series  of  characteristic 
outlines.  It  is  at  first  pear-shaped,  Fig.  89,  A,  the  anterior  end  be- 
ing pointed ;  it  extends  a  short  distance  only  in  front  of  the  primitive 
streak,  and  is  widest  a  little  distance  behind  the  area  pellucida,  ap. 
The  same  stage  is  found  in  mammals,  see  Kolliker,  ("Grundriss," 
p.  93  and  Fig.  71.)  The  condition  in  the  chick  at  about  the  twentieth 
hour  of  incubation  is  indicated  by  Fig.  89,  B,  drawn  on  the  same 


FIG.  89.  — Diagrams  of  the  embryonic  area  of  the 
chick:  Ao,  areaopaca;  Ap,  area  pellucida ;  pr,  prim- 
itive groove;  mes,  mesoderm.  After  Duval. 


FIG.  90.  — Diagram  of  the  embryonic 
area  of  a  chick :  Ao,  area  opaca ;  Ap, 
area  pellucida;  pr,  primitive  groove; 
mes,  mesoderm.  After  Duval. 


scale  as  A,  and  at  the  close  of  the  first  day  by  Fig.  90.  In  the  last 
mentioned  figure  it  will  ba  noticed  that  the  mesoderm  is  expanding 
unequally  in  front,  having  sent  out  two  lateral  wings  which  leave  a 
median  space  between  them  without  mesoderm.  These  wings  con- 
tinue their  growth  and  finally  meet  in  front,  so  that  in  the  anterior 
part  of  the  area  pellucida  there  is  a  small  tract  without  any  mesoderm, 
although  there  is  mesoderm  all  around  it ;  this  tract  is  ihQproamnion, 
of  which  a  fuller  history  is  given  in  Chapter  XV.  The  expansion 
does  not  take  place  by  any  means  with  the  exact  regularity  indicated 
by  Figs.  89,  90,  but,  on  the  contrary,  in  birds,  as  shown  by  Zum- 
stein,  87. 1,  the  outline  of  the  middle  layer  is  always  irregular  and 
more  or  less  asymmetrical.  Although  there  are  not  yet  many  ob- 
servations available  as  to  the  outline  of  the  growing  mesoderm,  yet 
it  is  probable  that  the  preceding  description  is  essentially  correct,  not 
merely  for  birds  but  for  all  amniota.  It  is  certainly  so  for  the  rabbit, 
Van  Benedenet  Julin,  84. 1. 


II.  FORMATION  OF  THE  CCELOM  AND  MESOTHELIUM. 

Early  in  the  course  of  development  there  appears  in  the  mesoderm 
a  complex  series  of  cavities,  which  very  soon  become  united  so  as  to 
form  two  large  cavities,  one  on  each  side,  which  together  constitute 
the  ccelom  or  embryonic  body  cavity.  In  the  adult  mammal  the 
ccelom  is  represented  by  the  pericardial,  pleural,  and  abdominal 
cavities ;  the  coelom  also  includes  the  cavities  of  the  muscular  seg- 


FORMATION   OF   THE    CCELOM   AND    MESOTHELIUM. 


151 


nients  (proto  vertebra?)  and  also  certain  tubular  parts  of  the  urogeni- 
tal  system.  But  although  its  subsequent  changes  are  complex,  the 
ccelom  consists  at  an  early  stage  of  a  pair  of  fissures  in  the  meso- 
derm.  As  the  coelomatic  cavities  appear  the  cells  bounding  them 
take  on  a  distinctly  epithelial  character.  The  mesodermic  epithelium 
bounding  the  ccelom  is  termed  the  mesothelium,  and  it  is  probable 
—if  we  judge  from  our  present  imperfect  knowledge — that  the  entire 
mesoderm  is  in  all  vertebrates  first  converted  into  mesothelium,  be- 
fore undergoing  differentiation. 

Only  one  precise  account  of  the  mode  of  development  of  the  ccelom 
in  mammals  is  known  to  me,  namely,  that  of  Bonnet,  84.1,  202, 
for  the  sheep  at  about  thirteen  days.  Around  the  embryo  at  some 
distance  from  the  axis  there  appear  a  series  of  irregular  fissures  of 
rounded  or  elongated  form,  which  may  in  part  open  on  the  mesoder- 
mic surface ;  gradually  the  fissures  enlarge  and  fuse,  at  the  same  time 
becoming  more  closely  bounded  by  the  mesodermic  cells ;  thus  there 
arises  a  continuous  cavity  in  the  mesoderm  which  is  for  a  time  crossed 
by  cells  and  cell  processes ;  similar  connections  between  the  two  leaves 
of  the  mesoderm  while  the  ccelom  is  forming  and  vtheir  subsequent 
rupture  have  been  noticed  in  Amphibia  by  B.  Solger,  85.1,  383,  in 
Elasmobranchs  by  E.  Ziegler,  88.1,  383,  and  I  find  similar  phases 
with  great  distinctness  in  the  chick ;  meanwhile  the  cells,  which  are 
loosely  put  together,  form  a  compact  layer  of  epithelium  bounding  the 
cavity,  which  we  can  now  designate  as  the  ccelom  or  primitive  body 
cavity.  By  similar  processes  the  ccelom  grows  toward  the  axial 
region,  but  never  penetrates  it,  the  primitive  streak  and  head-process 
never  developing  a  median  coelom. 

Albrecht  Budge,  87. 1,  has  made  a  very  exact  study  of  the  arrange- 
ment of  the  fissures  in  the  mesoderm  of  the  chick  by  means  of  in- 
jections of  Prussian  blue.*  The 
fissures  form  a  network  of  channels 
and  by  their  fusion  produce  the 
coelomatic  cavities.  The  channels 
appear  first  around  the  periphery 
of  the  area  vasculosa,  and  thence 
their  development  progresses  cen- 
trifugally,but  most  rapidly  toward 
the  head;  the  channels  fuse  first 
around  the  head  to  make  the  am- 
nio-cardial  coelom  (Parietalhohle 
of  His) ;  now  appears  a  circular 
sinus  just  inside  the  vena  termi- 
nalis;  the  ccelom  grows  back 
through  the  embryo  and  forms  the 
body  cavity  of  the  rump ;  alongside 
the  rump,  as  shown  in  Fig.  91, 
appears  a  network  of  channels, 
which  soon  fuse  to  create  the  ccelom 
under  the  lateral  amniotic  fold,  and  this  unites  with  the  ccelom  of 
the  rump,  forming  the  completed  ccelom  continuous  with  that  of  the 


FIG.  91.— The  mesodermal  cavities  of  the 
germinal  area  of  a  chick  of  the  third  day,  in- 
jected with  Prussian  blue.  After  A.  Budge. 


*  The  injections  are  made  either  into  the  amnio-cardial  vesicles  or  into  the  circular  fissure  just 
inside  the  vena  t^rminalis. 


152  THE   GERM-LAYERS. 

pericardium.  The  network  of  channels  Budge  regards  as  primary 
lymph  spaces.  Compare  Chapter  XIX. 

Whether  in  all  vertebrates  the  coelom  results  from  the  fusion  of 
numerous  small  spaces  or  not,  is  not  yet  determined  by  actual  obser- 
vation. It  is  probable  that  it  does  so,  and  we  may,  therefore,  say 
that  the  vertebrate  coelom  is  what  Huxley  terms  a  schizocoele,  i.e.,  a 
cavity  produced  by  splitting  the  mesoderm,  compare  p.  155.  I  con- 
sider it  also  probable  that  the  coelom  always  begins  to  appear  at  a 
little  distance  from  the  axis  of  the  embryo  and  spreads  both  centrip- 
etally  and  centrifugally. 

Additional  and  important  points  in  the  earliest  history  of  the 
coelom  are  treated  in  Chapter  IX.  We  must  add  here  that  the 
coelomatic  fissure  divides  the  mesoderm  on  each  side  into  an  upper 
or  outer  leaf  (Hautfaserblatt)  and  a  lower  or  inner  leaf  (Darmfaser- 
blatt),  Fig.  92.  The  upper  leaf  may  be  called  the  somatic  meso- 
derm, Som,  the  lower  leaf  the  splanchnic  mesoderm,  Spl,  as  pro- 
posed by  Balfour.  The  upper  leaf  lies  close  against  the  ectoderm ; 
the  two  layers  together  form  the  somatopleure  or  body- wall.  The 
lower  leaf  lies  close  against  the  entoderm ;  these  two  layers  together 

I* i  _  irffiCKT^r'.i uyirAtiss.       )**+•  & . 

W. 


EVit—-~ 


FIG.  92.— Section  of  a  chicken  embryo  of  about  thirty-six  hours :  EC,  ectoderm;  Som,  som- 
atic mesoderm ;  Spl,  splanchnic  mesoderm ;  Ent,  entoderm ;  W,  Wolffian  duct ;  m,  mesenchymal 
cells:  Md.  medullary  groove:  v,  vein:  Cce.  coalom:  MS,  primitive  segment;  Ch,  notochord. 
After  W.  Waldeyer. 

form  the  splanchnopleure  or  wall  of  the  alimentary  tract.  Both 
the  somatic  leaf  of  mesoderm  and  the  splanchnic  consist  at  first 
solely  of  mesothelium,  but  very  soon  each  contains  mesenchyma  also ; 
the  latter  arises  from  the  mesothelium ;  axially  the  two  layers  be- 
come continuous  both  with  one  another  and  with  the  axial  meso- 
derm. 

The  morphology  of  the  coelom  is  so  important  that  it  is  difficult 
to  understand  why  so  many  investigators  have  slurred  over  the 
question  of  its  embryonic  development.  Exact  observations  on  its 
first  appearance  and  on  the  first  stages  of  its  expansion  in  various 
types  are  urgently  needed,  and  would  certainly  do  more  than  any- 
thing else  to  throw  light  on  the  still  obscure  problem  of  the  origin  of 
the  mesoderm. 

The  histogenesis  of  the  mesothelium  varies  somewhat  in  the 
different  types.  Primitively  (marsiobranchs,  amphibians),  the  cells 
are  rounded  in  form,  contain  considerable  yolk,  and  are  at  first 
loosely  aggregated,  compare  Fig.  88.  When  the  coelom  appears 
the  cells  become  more  closely  appressed  and  so  gradually  assume 
more  and  more  the  characteristics  of  a  cuboidal  epithelium.  In  the 
amniota,  on  the  other,  hand,  the  mesodermal  cells  contain  very  little 


THE    MESENCHYMA — THE    MESOPERM.  153 

yolk,  Fig.  81,  in  which  the  yolk  grains  are  shown  a.>  black  dots;  the 
cells  are  connected  by  their  processes ;  as  the  ccelom  develops  the 
processes  are  shortened,  and  the  cells  become  more  closely  packed, 
and  thus  gradually  arrange  themselves  into  a  cuboidal  mesothelium. 

III.  ORIGIN  OF  THE  MESENCHYMA. 

The  genesis  of  the  mesenchyma  is  treated  in  Chapter  IX.,  as  it 
cannot  be  understood  without  a  knowledge  of  the  development  of 
the  primitive  segments.  I  will,  therefore,  merely  state  here  the 
general  methods  of  its  production  in  order  to  render  intelligible  the 
following  discussion  of  the  theories  of  the  mesoderm. 

By  mesenchyma  we  understand  the  whole  of  the  mesoderm  of  the 
embryo,  except  the  mesothelial  lining  of  the  coelom.  So  far  as  at 
present  demonstrated  it  arises  solely  from  the  mesothelium.  Single 
cells  leave  the  mesothelium  on  the  side  away  from  the  coelom ;  these 
cells  remain  connected  with  one  another  and  with  the  mesothelial 
cells  by  protoplasmatic  processes,  but  they  do  not  lie  close  together 
as  in  an  epithelium,  so  there  is  a  considerable  though  variable 
amount  of  intercellular  space.  By  the  migration  of  the  cells  and 
their  multiplication  a  large  amount  of  mesodermic  tissue  is  pro- 
duced, which  fills  up  all  the  room  between  the  mesothelium  and  the 
two  primary  germ-layers.  At  first  no  definite  distinction  between 
the  mesothelium  and  the  mesenchyma  can  be  established,  but 
ultimately  they  become  and  remain  distinct  tissues,  with  divergent 
histories. 

IV.  THEORIES  OF  THE  MESODERM.* 

From  the  time  of  Von  Baer's  "  Entwickelungsgeschichte, "  of  which 
the  first  part  appeared  in  1828,  until  1868,  when  W.  His'  great 
monograph  on  the  chick,  68.1,  was  published,  embryologists  recog- 
nized the  three  layers',  and  regarded  the  mesoderm  as  a  natural  unit. 
His  led  the  way  to  our  present  conception  by  a  little-known  article, 
6  5. 1 ,  on  the  membranes  and  cavities  of  the  body,  and  his  monograph, 
68.1,  above  mentioned  fully  established  the  necessity  of  recogniz- 
ing two  main  groups  of  mesodermic  tissues;  accordingly  he  divided 
the  mesoderm  into  two  parts,  the  archiblast  and  parablast,  corre- 
sponding respectively  essentially  to  mesothelium  and  mesenchyma. 
Under  archiblast  His  included  not  only  the  mesothelial  tissues  proper, 
but  also  the  smooth  or  organic  musculature;  under  parablast  the 
mesenchymic  tissue  except  the  smooth  muscle.  The  terms  used  cor- 
responded to  his  theory  of  the  origin  of  the  two  parts  of  the  meso- 
derm, for  he  believed  that  the  archiblast  arose  in  the  axial  region 
and  was  contained  in  the  embryo  from  the  start,  while  the  parablast 
arose  peripherally  and  grew  in  toward  the  embryo,  a  conception 
which  was  perhaps  suggested  by  the  appearance  of  the  blood-vessels, 
first,  outside  the  embryo  proper.  Seeking  still  farther  for  the  source 
of  the  supposed  peripheral  parablast  he  believed  he  had  found  it  in 
the  germinal  wall.  The  study  of  the  relations  of  the  wall  in  the 
chick  induced  him  to  think  that  the  elements  of  the  white  yolk  be- 

*  Cf.  ante,  p.  149. 


154  THE    GERM-LAYERS. 

came  parablast  cells;  moreover,  the  study  of  the  hen's  ovary  led  him 
to  the  conclusion  that  the  white  yolk  was  developed  from  the  granu- 
losa  cells,  and  that  these  cells  arise  from  leucocytes.  He  thus  traced 
back  the  parablastic  cells  to  maternal  leucocytes.  As  subsequent 
chapters  will  show  more  fully,  the  granulosa  cells  are  not  leucocytes ; 
in  Chapter  III.  it  has  already  been  shown  that  the  granulosa  cells 
do  not  enter  the  ovum ;  the  white  yolk  grains  never  become  cells, 
for  it  has  been  proved  that  all  nuclei  of  the  segmenting  ovum  come 
from  previous  nuclei  and  lie  in  protoplasm,  not  in  the  yolk  grains ; 
and,  finally,  it  has  been  shown  in  this  chapter  that  the  mesoderm 
arises  as  a  whole,  not  from  double  sources.  Professor  His'  views  as 
to  the  origin  of  the  parablast  must  be  given  up,  but  this  is  no  reason 
for  overlooking,  as  certain  writers  have  done,  the  fundamental 
significance  of  the  distinction  drawn  between  the  two  primary  groups 
of  mesodermic  tissues.  Subsequent  research  has  made  only  one  im- 
portant change  necessary — namely,  the  transfers  of  smooth  muscula- 
ture from  one  group  to  the  other.  In  view  of  this  change,  of  the 
fact  that  parablast  has  been  used  with  various  other  meanings,  and 
of  the  unaptness  of  His'  names — since  we  renounce  the  theory  they 
correspond  to — it  will  be  well  to  use  exclusively  the  newer  terms, 
mesothelium  and  mesenchyma. 

The  parablast  theory  has  been  defended  by  His,  76.2,  and  modified 
by  him,  82.1.  At  present  he  holds  to  the  distinction  originally 
drawn,  but  is  inclined  to  withdraw  his  hypothesis  of  the  origin  of 
the  parablast.  A  number  of  writers  have  agreed  with  His  as  to  the 
separate  peripheral  development  of  the  mesenchyma  (parablast). 
Among  these  maybe  mentioned  Rauber,  77.1,  83.4,  and  several 
authors  who  have  dealt  with  the  development  of  the  blood,  see  Chap- 
ter X.  The  most  important  of  the  disciples  of  His  is  Kollmann,  who, 
in  a  series  of  articles,  84.1,  3,  85.1,  2,  has  maintained  the  double 
origin  of  the  mesoderm.  Of  these  papers  the  most  important  is  that 
on  the  "Randwulst,"  or  germinal- wall,  of  the  structure  of  which  in 
the  chick  it  gives  an  excellent  description.  Kollmann  regards  the 
germinal-wall  not  as  part  of  the  entoderm,  but  as  a  distinct  organ 
composed  of  segmentation  spheres,  and  destined  to  produce  blood- 
vessels with  blood,  and  probably  also  connective  tissue ;  this  peripheral 
anlage  (Eandkeim)  he  designates  as  acroblast,  and  the  single  cells 
derived  from  it  he  names  poreuten.  Waldeyer,  83.1,  has  ac- 
cepted the  parablast  theory,  but  with  a  modification  by  which  he 
seeks  to  reconcile  conflicting  observations.  His  article  is  written 
with  characteristic  clearness  and  exhaustive  mastery  of  the  literature, 
and  will  be  found  especially  valuable  by  those  who  wish  to  pursue 
this  subject  farther.  Waldeyer  distinguishes  between  the  primary 
and  secondary  segmentation ;  the  former  producing  the  ectoderm, 
entoderm,  and  archiblastic  mesoderm,  the  latter  occurring  later  and 
giving  rise  to  the  parablast.  According  to  Waldeyer  this  remnant 
of  the  ovum  (which  in  holoblastic  ova  consists  of  cells,  in  meroblastic 
ova  of  egg  protoplasm)  has  its  cell  division  (segmentation)  retarded, 
and  the  cells  thus  tardily  produced  immigrate  into  and  between  the 
germ-layers  already  developed. 

The  opposition  to  the  parablast  theory  is  the  sum  of  numerous  ob- 
servations, which,  as  pointed  out  in  the  previous  part  of  this  chapter,. 


THEORIES   OF   THE   MESODERM.  155 

prove — it  seems  to  me — that  the  mesoderm  arises  in  all  vertebrates 
(except  mammals?)  as  a  unit,  and  subsequently  separates  into  meso- 
thelium  and  mesenchyma.  The  leading  opponent  of  the  separate 
origin  of  the  parablast  is  Kolliker  in  both  his  text-books  ("  Entwick- 
elungsgeschichte, "  etc.,  and  "  Grundriss"),  and  in  separate  articles, 
see  especially,  84.2,  4,  and  his  criticism,  85.3,  of  Kollmann.  I  agree 
with  Kolliker  that  it  has  been  sufficiently  demonstrated  that  the 
"  akroblast"  belongs  to  the  entoderm,  and  after  the  delamination  of 
the  mesoderm  is  transformed  into  the  epithelium  of  the  yolk-sac ;  for 
a  conclusive  demonstration  that  this  is  so  in  reptiles,  see  H.  Strahl, 
87.1. 

The  cceloin  theory  of  the  brothers  Hertwig  includes  a  fundamental 
modification  of  the  parablast  theory.  The  main  features  of  the 
coelom  theory  are  not  original  with  the  Hertwigs,  but  may  be  found 
in  previous  writers ;  nevertheless  they  were  the  first  to  present  the 
theory  in  a  complete  formula  and  with  a  backing  of  facts,  both  new 
and  collated,  from  others  so  extensive  as  to  compel  attention.  In 
justice  to  E.  Ray  Lankester  it  must  be  stated  that  he  is  really  the 
author  of  the  ccelom  theory,  having  in  1877  (77.1)  published  the 
hypothesis  that  the  ccelom  is  derived  from  the  archenteron,  and  that 
the  mesoderm  of  vertebrates  represents  solid  entodermal  diverticula. 
It  is  unfortunate  that  the  Hertwigs  have  not  made  due  acknowledg- 
ment of  what  they  owed  to  Lankester  and  others.  They  made  a 
series  of  investigations  on  the  germ-layers  of  various  representatives 
of  the  animal  kingdom,  and  presented  their  general  results  in  a  com- 
prehensive article  (O.  and  R.  Hertwig,  81.1),  and  O.  Hertwig  has 
again  expounded  the  theory  in  his  text-book  of  embryology.  The 
cceloin  theory  consists  of  two  parts :  1,  the  ccelom  is  formed  by  diver- 
ticula of  the  archenteron  and  its  lining,  the  mesothelium,  is  part  of 
the  entoderm ;  2,  the  mesenchyma  consists  of  cells  thrown  off  by 
the  other  germ-layers  and  is  essentially  distinct  from  the  mesothe- 
lium. The  value  of  this  theory  lay  in  the  clearness  of  its  formula- 
tion, thus  facilitating  discussion,  and  also  in  its  bringing  out  the 
difference  more  clearly  between  the  epithelial  and  the  non-epithelial 
portions  of  the  mesoderm.  As  we  have  seen,  there  is  no  evidence  of 
a  character  to  render  even  probable  that  the  mesoderm  of  vertebrates 
represents  archenteric  diverticula,  and  the  whole  mesoderm  appears 
as  a  single  germ-layer,  which  is  subsequently  differentiated  into 
mesenchyma  and  mesothelium.  Hence,  both  essential  parts  of  the 
coelom  theory  are  inapplicable  to  vertebrates,  at  least  in  the  present 
state  of  our  knowledge.  For  further  discussion  of  the  difficulties  of 
the  Hertwigs'  theory,  see  Rabl,  89.2,  198-202,  also  Alex.  Goette, 
30ol,  18,  as  well  asp.  140.  The  Hertwigs  recognized  the  signifi- 
cance of  the  parablast  and  added  the  important  rectification,  which 
Flemmings'  observations,  78.2,  had  already  rendered  necessary,  of 
separating  the  smooth  muscles  from  the  striated  skeletal  muscles — a 
separation  the  propriety  of  which  was  wrongly  questioned  by  Bal- 
four,  "Comp.  Embryol.  II.,"  351).  By  this  advance  the  two  groups 
of  mesodermal  tissues  became  properly  delimited. 

C.  Rabl's  theory  of  the  mesoderm  is  based,  it  seems  to  me,  wholly 
upon  his  failure  to  understand  the  law  of  concrescence.  That  the 
mesoderm  appears  (perhaps  in  all  vertebrates)  while  concrescence  is 


156 


THE    GERM-LAYERS. 


going  on  is  well  ascertained ;  consequently,  there  is  an  axial  meso- 
derm (Rabl's  "  gastrales  mesoderm")  where  concrescence  has  taken 
place,  and  a  lateral  mesoderm  (Rabl's  "  peristomales  mesoderm")  in 
the  part  of  the  blastodermic  rim  which  has  not  concresced.  Until 
Rabl  proves  that  his  "  peristomales"  mesoderm  does  not  become  axial 
mesoderm  in  later  stages  his  theory  can  have  no  standing.  Davidoff, 
90.1,  613,  makes  the  best  criticism  of  Rabl's  theory  which  I  have 
seen.  Rabl's  memoir  brings  out  one  point  of  very  great  importance 
for  the  elucidation  of  the  early  stages  of  vertebrates — namely,  that  the 
"  peristomal"  mesoderm,  in  other  words,  that  of  the  blastodermic  rim 
in  selachians,  and  of  the  lips  of  the  anus  of  Rusconi  in  amphibians, 
is  represented  in  the  amniota  by  the  mesoderm  of  the  primitive 
streak.  If  this  interpretation,  which  is  much  strengthened  by  L. 
Will's  researches  on  the  gecko,  89.1,  be  verified,  then  the  primitive 
streak  is  the  homologue  in  amniota  of  the  anus  of  Rusconi,  and  is 
the  region  where  concrescence  is  incomplete ;  the  head-process  is  then 
the  part  where  concrescence  is  finished ;  this  concords  with  the  ob- 
served fact  that  the  head-process  grows  at  the  expense  of  the  primitive 
streak,  as  it  "would  do  if  concrescence  continued. 

Alexander  Goette's  theory,  90.1,  24-33,  is  that  the  walls  of  the 
archenteron  in  Amphioxus  and  the  true  vertebrates  comprise  a 
dorsal  region  which  develops  the  notochord  and  mesoderm,  and  a 
ventral  region  which  develops  the  digestive  tract.  Owing  to  the 
great  amount  ,of  yolk  in  true  vertebrates  the  dorsal  region  is  spread 
so  as  to  lie  upon  the  yolk,  hence  it  is  separated  from  the  yolk  or 
entoderm  by  delamination  instead  of  forming  a  true  evagination  as 
in  Amphioxus.  It  occurs  to  me  that  Goette's  theory  may  be  perhaps 
verified  with  the  modification  that  the  notochordal  canal  corresponds 
to  his  dorsal  region,  the  yolk  cavity  to  his  ventral  region  of  the 
archenteron. 

HatscheWs  germ-band  theory  offers,  to  my  mind,  the  best-founded 
explanation  of  the  vertebrate  mesoderm,  because  it  connects  it  with  the 

mode  of  development  of  the  middle  layer 
in  the  annelids  and  other  invertebrates. 
To  understand  the  theory  we  must  first 
consider  the  formation  of  the  mesoderm 
in  Amphioxus. 

The  ovum  of  Amphioxus  is  discharged 
from  the  body  and  impregnated  external- 
ly; it  is  about  0.105  mm.  in  diameter, 
and,  as  it  contains  only  a  small  amount 
of  yolk,  undergoes  a  holoblastic  segmen- 
tation, which  results  in  a  well-marked 
blastula  stage,  Fig.  60,  followed  by  a 
gastrula  stage.  The  gastrula  elongates, 
the  blastopore  remaining  open  at  the  pos- 
terior extremity.  Differentiations  now 
take  place  by  which  the  ectoderm  forms 
fae  axial  anlage  of  the  nervous  system, 
and  the  entoderm  produces  the  notochord 
and  the  mesoderm — the  three  processes  going  on  simultaneously. 
The  accompanying  Fig.  93  represents  a  cross-section  of  a  larva. 


Md. 


,Ins. 


EC. 


Fia.  93.  — Transverse  section  of  an 
Amphioxus  embryo:  Md,  medullary 
plate;  Ms,  primitive  segment;  Ch. 
notochord;  Ent,  entoderm;  EC,  ecto- 
derm; In,  archenteric  cavity.  After 
B.  Hatschek. 


THEORIES    OF   THE   MESODERM. 


157 


The  ectoderm,  EC,  everywhere  bounds  the  section ;  on  the  dorsal  side 
a  portion  of  the  ectoderm  has  been  separated  off  to  form  the  medul- 
lary plate,  Md,  above  which  is  a  small  cavity.  The  cavity,  In,  of 
the  archenteron  is  irregular,  but  symmetrical  in  outline ;  the  entoderm 
bounding  it  can  be  separated  into  four  parts:  1,  the  lower  portion, 
which  forms  the  permanent  entoderm,  Ent;  2,  the  upper  median  por- 
tion, which  becomes  the  notochord,  Ch,  compare  Chapter  VII. ;  3, 
4th,  two  lateral  portions  constituting  the  diverticula,  Ms;  each  diver- 
ticulum  is  a  separate  pouch,  and  as  the  development  progresses  there 
are  formed  a  series  of  pairs  of  pouches,  stretching  on  either  side  along 
t'he  notochord ;  later  the  pouches  separate  altogether  from  the  archen- 
teron, each  becoming  a  closed  sack ;  the  first  pair  of  pouches,  how- 


EC. 


Mes,. 


Mb: 


FIG.  94.  — Amphioxus  embryo:  A,  side  view;  B.  ventral  view.  EC,  ectoderm;  En,  ento- 
derm: a,  neuropore;  X.  IKTVUUS  system;  Mes,  mesoderm;  A/6,  mesoblast;  1-5,  segments. 
After  B.  Hatschek. 

ever,  retain  their  connection  for  a  considerable  period  with  the 
archenteron,  and  have  been  described  by  older  writers  as  glandular 
organs.  The  development  of  the  pouches  is,  with  the  exception 
noted,  most  advanced  anteriorly,  and  as  we  go  tailward  the  pouches 
are  less  and  less  advanced  in  development,  until,  as  shown  in  Fig. 
04,  they  merge  into  the  general  entoderm  as  a  band  of  cells,  Mes,  the 
last  of  which  is  the  mesoblast,  Mb,  a  large  granular  cell  quite  dis- 
tinct from  the  remaining  cells  of  the  band  or  pouches.  The  pouches 
are  the  primitive  segments  (Urseymente,  mesoblastic  somites  of  Bal- 
four).  In  Amphioxus,  then,  the  mesoderm  arises  from  the  entoderm 
along  two  lines,  and  is  divided  into  paired  hollow  segments  before 
it  is  separated  from  the  entoderm.  Some  writers,  especially  the 
brothers  Hertwig,  think  this  process  of  development  to  be  primitive, 
and  that  the  vertebrate  type  is  derived  from  it.  In  true  vertebrates 
the  mesoderm  arises  on  each  side,  but  also  in  the  axis,  and  becomes 
two  masses  when  the  medullary  groove  and  notochord  appear ;  in 


158  THE    GERM-LAYERS. 

Amphioxus  the  medullary  plate  and  notochord  appear  very  early,  and 
the  division  of  the  mesoderm  may  be  due  to  that  fact.  Amphioxus 
is  undoubtedly  a  lower  type,  but  whether  it  really  preserves  the 
older  type  of  development  in  its  purity  is  doubtful ;  indeed  it  is  prob- 
ably a  tunicate  rather  than  a  vertebrate. 

Hatschek,  in  a  series  of  brilliant  investigations,  has  shown  that  in 
many  bilaterally  symmetrical  invertebrates  the  mesoderm  arises  as 
two  bands  of  cells,  which  subsequently  divide  into  a  series  of  closed 
sacks  (segments) ,  and  which  during  their  own  formation  terminate 
each  in  a  single  large  posterior  cell  (mesoblast) ,  which  throws  off  cells 
to  add  to  the  mesodermal  band  (germ-band,  Keimstreif).  This 
mesoblast,  by  its  appearance  and  position,  appears  to  be  a  derivative 
of  the  entoderm.  As  a  matter  of  speculation  we  may  assume  that 
in  Amphioxus  we  have  the  germ-bands,  but  characterized  by  an  ex- 
ceedingly precocious  segmentation.  We  can  further  assume  that  in 
vertebrates  we  have  the  germ-bands  also,  but  that  they  are  modified, 
1,  by  the  loss  of  the  distinct  terminal  mesoblast;  2,  by  precocious 
fusion  in  the  axial  line,  and  3,  by  extremely  retarded  segmentation. 
A  great  deal  may  undoubtedly  be  said  in  favor  of  these  three  assump- 
tions, which  together  constitute  that  theory  of  the  vertebrate  meso- 
derm," which,  of  the  many  theories,  that  have  been  advanced,  is 
most  likely,  in  my  opinion,  to  prove  of  permanent  value, 


CHAPTER  VII. 
GENERAL  REMARKS  ON  THE  GERM-LAYERS. 

IN  this  chapter  the  general  morphology  and  role  of  the  germ- 
layers,  the  history  of  the  theory  of  the  germ-layers,  and  the  laws  of 
differentiation  are  briefly  considered. 

I.  ROLE  OF  THE  GERM-LAYERS. 

It  has  long  been  known  that  the  bodies  of  embryos  consist  of  dis- 
tinct layers,  which,  in  many  cases,  are  separable  from  one  another, 
so  as  to  be  recognized  in  gross  as  discrete  membranes.  It  is  now 
known  that  all  such  layers  may  be  reduced  to  three  primitive  ones, 
named  the  ectoderm,  mesoderm,  and  entoderm  (by  certain  writers, 
epiblast,  mesoblast,  and  hypoblast).  The  ectoderm  is  a  layer  of 
epithelium ;  so  also  is  the  entoderm ;  the  mesoderm  is  more  complex, 
and  as  we  ascend  the  animal  scale  the  mesoderm  gradually  acquires 
a  greater  predominance  until  in  mammals  *  nearly  the  whole  bulk 
consists  of  mesoderm.  But  in  spite  of  this  change,  the  three  layers 
are  preserved  throughout,  and  their  essential  relations  are  not  altered, 
so  that  we  are  able  to  assert  that  unity  of  organization  without  which 
it  would  be  impossible  to  accept  the  doctrine  of  evolution.  The  dem- 
onstration of  the  homologies  of  the  germ-layers  is  the  most  important 
morphological  generalization  since  the  establishment  of  the  cell- 
doctrine. 

As  the  history  of  all  the  organs  is  given  in  detail  in  other  chapters, 
it  is  unnecessary  to  do  more  here  than  classify  the  tissues  and  organs 
of  the  human  body  according  to  the  germ-layers  from  which  they 
arise.  Now,  in  classifying  organs,  it  is  best  to  rank  them  as  belong- 
ing to  that  layer  from  which  their  functionally  essential  and  char- 
acteristic part  is  derived.  Thus,  although  the  pancreas,  ovary,  and 
spinal  cord  all  contain  connective  tissue,  we  do  not  call  them 
mesenchymal,  but  respectively  entodermal,  mesothelial,  and  ecto- 
dermal.  The  gland  cells  of  the  pancreas  come  from  the  entoderm ; 
the  ova  and  the  Graafian  follicles  come  from  the  mesothelium ;  the 
ganglion  cells  and  nerve  fibres  (axis  cylinders)  from  the  ectoderm. 
Adopting  this  principle  we  may  classify  the  organs  of  the  human 
body  as  follows : 


160 


THE    GERM-LAYERS. 


ECTODERMAL. 

Skin  (epidermis). 
Epidermal  structures : — 
Hairs. 
Nails. 
Glands : — 
Sebaceous. 
Sudorific. 
Salivary. 
Mammary. 
Corneal  epithelium. 
Lens  of  eye. 

Central  nervous  system  : 
Ganglia. 
Nerves. 
Eye:— 
Optic  vesicle. 
Optic  nerve. 
Olfactory  organ. 
Auditory  organ. 
Mouth  cavity : — 
Teeth. 

Hypophysis. 
Anus. 

Chorion : — 
Placenta. 
Amnion. 


MESODERMAL. 

.  Mesothelium. 
Peritoneum. 
Pleurae. 
Pericardium. 
Urogenital. 

Wolffian  body. 

Kidney. 

Testes. 

Ovary. 

Oviduct. 

Uterus. 

Vagina,  etc. 
Striated  muscle. 
.  Mesenchyma. 
Connective  tissues. 
Blood. 

Blood-vessels. 
Lymphatics. 
Spleen. 

Smooth  muscle. 
Fat- cells. 
Marrow. 
Skeleton. 


ENTODERMAL. 

Epithelium    (of    digestive 

tract) . 
Thyroid. 
Thymus. 
Tonsils. 

Trachea  and  lungs. 
CEsophagus. 
Stomach. 
Liver. 
Pancreas. 
Intestine. 
Yolk- sack. 
Caecum. 
Vermix. 
Colon. 
Rectum. 
Allantois : — 

(Bladder) . 
Notochord. 


The  human  body  may  be  defined  as  two  tubes  of  epithelium,  one 
inside  the  other ;  the  outer  tube  (epidermal  or  ectodermal)  is  very 
irregular  in  its  form ;  the  inner  tube  (entodermal)  is  much  smaller 
in  diameter,  but  much  longer  than  the  outer  and  has  a  number  of 
branches  (lung,  pancreas,  etc.),  and  is  placed  within  the  ectodermal 
tube.  Between  these  two  tubes  is  the  very  bulky  mesoderm,  which 
is  divided  by  large  cavities  (abdominal  and  thoracic)  into  two  main 
layers,  one  of  which  is  closely  associated  with  the  epidermis  and 
forms  the  body- wall,  the  somatopleure  of  embryologists ;  the  other 
joins  with  the  entoderm  to  complete  the  walls  of  the  splanchnic 
viscera,  and  constitutes  the  splanchnopleure  of  embryologists.  The 
mesoderm  is  permeated  by  two  sets  of  cavities:  1,  the  heart  and 
blood-vessels ;  2,  the  lymphatic  system.  It  is  also  differentiated  into 
numerous  tissues,  muscle,  tendon,  bone,  etc.,  and  organs,  urogenital 
system.  The  nervous  system,  although  developed  from  the  ecto- 
derm, is  found  separated  from  its  site  of  origin,  and  completely  en- 
cased in  mesoderm. 

As  we  ascend  the  animal  scale,  we  discover  in  all  parts  an  increas- 
ing complexity ;  especially  in  the  nervous  system  is  this  marked,  but 
it  is  even  more  strikingly  shown  by  the  evolution  of  the  mesoderm  in 
relative  size  and  differentiation.  This  important  correspondence 
between  the  organization  of  the  mesoderm  and  the  degree  of  evolu- 
tion of  animals  has  not,  to  my  knowledge,  hitherto  attracted  express 
attention. 

II.  DIFFERENTIATION. 

The  fundamental  law  of  embryology  is  that  the  simple  precedes  the 
complex,  the  general  and  typical  the  special.  Each  germ-layer  is  at 


DIFFERENTIATION.  161 

first  a  simple  layer  of  cells  of  nearly  uniform  character.  In  order  to 
develop  out  of  the  germ-layers  the  complex  organs  of  the  adult  the 
layers  have  to  be  folded  into  various  forms  by  unequal  growth  of  their 
parts,  and  the  cells  composing  them  have  to  be  specialized  some  in 
one  way,  some  in  another.  This  double  process  results  in  the  differ- 
entiation of  the  organs.  Differentiation  may  be  defined  as  the  proc- 
ess of  change  from  homogeneous  to  heterogeneous  structure,  or  as 
an  increase  of  heterogeneity,  since  in  living  organisms  there  is  no  real 
homogeneity.  From  what  has  been  just  said  it  will  be  understood 
that  under  the  present  head  we  have  to  consider,  1,  the  laws  of  un- 
equal growth;  2,  the  general  laws  of  cellular  differentiation,  or,  as  it 
is  called,  histogenesis — the  development  of  tissue. 

The  Relations  of  Surface  to  Mass. — However  much  the 
weight  of  an  animal  increases  during  its  development,  the  ratio  of 
the  free  surface  to  the  mass  alters  but  slightly  from  the  ratio  estab- 
lished when  the  embr}'o  begins  to  take  food  from  outside.  It  is  only 
for  convenience  that  I  express  this  law  in  this  precise  form;  in 
reality,  about  it  our  knowledge  is  scanty  and  our  conceptions  vague. 
According  to  a  geometrical  principle,  when  the  bulk  of  a  body  bounded 
by  a  simple  surface  increases,  the  surface  enlarges  less  than  the 
mass — in  the  simplest  case  of  a  cube,  the  surface  increases  as  the 
square,  the  mass  as  the  cube,  of  the  diameter.  If  in  a  cube  of  unit 
diameter  one  unit  of  surface  bounds  one  unit  of  mass,  then  in  a  cube 
of  three  units  diameter  nine  units  of  surface  will  bound  twenty-seven 
units  of  mass;  the  proportion  in  the  first  cube  is  1 :  1,  in  the  second 
1  :  :>.  To  maintain  the  proper  proportion  in  the  embryo,  simple  en- 
largement is  insufficient,  therefore  the  surface  increases  by  becoming 
more  and  more  irregular.  The  irregularities  are  characteristic  of 
each  organ  and  part,  and  may  be  either  large  or  microscopic.  They 
may  be  conveniently  grouped  under  two  main  heads — projections 
and  invaginations. 

Projection*  are  illustrated  by  the  limbs,  filaments  of  the  gills  in 
fishes,  the  villi  of  the  intestine,  folds  of  the  stomach  in  ruminants, 
etc.  In  every  case  the  projection  is  covered  by  an  epithelium  and 
has  a  core  of  mesodermic  tissue. 

Tn  ruminations  exist  in  much  more  varied  form  and  play  the 
principal  part  in  the  differentiation  of  the  animal  body.  They  may 
be  classified  under  four  principal  heads:  1,  Dilatations:  2,  Diver- 
ticula;  3,  Glands;  4,  Vesicles.  Dilatations  have  considerable  im- 
portance in  embryology;  the  stomach,  lungs,  bladder,  and  uterus 
arise  as  gradual  dilatations  of  canals  or  tubes  of  originally  nearly 
uniform  diameters.  Diverticula  in  the  sense  of  relatively  large 
blind  pouches  also  form  important  organs,  such  as  the  caecum  and 
appendix  vermiformis,  or  the  gall  bladder;  these  structures  arise, 
each  as  a  blind  outgrowth  of  a  canal,  the  walls  of  which  at  a  certain 
point  rapidly  grow  to  form  the  pouch.  Glands,  which  are,  as  first 
shown  by  Johannes  Miiller's  classic  researches,  only  small  diverticu- 
la,  which  end  blindly  and  appear  in  an  immense  variety  of  modifica- 
tions ;  the  manifold  types  of  glands  are  discussed  below  in  a  separate 
paragraph ;  they  constitute  the  largest  class  of  organs  with  which  we 
have  to  deal.  The  glands  are  developed  from  epithelium  and  push 
their  way  into  the  mesoderm  upon  which  the  epithelium  rests,  while 
11 


162  THE   GERM-LAYERS. 

in  dilatations,  and  in  diverticula,  the  epithelium  and  mesoderm  expand 
together.  Vesicles  we  call  those  epithelial  sacs,  which  develop 
somewhat  like  glands  by  growing  into  the  mesoderm,  but  the  mouth 
of  the  invagination  closes  by  the  coalescence  of  the  epithelium,  thus 
shutting  the  cavity.  The  closed  sac  separates  from  the  epithelium 
from  which  it  arose,  and  connective  tissue  grows  between  the  two ;  the 
sac  may  then  undergo  various  modifications.  The  membraneous 
labyrinth  of  the  ear  is  developed  from  the  ectoderm  in  this  way,  as 
is  also  the  lens  of  the  eye.  We  might  perhaps  also  class  the  medul- 
lary canal  under  this  head  (cf.  Chap.  VIII.)  if  we  choose  to  consider 
it  as  a  vesicle  so  much  lengthened  that  it  has  become  a  tube. 

The  Law  of  Unequal  Growth.— The  changing  shapes  of  the 
embryo  and  the  development  of  those  irregularities — projections  and 
invaginations,  which  preserve  the  proper  proportion  between  the  sur- 
face and  mass  of  the  body,  both  depend  upon  the  unequal  growth  of 
the  germ-layers,  especially  in  superficies.  The  expansion  of  a  germ- 
layer  having  the  epithelial  type  of  structure*  may  take  place  by  three 
means:  1,  the  multiplication  of  the  cells;  2,  the  flattening  out  of 
the  cells ;  3,  enlargement  of  the  cells.  In  the  early  stages  of  de- 
velopment the  influence  of  the  first  two  factors  predominates ;  during 
the  later  stages,  especially  after  birth,  the  latter.  Of  the  three  factors 
the  first  is  the  most  important. 

The  unequal  multiplications  of  the  cells  in  all  embryonic  epithelia 
is  the  fundamental  factor  of  development,  and  we  see  it  shaping  out 
the  embryo,  its  organs,  and  the  parts  of  organs,  before  histological 
differentiation  really  begins.  The  distinct  areas  and  centres  of 
growth  which  are  necessary  to  develop  the  human  body  out  of  the 
germ-layers  are  innumerable,  and  their  distribution,  limitations,  and 
interactions  make  up  a  large  part  of  the  subject-matter  of  embryology. 
At  every  turn  of  our  studies  we  encounter  fresh  illustrations.  If  in 
a  limited  area  of  a  cellular  membrane  there  occurs  a  growth  or  ex- 
pansion more  rapid  than  in  the  neighboring  parts,  then  that  area 
is,  as  it  were,  bounded  by  a  fixed  ring,  and  can,  therefore,  find  room 
for  its  own  expansion  only  by  rising  above  the  level  of  the  mem- 
brane ;  thus  when  in  the  embryonic  region  of  the  blastodermic  vesicle 
the  growth  becomes  more  rapid,  the  embryo  begins  to  rise  above  the 
level  of  the  vesicle ;  when,  at  a  certain  point  of  the  surface  of  the 
embryo,  a  steady  and  long-continued  growth  occurs,  the  limb  ap- 
pears, gradually  lengthens  out,  and  enlarges  from  a  small  bud  at  first 
to  a  complete  arm  or  leg.  If  the  departure  takes  place  the  other  way 
we  have  an  invagination  produced ;  thus  for  every  hair  and  every 
gland  of  the  intestine  there  is  a  separate  centre  of  growth. 

The  reason  for  the  unequal  growth  is  unknown.  We  have  not 
even  an  hypothesis  to  offer  as  to  why  one  group  of  cells  multiplies  or 
expands  faster  than  another  group  of  apparently  similar  cells  close 
by  in  the  same  germ-layer.  It  is  no  real  explanation  to  say  that  it  is 
the  result  of  heredity,  for  that  leaves  us  as  completely  in  the  dark  as 
ever  as  to  the  physiological  factors  at  work  in  the  developing  in- 
dividual. 

The  conception  that  the  development  of  an  animal  depends  funda- 
mentally upon  the  unequal  expansion  and  consequent  foldings  and 

*  By  this  limitation  we  exclude  the  mesenchyma,  but  not  the  mesothelium. 


DIFFERENTIATION.  K5o 

bendings  of  the  germ-layers  was  first  suggested  by  the  researches  of 
C.  F.  Wolff  on  the  development  of  the  intestine,  and  was  more 
clearly  recognized  by  Pander,  who  definitely  asserted  that  the  forma- 
tion of  the  embryo  is  affected  by  foldings  of  the  germ-layers.  In  re- 
cent times  His  has  studied  the  problem  very  intently,  and  in  his 
memoir  on  the  chick,  68. 1,  discussed  it  minutely.  In  this  memoir  is 
to  be  found  most  of  what  little  we  know  concerning  embryological 
mechanics. 

The  Classification  of  Glands. — For  a  long  time  it  has  been 
customary  to  divide  glands  into  tubular  and  acinous.  W.  Flem- 
ming,  in  an  admirable  article,  88. 1,  has  shown  that  this  classification 
as  currently  applied  is  untenable,  and  he  proposes  in  its  stead 
another,  the  basis  of  which  is  the  branching  of  the  glands ;  he  makes 
three  primary  divisions:  Single  glands  (Einzeldriisen) ,  which  are 
unbranched;  Single  branching  glands  (verastelte  Einzeldriisen), 
with  a  single  duct  and  the  secretory  portion  branched ;  Compound 
glands  (zusammengesetzte  Driisen),  with  both  the  ducts  and  the 
secretory  portions  branched.  Under  the  first  head  he  includes  the 
follicles  of  the  ovary,  under  the  last  the  seminiferous  tubules ;  but 
the  so-called  sexual  glands  are  not,  properly  speaking,  glands  at  all, 
since  their  products  arise  as  differentiations  of  the  cells,  not  as 
secretions ;  it  can,  I  think,  only  perpetuate  confusion  to  class  them 
with  the  true  glands.  So,  too,  with  regard  to  the  principal  organs 
of  excretion — the  lungs  and  the  kidneys;  the  former  can  certainly  not 
be  regarded  as  a  gland,  since  it  produces  no  secretion,  for  the  water 
and  gases  given  off  by  the  respiratory  organs  are  not  produced  by 
the  pulmonary  epithelium.  The  kidneys  have  more  claim  to  be 
classed  with  the  glands,  since  their  excretion  is  the  direct  product  of 
the  epithelium  of  the  renal  tubules ;  the  ureter  represents  the  duct 
and  the  secretory  portions  (collecting  tubules)  branching,  thus  bring- 
ing them  under  the  second  of  Flemming's  headings.  It  seems  to  me 
more  convenient  to  give  the  kidneys  a  place  apart.  Under  the  head 
of  compound  glands  Flemming  ranks  the  liver,  but  inasmuch  as  the 
gland  cavities  (gall-capillaries)  of  the  liver  form  an  anastomosing 
system  of  canals,  it  is  better  to  put  the  liver  in  a  class  by  itself,  es- 
pecially as  its  development  is  unlike  that  of  any  other  gland.  For 
the  sake  of  completeness  we  may  add  also  the  unicellular  glands, 
such  as  are  found  in  the  lower  vertebrates  and  in  many  invertebrates ; 
these  constitute  a  group  by  themselves,  distinct  from  the  multi cellular 
glands.  The  latter  may  be  divided  into  four  sub-groups :  Simple, 
Branching,  Compound,  Anastomosing.  A  simple  gland  is  one 
consisting  of  a  single  unbranched  epithelial  tube,  ending  blindly  and 
opening  upon  the  epithelial  surface  from  which  the  gland  has  been 
developed ;  a  simple  gland  may  be  tubular,  that  is,  a  canal  of  ap- 
proximately even  diameter;  or  alveolar,  that  is,  with  the  blind  end 
somewhat  dilated ;  or  vesicular,  that  is,  with  the  opening  small,  but 
the  rest  of  the  gland  distended  like  a  cyst.  Even  in  the  simple 
glands  we  usually  find  the  portion  of  the  epithelial  tube  near  the 
orifice  acting  simply  as  a  duct,  while  the  deeper  part  alone  performs 
the  secretory  office,  or  acts  as  the  gland  proper.  The  differentiation 
of  the  duct  is  to  be  regarded,  generally  speaking,  as  the  earliest  and 
most  primitive  specialization  of  a  gland.  A  branching  gland  is  a 


164  THE    GERM-LAYERS. 

simple  gland  with  the  addition  of  branching  of  the  secretory  portion 
proper;  under  this  head  also  we  have  tubular  and  alveolar  glands. 
A  compound  gland  is  a  branching  gland  with  the  addition  of  branch- 
ing of  the  duct.  An  anastomosing  gland  is  a  compound  gland  with 
the  additional  feature  of  the  branches  of  the  secretory  portion  united 
together  so  as  to  form  a  network. 

If  we  apply  this  classification  to  the  glands  of  man,  the  result  may 
be  presented  in  a  tabular  form,  as  follows : 

GLANDS 

A.  UNICELLULAR 

(Found  in  ichthyopsida  and  invertebrata) . 

B.  MULTICELLULAR 

1.  Simple  glands. 

a.  Tubular. 

1.  Lieberkiihn's  follicles. 

2.  Peptic  glands. 

3.  Sweat  glands. 

b.  Alveolar. 

Small  sebaceous  glands. 

c.  Vesicular. 

(Sub-epidermal  glands,  amphibia). 

2.  Branching  glands. 

a.  Tubular.* 

1.  Pyloric  glands. 

2.  Brunner's  glands. 

3.  Mucous  glands. 

4.  Uterine  gland. 

b.  Alveolar. 

1.  Large  sebaceous  glands. 

2.  Meibomian  glands. 

3.  Compound  glands. 

a.  Tubular. 

1.  Salivary  glands. 
2    Pancreas. 

3.  Tear  glands. 

4.  Cowper's  glands. 

5.  Prostate  glands. 

b.  Alveolar,  f 

Milk  glands. 

4.  Anastomosing  glands. 

Liver. 

This  classification  cannot  be  regarded  as  final,  since  it  is  based 
solely  on  the  general  shape  of  the  epithelial  imagination  forming  the 
glands.  We  may  expect  in  its  stead  a  better  classification,  based  on 
other  and  more  essential  characteristics.  The  defects  of  the  above 
arrangement  are  serious,  as  is  strikingly  illustrated  by  the  unnatural 
separation  of  large  and  small  sebaceous  glands.  The  basis  of  classi- 
fication ought  to  be  the  phylogeny  of  the  glands. 

Histological  Differentiation. — The  genesis  of  the  tissues  de- 
pends upon — 1,  the  multiplication  of  cells;  2d,  the  specialization  of 
cells ;  3d,  the  development  of  intercellular  substance.  The  first  of 
the  factors  will  be  discussed  in  a  later  chapter.  The  second  and 
third  are  to  be  considered  here. 

The  first  tissue  to  appear  is  the  epithelium  of  the  ectoderm  and 

*  If  the  kidneys  be  considered  as  glands  they  would  come  under  this  head,  as  branching 
tubular  glands. 

t  If  we  consider  the  lung  as  a  gland  and  the  bronchi  as  ducts,  the  lung  would  come  under 
this  head  as  a  compound  alveolar  gland. 


DIFFERENTIATION.  165 

entoderm;  the  second  form  of  tissue  is  the  mesenchyma,  for  the 
mesothelial  portion  of  the  mesoderm  is  also  epithelium.  Histological 
differentiation,  therefore,  begins  with  epithelium  and  mesenchyma ; 
these  two  primitive  tissues  we  must  consider  separately. 

A.  Epithelium. — In  invertebrates  the  ectoderm  and  entoderm  as 
soon  as  they  become  cellular  consist  each  of  a  single  row  of  polyhedral 
cells,  which  in  the  most  primitive  type  are  of  equal  height.     The  cells 
when  viewed  from  the  surface    are  always  irregular  in    outline, 
usually  five  and  six-sided,  sometimes  seven-sided  or  more,  but  prob- 
ably never  four-sided,  except  occasionally  isolated  cells,  which  as- 
sume that  outline.     When  the  cells  are  not  modified  by  the  pres- 
ence of  yolk,  the  round  or  nearly  round  nucleus  lies  in  the  centre  of 
each  cell.     In  every  epithelial  cell  three  axes  may  be  distinguished, 
two  parallel,  with  one  perpendicular  to  the  surface  of  the  layer,  of 
which  the  cell  forms  a  part.     In  the  primitive  epithelium  the  three 
axis  are  approximately  equal  in  length,  hence  the  tissue  is  said  to  be 
composed  of  "  cubical"  (cuboidal)  cells.     There  is  very  little  substance 
between  the  cells,  and  it  always  remains  relatively  insignificant  in 
epithelium  in  marked  contrast  to  its  development  in  the  mesenchyma. 

In  probably  all  vertebrates  the  ectoderm  and  entoderm  during  seg- 
mentation are  both  several-layered,  but  after  the  close  of  segmenta- 
tion they  soon  become  each  single-layered,  as  we  have  seen.  The 
significance  of  this  modification  of  the  course  of  development  is  un- 
known. 

The  further  differentiation  of  the  epithelial  germ-layers  depends 
on — 1,  the  formation  of  folds,  already  discussed,  p.  161;  2,  changes 
in  the  proportion  of  the  cellular  axes ;  3,  structural  changes  in  the 
cells;  4,  arrangement  of  the  cells  in  several  strata.  Concerning 
the  latter  factors  a  few  words  are  necessary.  The  horizontal  axis 
usually  remain  approximately  equal  in  length,  while  the  perpendicu- 
lar axis  varies  independently  and  to  a  much  greater  extent.  That 
epithelial  cells  are  primitively  equiaxial  may  be  accepted  as  an  axiom. 
Yet  in  vertebrates  there  are  marked  departures  from  this  type  during 
very  early  stages.  From  the  cuboidal  type  arise  the  principal  modi- 
fications known  as  the  "  cylinder"  epithelium  and  the  "  pavement" 
epithelium — names  which  are  unfortunate.  As  regards  the  struct- 
ural differentiation,  we  must  distinguish  between  the  specializa- 
tion of  single  cells  and  that  of  groups  of  cells.  The  former  is 
presumably  the  primitive  form,  since  it  predominates  in  ccelen- 
terates;  the  later  has  been  evolved,  we  must  assume,  by  the  grouping 
of  specialized  cells ;  but  in  the  development  of  a  vertebrate  we  see  al- 
ways a  cluster  of  cells  gradually  differentiated  from  their  fellows, 
and  never  the  cells  first  specialized  and  then  collected  by  migration 
or  otherwise.  Speaking  generally  we  may  say  that  the  higher  we 
ascend  the  animal  scale  the  less  specialization  do  we  find  of  isolated 
cells,  and  the  more  of  groups  of  cells.  This  noteworthy  fact  will,  I 
think,  be  ultimately  found  to  possess  an  important  significance  at 
present  hidden  from  us.  The  development  of  additional  strata, 
which  is  especially  characteristic  of  the  vertebrate  ectoderm,  is  de- 
scribed in  the  chapter  on  the  epidermis. 

B.  Mesenchyma. — The  first    histological    differentiation  of    the 
mesenchyma  in  vertebrates  is  the  separation  of  a  certain  number  of 


160  THE    GERM- LAYERS. 

cells  from  all  attachment  to  their  fellows;  these  cells  are  capable  of 
changing  their  site,  and  during  further  development  they  increase 
in  number  and  variety.  The  first  of  these  cells  to  appear  are  the 
blood-cells  of  the  so-called  blood  islands.  For  all  mesodermic  cells 
not  mechanically  united  to  others,  but  capable  of  change  of  site,  I 
have  assumed  that  the  primitive  type  was  a  cell  capable  of  indepen- 
dent amoeboid  movements,  and  have  proposed  for  them  (Minot,  23, 
207) ,  the  collective  name  of  Mesamceboids — as  a  term  at  once  appro- 
priate and  corresponding  to  a  natural  class  of  tissues.  The  mesa- 
moeboids,  then,  I  regard  as  a  primitive  form  of  the  cells  of  the 
mesoderm,  thus  implying  that  when  amoeboid  cells  are  found  in  the 
higher  metazoa  we  are  dealing  with  those  free  mesodermic  elements 
which  have  been  least  modified  in  the  course  of  development.  Ac- 
cording to  this  view  the  wander  cells  and  white  corpuscles  in  verte- 
brates represent  one  of  the  earliest  tissues  of  the  mesoderm.  As 
already  pointed  out,  the  essential  feature  of  the  mesenchyma  is  that 
its  cells  lie  somewhat  apart  and  are  connected  together  by  protoplas- 
matic processes  running  from  cell  to  cell ;  the  space  between  the  cells 
is  filled  with  a  homogeneous,  structureless,  transparent  substance, 
which  is  at  first  perhaps  merely  a  serous  fluid,  and  which  is  known 
as  the  basal  substance  (Grundsubstanz)  or  matrix.  The  mesenchy- 
mal  matrix  is  the  seat  of  numerous  modifications,  varying  according 
to  the  special  tissue  formed  out  of  the  mesenchyma ;  each  modifica- 
tion of  the  matrix  is  associated  with  the  corresponding  specific  change 
of  the  cells. 

III.  HISTORY  OF  THE  THEORY  OF  THE  GERM-LAYERS. 

The  fundamental  facts  of  the  construction  of  the  vertebrate  body 
out  of  distinct  layers  of  cells  are  collectively  designated  as  the  theory 
of  the  germ-layers.  The  theory  is  as  important  as  the  cell  theory 
for  the  comprehension  of  the  morphology  of  animals.  The  establish- 
ment of  it  is  due  principally  to  Carl  Ernst  von  Baer,  although  it  was 
first  suggested  half  a  century  earlier  by  C.  F.  Wolff,  and  more 
clearly  developed  by  Pander,  from  whom  Von  Baer  drew  his  im- 
mediate inspiration.  Since  Von  Baer's  time  numerous  investigators 
have  contributed  to  our  knowledge  of  the  germ-layers.  If  we  leave 
out  of  consideration  the  introduction  of  the  cell  doctrine,  which  had 
a  profound  influence  on  embryology,  as  upon  every  department  of 
biology,  we  may  distinguish  three  principal  steps  in  the  acquisition 
of  our  present  notions  concerning  the  germ-layers ;  the  first  step  was 
the  recognition  by  Huxley  that  the  coelenterates  are  built  up  of  two 
layers,  and  the  suggestion  that  these  two  layers  are  homologous  with 
the  germ-layers  of  the  higher  animals ;  the  second  step  was  the  formu- 
lation of  the  gastrula  theory  by  Kowalewsky,  and  the  third  step  was 
the  discovery  by  His  that  the  middle  germ-layer  comprises  two  dis- 
tinct groups  of  tissues. 

C.  F.  Wolff  was  the  first  investigator  to  recognize  the  embryonic 
germ-layers,  which  he  did  in  the  course  of  his  study  of  the  develop- 
ment of  the  digestive  canal  of  the  chick.  His  article  was  published 
in  Latin  in  the  "Commentaries  of  St.  Petersburg  Acad.,"  XII., 
XIII.,  1768-1769,  and  shows  that  he  suspected  the  far-reaching 


HISTORY    OF    THE    THEORY    OF   THE    GERM-LAYERS.  167 

significance  of  the  observations  which  taught  him  that  the  intestine 
is  evolved  out  of  a  leaf -like  sheet  in  the  embryo.  Wolff's  article  se- 
cured very  little  notice  from  his  contemporaries,  nor  was  it  until  it 
was  translated  into  German  by  the  elder  Meckel,  and  published  at 
Halle,  in  1812,  that  its  extraordinary  merit  became  recognized.  The 
translation  seems  to  have  awakened  the  interest  of  Dollinger,  a  pro- 
fessor at  Wurzburg  in  the  early  part  of  this  century,  who,  though 
little  known  by  his  own  works,  has  nevertheless  become  distinguished 
through  his  pupils,  foremost  among  whom  are  Pander  and  Von  Baer. 
The  former  in  his  dissertation  (Wurzburg,  1817)  gives  a  history  of 
the  metamorphosis  of  the  hen's  ovum  during  the  first  five  days  of 
incubation,  and  shortly  after  published  his  chief  work  ("  Beitrage  zur 
Entwickelungsgeschichte  des  Huhnchens  im  Eie,"  Wurzburg,  1817), 
the  beautiful  plates  of  which  were  prepared  by  his  friend,  D' Alton. 
Pander  distinguished  in  the  blastoderm  at  first  a  single  layer,  das 
Schleimblatt,  external  to  which,  after  the  twelfth  hour,  appears  the 
serous  layer,  which  is  thinner  and  more  transparent,  and  finally,  at 
the  end  of  the  first  day,  a  third  layer,  the  Gefassschicht,  between 
the  mucous  and  the  serous  layers.  Pander  appears  not  to  have  con- 
tinued his  embryological  researches,  but  to  have  left  that  to  his 
friend  and  fellow-student,  Von  Baer,  who  began  his  own  studies  in 
1819,  and  continued  them  with  some  interruptions  for  ten  years, 
extending  them  gradually  to  other  vertebrates.  In  Von  Baer's 
work  we  have  the  most  profound,  exhaustive,  and  original  contri- 
bution to  embryology,  which  has  ever  been  made,  and  it  is  un- 
questionably one  of  the  greatest  achievements  in  the  history  of 
science.  It  ought  to  be  read  and  pondered  upon  by  every  embryolo- 
gist.  The  work  itself  was  entitled  "  Ueber  Entwickelungsgeschichte 
der  Thiere,  Beobachtung  und  Reflexion. "  Never  again  have  observa- 
tion and  thought  been  so  successfully  combined  in  embryological 
work.  The  first  part  of  Von  Baer's  "Entwickelungsgeschichte" 
appeared  in  1828,  the  second  part  in  1837.  The  second  part  was, 
however,  incomplete  and  appeared  with  the  announcement  of  the 
publishers,  stating  that  they  had  begun  to  print  the  work  in  1829, 
and  after  waiting  five  years  for  manuscript  had  carried  the  printing 
to  the  315th  page,  and  finally,  after  three  years  more  waiting,  pub- 
lished the  incomplete  second  part.  In  1888  the  missing  termination 
of  Von  Baer's  work  was  published  by  Stieda.  It  seems  that  Von 
Baer  had  kept  it  back  in  the  hope  of  filling  up  some  gaps ;  not  suc- 
ceeding in  this  he  waited  too  long,  and  after  the  incomplete  work 
had  been  issued,  Von  Baer  seems  to  have  lost  his  interest  and  to 
have  laid  aside  his  manuscript  for  the  remainder  of  his  long  life. 
Von  Baer  worked  out,  almost  as  fully  as  was  possible  at  this  time, 
the  genesis  of  all  the  principal  organs  from  the  germ-layers,  instinct- 
ively getting  at  the  truth  as  only  a  great  genius  could  have  done. 
Von  Baer  recognized  the  somatopleure,  which  he  called  animates 
Blatt,  and  splanchnopleure,  which  he  called  vegetatives  Blatt, 
and  further  (as  each  of  these  Blatter  consists  of  two  layers)  the 
animales  Blatt  had  a  Hautschicht  (ectoderm)  and  a  Fleischschicht 
(mesoderm),  while  the  vegetatives  Blatt  had  its  Schleimschicht 
(entoderm)  and  Gefassschicht  (mesoderm).  With  this  generaliza- 
tion, and  with  the  detail  of  development  which  he  added,  Von  Baer 


168  THE    GERM-LAYERS. 

created  modern  embryology.  It  was  not  until  after  the  cell  doctrine 
was  announced  in  1838  by  Schwann  that  any  important  progress 
was  made;  0.  B.  Reichert,  40.1,  43.1,  added  something  to  our 
knowledge,  but  the  value  of  his  work  is  greatly  diminished  by  the 
imperfections  of  hi^  observations,  and  still  more  by  his  errors  of 
interpretation.  Perhaps  his  greatest  importance  was  in  his  influ- 
ence upon  Remak,  whose  masterly  investigations  upon  the  differen- 
tiation of  the  uniform  embryonic  cells  into  the  tissues  of  the  adult  at 
once  converted  embryology  into  a  science  closely  allied  to  histology ; 
to  Remak  we  owe  also  the  recognition  of  the  mesoderm  as  a  unit, 
he  having  discovered  that  Von  Baer's  Fleischschicht  and  Gefdss- 
schicht  are  really  parts  of  the  same  layer.  There  followed  next  a 
series  of  minor  investigations  by  sundry  authors,  which,  though  not 
very  numerous,  nevertheless  by  their  gradual  accumulation  afforded 
much  knowledge.  It  is  not  until  1868,  when  His  published  his 
monograph  on  the  chick,  that  anything  fundamentally  new  was 
added  to  our  notion  of  the  germ-layers ;  in  that  work  His  draws  the 
distinction  between  the  archiblast  and  parablast,  see  p.  153. 

From  another  side  progress  was  being  made  by  gathering  materials 
by  the  comparative  study  of  the  germ-layers  throughout  the  animal 
kingdom ;  here  Huxley  led  the  way  by  discovering  the  two  layers 
which  compose  the  body  of  coelenterates — a  discovery  which  he 
announced  in  1849,  adding  at  the  same  time  the  fortunate  suggestion 
that  the  two  layers  are  homologous  with  the  two  primary  germ- 
layers  of  vertebrates.  Four  years  later  (1853)  Allman  proposed  for 
the  two  layers  of  coelenterates  the  terms  ectoderm  and  entoderm, 
which  have  since  come  into  general  use,  not  only  for  these  layers,  but 
for  the  corresponding  germ-layers  throughout  the  animal  kingdom. 
Beginning  about  1845  we  have  a  series  of  researches  on  the  embry- 
ology of  invertebrates,  especially  of  marine  forms.  The  leader  in 
these  studies  was  Johannes  Muller,  whose  memoirs  are  classic  and 
were  published  for  the  most  part  by  the  Berlin  Academy,  1846-1854. 
He  had  numerous  followers,  among  whom  Alexander  Agassiz  and 
Metschnikoff  may  be  mentioned.  The  naturalist,  to  whose  work  in 
this  field  we  owe  most  as  far  as  the  development  of  the  theory  of 
the  germ-layers  is  concerned,  is  Anton  Kowalewsky,  who,  by  a  long 
series  of  well-known  investigations  accumulated  a  vast  amount  of 
evidence  in  favor  of  the  homology  of  the  germ-layers  throughout 
the  animal  kingdom.  Kowalewsky's  investigations  culminated  in 
the  theory  that  the  planula,  or,  as  it  is  now  called,  the  gastrula,  is 
the  primitive  embryonic  type;  he  is  the  originator  of  the  gastrula 
theory,  an  account  of  which  has  already  been  given,  p.  112.  Ernst 
Haeckel's  two  essays,  74.2,  75.1,  contain,  as  already  stated,  exceed- 
ingly little  that  is  really  original  and  valuable.  Lankester's  two 
essays,  73. 1,  77. 1,  are  more  scientific,  and  are  also  noteworthy  from 
having  furnished  us  with  a  considerable  number  of  terms,  which 
have  since  become  current  in  embryology.  Lankester's  essays  are 
further  remarkable  for  containing  the  first  enunciation  of  the  coelom 
theory.  It  will  be  remembered  that  Von  Baer  conceived  the  body 
cavity  to  be  bounded  by  two  distinct  layers,  the  Fleischschicht  and 
Gefassschicht ;  Remak  showed  that  the  coelom  is  bounded  by  one 
layer  only,  the  mesoderm;  Huxley,  75.1,  p.  54,  attempted  to  make 


HISTORY    OF   THE   THEORY    OF   THE   GERM-LAYERS.  169 

clear  the  morphology  of  the  body  cavity  by  distinguishing  three 
types  thereof — 1,  the  enterocoele  or  body  cavity,  arising  as  a 
diverticulum  of  the  alimentary  canal,  such  as  was  then  shown  to  be 
the  case  in  the  echinoderms  and  Sagitta ;  2,  schizoccele,  formed  by 
simple  splitting  of  the  mesoderm ;  3,  epiccele,  formed  by  invagina- 
tion  of  the  outer  wall  of  the  body  like  the  atrial  chamber  of  Tunicata. 
Huxley  suggests,  p.  56,  that  the  coelom  of  vertebrates  might  be  an 
epicoele.  Lankester,  77.1,  maintained  the  opposite  view,  that  the 
vertebrate  coelom  is  an  enterocoele;  for  the  subsequent  history  of 
Lankester's  theory,  especially  as  modified  by  the  Hertwigs,  81.1, 
see  Chapter  VI.,  p,  155. 


PART  III. 

THE   EMBRYO 


CHAPTER  VIII. 

THE    MEDULLARY    GROOVE,    NOTOCHORD,     AND    NEURENTERIC 

CANALS. 

IN  all  vertebrates  there  occur  two  primary  axial  structures  in  very 
early  embryonic  stages:  one  is  the  medullary  canal,  derived  from  the 
ectoderm ;  the  other  is  the  notochord,  derived  from  -^ 
the  entoderm :  as  soon  as  these  two  anlages  have  =| 
appeared  the  mesoderm  disappears  from  the  median 
line,  and  the  previously  continuous  sheet  of  meso-  ?  |  ti^ 
derm  becomes  divided  into  two  wings.  Connected  >g  o  £ 
with  the  early  history  of  the  medullary  canal  and  ||| 
notochord  are  the  temporary  passages  known  as  Jg*| 
the  neurenteric  canals.  For  these  reasons  these  IrS-M 
three  subjects  are  best  treated  together.  p^l 

I.  THE  MEDULLARY  GROOVE.  |c 

I.  The   Medullary  Plate. — By  this  name  we 
designate  the  central  axial  portion  of  the  ectoderm,        £•<! 
which   early  becomes  distinguished  by  its  greater       'g  | 
thickness  from  the  remaining  portions  of  the  layer       &g 
and  which  gives  rise  later  to  the  nervous  system.        5* 
The  ectoderm  of  the  mammalian  embryonic  shield 
and  of  the  sauropsidan  embryonic  area  has  at  first,        |^ 
it  will  be  remembered,  a  considerable  thickness,  for       JT| 
it  consists  of  cuboidal  or  low  cylinder  epithelial       ^|      £ 
cells.     The  stage  which  follows  next  after  the  for- 
mation of  the  primitive  axis  is  characterized  by  the 
gradual  thinning  out  of  the  ectoderm  over  the  peri-       g. 
pheral  portions  of  the  shield  or  area,  while  in  the 
neighborhood  of  the  axial  line  the  full  diameter  of 
the  outer  germ-layer  is  not  merely  retained,  but  is 
actually  increased.     For  a  time  there  is  a  gradual 
passage  between  the  thicker  and  thinner  parts,  but 
as  development  progresses  the  demarcation  rapidly 
becomes  sharper,  Fig.  95,  Md.     Soon  after  its  for-        ~ ! 
mation,  the  interval  varying  according  to  the  spe-       ^ 
cies,    the  medullary   plate  increases  its  thickness       g* 
everywhere  except  along  the  median  line,  thus  be- 
coming double ;  the  thin  median  part  often  shows 
a  slight  groove  which  is  known  as  the  dorsal  fur- 
row (Riickenfurche}.*    This  furrow  does  not  extend       .^j 
clear  to  the  cephalic  end  of  the  plate,  because  there 
the  lateral  thicker  bands  are  continuous  with  one 
another,  the  front  end  of  the  plate  being  rounded  and  clearly  limited. 

*  Riickenfurche  is  also  used  as  a  synonym  of  medullary  groove. 


174 


THE   EMBRYO. 


The  medullary  plate  appears  only  in  the  region  of  the  head-process 
in  amniota,  and  as  the  process  grows  backward  at  the  expense  of  the 
primitive  streak  the  medullary  plate  follows,  hence  it  is  unequally 
developed  throughout  its  longitudinal  extent,  being  always  more 
advanced  headward  and  less  advanced  tailward;  hence  it  is  that 
while  it  is  developing  its  posterior  extremity  is  always  vague  and 
fades  out  into  the  undifferentiated  ectoderm.  So  great  is  this 
inequality  in  mammals  that  we  find  the  front  end  of  the  plate  trans- 
formed into  the  medullary  groove  before  the  hind  end  is  differentiated. 
The  stage  of  development  in  which  there  is  a  well-marked  primi- 
tive streak  and  in  front  of  it  a  medullary  plate  overlying  the  head- 
process  occurs  in  the  rabbit  at 
the  beginning  of  the  eighth 
day.  At  its  hind  end  the  plate 
extends  so  as  to  partly  cover 
the  primitive  streak,  while  in 
front  its  edges  already  rise 
slightly,  so  that  it  constitutes 
a  minute  shallow  trough.  For 
figures  of  a  similar  stage,  age 
unknown,  in  the  mole,  see  W. 
Heape,  83. 1,  Figs.  13  and  14. 
In  older  writers  we  find  figures 
representing  the  medullary 
plate  (or  groove)  and  the  prim- 
itive streak  as  one  structure, 
and  the  dorsal  furrow  in  the 
middle  of  the  plate  as  the 
continuation  of  the  primitive 
groove.  To  illustrate  this  er- 
ror I  present  a  copy,  Fig.  96, 
of  one  of  Bischoff's  figures  of  the  rabbit's  ovum,  in  which  no  dis- 
tinction is  made  between  the  two  grooves,  although  in  reality  the 
dorsal  groove  stops  in  front  of  the  primitive  groove,  the  anterior 
end  of  which  is  often  bent  to  one  side. 

In  the  Sauropsida  the  medullary  plate  is  very  similar  to  that  of 
mammals.  In  both  birds  and  lizards  it  can  be  seen  that  not  the 
whole  of  the  axial  band  of  thicker  ectoderm,  but  only  the  parts 
nearest  the  median  line,  share  in  the  actual  formation  of  the  medul- 
lary groove.  The  differentiation  is  begun  as  in  mammals  by  the 
thinning  out  of  the  ectoderm  in  the  peripheral  regions,  until  it  becomes 
a  thin  pavement  epithelium,  while  about  the  axis  the  cells  become 
elongated  vertically;  pyramidal  cells,  with  the  apex  external, 
alternating  with  those  with  the  apex  internal,  thus  producing  a 
peculiar  appearance  on  sections  and  causing  the  nuclei  to  form  two 
layers ; .  the  single  cells  are,  of  course,  irregular  in  shape.  In  birds 
and  reptiles,  as  in  mammals,  the  medullary  plate  overlies  the  head- 
process  and  becomes  well  marked  off  in  front,  while  it  is  still  being 
differentiated  posteriorly,  compare  Fig.  97. 

The  Medullary  Groove. — Almost  or  quite  as  soon  as  the 
medullary  plate  is  formed,  its  edge  becomes  elevated  in  front  and 
on  each  side ;  hence  it  forms  an  open  trough,  known  as  the  medul- 


FIG.  96. —Blastoderm  of  Rabbit's  Ovum;  after 
Bischoff.  The  dorfeal  and  primitive  grooves  are  rep- 
resented as  a  single  continuous  line. 


THE   MEDULLARY   GROOVE. 


175 


lary  groove,  Fig.  97,  Md.gr.  During  this  process  the  medullary 
ectoderm  increases  considerably  in  thickness,  and  at  the  same  time 
the  nuclei  multiply  and  lie  irregularly  scattered  at  varying  heights. 
The  ectoderm  alongside  the  medullary  plate  or  groove  thins  out 
still  farther.  Inasmuch  as  the  development  is  most  rapid  in  the 


Seg 


FIG.  97.— Chicken  Embryo  with  Seven  Primitive  Segments  (Minot  Coll.,  Embryo  A  J,  sections 
811,  212,  172.  compare  Figs.  81  and  147).  A,  Section  through  one  of  the  segments;  B,  section 
posterior  to  the  segments;  C,  section  just  in  front  of  the  primitive  streak.  Md.  gr,  medullary 
groove ;  nch,  notochord ;  EC,  ectoderm ;  mes,  mesoderm ;  En,  entoderm.  x  about  230  diams. 

head  end  of  the  embryo,  there  comes  a  stage  in  which  there  is  a  well- 
marked  medullary  groove  in  front,  a  medullary  plate  behind  that, 
and  a  primitive  streak  at  the  hind  end  of  the  embryo ;  but  when 
the  streak  has  disappeared  the  medullary  groove  is  found  to  extend 
the  entire  length  of  the  embryo.  There  is  then  a  stage  in  which, 


176 


THE   EMBRYO. 


••Ent. 


by  means  of  a  series  of  transverse  sections,  Fig.  97,  of  the  embryo, 
we  may  study  the  successive  steps  in  the  development  of  the 
medullary  groove.  This  stage  is  found  in  the  rabbit  at  nine  days ; 
in  the  chick  at  thirty  to  forty  hours  of  normal  incubation. 

The  medullary  groove  gradually  deepens,  its  sides  rising  higher 
and  higher  and  arching  more  and  more  toward  one  another  until 
the  edges  meet  and  coalesce,  thus  changing  the  groove  into  a  tube. 
The  process  is  illustrated 
by  the  series  of  sections 
through  a  chicken  embryo 
with  seven  segments 
shown  in  Fig.  97. 

In  some  mammals  the 

medullary  grOOVe  becomes  FlG.  98._Part  of  a  Transverse  Section  of  a  Young  Mole 
Well  developed,  Fig.  98,  Embryo.  After  Heape.  Md,  Medullary  groove;  EC,  ecto- 
,  „  ',  &  ,  .  '  derm ;  Mes,  inesoderm ;  Ent,  entoderm. 

before  the  medullary  plate 

is  clearly  marked  off  by  the  thinning  out  of  the  ectoderm  alongside 
of  it;  the  groove  is  also  much  larger,  Fig.  98,  in  proportion  to  the 
size  of  the  embryo,  Fig.  99,  than  is  the  case  in  the  large  ova  of  birds 
and  reptiles.  The  anterior  end  of  the  groove  is  wide  open  and  ex- 
panded on  each  side;  this  lateral  spreading  is  the  anlage  of  the  optic 
diverticulum,  Fig.  99,  op,  and  is  transformed  later  into  the  optic 
vesicle,  which  is  an  essential  component  of  the  future  eye.  A  sec- 
tion through  the  optic  grooves  of  a  mole  embryo  a  trifle  older  than 
Heape's  stage,  F,  Fig.  99,  is  shown  in  Fig. 
»"°P--  100.  The  medullary  plate  is  thickened  and 

shows  a  median  lesser,  and  two  lateral  greater 
depressions ;  the  former,  M d,  is  the  medullary 


FIG.  99. — Surface  View 
of  a  Young  Mole  Embryo 
(stage  F,  196mm.).  After 
Heape.  op,  Optic  diver- 
ticula;  Mr,  medullary 
ridge  or  edge  of  medul- 
lary groove;  Md,  medul- 
lary groove  widely  open. 


FIG.  100.  — Tranverse  Section  of  a  Mole  Embryo 
(Heape's  stage  F).  Md,  Medullary  groove  proper; 
op,  optic  groove ;  EC,  ectoderm ;  Mes,  mesoderm ;  En, 
entoderm;  nch,  notochord. 


groove  proper ;  the  latter,  op,  do  not  participate  in  the  brain  forma- 
tion, but  in  that  of  the  eye ;  at  the  edge  of  the  optic  anlage  the  plate 
passes  abruptly  into  the  much  thinner  entoderm.  For  some  distance 
behind  the  optic  anlage  the  edges  of  the  medullary  groove  are  almost 
in  contact,  Fig.  99,  but  farther  back  the  grove  is  again  wide  open; 
this  widely  open  part  is  known  as  the  sinus  rhomboidalis,  which  is 
not  to  be  confused  with  the  sinus  rhomboidalis  of  the  neck,  for  the 
term  is  also  applied  to  the  cavity  of  the  embryonic  fourth  ventricle 
of  the  brain ;  the  sinus  here  described  belongs  to  the  future  lumbar 
region.  The  swelling  in  the  floor  at  the  hind  end  of  the  sinus  is 
caused  by  the  mesoblast  of  the  front  end  of  the  primitive  streak. 


THE   MEDULLARY   GROOVE. 


177 


On  either  side  of  a  rabbit  or  opossum  embryo,  in  a  stage  a  little 
more  advanced  than  in  Fig.  99,  just  behind  the  open  anterior  end 
of  the  canal,  there  extends  a  longitudinal  ridge  corresponding  to  the 
anlage  of  one  of  the  two  tubes  which  will  eventually  form  the  heart, 
see  Chapter  XI.  The  lateral  heart  anlage  of  the  opossum  is  shown 
in  section,  Fig.  95,  Ht. 

In  a  mole  embryo,  a  little  older  than  Fig.  99,  the  hinder  portion  of 
the  medullary  canal  is  much  the  same  as  before ;  anteriorly,  how- 
ever, development  has  progressed  and  the  edges  of  the  medullary  folds 
have  come  together  and  partially  fused  at  the  anterior  end  of  the 
embryo,  owing  to  the  more  rapid  growth  of  the  sides  than  of  the 
floor  of  the  canal  as  pointed  out  above.  At  the  extreme  end,  how- 
ever, a  pore  is  left.  At  this  stage,  therefore,  the  neural  canal  is  still 
open  to  the  exterior,  both  anteriorly  and  posteriorly.  The  optic 
grooves  are  now  closed,  and  have  given  rise  to  the  optic  vesicles ; 
these  are  shown  as  two  bud-like  vesicles  projecting  outward  and  back- 
ward and  slightly  downward  from  the  front  end  of  the  neural  tube ; 
behind  them  the  swelling  of  the  fore-brain  is  discernible,  while  still 
farther  backward  and  at  the  edge  of  the  body  of  the  embryo  the 
two  tubes  of  the  heart  are  indicated.  The  folding  off  of  the  embryo 
from  the  yolk-sac  has  at  this  stage  made  some  progress,  and,  indeed, 
the  whole  of  the  head  of  the  embryo  now  projects  freely  above  the 
blastodermic  vesicle.  In  the  next  stage  (H,  embryo  2.2  mm.)  of  the 
mole  the  edges  of  the  medullary  plate  have  met  and  united,  making 
the  medullary  groove  in  front  into  a  canal,  but  the  sinus  rhomboidalis 
is  still  open,  though  beginning  to  close.  The  closure  of  the  groove 
begins  in  the  cervical  region  and  spreads  forward  and  more  slowly 
backward;  where  the  closure  takes  place  last  in  front  is  known  as 
the  neunntonis;  the  position  of  the 
neuroporus  is  presumably  the  same  in 
all  amniota  if  not  in  all  vertebrates. 
Van  Wijhe,  84.1,  finds  that  in  the 
duck  the  Connection  with  the  ectoderm 
is  retained  in  front  longest  in  the  re- 
gion of  the  first  cerebral  vesicle,  and 
not  in  that  of  the  mid-brain,  so  that 
it  has  nothing  to  do,  as  some  have 
suggested,  with  the  development  of  the 
pineal  gland  (epiphysis) .  This  connec- 
tion represents  the  final  anterior  clos- 
ure; Van  Wijhe  speculates  that  it 
was  an  opening  in  the  ancestors  of 
vertebrates  and  terms  it  the  anterior 
neuroporus. 

The  medullary  groove  of  Amphi- 
bia has  been  more  fully  studied  than 

that  of  any  Other  claSS.      The  most  COm-     medullary  plate  f  m/c,   cepKalic  portion 
1    j       v  •   i  ,  T      ,  i          A  ••  of  medullary  fold.     After  S  F.  Clarke 

plete  history  is  that  given  by  Alex. 

Goette  for  Bombinator,  75.1,  158-176;  see  also  Scott  and  Osborn, 
79.1,  S.  F.  Clarke,  80.1,  Rusconi,  Moquin-Tandon,  76.1,  Ecker's 
"  Icones,"  Taf.  XXIII.,  and  others.  In  all  Amphibia  the  medullary 
plate  is^  very  wide,  indeed,  Fig.  101,  being  broadest  in  front. 


178 


THE   EMBRYO. 


the  surface  of  the  central  canal. 


mes 


Its  margin  is  thrown  up  into  a  slight  but  broad  ridge ;  when  the 
plate  closes  to  form  a  canal  the  surfaces  of  the  marginal  ridges  grow 
together  and  the  surface  of  the  plates  within  the  ridges  becomes 

In  all  Amphibia  the  central  dorsal 
groove,  mg,  is  very  distinct. 
As  the  ectoderm  of  the  amphib- 
ian ovum  very  early  becomes 
distinctly  two-layered,  it  results 
that  in  the  medullary  plate  the 
two  layers  can  be  recognized 
from  the  start ;  the  outer  layer 
(Goette's  Deckschicht,  Bal- 
four's  epidermic  stratum)  of 
course  lines  the  medullary  cav- 
ity and  alone  forms  the  epithe- 
lium of  the  cen  tral  canal.  When 
the  groove  closes  the  lumen  of 
the  canal  is  nearly  circular  in 
section,  but  it  soon  changes  into 
a  narrow  vertical  slit  similar  to 

FIG.  102. -Part  of  a  Transverse  Section  of  an  the  lumen  °f  the  amniote  Canal. 
Axolotl  Embryo.  After  Bellonci.  Mes,  Mesoderm;  The  round  Cavity  IS  due  to  the 
coe,  coelom ;  Ma,  medullary  groove ;  EC,  ectoderm ; 
ti  entodermal  or  archenteric  cavity :  Ch,  noto- 


OK' 


way  in   which   the   medullary 
cate'd  lnrafewyofktheeSiis.The  deutoplasm  is  indi'  plates  curl  up,  as  shown  in  Fig. 

102.     As.  pointed  out  by  Alex. 

Goette,  75.1,  160,  the  lateral  portion  of  the  medullary  plate  arises 
by  delamination,  a  peculiarity  which  has  an  important  bearing,  I 
think,  on  the  discussion  of  the  evolution  of  the  medullary  canal,  see 
p.  179.  Finally,  in  the  Amphibia,  the  medullary  plate  extends  to 
the  middle  of  the  blastopore,  and,  it  is  maintained  by  some  writers, 
extends  beyond  it,  so  as  to  completely  surround  it.  This  point  is 
recurred  to  in  connection  with  the  history  of  the  anus. 

The  Medullary  Canal. — The  medullary  canal,  as  stated,  arises 
by  the  closure  of  the  groove.  The  canal  closes  in  the  cervical  region 
first,  hence  it  has  at  one  time  two  free  openings ;  as  the  closure  pro- 
gresses the  anterior  region  is  completed,  while  the  sinus  rhomboidalis 
is  still  open ;  moreover,  we  see  that  the  anterior  end  achieves  consider- 
able differentiation  before  the  posterior  end  of  the  canal  is  closed. 
Of  the  entire  length  of  the  primitive  canal  about  one-half  is  the 
anlage  of  the  brain,  while  the  other  half  forms  the  spinal  cord.  In 
the  development  of  the  brain  the  transverse  expansion  of  the  canal  is 
most  conspicuous,  while  in  the  development  of  the  spinal  cord  the 
elongation  of  the  canal  predominates.  The  dilatation  of  the  brain 
part  begins  very  early,  and  comprises  at  first  a  general  dilatation  of 
the  whole  anlage,  and,  second,  special  and  greater  dilatation  of  three 
regions ;  the  three  dilatations  are  known  as  the  three  primary  cere- 
bral vesicles  (Hirnblasen) ,  and  are  designated  as  fore-brain  (Vor- 
derhirn,  prosencephalon) ,  mid-brain  (Mittelhirn,  mesencephalon) , 
and  hind-brain  (Hinterhirn,  metecephalon) ,  respectively.  The  first 
vesicle  is  much  the  widest,  and  appears  in  mammals  and  probably  in 
all  vertebrates  very  early ;  in  mammals  it  shows  itself  plainly  in  the 
medullary  groove  as  already  noted.  When  the  groove  closes  the  canal 


THE   MEDULLARY    GROOVE. 


179 


is  of  course  attached  to  the  ectoderm,  Fig.  9-2,  but  this  connection  is 
soon  severed,  and  the  medullary,  or,  as  it  also  called,  neural  canal, 
becomes  an  independent  structure  lying  inside  the  external  ectoderm 
of  the  embryo,  and  surrounded  by  mesodermic  cells,  which  subse- 
quently grow  in  between  the  canal  and  the  ectoderm  so  that  the 
canal  comes  to  lie  farther  and  farther  away  from  the  surface,  Fig.  103. 
The  structure  of  the  medullary  canal  in  early  stages  has  been  as 
yet  but  imperfectly  studied.  The  wall  increases  steadily  in  thick- 
ness, except  in  certain  parts  of  the  brain.  Where  it  thickens  its  nu- 
clei multiply  and  form  several  irregular  layers ;  the  cell  bodies  around 
the  nuclei  are  small  and  connected  by  numerous  processes,  so  as  to 
produce  a  protoplasmic  network ;  the  protoplasm  and  nuclei  next  the 
lumen  early  assume  the  character  of  epithelial  cells,  so  that  the  cavity 
of  the  medullary  canal  is  lined  by  a  distinct  epithelial  layer ;  this  layer 
corresponds  to  the  outside  layer  of  the  ectoderm  (epidermis) ;  in 
some  parts — as,  for  instance,  the  dorsal  wall  of  the  fourth  ventricle — 
the  single  epithelial  layers  constitute  the  entire  medullary  wall.  The 


Cho 


FIG.  103.— Transverse  Section  of  a  Rabbit  Embryo  of  Eight  Days  and  Two  Hours.  Afd,  Medul- 
lary canal;  Seg,  primitive  segment:  Cho,  chorion;  Am,  amnion:  Som,  somatopleure ;  Coe,  coe- 
lom:  Spl,  splanchnopleure ;  Ent,  entoderm;  Ch,  notochord;  Ao,  aorta. 

non-epithelial  cells  of  the  canal  become,  as  described  in  Chapter 
XX VII.,  ganglion  cells.  The  nuclei  of  the  medullary  canal  wall  are 
oval,  their  long  axis  being  more  or  less  nearly  perpendicular  to  the 
surface  of  the  canal ;  each  nucleus  contains  one  or  several  nucleoli. 
The  canal  is  primarily  oval  in  section,  but  its  lumen  is  a  narrow 
fissure,  Fig.  103,  hence  the  walls  are  thickest  at  the  sides,  and  thinner 
dorsally  and  ventrally ;  this  peculiarity  dominates  to  a  marked  de- 
gree the  subsequent  development  of  the  brain  and  spinal  cord. 

Evolution  of  the  Medullary  Canal.* — Under  this  head  we 
have  to  consider,  first,  what  is  the  primitive  vertebrate  type  of  the 
central  nervous  system  ;  second,  what  genetic  relation  existed  between 
the  vertebrate  and  the  invertebrate  type. 

The  opinion  generally  accepted  by  embryologists  is  that  the  typical 
vertebrate  canal  is  formed  by  the  closure  of  the  medullary  groove. 

*  Originally  published  in  the  American  Naturalist,  Nov.,  1889. 


180  THE   EMBRYO. 

This  view  is  advocated  by  Balfour,  and  has  been  so  thoroughly 
accepted  by  Adam  Sedgwick,  that  he  has  made  it  the  basis  of  a  spec- 
ulation, 83.1,  on  the  original  function  of  the  canal;  he  supposes  that 
it  was  open  behind  and  excretory  ;  the  cilia  which  are  found  in  the 
central  canal  of  the  spinal  cord  originally  served  to  produce  the  excre- 
tory current.  Van  Wijhe,  84.  1,  has  advanced  independently  almost 
the  same  hypothesis.  Both  of  these  speculations  overlook  the  serious 
difficulty  of  assuming  that  the  canal  is  primitive,  while  in  the  lowest 
vertebrates  it  is  clearly  a  secondary  modification.  In  Petromyzon, 
Lepidosteus,  and  Teleosts,  the  medullary  plate,  instead  of  becoming 
the  floor  of  an  external  groove,  forms  a  solid  keel-like  projection 
toward  the  ventral  surface.  This  keel  subsequently  becomes  sepa- 
rated from  the  superficial  layers  of  the  ectoderm,  and  afterward  a 
central  canal  is  developed  in  it.  In  the  ganoids,  which  approach  the 
elasmobranchs  in  structure,  there  is,  as  shown  by  Salensky,  81.1,  a 
medullary  groove  of  peculiar  form,  which  suggests  a  transition  from 
the  solid  keel  to  the  open  groove  ;  again  in  Amphibia  there  is  evidence 
that  the  delamination  is  still  preserved  to  a  slight  extent  in  that 
group.  These  considerations  lead  me  to  the  hypothesis  that  the  ner- 
vous system  of  vertebrates  was  primitively  a  solid  axial  thickening  of 
the  ectoderm,  and  within  the  class  of  ganoids  became  modified  into  a 
groove  perhaps  simply  by  more  precocious  development  of  the  central 
canal  ;  the  groove  type  has  been  kept  in  elasmobranchs,  amphibians, 
and  amniota.  Balfour,  "Comp.  Embryo!.,"  II.,  303,  thus  defends 
the  opposite  view  :  "  It  seems  almost  certain  that  the  formation  of  the 
central  nervous  system  from  a  solid  keel-like  thickening  of  the 
epidermis  is  a  derived  and  secondary  mode,  and  that  the  folding  of 
the  medullary  plate  into  a  canal  is  primitive.  Apart  from  its  greater 
frequency  the  latter  mode  of  formation  of  the  central  nervous  system 
is  shown  to  be  the  primitive  type  by  the  fact  that  it  offers  a  simple 
explanation  of  the  presence  of  the  central  canal  of  the  nervous  sys- 
tem ;  while  the  existence  of  such  a  canal  cannot  easily  be  explained 
on  the  assumption  that  the  central  nervous  system  was  originally  de- 
veloped as  a  keel-like  thickening  of  the  epiblast"  (epiblast-ectoderm)  . 

It  is  not  possible  at  present 
to  decide  positively  between 
the  two  views,  but  the  view 
which  I  am  inclined  to  adopt 
is  further  justified  by  the 
development  of  the  central 
nervous  system  in  Annelids, 
which  is  formed  by  the  co- 
alescence of  a  pair  of  linear 
cords  ;  these  cords  arise  each 
*  side  of  a  ciliated  longitudinal 

FIG.  104.—  Part  of  a  Transverse  Section  of  an  Embryo     /?  r> 

of  Lumbricus  Trapezoides.    After  Kleinenberp.     En.     turrOW,   first  as    a  Single  TOW 


Entoderm;  EC,  ectoderm;  nn.  anlages  of  the  nervous  nf     pp+nrlprrrml      rv>ll« 

system;  F,  cells  of  the  ciliated  band   separating  the  (  LiS' 

two  parts  of  the  nervous  system;  c,  c',  parts  of  the  queiltlv  as  Several  TOWS  I 
coelom;  m.c,  mesodermal  cords.  T_*I  j_'n  -±  j  A.  j.i 

while  still  united  to  the  ex- 

ternal ectoderm  they  extend  toward  one  another  inside  the  ciliated 
cells  of  the  furrow,  and  unite  in  a  single  nervous  band.  The  origin 
of  the  annelidan  nerve  cord  is  illustrated  by  Fig.  104,  which  repre- 


THE   XOTOCHORD.  181 

sents  a  transverse  section  of  the  embryo  of  an  earthworm,  at  a  stage 
during  which  the  cells,  n  n,  that  are  to  form  the  nerve  cords  are 
still  part  of  the  superficial  ectoderm,  EC,  though  their  future  separa- 
tion is  already  indicated.  In  leeches  and  arthropods  the  develop- 
ment is  very  similar.  In  all  these  cases  the  bands  split  off  from  the 
ectoderm.  It  appears  then  that  in  the  nearest*  invertebrate  allies  of 
the  vertebrates,  the  nervous  system  develops  as  a  thickening  along 
the  inner  surface  of  the  ectoderm,  and  delaminates  from  that  layer. 
It  seems  to  me  very  natural  to  suppose,  therefore,  that  the  strikingly 
similar  process  in  the  lowest  vertebrates  is  the  primitive  one,  and 
that  the  canalization  of  the  medullary  plate  was  evolved  within  the 
vertebrate  series. 

I  have  assumed  that  the  ventral  nerve  cords  of  annelids  are  homolo- 
gous with  the  medullary  canal,  a  view  that  is  now  generally  accepted 
by  embryologists.  Balfour  (Works,  I.,  393,  and  "  Comp.  Embryol." 
II.,  311)  has  suggested  a  more  complicated  relation  in  his  hypothesis 
that  the  lateral  nerve  trunks,  which  are  known  in  many  of  the  lower 
worms  (e.  </.,  nemerteans,  have  fused  on  the  ventral  side  in  annelids, 
but  on  the  dorsal  side  of  the  body  in  the  vermian  ancestors  of  verte- 
brates. In  favor  of  this  ingenious  surmise  no  evidence  has  since 
been  found.  Hubrecht  denies  the  homology  of  the  annelidan  nerve 
chain  and  the  vertebrate  medulla;  he  considers,  87.1,  620-624,  that 
the  more  primitive  condition  is  represented  by  certain  nemertean 
worms,  which,  beside  two  main  lateral  nerves,  have  a  small  longi- 
tudinal median  nerve;  the  lateral  nerves  gave  rise  to  the  nerve 
chain  of  annelids  by  their  fusion,  the  median  nerve  to  the  medulla 
of  the  ancestors  of  vertebrates.  As  no  intermediate  forms,  either 
adult  types  or  embryonic  stages,  are  known  to  represent  any  phases 
of  this  double  metamorphosis,  I  cannot  admit  that  Hubrecht's  bold 
speculation  invalidates  what  seems  to  me  the  well-established  ho- 
mology between  annelids  and  vertebrates. 

The  remarkable  hypothesis  of  W.  H.  Gaskell,  90. 1,  that  the  med- 
ullary canal  is  homologous  with,  and  derived  from,  the  entoder- 
mal  canal  of  Crustacea,  seems  to  me  unwarrantable. 

II.  THE  NOTOCHORD. 

As  the  notochord  is  a  purely  embryonic  structure,  I  present  its 
complete  history  here. 

The  notochord  (chorda  dorsalis,  Wirbelsaite)  is  a  rod  of  peculiar 
tissue,  constituting  the  primitive  axial  skeleton  of  vertebrates.  It 
begins  immediately  behind  the  pituitary  body  (hypophysis)  and 
extends  to  the  caudal  extremity.  It  occurs  as  a  permanent  structure 
in  the  lower  type,  and  as  a  temporary  one  in  the  embryos  of  amphibia 
and  amniota,  including  man.  Comparative  embryology  has  shown 
that  it  is  a  greatly  modified  epithelial  band  which  arises  in  the 
median  dorsal  line  of  the  entoderm,  being  in  position  and  mode  of 
development  analogous  to  the  ectodermal  medullary  canal,  or  primi- 
tive tubular  nervous  system . 

Numerous  embryological  articles  contain  observations  on  the  noto- 
chord. The  following  references  may  assist  students.  The  best 

*  With,  of  course,  the  possible  exception  of  Amphioxus. 


182  THE    EMBRYO. 

general  discussion  is  by  Balfour,  in  his  "  Comparative  Embryology;" 
the  best  observations  on  its  origin  in  mammals  is  by  Heape,  83.1, 
for  descriptions  of  the  chorda  canal  see  Lieberkuhn,  82.1,  84.1; 
Carius,  88.1,  and  VanBeneden,  88.3;  for  its  histology,  W.  Muller, 
71.2;  for  its  histogenesis,  A.  Goette,  75. 1,  349-361 ;  for  its  anterior 
anatomical  relations  see  Mihalkowics,  74.1,  75.1,  Froriep,  82.1, 
Rabl-Riickhard,  80. 1 ,  and  Romiti,  86.1;  for  its  atrophy  in  mammals 
see  Leboucq,  80.1;  for  its  evolution  see  Ehlers,  85.1. 

Origin  from  Notochordal  Canal. — The  notochord  appears 
very  early  in  the  course  of  development;  its  differentiation  from  the 
median  dorsal  wall  of  the  notochordal  canal  begins  at  the  time  when 
the  medullary  groove  is  not  fully  marked  out  posteriorly,  and  is 
nowhere  closed.  The  notochordal  anlage  can  be  first  detected  just 
in  front  of  the  primitive  streak  as  an  axial  band  of  cells,  which  at 
first  is  not  well  marked  off  from  the  mesoderm ;  this  band  forms  the 
median  dorsal  wall  of  the  blastoporic  canal  in  all  vertebrates  in  which 
that  canal  has  been  identified.  The  differentiation  of  the  notochordal 
cells  begins  usually  at  the  anterior  end  of  the  canal  and  progresses 
backward,  as  the  blastopore  moves  backward  during  concrescence. 
The  differentiation  varies  as  to  the  time  of  its  beginning ;  it  may 
begin  in  the  unconcresced  embryonic  rim,  as  in  Scyllium,  or  much 
later,  as  in  Lacerta. 

As  the  medullary  groove  (or  keel)  deepens,  it  pushes  down  toward 
the  mid-gut  until  it  comes  into  actual  contact  with  the  notochordal 
epithelial  band,  thus  dividing  the  mesoderm  into  two  lateral  masses, 
Fig.  97,  one  on  each  side;  this  also  leads  to  the  temporary  transverse 
stretching  of  the  notochord. 

Lieberkuhn,  82. 1 ,  84. 1 ,  has  directed  attention  to  a  special  peculiar- 
ity in  the  early  development  of  the  notochord  in  mammals.  The  noto- 
chordal canal  is  formed  throughout  its  length  and  then  breaks  through 
at  various  points  to  fuse  with  the  yolk  cavity,  so  that  it  may  be 
described  as  a  tube  running  along  the  median  line,  and  having  an 
irregular  series  of  openings  on  its  ventral  side.  The  canal  is  lined 
by  epithelium,  which  is  thickened  on  the  dorsal  side  to  form  the 
anlage  of  the  notochord.  In  transverse  section  the  chorda  appears 
according  to  the  level  of  the  section  to  constitute  part  of  a  furrow  or 
a  canal  (compare  also  Heape,  /.  c.,  p.  441,  Fig.  40,  41).  Lieberkuhn 
calls  this  canal  mesoblastic,  and  Kolliker  follows  him  in  so  doing, 
but  this  opinion  seems  to  me  based  upon  misconception.  Indeed, 
C.  Giacomini's  researches,  88.1,  show  that  the  canal  terminates  in 
the  rabbit  in  a  blastopore,  and  Van  Beneden,  88.3,  has  emphasized 
the  fact  that  the  canal  helps  to  form  the  definitive  archenteron. 
After  the  notochordal  canal  has  fused  with  the  yolk  cavity,  the  noto- 
chordal anlage  is,  of  course,  incorporated  in  the  entoderm  of  the 
main  archenteric  cavity,  and  appears  as  the  median  dorsal  portion 
of  the  entoderm.  It  early  acquires  a  sharp  demarcation  and  becomes 
considerably  thicker,  Fig.  105,  than  the  adjoining  entoderm,  and 
forms  a  distinct  though  shallow  groove. 

Separation  from  the  Entoderm. — The  notochordal  band  sep- 
arates off  and  the  entoderm  proper  closes  across  under  it,  so  that 
the  notochordal  band  lies  between  the  entoderm  and  the  floor  of  the 
medullary  groove  (or  later  canal)  as  shown  in  Figs.  106,  103,  and  97 


THE    NOTOCHORD. 


183 


Vta. 


FIG.  105.  —Transverse  Section  of  a  Mole  Embryo,  Stage  H. 
After  Heape.  am,  Amnion;  Md,  medullary  groove;  My, 
myotome ;  Coe,  coelom  or  body  cavity ;  En,  entoderm ;  nch, 
notochord;  ao,  aorta;  vt.a,  vitel line  artery;  Som,  somatic 
mesoderm ;  Spl,  splanchnic  mesoderm. 


A.  This  separation  does  not  take  place  at  the  anterior  extremity  of 
the  chorda  until  somewhat  later,  so  that  for  a  considerable  period  its 
front  end  remains  fused  with  the  walls  of  the  archenteron,  Fig.  106. 
Selenka,  87. 1,  observed  that  this  front  end  of  the  notochord  becomes 

dilated   in    the   opossum 

Md-  and   hollow;   the  hollow 

end  subsequently  forms 
an  irregular  sac  opening 
into  the  anterior  end  of 
the  intestinal  cavity ; 
Selenka  names  the  sac 
the  Gaumenst asche ;  it 
opens  behind  the  partition 
which  closes  the  mouth 
and  is  entirely  distinct 
from  the  hypophysal  eva- 
gination.  Further  inves- 
tigations led  to  the  dis- 
covery of  traces  of  a 
similar  canalization  of  the  front  end  of  the  notochord  in  other  verte- 
brates (Selenka,  88.1).  The  peculiarity  shows  conclusively  that  the 
connection  of  the  notochord  with  the  hypophysis  is  secondary,  and 
that,  therefore,  Hubrecht's  hypothesis  as  to  the  evolution  of  the  noto- 
chord is  untenable. 

The  separation  from  the  entoderm  is  effected,  at  least  in  mammals, 
by  the  entoderm  proper  showing  itself  under  the  notochord  toward 
the  median  line,  and  when  the  cells  from  one  side  meet  those  of  the 
other  they  unite  with  them  and  form  a  continuous  sheet  of  entoderm 
below  the  notochord  cells.  It  is  probable  that  the  separation  begins 
in  all  vertebrates, 
as  it  has  been 
shown  to  do  in 
several  cases,  be- 
fore the  whole 
length  of  the  noto- 
chord is  formed, 
and  progresses 
head  ward ;  see,  for 
example,  Mclntosh 
and  Prince's  ac- 
count of  the  pro- 
cess in  teleosts, 

90.1,  743.      So, 
also,  in  Triton  al- 
pestris,   Bambeke, 

80.2,  90,    found 

that  the  separation  of  the  notochord  from  the  entoderm  takes  place 
earlier  than  in  the  Urodela,  and  progresses  from  in  back  forward. 
After  the  separation  pigment  granules  appear  in  the  central  portion 
of  the  chorda,  an  important  observation,  since  certain  writers  have 
held,  I  believe  erroneously,  that  the  presence  of  pigment  proves  that 
the  notochord  must  be  derived  from  the  ectoderm,  which  is  usually 
pigmented  in  amphibian  ova. 


pro.am, 


FIG.  106. —Longitudinal  Section  of  the  Head  End  of  a  Mole  Embryo, 
Stage  H.  After  Heape.  EC,  Ectoderm ;  En .  entoderm ;  pro.  am. ,  pro- 
amnion;  m.b.,  mid-brain;  f.b.,  fore-brain;  Ent,  entodennic  cavity ; 
/<f,  heart;  mes,  mesoderm;  nch,  notochord. 


184  THE   EMBRYO. 

After  its  separation  the  chorda  is  a  narrow  band  of  cells  starting 
anteriorly  from  the  wall  of  the  alimentary  tract  and  running  back- 
ward to  the  blastopore.  So  long  as  the  blastoporic  canal  is  open, 
the  chorda  terminates  in  the  entodermic  epithelium  lining  the  canal. 
For  a  certain  period  the  chorda  continues  growing  tailward  by 
accretions  of  cells  from  the  walls  of  the  blastoporic  passage,  and 
after  the  canal  is  permanently  obliterated  the  chorda  may  still  con- 
tinue its  lengthening  by  acquisitions  at  its  caudal  end  of  additional 
cells  from  the  primitive  streak ;  such  cells  may,  however,  properly 
be  regarded  as  coming  from  the  entodermic  lining  of  the  blastopore. 
We  can,  then,  distinguish  two  portions  of  the  notochord ;  the  first 
arising  from  the  epithelium  of  the  notochordal  canal,  the  second  pre- 
sumably from  the  cellular  wall  of  the  obliterated  blastopore.  Braun 
and  others  have  sought  to  attribute  essential  importance  to  these 
differences,  but  it  seems  to  me  improperly.  It  is  more  reasonable  to 
say  that  the  chorda  arises  in  the  amniota,  as  in  the  lower  forms,  di- 
rectly from  the  entoderm,  but  presents  certain  secondary  modifications 
in  its  development. 

After  it  is  once  formed  as  a  band  of  cells  the  notochord  passes 
through  various  changes  of  form,  but  ultimately  becomes  a  cylin- 
drical rod  with  tapering  extremities.  It  attains  considerable  size  in 
the  embryos  of  most  vertebrates,  but  in  those  of  placental  mammals 
it  is  always  small,  particularly  so  in  the  mole  (Heape,  83. 1).  It  is 
probable  that  in  mammals  the  notochord,  when  first  separated  from 
the  entoderm,  is  a  broad  flat  band,  as  if  compressed  between  the  medul- 
lary canal  and  entoderm  (cf.  Kolliker,  "  Entwickelungsgesch.,"  Figs. 
194  to  197,  and  also  Heape,  86.2,  PL  XIII.,  Figs.  36  to  42).  The  band 
then  draws  together,  diminishing  its  transverse  and  increasing  its 
vertical  diameter,  until  it  has  acquired  a  rounded  form  ;*  finally  its 
outline  becomes  circular  in  cross-section.  This  series  of  changes 
begins  near  the  anterior  end  of  the  chorda,  and  progresses  both  for- 
ward and  backward.  The  nuclei  of  the  notochord  tend  to  gather  at 
first  in  the  central  portion  of  the  chorda,  but  in  later  stages  (shark 
embryos  with  fifty  and  sixty  myotomes)  the  nuclei  are  found  situated 
peripherally,  Rabl,  89.2,  249.  The  mesoderm  early  grows  in  between 
the  entoderm  and  the  notochord,  which,  however,  for  a  considerable 
time  remains  close  to  the  medullary  tube.  Later  the  mesoderm  pen- 
etrates also  between  the  notochord  and  medulla.  The  layer  of  meso- 
dermic  cells  immediately  around  the  notochord,  which  are  of  the 
well-known  mesenchymal  type,  forms  a  special  sheath,  which  at  first 
comprises  only  a  single  layer  of  cells,  at  least  in  batrachia  (Goette, 
75.1,  357,  Fig.  187).  This  is  the  commencement  of  the  so-called 
outer  chorda  sheath ;  it  subsequently  becomes  much  thicker.  In  the 
lower  types  it  is  sometimes  an  important  axial  structure ;  but  in  most 
cases  it  is  replaced  by  cartilage,  and  in  all  the  amniota  the  cartilage 
is  replaced  by  the  osseous  vertebrae,  the  intervertebral  ligaments,  etc. 
The  formation  of  the  vertebral  column  involves  the  disappearance  of 
the  notochord  as  described  below. 

Notochord  of  Teleosts. — The  medullary  keel  or  great  neural 
axial  thickening  of  teleosts  extends  to  the  entoderm ;  the  cells  at  the 
bottom  of  this  keel  next  the  entoderm  give  rise  to  the  chorda.  There 

*  A  splendid  description  of  the  selachian  notochord  at  this  stage  is  given  by  C.  Rabl,  89.2, 
213,  214. 


THE    NOTOCHORD. 


185 


being,  it  is  said,  no  open  blastoporic  canal  in  the  bony  fishes,  we  can 
only  trace  the  cells  back  into  the  undifferentiated  mass  of  cells  with 
which  ectoderm  and  entoderm  also  fuse,  and  which  lies  at  the  hind 
end  of  the  embryo.  According  to  the  most  generally  received  opin- 
ion, the  cells  of  the  notochord  arise  from  the  entoderm,  and  their 
fusion  with  the  ectoderm  of  the  medullary  keel  is  temporary  only. 
The  teleostean  chorda  separates  first  from  the  mesoderm,  second 
from  the  entoderm,  and  third  from  the  ectoderm.  The  development 
in  Lepidosteus  is  similar.  The  modifications  we  here  encounter  will 
probably  be  traced  bark  to  the  general  vertebrate  type.  For  discus- 
sion of  the  subject  and  citations  of  earlier  authorities,  see  Mclntosh 
and  Prince,  90.1,  740-715. 

Shape  and  Relations  to'Other  Parts. — As  soon  as  the  head- 
bend  (first  cerebral  flexure)  appears,  Fig.  107,  the  notochord  becomes 
correspondingly  bent,  and  its  bend  lies  close  to  Rathke's  pocket,  Fig. 
107,  hy.  From  Selenka's  Guamen- 
tasche  there  now  runs  upward  and  for- 
ward a  short  limb  of  the  notochord, 
which  subsequently  atrophies.  This 
limb  may  remain  regular  or  it  may 
grow  and  become  somewhat  irregular 
before  it  atrophies;  after  it  is  gone  the 
chorda  has  a  new  or  secondary  ante- 
rior extremity,  which  Romiti,  86.1, 
finds  in  the  chick  embryo  at  the  end  of 
the  fourth  and  during  the  fifth  day  of 
incubation  to  be  united  with  an  irregu- 
lar solid  cord  of  cells  which  grows  out 
from  the  epithelium  of  the  hypophysis. 
The  cord  soon  disappears.  Its  signifi- 
cance is  quite  unknown.  Romiti  sug- 
gests that  it  may  produce  a  strain  re- 
sulting in  the  pulling  out  of  the  hy- 
pophysal  evagination.  This  notion  seems  to  me  untenable,  since 
the  hypophysal  invagination  begins  before  there  is  any  union  with 
the  notochord.  The  cranial  portion  of  the  notochord  has  not  only 
the  bend  shown  in  Fig.  107,  but  also  follows  the  other  curves  of 
the  head ;  it  takes  a  sinuous  course  besides  within  the  base  of  the 
cranium ;  finally,  in  the  region  corresponding  to  the  middle  third  of 
the  spheno-occipital  cartilage,  it  makes  a  great  dip  ventral  ward. 
The  sheath  of  the  notochord  in  the  cranial  region  is  converted  into 
the  spheno-occipital  cartilage ;  at  the  dip  just  mentioned,  however, 
the  notochord  lies  entirely  below  the  cartilage  close  against  the  wall 
of  the  pharynx  (Froriep,  82.1,  Romiti,  86.1).  Writers  before 
Froriep  had  represented  the  chorda  as  having  disappeared  at  the 
bottom  of  the  dip. 

The  anterior  termination  of  the  notochord  has  been  carefully  stud- 
ied by  Prenant,  91.2,  203,  who  finds  that  it  has  (pig  and  rabbit)  no 
connection  with  the  hypophysis,  but  may  have  a  secondary  tem- 
porary connection  with  the  entoderm  just  behind  Seesel's  pocket, 
and  that  the  part  of  the  notochord  nearest  the  hypophysis  very  early 
degenerates,  leaving  the  notochord  to  terminate  above  Seesel's  pocket ; 


Pro.am. 


FIG.  107.  —Rabbit  Embryo  of  6  mm. ; 
Median  Longitudinal  Section  of  the 
Head.  The  connection  between  the 
mouth  M,  and  pharynx  ent,  is  just  es- 
tablished; nch.  notochord;  Tib,  hind- 
brain;  w6,  mid-brain;  fb,  fore-brain; 
n-ti.aiii,  proamnion:  hy,  hypophysis 
lit.,  heart.  After  Mihalkovics. 


180 


THE   EMBRYO. 


according  to  this  view  the  so-called  prse-chordal  region  primitively 
contains  the  notochord.  The  secondary  anterior  termination  of  the 
notochord  is  close  to  the  infundibulum  (and  future  pituitary  body), 
and  it  is  customary  for  subsequent  stages  to  divide  the  head  and 
skull  into  a  prce-pituitary  and  a  post-pituitary  region;  the  latter 
region  alone  contains  the  notochord,  after  very  early  stages. 

Histogenesis. — After  the  notochord  has  been  formed  as  a  rod  of 
cells,  its  cells  undergo  a  process  of  histological  differentiation,  unique 
in  vertebrates.  The  cells  at  first  become  greatly  compressed  in  the 
line  of  length  of  the  chorda,  and  hence  appear  quite  thin  in  longitu- 
dinal sections,  Fig.  108,  hardly  greater  in  diameter  than  their  own 
nuclei.  The  flattened  cells  are  next  converted  into  a  highly  charac- 
teristic reticulum  by  vacuolization.  Thus,  in  the  chick,  by  the  third 
day  some  of  the  central  cells  become  vacuolated,  while  the  peripheral 
cells  are  still  normal;  at  first,  as  in  the  frog,  there  seems  to  be  only 
one  large  vacuole  in  each  cell,  Fig.  108,  B.  Around  the  vacuole  is 


FIG.  108.  —Longitudinal  Sections  of  the  Notochord  of  Bombinator.  After  Goette.  A,  before 
the  appearance  of  the  vacuoles :  B,  after  the  appearance  of  the  vacuoles ;  nch,  notochord ;  En, 
entoderm.  The  cells,  as  is  usual  in  amphibian  embryos,  are  changed  with  yolk-granules. 

a  peripheral  layer  of  granular  protoplasm,  in  which  the  nucleus  lies 
imbedded,  while  the  vacuoles  themselves  are  filled  with  a  perfectly 
clear  and  transparent  material,  which  is  supposed  to  be  fluid  in  its 
natural  condition.  During  the  fourth  day  (chick)  all  the  cells  be- 
come vacuolated,  with  the  exception  of  a  single  layer  of  flattened 
cells  at  the  periphery.  (In  the  anura,  it  is  said,  there  is  no  distinct 
peripheral  layer  of  protoplasmatic  cells.)  The  vacuoles  go  on  en- 
larging until  by  the  sixth  day  they  have  so  much  increased  at  the 
expense  of  the  protoplasm  that  only  a  very  thin  layer  of  the  latter  is 
left  at  the  circumference  of  the  cell,  at  one  part  of  which,  where 
there  is  generally  more  protoplasm  than  elsewhere,  the  remains  of  a 
nucleus  may  generally  be  detected.  Thus  the  notochord  becomes 
transformed  into  a  spongy  reticulum,  the  meshes  of  which  correspond 
to  the  vacuoles  of  the  cells  and  the  septa  to  the  remains  of  their  cell 
walls  (Foster  and  Balf our) .  As  Goette  has  pointed  out,  the  process 
is  accompanied  by  an  expansion  of  the  cells  which  is  the  main  factor 
in  the  widening  and  lengthening  of  the  notochord,  which  goes  on 
pari  passu  with  the  growth  of  the  surrounding  tissue. 

The  histogenetic  process  is  stated  to  be  essentially  similar  in  mam- 
mals (W.  Miiller,  71.2,337,  338).  There  is  the  central  layer  of 
vacuolated  cells  and  the  peripheral  layer  of  protoplasmatic  cells. 


THE   NOTOCHORD.  187 

The  latter  are,  however,  ultimately  converted  into  vacuolated  cells. 
The  cell  walls  are  perforate,  having  fine  pores  that  correspond  prob- 
ably to  intercellular  bridges  of  protoplasm.  The  inner  chorda 
sheath  appears  early,  and  is  to  be  regarded  as  an  anhistic  basement 
membrane  secreted  by  the  notochordal  cells. 

Disappearance. — The  disappearance  of  the  notochord  in  man 
commences  with  the  second  month  of  foetal  life.  The  first  step  is  an 
alteration  of  the  characteristic  histological  structure,  accompanied 
by  shrinking  of  the  tissues,  so  that  a  clear  space  appears  around  it. 
The  inner  chorda  sheath  is  lost.  The  cell  walls  disappear,  the  tissue 
becomes  granular,  and  breaks  up 
into  multinucleate,  irregularly 
recticulate  masses,  Fig.  109, 
which  are  gradually  resorbed 
(Leboucq,  80.1).  In  mammals 
the  resorption  progresses  more 
rapidly  in  the  cores  of  the  verte- 
bra than  in  the  intervertebral 
spaces,  and  again  more  rapidly 

at  the  ends  than  in  the  Centre  Of  FIG.  H>,.-1  venerating  Notochord  Tissue.from 
onr4i  \-^vf ^V»T-n  •  VIATICA  tlio  r»VinT-fla  the  Central  Portion  of  the  Intervertebral  Disk  of 

TUd,    a  QOW»S  Embryo.     After  Leboucq. 

persists  a  little  longer  in  the  cen- 
tre of  the  vertebra,  and  considerably  longer  in  the  intervertebral 
spaces ;  in  these  last  the  final  remnants  of  the  chorda  may  be  de- 
tected in  man  even  after  birth.  The  cavity  between  the  vertebral 
cartilages  is  a  new  structure  and  is  not  the  space  left  by  the  noto- 
chord, as  has  been  sometimes  asserted.  It  appears,  however,  that 
the  resorption  of  the  chorda  may  leave  a  small  space,  which  becomes 
included  in  the  intervertebral  cavity.  A  peculiar  feature  is  the  fre- 
quent persistence  of  calcified  cartilage  immediately  around  the 
notochord  in  ossifying  vertebrae. 

Morphology. — The  notochord  was  for  a  long  time  supposed  to 
be  exclusively  characteristic  of  vertebrates.  It  is  now  known  to 
exist  in  Amphioxus,  which  is  not  a  true  vertebrate,  and  in  the  Tuni- 
cata.  Morphologists  have  long  believed  that  it  must  have  some 
homologue  among  the  organs  of  invertebrates.  The  development  of 
the  notochord  in  the  lower  vertebrates  indicates  very  plainly  what 
must  have  been  the  general  character  of  such  an  homologous  inver- 
tebrate organ.  In  certain  fishes  and  amphibia  the  notochord  has 
been  asserted  to  arise  as  a  furrow  along  the  median  dorsal  line  of 
the  entoderm ;  the  furrow  deepens  and  then  closes  over  to  form  a  rod 
separate  from  the  entodermic  canal  proper.  The  notochordal  rod 
retains  for  a  time  its  anterior  and  posterior  connections  with  the 
entoderm.  It  is  usually  regarded  as  morphologically  a  solid  canal, 
a  view  very  open  to  doubt.  Ultimately  the  ends  become  detached, 
and  so  arises  the  solid  isolated  chorda.  In  the  higher  vertebrates 
the  course  of  development  is  similar,  although  several  of  the  primi- 
tive features  in  the  formation  of  the  chorda  are  obscured.  Ehlers, 
85. 1,  has  pointed  out  that  in  various  invertebrates  there  is  a  similar 
canal,  the  "  Nebendarm  "  of  German  writers,  which  is  derived  from 
the  entoderm  and  connected  anteriorly  and  posteriorly  with  the  en- 
todermal  cavity.  It  is  a  very  plausible  suggestion,  which  homolo- 


188 


THE   EMBRYO. 


gizes  the  vertebrate  notochord  with  the  invertebrate  "  Nebendarm." 
Hubrecht  has  sought  to  homologize  the  notochord  with  the  proboscis 
of  nemertean  worms.  There  is  not  a  single  fact  which  seems  to 
me  to  justify,  even  remotely,  this  attempt  at  guess-work  phylogeny, 
nor  can  I  find  any  resemblance  of  the  notochord  with  the  structure 
in  Balanoglossus  with  which  Bateson  has  sought  to  homologize  it. 


III.  NEURENTERIC  CANALS. 

The  term  neurenteric  canal  is  used  to  designate  an  open  commu- 
nication between  the  archenteric  cavity  and  the  medullary  canal. 
Such  communications  are  found  only  in  early  stages  ;  they  always 
pass  through  the  anterior  end  of  the  primitive  streak  and  lead, 
therefore,  into  the  posterior  end  of  the  medullary  canal  or  groove  ; 
they  are  present  only  during  a  short  period.  Much  confusion  has 
existed  in  regard  to  these  canals,  of  which  as  many  as  three  have 
been  distinguished  by  M.  Braun,  8S.3,  while  several  writers  recog- 
nize two. 

The  true  Neurenteric  Canal  is  probably  the  blastoporic  canal 
proper,  and  is  to  be  identified  by  the  notochord  terminating  in  its 

_  ^vall.  As  stated  in  Part  II.  of 
this  chapter,  the  "  chorda-canal" 
of  mammals  is  the  "  blastoporic" 
canal,  k  and  therefore  also  in- 
cludes the  neurenteric  canal. 
As  previously  described,  the 
blastopore  is  the  opening  of  the 
notochordal  canal  at  the  ante- 
rior end  of  the  primitive  streak. 
The  neural  ridges  or  medullary 
folds  extend  around  and  behind 
or  across  the  blastopore,  which, 
therefore,  opens  into  the  poste- 
rior extremity  of  the  medullary 
groove.  If  now  the  canal  is 
open  at  the  period  of  develop- 
ment when  the  medullary 
groove  is  deep  or  has  already 
closed  over,  making  the  medul- 
lary canal,  then  there  is  a  direct 
communication  between  the  en- 
todermal  canal  on  the  one  hand 
and  the  spinal  canal  on  the 

othpr        \Vp    OWP  to  T^alfonr  thp 
\ 

identification     Ol     thlS    Canal    as 

the  blastopore.  It  may  with 
Propriety  .  be  termed  the  true 
neurenteric  canal,  or  the  canal 
of  Kowalewsky  from  its  discoverer.  Kowalewsky  first  found  it  in 
Amphioxus,  and  subsequently  demonstrated  its  occurrence  in  vari- 
ous fish  types. 

This  canal  is  well  known  in  Elasmobranchs  and  Sauropsida  under 


FiG.llO.—  Longitudinal  Section  of  a  Frog's  Ovum 
shortly  after  the  Closure  of  the  Medullary  Groove. 
The  anal  canal,  bl,  is  only  partially  cut,  but  was 


NEURENTERIC    CANALS.  189 

the  name  of  the  blastoporic  canal.  It  has  recently  been  shown  to  be 
present  in  Petromyzon  by  A.  Goette,  90.1.  In^teleosts  it  is  rudi- 
mentary, the  passage  being  only  imperfectly  indicated  (see  Mclntosh 
and  Prince,  90. 1,  734-736).  In  the  Amphibia  its  relations  are  more 
clearly  understood  than  in  any  other  type.  According  to  H.  E. 
Durham,  86. 1,  it  can  be  well  seen  in  longitudinal  sections  of  early 
stages  of  the  frog,  Fig.  110,  as  a  short  canal,  ne,  opening  widely 
into  the  entodermic  cavity.  This  canal  has  also  been  described  in  Bom- 
binator  by  A.  Goette,  75.1,  and  in  Triton  and  Rana  by  F.  Schanz, 
87. 1.  Schanz  was  the  first  to  clearly  discriminate  between  the  anal 
or  false  blastoporic  and  the  neurenteric  or  true  blastoporic  canal.  In 
birds  the  neurenteric  canal  was  first  described  by  Gasser,  79.1,  in 
goose  embryos,  and  since  then  has  been  found  by  Braun  in  several 
other  birds,  though  as  an  open  passage  it  appears  to  be  usually  oblit- 


FIG.  111.  —Transverse  Section  of  an  Embryo  Paroquet  (Melopsittacus)  to  show  the  Anterior 
( >r  t  rue  Neurenteric  Canal.  EC,  Ectoderm ;  My,  myotome ;  Md,  medullary  canal ;  C7i,  notochord 
pierced  by  the  short  neurenteric  canal,  ne;  Ent,  entoderm;  mes,  mesodenn.  After  Max 
Braun. 

erated,  as  in  the  chick.  Fig.  Ill  represents  a  transverse  section 
which  passes  through  the  Gasserian  or  neurenteric  canal  of  the 
paroquet. 

Braun  has  maintained  that  in  various  birds  there  are  two  neuren- 
teric canals  recognizable,  which  lie  near  together.  Braun  states 
that  in  the  duck  and  Motacilla  the  two  canals  are  separated  both  in 
the  times  and  position  of  their  occurrence,  and  that  in  the  Australian 
paroquet  they  are  present  simultaneously.  D.  Schwarz,  89.1,  criti- 
cises Braun 's  observations  and  concludes  that  there  is  really  no  sec- 
ond canal.  I  am  inclined  to  accept  this  conclusion. 

In  mammals  the  open  blastoporic  canal  has  been  seen  by  very  few 
observers ;  it  has  been  carefully  studied  in  the  rabbit  by  C.  Giaco- 
mini,  88. 1,  who  shows  that  the  front  portion  persists  for  a  time  as 
the  chorda-canal,  while  the  hind  portion,  running  through  the  prim- 
itive streak,  corresponds  to  the  neurenteric  canal  and  is  obliterated 
quite  early. 

The  Anal  Canal  is  also  sometimes  called  the  neurenteric  canal, 
and  seems  to  have  been  especially  the  subject  of  misconception. 
Its  full  history  is  given  later.  Its  morphological  relations  are  prob- 
ably correctly  indicated  by  the  observations  of  Fr.  Schanz,  87.1, 
which  have  been  confirmed  by  T.  H.  Morgan,  90.2,  by  Robinson  and 


190  THE   EMBRYO. 

Assheton,  91.1,  and  others.  To  explain  these  relations  we  may 
start  from  the  stage  in  amphibian  ova,  in  which  the  anus  of  Rusconi 
is  almost  closed  over ;  the  true  blastopore  lies  at  its  front  edge.  As 
the  anus  of  Rusconi  contracts  its  aperture  appears  more  and  more  as 
a  mere  enlargement  of  true  blastopore,  and  it  is  at  this  stage  com- 
monly spoken  of  as  the  blastopore ;  to  preserve  the  distinction  we 
may  name  this  opening  the  secondary  blastopore.  Alice  Johnson, 
84. 1,  had  shown  that  the  permanent  anus  is  derived  in  Triton  from 
this  secondary  blastopore.  H.  E.  Durham,  86.1,  observed  that 
there  are  two  passages  in  the  frog  at  a  little  later  stage,  Fig.  110. 
Schanz,  /.  c.,  found  that  the  medullary  ridges  meet  at  their  hind 
ends  across  the  secondary  blastopore  and  divide  it  into  two  openings, 
the  anterior,  the  true  blastoporic  or  neurenteric,  and  the  posterior, 
the  anal  opening.  In  many  Amphibia  the  anal  canal  is  often  tem- 
porarily closed  by  the  tissue  growing  across  it ;  in  later  stages  the 
ectoderm  forms  a  slight  invagination  to  develop  the  anus  proper,  the 
partition  between  it  and  the  archenteron  breaking  through.  The 
partition  is  called  the  anal  plate. 

The  relations  of  the  anal  canal  in  Sauropsida  are  not  yet  well  as- 
certained. It  is  represented  by  the  anal  plate,  consisting  only  of 
ectoderm  and  entoderm.  Although  this  anal  plate  (Afterhaut)  has 
not  been  actually  proved  to  be  homologous  with  the  tissue  which  tem- 
porarily closes  the  anal  canal  in  amphibians,  yet  it  is  hardly  possible 
to  question  the  correctness  of  the  homology,  for  it  separates  the  anal 
invagination  from  the  archenteron  and  subsequently  ruptures.  The 
anal  membrane  recurs  in  mammals,  and  if  it  represents  the  anal 
canal  in  one  case  it  does  also  in  the  other.  C.  Giacomini,  88.1, 
287,  288,  states  that  in  rabbit  embryos  with  several  myotomes  the 
anal  plate  is  grown  into  a  short  cord  of  cells,  in  which  there  appears 
a  temporary  lumen — this  lumen  he  calls  the  anal  canal.  After  the 
canal  has  disappeared  the  anal  membrane  is  again  found  to  consist 
of  two  epithelial  plates,  the  rupture  of  which  forms  the  true  anal 
perforation. 

Braun's  Third  Canal. — The  third  canal,  which  was  first  de- 
scribed by  Braun,  82.3,  is  said  to  occur  in  older  embryos.  D. 
Schwarz,  89.1,211,  denies  its  existence  altogether.  The  "End- 
darm"  of  Gasser  and  Kolliker  becomes  the  "  Schwanzdarm"  (post- 
anal  gut,  Balfour)  of  older  embryos,  which  soon  becomes  divided,  at 
least  in  birds,  into  a  dilated  terminal  portion  and  a  narrower  neck 
communicating  with  the  intestine  proper.  The  posterior  section  then 
subdivides,  and  its  narrow  end-segment  lengthens  out  and  unites 
with  the  spinal  cord.  This  passage  we  may  designate  as  Braun's 
canal.  It  is  not  improbable  that  it  is  homologous  with  the  amnio- 
allantoic  canal  of  Gasser,  82.2,  which  Rauber,  83.2,  has  nicknamed 
Cochin-China  canal,  after  a  breed  of  hens  in  which  it  seems  most 
constant.  In  the  one  case  we  may  suppose  the  canal  to  open  after, 
in  the  other  before,  the  closure  of  the  posterior  end  of  the  medullary 
groove.  If  the  homology  is  correct  it  may  be  further  said  that  the 
canal  is  identical  with  Kupffer's  myelo-allantoidean  canal;  it  can- 
not be  brought  into  relation  with  the  development  of  the  allantois, 
as  believed  by  Kupffer,  82.2,  83.1,  as  the  allantois  and  end-darm 
are  both  formed  before  the  canal  appears. 


NEURENTERIC    CANALS.  191 

Significance  of  the  Neurenteric  Canal. — As  to  the  mor- 
phology and  physiology  of  the  canal  we  know  almost  nothing.  The 
suggestion  of  Sedgwick  and  Van  Wijhe,  that  it  is  the  excretory 
opening  of  the  tubular  nervous  system,  has  already  been  noticed,  p. 
179 ;  it  will  suffice  to  recall  here  that  no  valid  evidence  in  favor  of 
their  hypotheses  has  been  found  yet.  There  is  no  adult  form  known 
in  which  the  neurenteric  canal  persists ;  were  there  such  an  animal 
we  might  hope  to  discover  the  function  of  the  canal  by  observation. 
Morphologically  the  neurenteric  canal,  so  far  as  I  can  judge  from 
present  evidence,  is  part  of  the  persistent  blastoporic  canal,  which 
is  included  in  the  medullary  groove,  and  by  the  closure  of  the  groove 
becomes  shut  off  from  the  exterior.  Why  the  secondary  blastopore 
(prostoma)  should  be  divided  into  the  two  openings,  the  neurenteric 
.and  anal,  we  do  not  know. 

It  seems  not  impossible  that  a  persistent  neurenteric  canal  may 
occur  as  an  excessively  rare  anomaly  in  the  adult. 


CHAPTER   IX. 

THE   PRIMITIVE    DIVISIONS   OF   THE   CCELOM ;    ORIGIN    OF   THE 

MESENCHYMA. 

IN  all  true  vertebrates  the  ccelom  presents  the  peculiarity  of  con- 
sisting of  an  upper  or  dorsal  segmented  portion  and  a  lower  or  ven- 
tral continuous  unsegmented  portion.  The  segmented  co3lom  consists 
of  a  series  of  discrete  separate  cavities,  each  of  which  communicates 
with  the  ventral  coelom. 

Now,  in  annelids  (and  their  arthropodous  descendants)  the  ccelom 
consists  only  of  separate  paired  cavities,  so  that  the  mesothelium  is 
divided  into  distinct  parts,  each  inclosing  a  space ;  each  division  is 
known  as  a  mesomere  or  mesoblastic  somite.  'Hence  we  have  the 
morphological  question,  how  has  the  completely  segmented  ccelom  of 
annelids  become  transformed,  as  we  must  assume  it  has,  into  the 
partially  segmented  coelom  of  vertebrates?  The  answer  is  probably 
given  correctly  by  Hatschek's  investigation  of  the  changes  of  the 
mesothelium  in  Amphioxus,  88. 1.  In  Amphioxus  the  entire  meso- 
derm  becomes  segmented ;  the  ventral  cavities  of  the  segments  sub- 
sequently fuse,  while  the  dorsal  parts  remain  distinct.  In  the  lower 
vertebrates  the  segmented  ccelom  appears  first  and  the  unsegmented 
portion  later ;  whether  the  latter  is  temporarily  segmented  remains 
for  future  investigation  to  determine.  In  amniota  the  unsegmented 
portion  of  the  coelom  appears  first,  as  described  in  Chapter  VI. 
This  must  be  regarded  as  a  secondary  modification,  probably  con- 
nected with  the  evolution  of  the  amnion ;  as  explained  in  Chapter 
XV,  the  development  of  the  amnion  depends  upon  a  precocious  and 
exaggerated  development  of  part  of  the  coelom. 

With  these  general  notions  in  mind,  we  can  better  appreciate  the 
early  history  of  the  vertebrate  coelom.  We  consider — 1,  the  primi- 
tive segments ;  2,  the  unsegmented  coelom ;  3,  division  of  the  primi- 
tive segments;  4,  the  differentiation  of  the  myotome;  5,  origin  of 
the  mesenchyma ;  6,  comparison  with  Amphioxus. 

The  Primitive  Segments.* — A  segment  consists  of  a  pair  of 
cavities  symmetrically  placed  and  bounded  by  mesothelium.  The 
segments  are  permanent  in  many  invertebrates,  but  they  are  greatly 
modified  in  all  adult  vertebrates,  and  so  much  modified  in  the  amni- 
ota, that  they  can  be  said  properly  to  exist  only  during  embryonic 
stages,  although  they  determine  a  large  part  of  the  adult  structure. 
I  have  selected  the  term  primitive  segments  as  unlikely  to  lead  to 
confusion,  but  numerous  other  names  have  been  proposed;  the  one 
most  generally  in  use  is protovertebra  (Urwirbel),  which  was  intro- 
duced long  ago  under  the  erroneous  notion  that  the  segments  were 
the  direct  precursors  of  the  vertebra,  which  they  are  not,  properly 
speaking.  The  term  protovertebra3  is,  however,  more  often  used  in 

*  On  the  segments  of  the  head,  see  p.  200. 


THE   PRIMITIVE   DIVISIONS   OF   THE   CCELOM. 


193 


a  more  restricted  sense,  viz.,  for  that  part  of  the  primitive  segment 
which  is  called  the  myotome  in  the  following  pages.  Other  terms 
are  mesoblastic  somites,  mesomeres,  metameres,  Ursegmente. 

The  primitive  segments  appear  very  early;  the  first  pair  can  be 
recognized  in  the  chick  at  twenty  to  twenty -two  hours,  in  the  rabbit 
at  the  beginning  of  the  eighth  day,  or  even  earlier ;  in  both  cases 
the  medullary  groove  is  still  nowhere  closed  and  the  primitive  streak 
is  still  present.  In  the  anamniota  the  first  segments  appear  at  about 
the  same  early  stage.  In  the  amniote  embryo  just  before  the  first 
segment  appears  the  mesoderm  forms  a  continuous  sheet;  surface 
views  show  that  it  forms  two  wings,  being  divided  by  the  median 
down-growth  of  the  medullary  groove  as  stated,  p.  149.  The  meso- 
derm on  each  side  is  considerably  thicker  alongside  the  axial  line 
than  farther  away  from  it ;  the  distinction  is  well  marked  and  ena- 
bles us  to  distinguish  two  zones,  namely,  the  thicker  segmental  zone 
near  the  axis,  and  the  thinner  but  much  wider  lateral  or  parietal 
zone;  the  segmental  zone  is  the  Stammzone  of  German  writers,  the 
Wirbelplatte  of  Remak,  or 
vertebral  plate  of  Balfour. 
The  first  noticeable  indica- 
tion of  the  formation  of  the 
primitive  segment  is  a  loos- 


Fio.  112.— Chicken  Embryo  with  one 
Segment.  Ap,  Area  pellucida;  vAf, 
anterior  crescent;  i-Kf,  head  fold; 
Ch,  notochord;  Pz,  parietal  zone;  Stz, 
Stammzone.  From  Kolliker.  (Com- 
pare text. ) 


FIG.  113.— Area  Vasculosa  and  Embryo  with  Eight  Seg- 
ments of  a  Hen's  Egg.  F7i,  Fore-brain;  vAf,  anterior 
amniotic  fold;  Pd,  fovea  cardiaca;  omr,  anterior  limit 
of  open  medullary  groove;  Rf,  rhomboidal  sinus  of 
medullary  groove;  Pr,  primitive  streak;  Rw,  margin 
of  medullary  groove;  Ap,  area  pellucida;  Ao,  area 
opaca ;  Uu\  first  segment ;  Hh,  hind-brain ;  Mh,  mid- 
brain.  From  Kolliker. 


ening  of  the  cells  in  the  segmental  zone  along  a  narrow  trans- 
verse line;  in  the  chick  this  occurs  about  0.14  mm.  in  front  of  the 
13 


194 


THE   EMBRYO. 


primitive  streak,  at  a  time  when  only  a  short  stretch  of  the  head- 
end of  the  medullary  groove  is  formed.  Very  soon  there  appears  a 
second  transverse  loosening  of  the  cells  and  cleavage  of  the  meso- 
dermic  segmental  zone  takes  place.  According  to  A.  Goette,  75.1, 
203,  the  cleavage  begins  in  teleosts  and  the  chick  and  probably  in 
other  vertebrates  with  a  small  depression  on  the  ectodermal  side, 
and  this  depression  gradually  deepens  to  a  cleft,  which  divides  the 

segmental  plate  completely.  The 
disposition  of  the  fissures  is  such  that 
they  include  on  ea,ch  side  of  the  axis 
a  cuboid al  block  of  mesoderm,  and 
this  block  with  its  fellow  on  the  op- 
posite side  constitutes  the  first  prim- 
itive segment,  Fig.  112.  The  site  of 


Ent 


FIG.  114.  —Rabbit  Embryo  with  Eight  Seg- 
ments, vh,  Fore-brain;  ab,  optic  vehicles; 
a,  heart ;  a/,  amniotic  fold ;  mh,  mid-brain ; 
hh,  hind-brain;  pz,  parietal  zone;  stz,  seg- 
mental zone ;  op,  area  pellucida ;  rf,  edge  of 
open  medullary  groove;  uw,  primitive  seg- 
ment; t?o,  venous  end  of  heart;  /i,  heart; 
ph,  pericardial  cavity ;  vd,  f ovea  cardiaca. 
From  Kolliker. 


FIG.  115.— Transverse  Section  of  a  Pristiu- 
rus  Embryo  with  Fourteen  Segments,  through 
the  Centre  of  the  Fourth  Segment,  md,  Me- 
dullary groove;  EC,  ectoderm;  msth,  meso- 
thelium;  Coe,  cavity  of  segment;  Ent,  ento- 
derm;  nc/i,  notochord.  After  C.  Rabl. 


the  first  segment  corresponds  to  the  posterior  occipital  region ;  the 
second  segment,  at  least  in  the  chick,  is  formed  immediately  in  front 
of  the  first;  these  two  segments,  according  to  Chiarugi,  90.1,  329, 
are  the  third  and  fourth  occipital  segments,  and  together  with  the 
first  and  second  segments — subsequently  found  in  front  of  them — 
abort  in  all  amniota  during  early  embryonic  life,  as  was  discovered 
by  Froriep.  According  to  Julia  B.  Platt,  89.1,  179,  the  first  divi- 
sion formed  in  the  chick  is  that  between  the  third  and  fourth  occipital 
segments,  and  two  segments  are  subsequently  produced  in  front  of 
this  division,  while  seven  are  forming  behind  it.  A  chick  with  one 
segment  is  shown  in  Fig.  112.  The  medullary  groove,  Rf,  is  short 
and  broad;  the  anterior  end  of  the  embryo,  vKf,  is  already  rising 


THE   PRIMITIVE    DIVISIONS    OF   THE    CCELOM.  195 

above  the  yolk ;  the  parietal,  Pz,  and  segmental  zones,  Stz,  are  dis- 
tinct even  in  the  region  of  the  primitive  streak,  along  which  the 
primitive  groove,  Pr,  is  well  marked ;  the  segment  is  well  advanced 
and  another  has  begun  to  form  in  front  of  it.  A  chicken  embryo 
with  eight  segments  is  shown  in  Fig.  113,  and  a  rabbit  embryo  also 
with  eight  segments,  in  Fig.  114;  comparison  shows  many  important 
differences  between  the  two  embryos. 

The  examination  of  transverse  sections  shows  that  the  primitive 
segments  in  all  anamniote  vertebrates  are  hollow  and  bound  by 
mesothelium  on  all  sides.  The  relations  can  be  understood  readily 
in  elasmobranchs.  Fig.  115  is  from  a  Pristiurus  embryo  and  shows 
the  cavity  of  the  segment  very  clearly ;  the  embryo  is  much  more 
separated  from  the  yolk  than  is  the  case  with  amniote  embryos 
at  a  corresponding  stage,  consequently  the  lateral  or  parietal  zone 
of  mesoderm  lies  nearly  vertical,  instead  of  resting  horizontally,  as 
it  does  upon  the  yolk  of  amniota ;  in  the  parietal  zone  there  is  as  yet 
no  cavity  (coelom) ;  the  ventral  or  unsegmented  ccelom  arises  later. 
It  is  probable  that  the  segmental  cavities  spread  down  into  the  pari- 
etal zone,  and  that  their  ventral  (i.  e.  lower  or  so-called  parietal) 
ends  fuse  together  and  form  one  large  main  body  cavity.  This 
probability,  as  C.  Rabl  has  said,  89.2,  rests  upon  the  analogy  with 
the  ascertained  process  in  Amphioxus,  and  upon  the  fact  that  the 
segmental  cavities  appear  first,  and  expand  outward  or  away  from 
the  axis.  Whether,  however,  they  do  actually  give  rise  to  the  main 
coelom  by  their  partial  lateral  or  ventral  fusion  or  not,  there  are  no 
observations  at  present  to  decide. 

Of  the  development  of  the  segments  in  the  primitive  vertebrates 
(marsipobranchs,  ganoids,  and  amphibians) ,  there  is  not  much  known, 
though  there  are  many  scattered  observations  recorded.  There  ap- 
pears, however,  to  be  a  distinct  thickened  zone  of  mesoblast  on  each 
side  of  the  axis,  and  from  this  zone  the  segments  are  developed  as 
pairs  of  cuboidal  blocks  of  mesothelium ;  the  central  cavity  of  the 
segment  is  very  small ;  its  mesothelium  is  thick.  The  main  ccelom 
is  at  first  a  fissure  farther  away  from  the  axis,  and  it  has  not  yet 
been  shown  that  there  is  from  the  start  a  communication  between 
the  segmental  and  the  main  coelom,  although  the  mesoblast  is  con- 
tinuous. In  Petromyzon,  if  I  understand  Goette  aright,  the  meso- 
derm is  at  first  solid;  Goette  states,  90.1,  48,  that  in  the  cervical 
region  the  main  body  cavity  appears  first,  but  in  the  rump  the  prim- 
itive segments  acquire  their  cavities  first ;  that  is,  while  the  mesoderm 
of  the  parietal  zone  is  still  solid.  This  is  important  as  foreshadow- 
ing the  precocious  development  of  the  cervical  coelom  (cavity  of  the 
amnio-cardial  vesicles)  in  amniota.  In  sections  the  primitive  seg- 
ments in  Petromyzon  and  Amphibia  are  triangular,  filling  out  the 
space  between  the  medullary  canal  and  the  adjacent  ectoderm  and 
entoderm  on  each  side.  In  Bombinator,  A.  Goette,  75.1,  202,  and 
Petromyzon,  A.  Goette,  90.1,  the  first  segment  appears  near  the 
middle  of  the  embryo,  and  new  segments  are  added  in  front  to  make 
the  cervical  region  and  a  much  larger  number  progressively  back- 
ward to  form  the  rump  of  the  adult. 

In  Sauropsida  transverse  sections  of  the  primitive  segments,  when 
they  are  first  formed,  show  no  cavity,  nor  does  any  appear  until  con- 


196  THE   EMBRYO. 

siderably  later.  The  development  begins  with  the  differentiation  of 
the  segmental  zone  (Remak's  Unvirbelplatte,  Balfour's  vertebral 
plate),  which  is  accomplished  by  the  thickening  of  the  mesoderm 
near  the  axis  of  the  embryo.  The  process  is  intimately  associated 
with  the  upward  movement  of  the  medullary  plate  to  form  the  med- 
ullary groove,  His,  68.1,  81,  and  Goette,  75.1;  the  space  between 
the  ectoderm  and  entoderm  is  enlarged  by  this  movement,  and  is 
always  nearly  filled  by  the  mesoderm  ;  consequently  the  segmental 
zone  appears  triangular  in  cross-sections,  the  base  of  the  triangle 
being  against  the  wall  of  the  medullary  groove,  its  two  sides  against 
the  ectoderm  and  entoderm  respectively,  and  its  apex  merging  into 
the  lateral  mesoderm,  which  is  very  much  thinner  than  the  segmental 
plate.  The  changes  just  described  show  a  very  exact  adjustment  of 
the  growth  of  the  mesoderm  to  changes  in  the  outer  germ-layers. 
Such  adjustments  occur  throughout  all  embryological  developments, 
and  are,  I  think,  due  to  methods  of  growth  rather  than  to  simple 
mechanical  conditions.  His  has  attributed,  68.1,  81,  93,  special  in- 
fluence to  the  attachments  of  the  mesoderm  to  the  other  layers,  and 
the  consequent  strain  upon  the  segmental  plate  as  the  medulla  rises  ; 
but  the  enlargement  of  the  plate  depends  upon  the  multiplication  of 
the  cells,  and  we  cannot  assume  that  the  strain  causes  cell  prolifera- 
tion. At  most,  one  might  say  that  the  strain  determines  the  shape 
of  the  segmental  plate.  But  is  it  not  more  natural  to  assume  that 
the  cells  of  the  mesoderm  simply  spread  out  until  they  fill  the  avail- 
able space? 

The  first  sign  of  the  mesomeres  is  the  assumption  by  the  cells,  that 
are  to  form  them,  of  a  more  distinctly  epithelial  arrangement,  the 

cells    radiating     in 

VT 


centre  a  core  or 
nucleus  (Urwirbel- 
kern)  of  cells,  Fig. 
116,  C,  which  have 
small  bodies  with 
anastomosing  proto- 
plasmatic processes  ; 
it  is  impossible,  at 

FIG.  116.—  Transverse  Section  through  a  Recently  Formed  Primi-  least  at  present,  to 
tive  Segment  of  a  Chick  with  Eighteen  and  Twenty  Segments.  My,  O.j.04.~  tTrl-,^4-V>/™  +1-,  i  o 
Myotome:  W.d,  Wolffian  duct;  Som,  somatic  mesoderm;  Coe,  ere-  State  Wnetner  tlllS 


lorn  (splanchnocoele)  ;  8pl,  splanchnic  mesoderm:  N,  nephrotome  pnrp  nf  Inncipr  nrm- 
or  intermediate  cell  mass;  C,  core  of  myotome.  X  227  diams.  . 

epithelial  tissue    is 

an  ingrowth  from  all  sides  or  only  from  one  or  two,  but  I  am 
strongly  inclined  to  think  that  it  is  probably  part  of  that  side  of 
the  primitive  segment  which  is  next  the  medullary  canal.  The 
cross-sections  further  show  that  the  mesomere  is  more  complex 
than  is  apparent  in  surface  views,  in  that  it  consists  of  a  wider  tri- 
angular part,  Fig.  116,  My,  next  the  medullary  groove,  and  a  nar- 
rower lateral  portion,  JV",  next  the  parietal  zone  ;  it  is  to  the  former 
that  the  term  protovertebra  (  Urwirbel)  is  restricted  by  most  writers, 
while  the  latter  is  termed  the  intermediate  cell  mass,  as  proposed 
by  Balfour  ;  both  are  parts  of  the  primitive  segment.  The  appear- 


THE    PRIMITIVE    DIVISIONS    OF    THE    CCELOM.  197 

ance  of  an  epithelial  arrangement  of  the  cells  is  confined  to  the 
"  protovertebra"  sensu  strictu.  The  square  blocks  seen  in  surface 
views  correspond  to  the  protovertebrse  only  and  not  to  the  whole 
segment. 

The  most  exact  observations  on  the  primitive  segments  of  mam- 
mals known  to  me  are  those  of  Heape,  86.2,  on  the  mole,  of  R. 
Bonnet,  89. 1,  on  the  sheep,  and  of  J.  Kollmann,  91.1,  on  the  human 
embryo.  The  vertebral  plate  thickens  as  the  medullary  plate  rises 
and  becomes  triangular  in  cross-section ;  the  mesodermal  cells,  which 
up  to  this  point  have  been  of  the  anastomosing  type,  become  elon- 
gated and  radiating,  and  gradually  assume  an  epithelioid  character, 
which  becomes  most  distinct  on  the  ectodermal  side ;  the  cells  grad- 
ually withdraw  from  the  centre  of  the  segment,  leaving  a  cavity.* 
The  cells  of  the  segment  multiply  rapidly,  most  of  the  divisions  tak- 
ing place  in  radial,  but  some  in  tangential,  planes.  The  segments 
have  the  triangular  form  already  noticed  in  other  classes.  The  cells 
have  branching  prolongations,  which  extend  out  to  the  primary 
germ-layers,  and  are  especially  marked  on  the  ectodermal  side. 
In  the  sheep  the  cavities  of  the  first  four  segments,  and  of  them  only 
(Bonnet,  /.c.,  50),  extend  through  the  lateral  portions  of  the  segments 
and  communicate  with  the  main  ccelom ;  these  four  segments  Bonnet 
assigns  to  the  occipital  region.  A  similar  series  of  communications 
have  been  recorded  for  the  chick  by  S.  Dexter,  90. 1. 

The  Ventral  or  Unsegmented  Ccelom.  f — This  portion  of  the 
ccelom,  which  persists  in  the  adult,  gives  rise  to  the  pericardial, 
pleural,  and  abdominal  cavities,  which  are  morphologically  parts  of 
one  continuous  cavity,  the  ventral  ccelom.  Many  terms  are  in  use 
to  designate  the  ventral  ccelom ;  by  English  embryologists  it  is  usu- 
ally called  the  pleiiro-peritoneal  space  or  cavity ;  or  often  simply 
body-cavity  (Leibeshohle,  cavite  somatique) ;  by  German  writers  it 
is  sometimes  termed  lateral  ccelom,  sometimes  the  Parietalhohle, 
although  the  latter  term  is  properly  used  only  for  the  pleuro-peri- 
cardial  division  of  it.  Hatschek  has  proposed  splanchnoccele,  which 
is  adopted  in  this  work. 

The  splanchnoccele  appears  in  all  cases  in  the  parietal  zone  of  the 
mesoblast  as  a  narrow  fissure,  the  method  of  origin  of  which  has 
already  been  described,  p.  151.  The  fissure  rapidly  widens  and  ex- 
tends toward  the  axis  until  it  almost  reaches  the  primitive  segments 
and  also  spreads  out  laterally  and  into  the  so-called  extra-embryonic 
region  of  the  amniota,  but  there  is  for  a  considerable  period  a  circu- 
lar area  inclosing  the  region  of  the  embryo  like  a  ring,  in  which  the 
mesoblast  contains  no  ccelom ;  this  mesodermic  ring  is  known  as  the 
vascular  area  (area  vasculosa,  G-efdsshof) ,  and  has  for  its  special 
function  the  production  of  the  first  blood-vessels  and  blood-corpuscles ; 
see  Chapter  X.  In  later  stages  the  coelom  extends  into  the  vascular 
area. 

The  splanchnoccele  is  developed  earlier,  and  acquires  a  greater  clis- 
tention  at  first  in  the  future  cervical  region.  A.  Goette,  90. 1, 
48-49,  states  that  in  Petromyzon  it  precedes  in  the  region  of  the 
heart  the  appearance  of  the  segmental  cavity.  In  the  Amphibia 

*See  Bonnet's  figure,  I.e.  Arch.  Anat.  und  physiol.  1889,  Taf.  v.  Fig.  5. 
t  On  the  splanchnoccele  of  the  head,  see  also  p.  199. 


I'.IS 


THE    EMBRYO. 


the  precocity  of  the  cervical  ccelom  appears  also,  and  it  is  perhaps 
true  of  other  anamniota.  The  development  of  the  main  ccelom  is  still 
more  hastened  in  all  the  amniota,  being  in  them  intimately  associ- 
ated with  the  development  of  the  amnion.  In  the  chick  this  is  very 
well  marked,  because  as  probably  in  all  sauropsida  the  splanchno- 
coele  enlarges  so  rapidly  in  the  cervical  region  that,  even  while  the 
number  of  primitive  segments  is  very  small,  we  can  recognize  a 
vesicular  space  in  the  mesoderm  on  either  side  of  the  head  of  the 
embryo ;  for  these  spaces,  which  are  the  Parietalhohlen  of  German 
embryologists,  I  propose  the  name  of  amnio-cardial  vesicles.  They 
are  shown  in  Fig.  117,  Ves.  Their  rapid  expansion  soon  brings 


FIG.  117.—  Section  of  a  Chick  with  about  Twenty  Segments.  Ht,  Heart;  Coc,  ccslom;  J/<7, 
medullary  tube ;  Ph,  pharynx;  £om,  somatopleure ;  Am,  amniou;  Fes,  amnion-cardial  vesicles; 
C/io,  chorion ;  <SpZ,  splanchnopleure.  X  about  40  diams. 

them  into  contact,  and  then  into  fusion  with  one  another  under  the 
neck  of  the  embryo.  The  heart  is  lodged  in  this  cavity,  of  which 
the  lateral  increase  produces  the  so-called  head-fold  of  the  amnion. 
It  is  on  account  of  this  double  destiny  that  the  name  amnio-cardial 
vesicles  is  proposed.  The  relations,  which  are  traced  out  through 
the  later  stages  in  Chapter  XV.,  may  be  more  fully  understood  from 
Fig.  117,  which  is  a  cross-section  of  a  chick  through  the  heart  region 
at  an  older  stage  than  we  are  now  considering.  The  posterior  limit 
of  the  amnio-cardial  ccelom  is  marked  by  the  course  of  the  omphalo- 
mesaraic  veins,  which  arise  later  and  establish  the  communication 
between  the  area  vasculosa  and  the  venous  end  of  the  embryonic 
heart.  The  topographical  relations  are  described  in  Chapter  XIII., 
on  the  germinal  area.  In  mammals  the  same  peculiarity  of  the 
precocious  dilatation  of  the  amnio-cardial  ccelom  probably  recurs, 
but  has  not  yet  been  properly  investigated.  In  the  sheep  (Bonnet, 
89. 1),  the  amnion  appears  extraordinarily  early,  and,  as  it  must  be 
preceded  by  the  formation  of  the  coelom,  we  find  in  the  sheep  a  huge 
ring  of  splanchnoccele  around  the  embryo  while  it  is  still  in  the 
primitive  streak  stage. 

The  splanchnoccele  of  the  body  proper — that  is,  of  the  region  behind 
the  neck  and  heart — appears  after  the  primitive  segments,  even  in  the 
sheep,  in  which  the  extra-embryonic  coelom  is  so  very  early  developed. 
Moreover,  in  the  body  the  main  coelom  expands  more  slowly  than  in 
the  neck.  The  expansion  takes  place  at  first  only  in  the  part  of  the 
mesoderm  next  the  primitive  segments,  Fig.  02.  Already  the  thick- 
ening of  the  segmental  plate  ( Urwirbelplatte) ,  which  accompanied 
the  uprising  of  the  medullary  plate,  has  marked  out  partially  the 


THE   PRIMITIVE   DIVISIONS   OF   THE   COELOM.  199 

region  of  the  embryo  from  that  of  the  yolk,  and  now  the  distention 
of  the  splanchnocoele  increases  and  finally  completes  the  demarcation 
of  the  embryonic  region  from  the  extra-embryonic.  The  splanchno- 
coele extends  in  all  amniota  only  part  way  through  the  mesoderm, 
until  quite  late  in  development,  so  that  at  a  gradually  increasing 
distance  from  the  embryo  there  is  a  layer  of  mesoderm  without  any 
cavity,  and  the  cells  of  which  preserve  the  mesenchymal  type.  This 
undivided  mesoderm  develops  the  first  blood  and  blood-vessels. 
After  the  first  vessels  of  the  area  have  appeared  the  splanchnocoele 
spreads  out  over  them,  so  that  the  first  vessels  lie  then  beloiv  the 
ccelom — i.  e.,  in  the  splanchnopleure. 

As  the  splanchnocoele  develops,  the  mesodermal  cells  assume  grad- 
ually a  more  and  more  distinctly  epithelial  character,  so  that  the  main 
ccelom  becomes  bounded  by  mesothelium,  as  described  in  Chapter 
VI.,  and  the  somatic  leaf  of  the  mesoderm  is  differentiated  from  the 
splanchnic ;  toward  the  axis  of  the  embryo  the  two  leaves  pass  into 
one  another,  and  also  at  the  distal  edge  of  the  ccelom  the  two  leaves 
pass  without  any  distinct  limit  into  the  uncleft  mesoderm  of  the 
area  vasculosa. 

In  conclusion,  I  wish  to  emphasize  the  fact  that  the  splanchnocoele 
(pleuroperitoneal  cavity)  is  almost,  if  not  quite,  from  the  start  di- 
vided into  a  precociously  enlarged  cervical  portion  (amnio-cardial 
vesicles,  Parietalhohle)  and  a  rump  portion  (abdominal  cavity) ;  the 
boundary  between  the  two  portions  is  marked  by  the  omphalo- 
mesaraic  veins,  which  run  from  the  area  vasculosa  into  the  embryo 
proper  at  nearly  right  angles  to  the  embryonic  axis.  This  primitive 
disposition  is  of  fundamental  morphological  significance. 

Coelom  of  the  Head. — No  thorough  investigation  of  the  history 
of  the  early  stages  of  the  mesoderm  in  the  head  has  yet  been  made 
for  any  vertebrate.  Until  this  is  done  we  cannot  hope  to  understand 
the  morphology  of  the  head,  because  the  progress  of  research  has 
demonstrated  more  and  more  clearly  that  the  head  is  made  up  of 
seriss  of  greatly  modified  segments,  but  the  number  and  metamor- 
phoses of  the  head  segments  can  be  determined  only  by  knowing  the 
entire  history  of  the  mesoderm.  Balfour  was  the  first,  78.3,  to 
demonstrate  the  existence  of  the  ccelom  in  the  head,  and  to  partially 
work  out  its  subdivisions.  The  subject  was  further  advanced  by 
A.  Millies  Marshall,  81.2,  Van  Wijhe,  82.1,  A.  Dohrn,  90.2,  and 
others.  Marshall  and  Van  Wijhe's  results  have  been  subjected  by 
Gegenbaur,  88. 1,  3-8,  to  criticism,  which  seems  to  me  by  no  means 
fortunate.  Gegenbaur 's  conclusion,  that  the  number  of  cephalic 
segments  gives  no  trustworthy  indication  of  the  ancestral  history  I 
must  entirely  dissent  from,  since  I  believe  that  the  number  of  meso- 
dermic  segments  in  the  head  of  the  embryos  of  the  lower  vertebrates 
is  the  only  trustworthy  clew  to  the  morphogeny  of  the  head,  which 
we  can  seek  at  present. 

A.  Dohrn,  90.1,  335,  made  the  discovery  of  a  large  number  of 
segments  in  the  head  of  vertebrate  embryo,  having  observed  seven- 
teen or  eighteen  in  the  head  of  Torpedo  marmorata  of  3  mm.  Killian, 
91.1,  confirms  and  rectifies  (/.  c.,  p.  103)  Dohrn 's  observations,  and 
describes  seventeen  to  eighteen  segments,  Fig.  118,  in  the  head  of 
Elasmobranchs,  as  follows:  Oral  zone  with  two  segments;  mandibu- 


200 


THE   EMBRYO. 


lar  zone  with  three;  spiracular  zone  with  three,  corresponding  to 
the  first  gill  cleft ;  hyoid  zone  with  four,  in  the  region  of  the  second 
gill  cleft;  glossopharyngeal  zone  with  two;  occipital  zone  with  four. 

Killian  observed  these  segments 
in  Balfour's  stages  F  and  J,  of 
Torpedo  ocellata. 

In  later  stages  Van  Wijhe, 
82.1,  whose  results  have  been 
verified,  found  only  nine  seg- 
ments. The  number  is  presuma- 
bly reduced  chiefly  by  abortion, 
but  partly  also  by  fusion.  Van 
Wijhe's  segments  are  as  follows : 
The  first  or  prae-oral  is  identical 
with  Balfour's  prse-mandibular 
cavity;  and  it  is  identified  by 
Killian  with  his  oral  zone;  it  is 
possible  that  the  first  segment  of 
the  oral  zone  is  identical  with 


the 


new 


FIG.  118.  —Head  of  an  Embryo  of  Torpedo  Ocel- 
lata,  in  Balfour's  Stage  J.  I-X,  Anlages  of  the 
cephalic  ganglia  and  nerves;  1-18,  cephalic 
primitive  segments;  1-3,  first  three  rump  seg- 
ments; o.pl,  oral  plate;  Sp,  "spiracular"  cleft, 
or  first  gin  cleft;  Hy,  hyoid  cleft.  The  dotted 
circle  below  I  and  II  indicates  the  optic  vesicle. 
After  Killian. 


head  cavity  described 
by  Julia  B.  Platt,  91.2;  Van 
Wijhe's  first  segment  is  small 
and  acquires  its  cavity  late,  being 
solid  after  the  remaining  eight 
myotomes  have  developed  their 
cavities;  it  is  connected  by  a 
short  band  of  cells  across  the 
median  line  with  its  fellow  of  the 
opposite  side;  this  band  subse- 
quently (in  Balfour's  stage  L) 
disappears ;  the  first  segment  pro- 
duces four  muscles,  the  rectus 

superior,  internus,  and  inferior,  and  the  obliquus  superior.  The 
second  or  mandibular  segment  (Balfour's  mandibular  cavity)  cor- 
responds with  Killian's  mandibular  zone;  its  cavity  disappears 
in  Balfour's  stage  O ;  it  produces  the  muscles  of  mastication ;  ac- 
cording to  Killian,  it  is  produced  by  the  fusion  of  three  segments. 
The  third  segment  seems  also  to  be  the  product  of  the  fusion  of  three 
primitive  segments  of  Killian's  spiracular  zone;  its  cavity  has  a 
communication  through  the  hyoid  arch  with  the  ventral  ccelom 
(pericardial  cavity).  The  fourth  segment  corresponds  in  position 
over  the  second  or  hyoid  gill  cleft  with  the  three  segments  of  Kil- 
lian's hyoid  zone  (Dohrn's  eleventh  to  thirteenth  segments).  The 
fifth  segment  corresponds  to  the  two  segments  of  Killian's  glosso- 
pharyngeal zone.  Killian's  four  occipital  segments  all  persist  inde- 
pendently of  one  another  to  constitute  Van  Wijhe's  sixth  to  ninth 
segments,  which  I  think  are  to  be  further  identified  with  the  four 
temporarily  present  hypoglossal  or  occipital  segments  which  Froriep 
has  discovered,  86.1,  in  amniote  embryos.  Van  Wijhe  regarded 
nine  as  the  total  maximum  number  of  segments  in  the  vertebrate 
head,  and  sought,  89.2,  to  identify  nine  corresponding  segments  in 
Amphioxus. 


THE    PRIMITIVE    DIVISIONS    OF   THE   CCELOM.  201 

That  a  series  of  coelomatic  cavities  exist  in  the  head  of  the  am- 
phibian embryo  was,  if  I  am  not  mistaken,  first  observed  by  Scott 
and  Osborn,  79. 1.  Houssay,  90. 1,  has  sought  to  identify  the  num- 
ber of  cephalic  myotomes  in  the  axolotl.  He  accepts  the  idea  of 
the  exact  correspondence  between  the  branchial  pouches  and  the  myo- 
tomes in  segmental  order ;  and  as  he  maintains  that  there  is  a  gill 
pouch,  which  corresponds  to  the  auditory  nerve  and  aborts  during 
embryonic  life,  and  further  regards  the  nose,  hypophysis,  and  mouth 
each  as  representing  a  separate  segment,  he  finds  that  there  must  be 
at  least  eleven  segments  in  the  head  of  the  axolotl,  as  follows:  1, 
nose;  2,  hypophysis;  3,  mouth;  4,  "event;"  5,  hyo-mandibular ;  6, 
hyoid;  7,  ear;  8,  first  branchial;  9,  second;  10,  third;  11,  fourth 
branchial ;  for  each  of  these  he  assumes  a  separate  myotome.  He 
has  actually  observed,  90. 1,  the  nine  somites  corresponding  to  those 
described  by  Van  Wijhe  (see  above),  and  further  claims  to  have 
found  evidence  that  the  second  and  third  of  these  are  both  really 
double,  thus  identifying  eleven  mesomeres,  which,  he  says,  91.1, 
58,  appear  in  the  following  order : 

In  position 1        23456789      10      11 

In  time 1        2      11      10        6345789 

Or  in  groups 1'       2'       3"      2"      1"      1'"     2'"     3'"     4'"     5'"     6'" 

.Van  Bemmelen,  89.1,  254,  in  a  superb  reconstruction  of  the  head 
of  a  snake  embryo,  shows  three  myotomes  belonging  to  the  eyeball, 
but  gives  no  information  concerning  them,  and  represents  no  other 
myotomes  in  the  head  until  the  hypoglossal  region  with  its  four 
myotomes  is  reached.  A.  Oppel,  90.1,  describes  the  cephalic  seg- 
ments in  Anguis  embryos;  he  has  recorded  the  presence  of  Van 
Wijhe 's  first  to  third  and  sixth  to  ninth  segments. 

The  splanchnoccele  of  the  head  becomes  the  pericardial  cavity  of 
the  adult ;  its  mesothelium,  where  it  covers  the  heart,  gives  rise  to 
the  cardiac  muscle,  and  it  is  supposed  to  extend  between  the  gill 
pouches  to  produce  the  muscles  of  the  branchial  arches.  Along  the 
level  of  the  branchial  pouches  the  splanchnoccele  becomes  in  part 
divided,  as  first  shown  by  Balfour,  78.3,  into  a  series  of  separate 
cavities  by  the  outgrowth  of  the  gill  pouches  and  the  union  of  the 
entoderm  of  each  pouch  with  the  ectoderm.  Each  of  these  cavities 
has  an  elongated  form  and  communicates  on  the  dorsal  side  with  a 
myotome,  and  on  the  ventral  side  with  the  pericardial  cavity  (Van 
Wijhe,  82.1,  Van  Bemmelen,  90.1).  We  may  distinguish,  there- 
fore, the  mandibular  ccelom,  the  hyoid  ccelom,  and  the  branchial 
ccelom  (one  cavity  in  each  gill  arch).  The  connection  of  the  cavities 
of  the  arches  with  both  the  myotomes  and  pericardial  cavity  is  ap- 
parently lost,  but  as  to  the  separation  there  are  no  definite  observa- 
tions. The  actual  cavities  in  the  arches  are  soon  obliterated,  but 
their  mesothelial  walls  persist  and  produce  the  branchial  muscles ; 
compare  Chapter  XXI. 

Division  of  the  Primitive  Segments.— The  primitive  seg- 
ments very  early  divide,  each  into  two  parts — the  myotome  (proto- 
vertebra  of  authors)  next  the  medullary  canal,  and  the  smaller 
nephrotome  (intermediate  mass,  Kolliker's  Mittelplatte) ;  next  the 
lateral  plates  or  mesothelium  of  the  splanchnocoale,  Fig.  116.  The 


202  THE    EMBRYO. 

division  is  evidently  indicated  as  soon  as  the  primitive  segments  are 
formed,  the  thicker  proximal  end  being  destined  for  the  myotome, 
the  thinner  distal  end  for  the  nephrotome ;  the  latter  originally  unites 
the  myotome  with  the  lateral  plates,  hence  its  name  of  u  intermediate 
cell  mass" ;  as  its  principal  function  is  to  develop  the  nephridia  it 
may  be  more  conveniently  named  the  nephrotome,  as  proposed  by 
Ruckert,  88.1. 

The  nephrotome  has  to  separate  from  the  splanchnoccelic  meso- 
thelium  (lateral  plates)  on  the  one  side  and  the  myotome  upon  the 
other  Unfortunately  this  double  separation  has  been  as  yet  very 
inadequately  studied,  except  in  the  case  of  elasmobranchs,  where 
the  development  of  the  nephridia  has  been  carefully  investigated; 
for  details  compare  Chapter  XI.  In  Bombinator  a  groove  appears 
on  the  ectodermal  side  and  gradually  deepens  until  it  separates  the 
myotome  from  the  rest  of  the  mesoderm ;  this  groove  does  not  pass 
through  in  the  shortest  direction,  but  extends  obliquely  upward,  A. 
Goette,  75.1,213.  The  nephrotome  loses  its  connection  with  the 
myotome  relatively  early,  but  retains,  at  least  in  some  segments,  the 
connection  with  the  lateral  plates  for  some  time  longer  in  most 
elasmobranchs  and  amphibians  throughout  life,  but  in  amniota  only 
during  embryonic  stages.  The  exact  histological  changes  by  which 
the  nephrotome  serves  its  double  connections  are  still  unknown.  A. 
Goette,  90. 1,  49,  states  that  in  Petromyzon  the  isolation  of  the  neph- 
rotome takes  place  in  the  front  end  of  the  body  when  the  mesoderm 
has  a  well-developed  coelom,  but  in  the  rear  part  while  the  meso- 
derm has  no  coelom  either  in  the  vertebral  or  lateral  plates. 

C.  Rabl,  89.2,  has  directed  especial  attention  to  the  fact  that  in 
elasmobranchs  there  is  a  special  outgrowth  of  the  wall  of  the  primi- 
tive segments  on  the  side  nearest  the  chorda  and  from  the  point 
where  the  nephrotome  joins  the  myotome,  Fig.  122.  This  out- 
growth* is  the  beginning  of  the  mesenchyma,  and  recurs,  of  course, 
segmentally,  so  that  the  term  sclerotome  may  be  applied  to  it,  but 
all  trace  of  segmental  division  is  very  soon  lost,  nor  does  the  seg- 
mental  origin  of  the  axial  mesenchyma,  which  is  developed  from 
these  outgrowths,  determine  the  subsequent  morphological  differen- 
tiation, so  far  as  yet  known.  Rabl  likens  this  outgrowth  to  an 
evagination,  and  points  out  that  the  cavity  of  the  nephrotome  pres- 
ents a  slight  diverticulum  at  first,  where  the  outgrowth  takes  place. 
He  compares  this  evagination  with  the  evagination  at  a  corresponding 
point  in  Amphioxus,  which  has  been  described  by  Hatschek,  88.1, 
and  is  said  to  grow  up  between  the  myotome  and  the  medulla;  in 
Amphioxus,  however,  the  cells  retain  an  epithelial  character,  while 
in  the  vertebrate  they  are  mesenchymal ;  but  as  no  strict  line  can  be 
drawn  between  these  two  types  of  tissue,  the  histological  difference 
cannot  be  held  to  invalidate  the  homology  drawn  by  Rabl. 

The  cavity  of  the  primitive  segment  varies  greatly  in  the  various 
classes  of  vertebrates.  In  the  primitive  forms,  Petromyzon,  Am- 
phibians, etc.,  the  myotomic  portion  is  wedge-shaped,  appearing 
triangular  in  cross-section,  and  considerably  wider  than  the  cavity 
of  the  nephrotome.  In  elasmobranchs,  cf.  C.  Rabl,  89.2,  Taf.  X., 
Figs.  1-6,  a  similar  difference  exists  at  first,  but  very  soon  the  two 

*  Compare  Rabl,  I.  c.,  "Morph.  Jb.  ,"xv. ,  Taf.  x.,  Fig.  4,  sk. 


THE    PRIMITIVE   DIVISIONS   OF   THE   CCELOM.  203 

walls  of  the  myotome  come  close  together,  Fig.  12*2,  obliterating  the 
cavity ;  the  nephrotomic  portion,  on  the  contrary,  widens  meanwhile. 
In  Lepidosteus  the  medullary  and  entodermal  sides  of  the  myotome 
are  represented  as  several  layers  of  cells  thick  by  Balfour  and 
Parker,  82.1,  PI.  23,  Figs  28,  29,  so  that  the  myotome  appears 
partly  filled  with  cells  belonging,  however,  to  its  inferior  wall.  We 
have  in  this  case  perhaps  a  transition  to  the  amniote  structure,  in 
which  the  encroachment  of  cells  is  so  great  that  no  distinct  cavity 
can  be  recognized  in  the  myotome,  Fig.  116;  and  since  the  nephro- 
tomic cavity  appears  very  late,  it  results  that  in  the  amniota  there 
is  no  distinct  cavity  whatsoever  in  the  primitive  segments,  though 
there  is  a  cavity  later  in  both  the  myotome  and  nephrotome. 

The  primitive  aortee  lie  close  below  the  myotomes  on  each  side, 
Figs.  119,  122,  161,  105;  a  glance  at  any  of  these  will  show  the 
reader  that  the  mesoderm  derived  from  the  myotome  from  the  very 
first  comes  into  contact  with  and  soon  envelops  the  medullary  tube, 
Mdy  the  notochord,  Ch,  and  the  aorta,  Ao,  and  also  reaches  over 
part  of  the  entodermal  wall  of  the  archenteron. 

Shape  of  the  Myotome. — As  described  above,  the  myotome, 
when  first  formed  and  even  before  it  is  separated  from  the  nephro- 
tome, appears  more  or  less  nearly  square  in  surface  views  and  trian- 
gular in  cross-section.  Very  soon  it  enlarges  in  Amphibia  and 
amniota,  so  as  to  appear  square  in  section  also,  Fig.  119.  The  cavity 
in  Amphibia  is  very  distinct  and  the  epithelial  character  of  the  walls 
well  marked;  but  in  all  amniota,  so  far  as  known,  the  cavity  at  this 
stage  is  still  obliterated  by  the  core  of  cells  (Remak's  Urwirbelkern) . 
By  the  assumption  of  the  cuboidal  shape  the  myotome  becomes  more 
sharply  marked  off  from  the  intermediate  mass  or  nephrotome,  and 
as  the  lateral  or  main  coelom  has  been  expanding  during  the  same 
period,  there  is  established  a  space  above  the  nephrotome  and  be- 
tween the  myotome  and  the  lateral  plates.  It  is  in  this  space  that 
the  primitive  longitudinal  duct  of  the  urogenital  system,  Fig.  116, 
IV.d.,  is  situated  as  soon  as  developed — a  fact  which  led  many  writers 
to  attribute  the  origin  of  the  duct  to  a  differentiation  of  the  interme- 
diate cell  mass. 

Differentiation  of  the  Myotome. — We  can  distinguish  three 
steps  in  the  differentiation :  1,  production  of  mesenchyma  from  the 
inner  wall  of  the  myotome,  Fig.  119;  2,  production  of  the  true 
muscle  plate,  Fig.  120;  3,  conversion  of  the  outer  wall  into  mesen- 
chyma to  form  the  dermal  layer,  Fig.  121. 

The  production  of  mesenchyma  from  the  inner  wall  begins  very 
early,  and  is  marked  by  a  loosening  and  moving  apart  of  the  meso- 
thelial  cells  until  the  entire  inner  wall,  at  least  in  amniota,  is  con- 
verted into  tissue  of  the  mesenchymal  t}-pe,  Fig.  119,  mes.  Owing 
to  the  moving  apart  of  the  cells  the  tissue  occupies  a  large  space  and 
fills  up  the  myotomic  cavity.  While  the  metamorphosis  is  going  on 
the  cells  multiply  rapidly.  The  course  of  this  change  of  the  inner 
wall  has  been  carefully  studied  by  W.  Heape,  86.2,  in  the  mole,  by 
R.  Bonnet,  89.1,  45-55,  in  the  sheep,  and  by  Erik  Muller,  88.1,  in 
the  chick.  Muller  has  further  demonstrated  that  the  muscular  en- 
velope of  the  aorta  comes  from  the  mesenchyma  produced  by  the 
inner  myotomic  wall.  In  elasmobranchs,  according  to  C.  Rabl, 


204 


THE   EMBRYO. 


89.2,  the  greater  part  of  the  inner  wall  of  the  myotome  very  early 
shows  the  differentiation  of  muscle  fibres,  the  cells  retaining  the 
mesothelial  type,  Fig.  122,  and  the  mesenchyma  is  produced  only 


from  that  part  of  the  inner  wall,  which  is  nearest  the  nephrotome 
(Mittelplatte  of  Remak) ;  in  elasmobranchs,  therefore,  the  mesen- 
chyma appears  more  as  an  outgrowth  from  one  point — a  fact  which 


THE    PRIMITIVE    DIVISIONS    OF   THE    CCELOM. 


205 


leads  Rabl  to  a  significant  comparison  with  Amphioxus,  as  stated 
above.  In  amniota  the  persistence  of  the  outgrowth  is  indicated  by 
the  fact  that  the  metamorphosis  of  the  mesothelium  of  the  inner  wall 
begins  near  the  nephrotome;  it  spreads,  however,  rapidly,  so  that 
nearly  the  entire  wall  undergoes  the  transformation.  My  own  obser- 
vations are  incomplete,  but  they  indicate  that  in  amniota  the  differ- 
entiation of  myotomic  muscles  invariably  follows  later.  Where  the 
inner  wall  joins  the  outer  the  cells  retain  the  mesothelial  arrange- 
ment for  a  very  considerable  period  (see  Figs.  119  and  121). 

The  muscle  plate  proper  arises  from  cells  of  the  inner  wall  next 
the  myotomic  cavity,  or  we  may  say — since  the  cavity  is  obliterated 


EC 


mes 


FIG.  120.—  Longitudinal  Horizontal  Section  through  a  Segment  of  a  Rabbit  Embryo  of  Ten 
and  One-half  Days.  3/rf,  Edge  of  the  medullary  wall:  EC.  ectoderm;  I. a.  i ntersegmental artery ; 
mes,  the  so-called  sclerotome  or  mesenchyma  of  the  inner  wall ;  Cu,  outer  wall  of  the  myotome 
(anlage  of  the  cutis;  ;  M,  muscle-plate,  x  296  diams. 

— from  the  cells  nearest  the  outer  wall.  The  cells  become  elongated 
parallel  with  the  longitudinal  axis  of  the  embryo,  Fig.  120,  M ;  the 
nuclei  also  elongate  in  the  same  direction,  thus  becoming  oval,  and 
as  shown  in  the  figure  they  are,  at  least  in  the  chick,  larger  than 
the  nuclei  both  of  the  neighboring  mesenchyma,  mes,  and  of  the 
outer  myotomic  wall,  Cu.  The  remainder  of  the  inner  wall,  mes,  is 
the  sclerotome  of  recent  German  writers;  it  consists  of  mesen- 
chymal  cells  which  are  now  entirely  separated  from  the  parts  of  the 


206 


THE   EMBRYO. 


myotome  which  are  still  mesothelial.  While  the  muscle-plate  is 
forming  the  mesenchyma  merges  with  it,  but  gradually  it  becomes 
sharply  marked  off  from  the  muscle  cells.  The  muscle-plate  is  con- 
tinuous at  its  edge  with  the  outer  wall,  Cu,  and  retains  the  continu- 
ity for  a  very  long  period.  The  muscle-plate  and  outer  mesothelium 
now  form  a  single  and  highly  characteristic  structure,  familiar  to 
all  embryologists ;  the  structure  is  a  double  plate,  which  takes  an 
oblique  position  in  the  embryo ;  as  seen  in  cross-sections  the  double 
plate  descends  from  near  the  dorsal  border  of  the  medullary  tube 
downward  and  outward  toward  the  somatopleure. 

The  next  change  is  the  production  of  mesenchyma  from  the  outer 
wall ;  the  cells  of  the  mesothelium  move  asunder  until  they  come  to 
lie  quite  far  apart,  Fig.  121,  Cu,  forming  from  the  start  a  much 


mes 


msth 


EC 


mes 


Mil 


FIG.  121.— Transverse  Section  through  the  Upper  Part  of  a  Myotome  of  a  Chick  of  about 
Seventy  Hours.  Mes,  mes',  Mesenchyma  of  inner  wall  of  myotome ;  Ep,  endothelial  layer  formed 
against  the  surface  of  the  medullary  tube ;  msth,  mesothelial  portion  of  the  myotcme ;  EC,  ecto- 
derm; Cu,  mesenchyma  (cutis)  from  outer  wall  of  myotome;  Mu,  muscle-plate,  the  limits  of 
winch  are  much  clearer  in  the  preparation  than  in  the  engraving,  x  296  diams. 

looser  tissue  than  did  the  mesenchyma  from  the  inner  wall ;  but  at 
this  stage,  Fig.  121,  the  inner  mesenchyma,  mes,  is  spreading  around 
the  medullary  canal,  and  as  it  spreads  assumes  also  a  looser  texture. 
The  mesothelium,  msth,  still  persists  around  the  four  margins  of 
the  double  plate,  apparently  as  an  organ  to  produce  cells  to  be  added 
on  the  one  hand  to  the  muscle  plate  proper,  Mu,  on  the  other  to  the 
cutis  (dermal  mesenchyma),  Cu.  In  sections  the  mesothelium  usu- 
ally makes  a  U-shaped  figure,  which  is  highly  characteristic  of  all 
vertebrate  embryos.  . 

In  the  primitive  vertebrates  as  exemplified  by  Petromyzon  (Goette, 


ORIGIN   OF   THE    MESEXCHYMA.  207 

90.1,  Taf.  VI.,  Figs.  60-63),  the  flattened  myotome  consists  of  two 
closely  appressed  epithelial  plates  with  a  narrow  fissure  between 
them  and  passing  over  at  their  edges  into  one  another ;  the  upper 
edge  of  the  myotome  is  nearly  on  a  level  with  the  dorsal  margin  of 
the  medulla ;  the  myotome  inclines  obliquely  outward  and  downward 
and  has  its  lower  edge  on  the  level  of  the  archenteric  cavity;  the 
outer  layer  of  epithelium  is  the  thinner,  while  the  inner  layer  is  con- 
siderably thickened ;  as  the  myotome  develops  farther  this  difference 
between  the  two  layers  increases. 

The  amphibian  myotomes  resemble  very  closely  those  of  Petromy- 
zon,  but  soon  come  to  differ  from  them  by  the  multiplication  of  cells 
of  the  inner  layer  (A.  Goette,  75.1,  211,  Figs.  13S-L40),  which  be- 
comes several  cells  thick  and  loses  at  the  same  time  its  distinctly 
epithelial  character  in  the  inner  part  of  the  layer,  though  it  retains  it 
in  the  outer  part,  there  remaining,  on  the  side  nearest  the  entoderm, 
a  single  row  of  cells  in  epithelial  form,  so  that  we  have  here  a  con- 
dition established  secondarily  which  in  the  amniota  exists  almost 
from  the  start — namely,  a  core  of  looser  cells  filling  the  myotomic 
cavity,  but  belonging  to  the  entodermal  side;  it  is  at  this  stage  that 
in  Bombinator  the  myotome  separates  from  the  remaining  mesoderm. 
In  later  stages  the  amphibian  myotome  gives  off  from  probably  all 
parts  of  its  waU  cells  to  form  part  of  the  mesenchyma.  while  the  cells 
which  remain  form  the  definite  muscle-plate. 

Origin  of  the  Mesenchyma. — The  first  author  to  trace  the 
origin  of  the  mesenchyma  to  the  primitive  mesothelium  was  Alex- 
ander Goette,  who  fully  demonstrated  the  fact  in  his  great  work  on 
the  "  Unke,"  75.1.  Goette  designates  the  mesenchyma  as  Bildungs- 
geicebe,  and  seems  to  me  to  have  been  the  first  to  fully  recognize  the 
morphological  significance  of  the  tissue.  But  his  work  has  not  hith- 
erto received  its  deserved  attention.  Scattered  through  numerous 
special  papers  are  isolated  observations  which  might  be  profitably 
collated,  and  which  suffice  to  show  that  the  mesenchyma  arises  from 
the  mesothelium.  In  spite  of  this  the  brothers  Hertwig  advanced, 
8 1 . 1,  as  stated  previously,  p.  155,  the  theory  that  the  two  mesodermal 
tissues  are  of  different  origin — a  theory  which  we  now  know  to  be 
false,  as,  indeed,  was  proved  by  Goette  six  years  before  the  Hertwigs' 
theory.  That  all  parts  of  the  mesoderm  have  a  common  origin  was 
the  view  of  the  older  embryologists,  and,  in  fact,  the  differentiation 
of  the  middle  layer  was  in  the  main  correctly  given  by  Remak, 
50. 1.  The  unity  of  the  mesoderm  has  always  been  maintained  by 
Kolliker  in  his  text-books  and  articles,  one  of  which,  84.4,  contains 
a  series  of  well-founded  criticisms  of  other  views  and  a  sufficient 
defence  of  his  own.  Recently  the  origin  of  the  mesenchyma  has 
been  specially  investigated  by H.  Ziegler,  C.  88.1,Rabl,  89.2,  and 
Van  Wijhe,  89.1,  in  elasmobranchs,  and  by  R.  Bonnet,  89.1,  in  the 
sheep. 

The  mesenchyma  rises  from  cells  thrown  off  from  the  mesothelium. 
The  entire  mesothelium  participates  in  this  process,  but  not  to  an 
equal  degree,  nor  at  the  same  time  throughout  its  whole  extent. 
The  first  part  to  produce  the  mesenchymal  cells  in  elasmobranchs  is 
the  splanchnic  leaf  at  the  point  where  the  nephrotome  unites  with 
the  myotome;  at  this  point,  as  stated  above,  there  are  traces  of 


208 


THE   EMBRYO. 


Md 


an  evagination.  A  little  later,  Fig.  122,  the  outer  wall  of  the 
myotome  throws  off  cells  throughout  its  whole  extent,  and  at  the 
same  time  a  much  less  active  emigration  is  going  on  from  the  ne- 
phrotome,  while  it  is  not  until  much  later  that  the  walls  of  the 
splanchnocoele  contribute  to  the  mesenchyoa.  Whether  the  meso- 
derm  of  the  area  vasculosa,  in  which  there  is  at  first  no  coelom,  con- 
tributes directly  to  the  mesenchyma 
is  uncertain;  it  certainly  produces 
(see  Chapter  X.)  the  blood-vessels, 
and  whether  the  vessels  ought  to  be 
considered  as  mesenchyma  or  as  a 
distinct  tissue  is  still  under  debate. 
An  excellent  diagram  illustrating 
the  mesothelial  sources  of  the  mes- 
enchyma is  given  by  H.  Ziegler, 
88. 1,  Taf.  XIII.,  Fig.  1.  For  am- 
phibians we  have  Goette 's  detailed 
account;  the  mesenchyma  arises 
from  all  parts  of  the  mesothelium, 
the  cells  moving  off  from  their  epi- 
thelial union  but  remaining  con- 
nected together  by  short  thick  pro- 
cesses, which  are  never  numerous, 
though  variable  in  number ;  the  cells 
all  contain  a  great  deal  of  deuto- 
plasm;  as  development  progresses 
the  yolk  grains  disappear,  the  cells 
become  entirely  protoplasmatic,  and 
the  number  of  intercellular  processes 
increases,  the  processes  at  the  same 
time  becoming  finer  and  longer. 
There  are  regional  distinctions  in 
the  density  of  the  tissue,  which  are 
constant.  The  tissue  increases  by 
additions  from  the  mesothelium  dur- 
ing a  certain  period,  and  continu- 
ously by  the  proliferation  of  its  own 

FIG.  122.-Pristiurus  Embryo  with  Forty-  Cells.       Goette    also,   75.1,  497-498, 

five  to  Forty-six  Segments ;  Cross-Section  of  oocp-pf«i    tViat     «ftpr     tViP     Pirpnlatinn 

the  Anterior  Part  of  the  Body.     Md,  Medul-  ' 

lary  groove ;  Gl,  anlage  of ^the  ganglion ;  My,  begins    leUCOCyteS    leave    the    blOOQ- 

vessels    and   are  transformed  into 


1*1  ron  g  *»ypocborda; 

After  C.  Rabl. 


notochord. 


Bildunqsqewebszellen;  he  does  not 

«.  j«s    •  £ 

seem  to  me  to  otter  sufficient  proor 
to  justify  this  assertion.  Goette  attributes,  /.c.,  493,  the  moving 
ing  apart  of  the  cells,  not,  as  seems  to  me  most  reasonable,  to  their 
own  growth,  but  to  the  accumulation  of  intercellular  fluid,  which  he 
assumes  to  be  produced  by  transfusion  from  the  archenteron.  In 
mammals  and  birds  the  manner  in  which  the  myotome  contributes 
to  the  mesenchyma  is  now  pretty  thoroughly  understood,  but  the 
share  taken  by  the  nephrotome  and  lateral  plates  has  still  to  be  ascer- 
tained. In  both  classes  the  metamorphosis  of  the  outer  wall  occurs 
much  later  than  that  of  the  inner  wall,  which  very  early  becomes 


ORIGIN   OF    THE    MESENCHYMA. 


209 


considerably  thickened  by  the  multiplication  of  its  cells.  Heape, 
86.2,  describes  the  process  in  the  mole  nearly  in  the  following  words : 
The  myotomes  at  Heape's  stage  H  commence  first  in  the  anterior 
region,  and  gradually  assuming  the  same  relations  posteriorly,  to 
divide  into  two  portions,  an  outer  arched  epithelial  portion  and  a 
thicker  inner  portion  composed  of  anastomosing  cells  of  distinctly 
mesenchymal  type,  which  give  rise  to  the  axial  mesenchyma,  and 
participate  in  the  formation  of  the  definite  muscle-plate.  The  myo- 
tomic  cavity  is  very  marked.  In  the  next  stage  (J)  the  anterior 
myotomes  exhibit  still  further  changes ;  the  inner  layer  has  grown 
very  considerably,  and  the  row  of  its  cells  next  the  cavity  are  more 
closely  packed  and  so  have  assumed  the  epithelial  form,  while  the  re- 
mainder of  the  layer  preserves  the  anastomosing  character  of  the  cells ; 
the  inner  layer  of  the  myotome  is  therefore  separated  into  its  two 
parts ;  the  epithelial  part  becomes  continuous  with  the  outer  layer, 
and  the  two  epithelia  together  constitute  the  so-called  double  muscle- 
plate.  Although  arising  from  separate  segments  the  axial  mesen- 
chyma loses  almost  immediately  every  trace  of  segmental  arrange- 
ment, and  there  is  no  real  proof  that  its  segmental  origin  has  direct 
influence  upon  the  segmental  arrangement  of  the  vertebral  and  other 
structures  differentiated  later  from  the  mesenchyma.  Ultimately,  as 
in  other  vertebrates,  the  entire  outer  layer  is  converted  into  mesen- 
chyma, which  forms  the  dermal  layer,  R.  Bonnet,  89.1,  54. 

Comparison  with  Amphioxus.— Hatschek's  observations, 
88. 1,  on  the  differentiation  of  the  mesoderm  of  Amphioxus  show 
that  there  are  many  striking  resemblances 
with  the  history  of  the  vertebrate  meso- 
derm as  given  above.  The  mesoderm  con- 
sists at  first  of  a  series  of  paired  mesothe- 
lial  sacs ;  the  ventral  portions  of  the  sacs 
fuse  into  a  continuous  splanchnoccele ;  in 
a  larva  several  weeks  old  the  inner  wall  of 
the  dorsal  segments  is  a  thick  epithelium, 
which  produces  the  muscles  on  the  inner 
or  entodermal  side  of  the  cavity  of  the 
segment  (myocosle  of  Hatschek) ;  the  me- 
sothelium  becomes  a  thin  pavement  epi- 
thelium. After  about  three  months  of 
pelagic-life,  the  larva  changes  into  Am- 
phioxus and  takes  to  the  sand.  At  this 
time  the  lower  edge  of  the  segment  is 
found  to  have  formed  a  diverticulum, 
which  stretches  upward  beween  the  mus- 
cles on  the  one  side  and  the  medulla  on 
the  other.  The  segments  have  also  ex- 
tended into  the  dorsal  and  ventral  fins  and 
have  there  formed  cavities.  These  rela- 
tions are  illustrated  by  the  accompanying 
diagram,  Fig.  123,  after  Hatschek.  The 
points  of  special  interest  to  us  are  four:  1,  the  formation  of  the 
splanchnoccele  by  the  fusion  of  segmental  cavities ;  2,  the  develop- 
ment of  the  muscles  exclusively  from  the  inner  layer  of  the  secon- 
14 


FIG.  123. — Diagram  of  a  Cross 
Section  of  a  Young  Amphioxus.  /, 
/,  7,  Parts  of  the  ccelom  of  the  seg- 
ment; //,  splanchnocoele ;  1,  outer 
layer  of  segment ;  2,  muscle  layer ; 
3,  4,  5,  6,  portions  of  sclerotomie 
diverticulum;  7,  splanchnic  meso- 
derm around  the  entoderm.  After 
Hatschek. 


210  THE   EMBRYO. 

dary  segments ;  3,  the  absence  of  differentiation  in  the  outer  layer 
of  the  segment ;  4,  the  outgrowth  of  mesothelium  passing  upward 
between  the  muscular  layers  and  the  axial  structures,  medulla,  and 
notochord.  It  is  probable  that  all  these  four  peculiarities  recur  in 
the  true  vertebrates,  though  masked  principally  by  the  fact  that  the 
outer  layer  of  the  segment  and  the  epiaxial  diverticulum  both  lose, 
the  former  gradually,  the  latter  almost  from  the  start,  all  trace  of 
epithelial  structure,  and  become  converted  into  mesenchyma.  Of 
course  the  assumption  that  the  vertebrate  splanchnocoele  arises  in  the 
same  way  as  in  Amphioxus,  is  at  present  entirely  hypothetical/ 


CHAPTER   X. 
ORIGIN    OF    THE    BLOOD,  BLOOD-VESSELS,  AND   HEART. 

THE  circulatory  system  is  developed  from  two  anlages  which  are 
at  first  independent.  The  heart  arises  in  the-  cervical  region  of  the 
embryo ;  the  blood-vessels  and  first  blood-cells  in  the  extra-embryonic 
area  vasculosa ;  the  blood-vessels  subsequently  grow  into  the  embryo 
and  unite  with  the  heart.  The  heart  begins  to  beat  before  the  vessels 
are  connected  with  it,  so  that  as  soon  as  the  connection  is  established 
the  circulation  begins.  The  heart  contains  at  first  only  a  clear  fluid ; 
after  the  circulation  has  begun  blood-cells  come  in  through  the  ves- 
sels from  the  area  vasculosa.  The  first  blood-cells  have  a  reddish 
color  and  a  round  nucleus.  Somewhat  later  the  colorless  granular 
leucocytes  appear,  but  where  they  arise  is  uncertain.  In  all  verte- 
brates except  mammals  the  red  cells  persist  throughout  life,  but  in 
mammals  they  are  confined  to  the  foetal  period,  during  which  they 
are  gradually  replaced  by  the  non-nucleated  red-blood  globules  (plas- 
tids) .  Much  confusion  exists  as  to  the  nature  and  development  of 
the  blood,  because  the  great  majority  of  writers  have  ignored  the 
important  fact  that  the  mammalian  adult  blood-globules  are  a  new 
acquisition  of  that  class  and  are  not  homologous  with  the  red-blood 
corpuscles  of  other  vertebrates.  Mammals  have  three  kinds  of  blood 
corpuscles :  red  cells,  leucocytes,  and  the  adult  red  globules;  all  other 
vertebrates  have  two  kinds  only. 

An  immense  deal  has  been  written  on  the  development  of  the 
blood  in  the  embryo,  and  there  is  perhaps  no  other  question  in  em- 
bryology which  has  been  so  much  studied  and  yet  left  with  such  a 
variety  of  opinions  as  to  its  right  answer.  In  the  following  pages  I 
have  endeavored  to  collate  what  seem  to  me  the  best-established 
results ;  but  until  some  one  subjects  the  literature  of  the  subject  to  a 
critical  revision,  based  on  a  thorough  comparative  investigation  of 
the  development  of  the  blood  and  blood-vessels  throughout  the  ver- 
tebrate series,  we  can  hardly  expect  a  satisfactory  history  of  the 
embryonic  blood. 

We  have  to  distinguish  between  the  primary  and  secondary  vas- 
cular anlages. 

I.  BLOOD-VESSELS  AND  BLOOD. 

Primary  Vascular  Anlages. — These  are  cords  of  cells  which 
appear  first  in  the  area  vasculosa  and  rapidly  extend  into  the  embryo ; 
the  cords  form  a  network ;  scattered  clusters  of  cells  in  the  cords  very 
early  assume  the  haemoglobin  color  and  appear  as  reddish-yellow 
spots  which  have  long  been  known,  and  are  described  by  Pander, 
VonBaer,  28.2,  Remak,  50.1,  Prevostet  Lebert,  44.1,  and  others. 
We  owe  to  His,  68.1,  95-103,  the  first  exact  account  of  the  origin 


212 


THE    EMBRYO. 


of  blood-vessels  in  the  chick;  since  then  the  studies  of  Disse,  79.1 
Gotte,  74. 1,  Kolliker  ("  Entwickelungsges."),  Balfour,73. 1,  J.  Koll- 
mann,  84.3,  Uskow,  87.1,  and  others  have  added  a  little  to  the 
descriptions  by  His.  It  is  now  demonstrated  that  the  blood  arises 
in  amniota  from  the  mesoderm  and  not  from  the  yolk,  as  was,  I  be- 
lieve, first  suggested  for  teleosts  by  Lereboullet  and  recently  by 
Ryder.  The  exact  history  of  the  first  blood-vessels  has  yet  to  be 
studied  in  other  amniota  than  the  chick. 

In  the  chick  the  distal  portion  of  the  mesoderm  has  no  ccelomatic 
cavity  when  the  development  of  the  blood  begins;  the  mesoderm 
lies  close  against  the  entoderm  or  germinal  wall  (Keimwall).  The 

juxtaposition  of  the  two 
layers  has  led  His  and 
others  to  consider  that 
the  entoderm  or  yolk  gave 
off  the  cells  which  form 
the  mesoderm  of  the  area 
vasculosa.  This  portion 
of  the  mesoderm  was 
early  distinguished  by 
German  writers  under 
the  name  of  Gefass- 
schicht  or  vascular  layer 
( / euillet  angioplas- 
tique),  and  has  been 
called  the  blood  -  germ 
(Blutkeim) ;  by  His  it  is 
identified  as  a  stage  of 
the  parablast,  see  Chap- 
ter VI.  The  first  indica- 
tion of  the  blood-vessels 
is  a  reticulate  appearance 
of  the  layer,  which  can  be 
recognized  in  surface  views  at  the  end  of  the  first  day  and  rapidly  in- 
creases in  extent  and  distinctness  during  the  second  day  of  incubation. 
As  soon  as  there  are  several  primitive  segments  the  network  shows 
traces  of  coloration  in  irregularly  shaped  reddish-yellow  spots  which 
are  largest  and  most  numerous  around  the  caudal  end  of  the  embryo ; 
these  spots  are  the  so-called  blood-islands,  Fig.  124.  The  network 
appearance  is  due  to  thickenings  of  the  mesoderm,  as  is  evident 
from  sections.  The  two  primary  layers  are  separated  so  that  the 
mesodermic  thickenings  lie  between  them.  Between  the  thickenings 
are  irregular  lacunaB,  Fig.  124,  b  &,  which  are  only  partly  filled 
with  mesodermic  cells ;  these  Iacuna3  by  their  subsequent  expansion 
and  fusion  develop  during  the  latter  half  of  the  second  day  the  ccelom 
of  the  area  vasculosa,  and  always  so  that  the  thickenings  (or  blood- 
vessels) are  on  the  entodermal  side,  Fig.  126.  In  other  words,  as 
soon  as  the  two  leaves  of  the  mesoderm  are  differentiated  in  the  area 
vasculosa  the  blood-vessels  are  found  exclusively  in  the  splanchnic 
leaf.  In  the  sheep  they  appear  also  in  the  somatic  leaf,  R.  Bonnet, 
89.1,  56,  or  future  amnion,  but  they  soon  disappear  and  never  con- 
tain any  blood-corpuscles.  The  network  of  blood-vessels  of  the 


FIG.  124.  —Surface  View  of  a  small  Part  of  the  Vascular 
Network  of  an  Embryo  Chick  of  two  Days,  a  a,  Blood-ves- 
sels ;  6  &,  mesoderm  between  the  vessels ;  c  c,  blood-islands. 
From  Kolliker. 


BLOOD-VESSELS    AND    BLOOD. 


213 


vascular  area  form  at  first  a  thick  network  without  distinction  of  stem 
or  branch,  and  are  all  in  one  layer,  none  overlying  the  others  (Kolliker, 
"  Grundriss,"  p.  GO),  Fig. 
1 25 ;  the  edge  of  the  area  is 
marked  by  a  single  large 
vessel  which  is  known  as 
the  vena,  or  better,  ,s /////.v 
terminalis,  Fig.  125,  v t. 
I  have  spoken  of  vessels, 
but  up  to  this  time  the 
vascular  anlages  are 
solid.  The  vena  termi- 
nalis  persists  for  some 
time  as  the  distal  boun- 
dary of  the  area,  while  it 
is  spreading  farther  and 
farther  over  the  yolk,  but 
by  the  end  of  the  fourth 
day  it  is  no  longer  dis- 
tinguishable as  a  distinct 
structure  (Prevost  et  Le- 
bert,  44.3,  240).  The 
vena  terminalis  ultimate- 
ly becomes  connected 
with  the  venous  system 
of  the  chick,  but  in  rab- 
bits with  the  arterial  sys- 
tem ;  for  this  reason  the  term  sinus  is  to  be  preferred  to  vena  as  ap- 
plied to  this  vessel. 

The  blood-islands  are  spots  where  there  is  a  cluster  of  cells  which 
remain  attached  to  the  Avails  of  the  vessels  in  the  area  vasculosa  (see 
Fig.  12G,  bl.  is).  The  cells  develop  hemoglobin  in  their  interior, 
hence  the  clusters  have  a  reddish  color,  which  renders  the  islands 
very  conspicuous  in  surface  views  of  fresh  specimens.  The  blood- 
islands  of  the  chick  appear  first  in  the  area  opaca,  and  almost  imme- 
diately after  in  the  pellucida  also.  They  have  at  first  a  rounded  or 
branching  form,  Fig.  124;  in  the  inner  part  of  the  layer  they  are 
small  and  stand  alone;  toward  the  periphery  they  are  larger,  closer 
set,  and  more  united  with  one  another;  their  development  is  greater 
around  the  caudal  end  of  the  embryo.  They  are  situated,  chiefly, 
at  the  nodes  of  the  vascular  network.  When  the  solid  vascular 
cords  acquire  a  lumen,  the  islands.  Fig.  12G,  bl.  is,  remain  attached 
to  one  side  of  the  vessel,  like  a  thickening  of  its  wall.  The  cells  of 
the  islands  ultimately  become  free  blood-corpuscles. 

The  growth  of  the  primary  anlages  takes  place  by  the  develop- 
ment of  buds  from  the  vessels  already  formed,  as  first  shown  accu- 
rately by  Prevost  et  Lebert,  44.3,  239;  these  buds  are  rounded  or 
pointed  and  elongated,  forming  as  it  were  spurs;  they  often  end  by 
meeting  one  another  and  uniting ;  they  are  usually  hollow  from  the 
first,  and  after  they  meet  one  another  or  an  adjacent  vessel,  the  cavi- 
ties become  continuous  and  thus  the  vascular  network  is  extended. 
A.  Goette,  however,  maintains,  75. 1,  497,  that  the  network  arrange- 


FIG.  125.— Vascular  Anlages  of  the  Area  Vasculosa  of  a 
(hick  of  Forty  Hours,  ps,  ps.  Blood-islands;  trt,  vena  ter- 
minalis. From  Kolliker.  x  26  diams. 


214 


THE    EMBRYO. 


ment  exists  from  the  start  in  all  vertebrates,  and  that  the  apparent 
budding  is  due  to  the  progress  of  vascular  differentiation  into  in- 
different mesenchymal  cells. 

In  mammals    the   solid   primary  anlages   appear  in  the   extra 
embryonic  area  vasculosa,  and  extend  later  into  the  embryo.     So 

far  as  known  to  me  there  has 
been  as  yet  no  exact  investiga- 
tion of  their  history.  They  pre- 
sent well-marked  blood-islands, 
which  are  thickenings  of  the 
mesoderm,  and  make  their  first 
appearance  in  rabbit  embryo  of 
the  eighth  day  just  before  the 
first  primitive  segments  (Kolli- 
ker,"  Entwickelungsges.,"  2G6). 
The  growth  of  the  network  in 
the  rabbit  by  the  formation  of 
solid  buds  which  become  hollow 
has  been  described  by  Wisso- 
sky,  77.1. 

Growth  of  the  Vessels 
into  the  Embryo.— The  fact 
that  the  vessels  penetrate  the 
embryo  after  they  have  appeared 
in  the  area  vasculosa  was  first 
discovered  by  His,  68.1,  09, 
and  is  now  a  familiar  pheno- 
menon. It  is  evident  that  this 
penetration  may  take  place  in 
two  ways :  it  may  be  a  progres- 
sive differentiation  of  cells  al- 
ready present  (cf.  Goette,  75. 1, 
539) ,  or  it  may  be  an  actual  in- 
growth of  vaso-formative  tis- 
sue ;  the  balance  of  evidence  is 
in  favor  of  the  latter  alterna- 
tive, which  accordingly,  follow- 
ing His  in  this  respect,  the  ma- 
jority of  embryologists  have 
adopted.  In  the  chick  the  vas- 
cular differentiation  extends 
from  the  area  opaca  to  the  area 
pellucida,  and  thence  into  the 
body  proper  of  the  embryo. 
But  in  the  lizard  (Strahl,  Mar- 
burg, Sitzber.,  1883,  60-71)  the 
vessels  appear  first  in  the  area 

pellucida  and  thence  extend  into  the  area  opaca  and  the  embryo. 
The  entrance  of  the  vessels  into  the  embryo  chick  begins  toward 
the  end  of  the  second  day.  It  is  effected,  according  to  His,  68.1, 
99,  by  buds,  which  are  at  first  solid  cords,  and  grow  toward  the 
embryo,  uniting  as  they  extend  into  a  network;  the  hollowing 


BLOOD-VESSELS   AND   BLOOD.  215 

out  of  the  cords  likewise  progresses  centripetally.  The  penetrating 
vessels  follow  certain  prescribed  paths.  A  part  of  the  vessels  run 
along  the  posterior  edge  of  the  amnio-cardial  vesicles,  and  enter  into 
connection  with  the  posterior  end  of  the  heart  which  has  meanwhile 
been  developed — owing  to  the  early  separation  of  the  head  end  of  the 
embryo  from  the  yolk  this  is  the  only  part  of  the  heart  the  vessels 
can  reach  directly.  While  the  vessels  are  approaching  the  heart 
their  differentiation  into  various  sizes  is  going  on,  the  smallest  ones 
to  remain  as  capillaries,  the  larger  ones  to  become  arteries  or  veins ; 
this  differentiation,  which  has  yet  to  be  followed  step  by  step,  leads 
to  there  being  only  two  main  vessels,  the  so-called  omphalo-mesaraic 
veins,  which  actually  open  into  the  hind  end  of  the  heart.  Another  set 
of  vessels  penetrates  along  the  splanchnopleure  of  the  rump  on  each 
side,  until  they  attain  the  small  space  between  the  notochord,  myo- 
tome,  and  entoderm,  where  they  fuse  (Turstig,  86.1),  so  as  to  form 
a  longitudinal  vessel,  the  anlage  of  the  aorta  descendens,  which  is 
primitively  double.  The  aorta  appears  first  at  the  head  end  of  the 
rump  and  hence  its  development  progresses  backward ;  it  also  grows 
forward  over  the  heart,  bends  over  ventrally  just  behind  the  mouth, 
and,  passing  around  the  blind  end  of  the  vorderdarm,  approaches 
the  median  line  and  unites  with  the  cephalic  end  of  the  tubular 
heart.  An  utterly  different  history  of  the  origin  of  the  aorta,  namely, 
from  the  median  dorsal  wall  of  the  archenteron,  is  asserted  by  C. 
K.  Hoffmann,  92.1,  for  the  dog-fish.  The  heart  begins  to  beat 
before  the  vessels  unite  with  it ;  the  first  blood-cells  have  already 
been  formed ;  hence  as  soon  as  the  union  is  accomplished  the  blood- 
circulation  starts  up,  the  blood  passing  through  the  aorta  to  the 
rump,  thence  by  numerous  lateral  branches  to  the  area  vasculosa, 
and  returning  by  the  two  omphalo-mesaraic  veins  to  the  heart.  The 
course  and  modifications  of  the  primitive  circulation  are  described 
and  figured  in  Chapter  XIV. ,  on  the  germinal  area. 

Origin  of  the  First  Red  Blood-Cells.* — I  consider  it  proba- 
ble that  the  red  blood-cells  of  all  vertebrates  arise,  as  has  been  main- 
tained byH.  Ziegler,  89.1,  by  proliferation  of  the  endothelial  lining 
of  the  vessels.  This  conclusion  is  based — 1,  upon  the  fact  that  in 
various  vertebrates,  notably  in  bony  fishes,  elasmobranchs,  and  all 
amniota,  certain  parts  of  the  vascular  system  are  at  first  solid  cords 
of  cells,  and  of  these  cord;?  the  central  portion  becomes  blood-cells, 
the  peripheral  portion  the  vascular  wall;  it  seems  to  me  that  the 
right  interpretation  is  to  regard  the  central  cells  as  belonging  with 
the  outer  cells,  and  therefore  equivalent  to  the  product  of  an  endo- 
thelial proliferation ;  2,  upon  the  origin  of  red  cells  from  the  walls 
of  the  venous  capillaries  of  the  bony  marrow  of  birds  (J.  Denys, 
87.1).  In  all  these  cases  the  blood-cell  is  a  liberated  specialized 
endothelial  cell.  A.  Goette  is  the  principal  opponent  of  this  view, 
and  has  maintained  that  in  Petromyzon,  90.1,  (30,  Amphibia,  75.1, 
538,  and  birds,  74. 1,  180-186,  the  blood-cells  of  the  embryo  have  an 

*It  is  singular  that  so  close  an  observer  as  Balfour  should  have  maintained,  as  he  did.  73.1. 
that  the  blood-cells  of  sauropsida  are  metamorphosed  nuclei,  and  this  view  is  still  adhered 
to  in  his  "Elements,"  2d  ed.  1883.,  Balfour's  error  was  due  to  the  fact  that  the  cells,  when  first 
set  free,  have  a  minimum  quantity  of  protoplasm  around  the  nucleus,  and  this  he  did  not  observe: 
the  nuclei  have  too  at  first  a  very  distinct  large  nucleolus,  which  Balfour  wrongly  assumed  to 
represent  the  nucleus  of  the  future  corpuscle. 


21 G  THE   EMBRYO. 

origin  different  from  the  endothelium,  the  former  arising  from  the 
yolk,  the  latter  from  the  mesoderm.  Although  Goette  is  one  of  the 
very  best  of  embryological  observers,  I  cannot  agree  with  him  on 
this  point,  for  I  feel  satisfied  that  he  is  in  error  as  regard  the  chick, 
while  in  regard  to  the  lamprey  and  the  land  frogs  it  is  possible  that 
Goette 's  observations  are  incomplete — certainly  his  descriptions  are 
less  clear  than  those  of  the  origin  of  the  blood-cells  within  the  vas- 
cular anlages.  It  must  be  added  that  Davidoff  has  maintained, 
84. 1,  that  in  the  salamander  the  blood-cells  arise  from  the  surface 
of  the  yolk;  but  his  statements  need,  I  think,  verification. 

The  blood-cells  of  teleosts  arise,  at  first  at  least,  in  certain  large 
vessels  within  the  embryo  (Wenckebach,  H.  Ziegler,  87.1,89.1, 
compare  also  H.  Ambert,  56. 1),  which  are  formed  as  solid  cords,  the 
central  cells  of  which  are  metamorphosed  into  blood-corpuscles.  At 
the  time  the  circulation  begins  there  are  no  blood-vessels  over  the 
yolk,  but  definite  blood- channels,  which  are  merely  grooves  on  the 
yolk  or  passages  between  the  yolk  and  the  ectoderm ;  these  channels 
subsequently  acquire  mesenchymal  walls  when  the  mesoderm  grows 
out  over  the  yolk.  Owing  to  this  peculiarity  of  the  early  vitelline 
circulation  blood-cells  appear  over  the  yolk  before  there  are  blood- 
vessels, and  the  observation  of  this  fact  seems  to  have  led  several 
observers  to  the  error  of  attributing  the  origin  of  the  blood-cells  to 
the  yolk  or  the  superficial  layer  thereof  (Kupffer's  periblast) .  For  a 
synopsis  of  the  various  opinions  see  Mclntosh  and  Prince,  90.1, 
782-783.  In  elasmobranchs  (J.  Kollmann,  85.1,  297)  there  are 
mesodermal  blood-islands,  which  expand  and  unite,  forming  a  net- 
work in  the  area  opaca ;  the  vessels  are  at  first  solid,  the  central  cells 
become  blood-cells,  the  peripheral  cells  endothelial  walls ;  so  far  as 
observations  go  it  is  possible,  however,  that  all  the  cells  of  the  blood- 
islands  become  blood-cells,  and  that  the  endothelium  is  simply  an 
overgrowth  of  mesenchyma,  but  in  view  of  the  development  in  other 
vertebrates  this  possibility  has  little  probability.  The  development 
of  the  blood  in  reptiles  and  mammals  needs  thorough  study,  but  we 
know  that  it  is  closely  similar  to  that  in  birds.  In  the  chick,  as 
stated  above,  the  cells  of  the  blood-islands  form  the  first  blood-cells, 
and  this  statement  probably  applies  also  to  all  amniota. 

For  the  origin  of  blood-cells  in  the  embryo  see  the  following 
section. 

Secondary  Vascular  Anlages. — These  are  buds  which  arise 
from  the  vessels  already  present  in  the  embryo,  similar  to  the  buds 
already  described  in  the  area  vasculosa.  There  being  no  real  division 
between  the  primary  and  secondary  anlages  the  distinction  is  used 
merely  for  convenience  of  description.  The  secondary  anlages,  like 
the  primary,  give  rise  to  the  endothelium  of  the  wall  only ;  when  a 
vessel  becomes  an  artery  or  a  vein  the  media  and  adventitia  are 
added  by  differentiation  of  the  surrounding  mesenchyma.  The  sec- 
ondary anlages  can  be  found  in  mammals  in  various  parts  of  the 
body  during  embryonic  life,  and  even  after  birth,  and  in  Ajnphibia 
may  be  studied  during  the  larval  period ;  the  tail  of  tadpoles  being 
a  favorite  object  for  this  purpose  (Golubew,  69.1).  The  second- 
ary anlages  were,  so  far  as  I  know,  first  accurately  described  in 
batrachians  by  Prevost  et  Lebert,  44. 1 ;  they  were  followed  two 


BLOOD-VESSELS   AND    BLOOD.  217 

years  later  by  Kolliker,  46.2,  see  also  Golubew,  69. 1,  Arnold,  71.1, 
and  Ranvier  8  '"  Traite  technique,"  018,  623.  In  mammals  they  have 
been  well  described  by  Ranvier,  74.2,  E.  A.  Schafer,  74.1,  Kolliker 
("Entwickelungsges.,"  171,  "  Grundriss,"  63),  and  others. 

The  secondary  anlages  appear  as  thorn-shaped  points  projecting 
more  or  less  nearly  at  right  angles  from  the  walls  of  capillaries  al- 
ready formed.  A.  Goette,  75. 1,  544,  has  maintained  that  these  are 
not  real  outgrowths,  but  differentiations  of  intercellular  processes 
present  ab  initio  in  the  mesenchyma.  These  points  rapidly  elongate 
into  fine  threads,  which  may  join  the  wall  of  another  capillary  or 
the  tip  of  another  point ;  Golubew  states  that  when  two  points  unite 
in  the  frog,  they  overlap  and  then  unite  by  their  sides ;  while  the 
point  is  growing  the  cavity  of  the  parent  capillary  extends  into  the 
base  of  the  point,  and  penetrates  farther  and  farther,  so  that  the 
thread-like  point  becomes  gradually  enlarged  into  a  capillary  blood- 
vessel. The  capillaries  formed  in  this  way  show  a  marked  tendency 
to  form  loops. 

Very  similar  is  the  account  quoted  below  by  E.  A.  Schaeffer  in 
Quain's  "  Anatomy,"  ninth  edition,  II.,  198,  199) :  "  Within  the  body 
of  the  embryo  vessels  are  formed  in  like  manner  from  cells  belonging 
to  the  connective  tissue.  One  of  the  most  favorable  objects  for  the 
study  of  the  development  of  the  blood-vessels  and  their  contained 
blood-corpuscles  is  afforded  by  the  subcutaneous  tissue  of  the  new- 
born rat,  especially  those  parts  in  which  fat  is  being  deposited. 
Here  we  may  observe  that  many  of  the  connective-tissue  corpuscles 
are  much  vacuolated,  and  that  the  protoplasm  of  some  of  them  pre- 
sents a  decided  reddish  tinge.  In  others  the  red  matter  has  become 
condensed  in  the  form  of  globules  within  the  cells,  varying  in  size 
from  minute  specks  to  spheroids  of  the  diameter  of  a  blood-corpuscle 
or  more.  At  some  parts  the  tissue  is  completely  studded  with  these 
cells,  each  containing  a  number  of  such  spheroids,  and  forming,  as 
it  were,  'nests'  of  blood -corpuscles  or  minute  'blood-islands.'  After 
a  time  the  cells  become  elongated  and  pointed  at  their  ends,  sending 
out  processes  also  to  unite  with  neighboring  cells.  At  the  same 
time  the  vacuoles  in  their  interior  become  enlarged,  and  coalesce  to 
form  a  cavity  with  the  cell  in  which  the  reddish  globules,  which  are 
now  becoming  disc-shaped,  are  found.  Finally,  the  cavity  extends 
through  the  cell  processes  into  those  of  neighboring  cells  and  into 
those  sent  out  from  pre-existing  capillaries,  but  a  more  or  less  exten- 
sive capillary  network  is  often  formed  long  before  the  connection 
with  the  rest  of  the  vascular  system  is  established.  Young  capilla- 
ries do  not  exhibit  the  well-known  lines  when  treated  with  nitrate  of 
silver  for  the  differentiation  of  the  hollowed  cells  and  cell-processes 
into  flattened  cellular  elements  is  usually  a  subsequent  process.  The 
mode  of  extension  of  the  vascular  system  in  growing  parts  of  older 
animals,  as  well  as  in  morbid  new  formations,  is  quite  similar  to 
that  here  described,  except  that  blood-corpuscles  are  not  developed 
within  the  cells  which  are  forming  the  blood-vessels." 

The  development  of  new  capillaries  in  the  manner  just  described 
also  takes  place  from  the  vessels  formed  by  vasoformative  cells. 

The  secondary  vascular  anlages  of  the  foetal  liver  have  been 
specially  studied  by  P.  Kuborn,  90. 1 ;  they  correspond  to  the  so- 


218  THE   EMBRYO. 

called  foetal  hepatic  giant  cells  of  early  authors,  and  give  rise  to 
vascular  walls,  red  cells,  and  later  (embryos  of  three  or  four  centi- 
metres) to  the  red  plastids,  compare  p.  221. 

Vasoformative  Cells. — In  all  secondary  anlages  of  the  vessels 
we  have  outgrowths  of  vessels  already  present ;  there  are  also  vessels 
developed  from  special  vasoformative  cells,  which  have  no  connection 
with  previous  vessels ;  the  origin  of  the  vasoformative  cells  has  still 
to  be  ascertained,  but  it  may  be  safely  asserted  that  they  are  derived 
from  the  mesenchyma.  These  cells  were,  I  believe,  discovered  by 
L.  Ranvier  (74.2,  and  "  Traite  technique,"  625),  who  studied  them 
in  the  omentum  of  the  rabbit  before  and  after  birth.  He  found  small 
spots  of  milky  appearance,  which  he  designates  as  "  taches  laiteuses," 
and  which  contain  ordinary  connective-tissue  corpuscles,  and  fibrillse, 
numerous  leucocytes,  and  vasoformative  cells.  The  last,  in  rabbits 
from  two  to  eight  weeks  old,  are  finely  granular,  branching  often 
anastomosing,  elongated  cells  with  elongated  nuclei;  earlier  they 
are  scattered,  spindle-shaped  cells.  Soon  a  capillary  from  the  neigh- 
borhood grows  in  and  unites  with  the  vasoformative  network,  and 
thereupon  the  excavation  of  the  network  begins,  the  lumen  of  the 
capillary  gradually  extending  throughout  the  cluster  of  vasoforma- 
tive cells. 

Primitive  Blood -Vessels. — The  first  vessels  consist  merely  of 
a  wall  of  protoplasm  with  scattered  nuclei,  and  accordingly  are  all 
essentially  alike  in  structure ;  the  first  differentiation  is  one  of  size 
only,  the  vessels  that  are  to  become  arteries  and  veins  rapidly  in- 
creasing their  calibre,  while  the  mesenchyma  around  them  is  still 
undifferentiated.  The  protoplasmatic  wall  in  cross-sections  of  a 
vessel  is  thick  enough  to  contain  a  nucleus.  The  next  step  in  devel- 
opment is  the  thinning  out  of  the  layer,  so  that  the  nuclei  become 
protuberant  as  in  the  adult  endothelium ;  at  the  same  time  the  pro- 
toplasm becomes  divided  into  distinct  cell  territories,  and  intercellu- 
lar lines  are  developed  and  may  be  impregnated  with  nitrate  of  silver, 
as  in  the  adult. 

The  vessels  grow  by  the  multiplication  of  the  cells  of  their  walls. 
W.  Flemming,  90. 1,  has  shown  that  in  the  capillaries  the  nuclei 
undergo  karyokinetic  division,  and  that  the  division  of  the  proto- 
plasm takes  place  later. 

The  distribution  and  metamorphoses  of  the  principal  vessels  are 
discussed  in  Chapter  XIV. 

The  red  blood-cells  are  the  only  elements  contained  in  the 
blood  during  the  earliest  stages  of  the  vertebrate  embryo.  When 
the  circulation  begins  the  number  of  corpuscles  is  small,  but  rapidly 
increases  thereafter.  The  cells  are  at  first  round  (in  probably  all 
vertebrates) ;  in  the  chick  they  measure  from  8.3  to  12.5  /*.  The  nu- 
cleus is  large,  more  or  less  nearly  spherical,  and  surrounded  by  a  layer 
of  protoplasm  (Minot,  122),  which  is  so  thin  as  to  have  been  often 
overlooked.  The  cells  at  first  are  granular  and  slightly  colored  (Pre- 
vost  et  Lebert,  44.3,  241;  Kolliker,  "Grundriss,"  63),  and  then  be- 
come more  colored  and  homogeneous,  scarcely  showing  the  nucleus 
during  life,  though  it  comes  out  very  clearly  as  soon  as  the  corpus- 
cles are  removed  from  the  vessels  or  acted  upon  by  hardening  re- 
agents. The  nucleus  in  hardened  corpuscles  stains  deeply.  In 


BLOOD-VESSELS    AND    BLOOD. 


219 


amphibians  the  young  blood-cells,  like  all  the  other  cells  of  the  em- 
bryo, contain  numerous  yolk  granules ;  as  the  granules  disappear  the 


fa 


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g.5 

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II 


nuclei  and  bodies  of  the  cells  both  acquire  a  more  homogeneous 
and  opalescent  appearance,  and  at  the  same  time  become  flattened, 
elongated,  and  colored  (A.  Goette,  75.1,  770). 


220  THE   EMBRYO. 

The  primitive  form  of  the  vertebrate  red-blood  cell  is  probably 
spherical,  or  at  least  spheroidal,  and  the  characteristic  mature  shape 
is  not  assumed  until  later,  as  I  have  learned  from  my  own  observa- 
tions on  a  considerable  variety  of  embryos.  This  statement  is  further 
supported  by  A.  Goette's  observations  on  Petromyzon,  90.1,  66,  and 
Bombinator,  75.1,538.  In  the  chick  the  mature  elliptical  form 
begins  to  predominate  during  the  fourth  day;  the  earlier  round 
form  is  still  encountered  for  several  days,  but  it  gradually  becomes 
rarer  (Prevost  et  Lebert,  44.3,  242). 

Minot,  122,  has  outlined  the  progressive  differentiation  of  the  red 
cells  in  sharks,  salamanders,  chicks,  and  rabbits.  The  following 
description  refers  primarily  to  the  chick :  By  following  the  develop- 
ment we  find  that  the  protoplasm  enlarges  for  several  days,  and  that 
during  the  same  time  there  is  a  progressive  diminution  in  size  of  the 
nucleus,  which,  however,  is  completed  before  the  layer  of  protoplasm 
reaches  its  ultimate  size.  The  nucleus  is  at  first  granular,  and  its 
nucleolus,  or  nucleoli,  stand  out  clearly ;  as  the  nucleolus  shrinks  it 
becomes  round  and  is  colored  darkly  and  almost  uniformly  by  the 
usual  nuclear  stains.  This  species  of  blood-corpuscle  occurs  in  all 
vertebrates  and  represents  the  genuine  blood-cells.  The  blood-cells 
of  mammals  pass  through  the  same  metamorphoses  as  those  of  birds. 
For  example,  in  rabbit  embryos,  the  cells  have  reached  the  ichthy- 
opsidan  stage  on  the  eighth  day ;  two  days  later  the  nucleus  is  already 
smaller,  and  by  the  thirteenth  day  has  shrunk  to  its  final  dimensions. 
According  to  the  above  description  we  can  distinguish  three  principal 
stages:  1,  young  cells  with  very  little  protoplasm;  2,  old  cells  with 
much  protoplasm  and  granular  nucleus;  3,  modified  cells  with 
shrunken  nucleus,  which  colors  darkly  and  uniformly,  Fig.  127.  I 
do  not  know  whether  the  first  form  occurs  in  any  living  adult  verte- 
brate, although  the  assumption  seems  justified  that  they  are  the 
primitive  form.  On  the  other  hand,  the  second  stage  is  obviously 
characteristic  of  the  Ichthyopsida  in  general,  while  the  third  form  is 
typical  for  the  Sauropsida.  Therefore,  the  development  of  the  blood- 
cells  in  amniota  offers  a  new  confirmation  of  Louis  Agassiz'  law 
(Haeckel's  biogenetisches  Grundgesetz) . 

Multiplication  of  the  red  cells  by  division  was  recorded  by 
Remak,  50. 1, 164,  and  has  since  been  frequently  observed.  Special 
attention  was  directed  to  its  occurrence  by  Peremeschko  in  1879, 
79.1,  81.1,  and  by  Bizzozero  (Cbl.  med.  Wiss.,  1881,  Moleschott's 
"Unters.  zur  Naturlehre,"  XIII.)  in  1881,  and  has  since  been  studied 
by  Bizzozero  et  Torre,  84.1,  Bizzozero,  84.1,  Funcke,  80.1,  Eberth 
and  Aly,  85.1,  A.  Mosso,  88.2,  and  others.  The  division  is  indi- 
rect or  karyokinetic,  and  takes  place  across  the  longitudinal  axis  of 
the  corpuscle,  with  which  the  nuclear  spindle  is  parallel.  The  process 
has  been  observed  in  bony  fishes,  amphibians,  adult  Sauropsida,  and 
in  amniote  embryos.  The  division  occurs  only  in  young  or  partly 
differentiated  corpuscles;  the  divisions,  for  example,  are  abundant 
in  the  blood  of  the  chick  of  from  three  to  five  days ;  the  sixth  day 
they  are  rarer,  the  tenth  seldom,  and  after  hatching  are  not  found  in 
the  circulating  blood  at  all  (Funcke,  I.  c.) .  It  is,  accordingly,  safe  to 
assume  that  the  proliferation  of  the  red  cells  is  typical  for  all  verte- 
brates. Their  number  is  further  increased  by  additions  from  various 


BLOOD-VESSELS   AND   BLOOD.  221 

sources  in  the  embryonic  and  (adult  non-mammalian)  vertebrates; 
but,  so  far  as  at  present  known,  the  mammals  have  only  the  red  cells, 
which  arise  directly  from  the  primary  vascular  anlages,  therefore 
the  discussion  of  the  maintenance  of  the  supply  of  red  cells  falls  out- 
side our  scope.  The  problem  has  been  much  debated ;  the  investiga- 
tion which  seems  to  me  to  have  led  to  the  best  results  is  that  of  J. 
Denys,  87. 1.  For  the  reader's  convenience  I  cite  also  the  following 
authorities,  but  the  list  is  very  incomplete :  Bayerl,  84.1,  W.  H. 
Howell,  88.1;  Lowit,  87.1,  91.1,  E.  Neumann,  74.1,  Malassez, 
82.1;  Obrastzow,  81.1;  G.  Pouchet,  80.1;  and  Rindfleisch,  80.1. 
For  additional  references  see  Quain's  "Anatomy,"  ninth  edition, 
II.,  40. 

Disappearance  of  the  Red  Cells. — The  red  cells  form  the 
permanent  red-blood  globules  in  all  vertebrates  except  the  mammals. 
In  mammals  they  disappear  during  embryonic  life  or  soon  after 
birth.  Although  they  persist  for  a  long  period,  it  will  be  convenient 
to  state  here  what  little  is  known  of  their  history.  How  they 
disappear  is  not  known,  although  several  authors  have  main- 
tained that  they  are  transformed  into  red  plastids,  but  this  opin- 
ion seems  to  me  ill  founded.  W.  H.  Howell,  90.1,  reports  the 
interesting  discovery  that  the  nucleus  of  the  mature  red  cells  is  ex- 
truded in  mammals  leaving  the  body  of  the  cell;  in  consequence  he 
maintains  the  plausible  conclusion  that  the  extrusion  is  the  means  of 
developing  the  non-nucleated  red  corpuscles,  but  I  am  more  inclined 
to  regard  it  as  a  step  in  the  degeneration  and  destruction  of  the  red 
cells.  In  the  human  embryo  at  one  month  the  red  cells  are  the  only 
blood-corpuscles;  at  two  months  they  are  the  most  numerous,  al- 
though the  plastids  have  begun  to  appear ;  at  three  months  they  form 
only  a  small  minority  of  the  corpuscles. 

Origin  of  Leucocytes. — The  origin  of  the  first  colorless  corpus- 
cles in  the  embryo  is  still  uncertain.  The  blood  is  found  to  contain 
for  some  time  only  the  red  cells,  the  leucocytes  appearing  in  the 
chick  (Prevost  et  Lebert,  44.3,  243),  about  the  eighth  day  of  incuba- 
tion; in  the  rabbit,  it  is  said,  about  the  ninth  day,  and  in  elasmo- 
branchs  not  until  the  embryo  is  well  advanced  in  development,  A. 
Mosso,  88.2.  It  is  to  be  noted  that  after  the  blood-vessels  and  red - 
blood-cells  the  leucocytes  are  the  first  cells  to  be  differentiated  from 
the  mesenchyma,  the  remaining  mesenchymal  tissues  (Chapter  XIX.) 
being  differentiated  gradually  and  to  a  large  extent  simultaneously. 
So  far  as  I  know,  the  subject  has  never  been  carefully  investigated, 
nor  is  there  even  any  exact  description  of  the  appearance  and  number 
of  the  first  leucocytes. 

After  the  lymph-glands  appear  they  probably  assume  the  function 
of  producing  leucocytes ;  but  the  process  in  embryonic  glands  has 
still  to  be  studied,  and  accordingly  for  further  information  the  reader 
is  referred  to  the  standard  histologies.  That  the  leucocytes  multiply 
by  direct  or  akinetic  division  has  been  recorded  by  several  observers, 
L.  Ranvier,  J.  Arnold,  84.1,  and  others. 

Origin  of  Mammalian  Blood -Globules  or  Red  Plastids.— 
There  are  many  opinions  as  to  the  origin  of  the  non-nucleated  red 
blood-globules  of  mammals.  The  best-founded  conclusion  is,  it 
seems  to  me,  that  of  E.  A.  Schafer,  who  traces  them  to  local  differ- 


222  THE   EMBRYO. 

entiations  of  the  protoplasm  of  the  vasif active  cells.  This  view 
makes  the  globules  comparable  to  the  plastids  of  botanists,  such,  for 
instance,  as  the  chlorophyll  granules.  As  the  terms  "  globules"  and 
"  corpuscles"  have  been  applied  indiscriminately  to  all  the  formed 
elements  of  blood,  and  as  it  is  desirable  to  have  a  simple  term  which 
shall  also  indicate  the  morphological  separation  from  the  other 
"blood-corpuscles,"  I  shall  apply  the  term  "red  plastids"  to  the 
non-nucleated  mammalian  adult  red  globules.  The  chief  opinion 
rivalling  Schafer 's  is  that  the  red  plastids  are  derived  from  nucleated 
corpuscles,  which  have  lost  their  nuclei  and  shrunk,  the  plastids 
being  always  much  smaller  than  the  red  cells.  This  view  has  been 
specially  advocated  by  Kolliker,  "  Gewebelehre, "  5te  Aufl.,  1867,  p. 
638,  is  found  in  several  subsequent  writers,  and  has  been  very  re- 
cently brought  forward  by  Casimoro  Mondino,  88.1,  but  sufficient 
observation  to  justify  it  has  not  been  furnished  in  my  judgment. 
The  strongest  evidence  in  favor  of  the  conversion  of  nucleated  corpus- 
cles into  plastids  is  that  which  is  presented  by  Howell,  90. 1 ,  and  men- 
tioned p.  221.  Similar  to  this  view  is  that  which  traces  the  plastids 
to  modifications  of  leucocytes  occurring  after  birth,  F.  Sanfelice, 
89.1;  the  white  cells  are  supposed  to  shrink,  lose  their  nuclei,  and 
become  charged  with  haemoglobin.  Yet  another  opinion  affirms 
that  the  marrow  of  bones  produces  from  certain  of  its  cells  the  red 
plastids,  but  the  defenders  of  this  opinion  are  by  no  means  agreed 
among  themselves  as  to  how.  For  a  good  synopsis  of  the  conflicting 
theories  see  Schafer  in  Quain's  "Anatomy,"  tenth  edition,  Vol.  I., 
Pt.  II. 

The  first  red  plastids  certainly  arise  in  the  vasifactive  cells  in 
various  parts  of  the  embryo.  Schafer  in  Quain's  "  Anatomy,"  ninth 
edition,  II.,  36-37,  gives  the  following  description  of  the  process :  "  A 
part  of  the  protoplasm  of  the  cell  acquires  a  reddish  tinge,  and  after 
a  time  the  colored  substance  becomes  condensed  in  the  form  of  glob- 
ules within  the  cells,  varying  in  size  from  a  minute  speck  to  a  sphe- 
roid of  the  diameter  of  a  blood-corpuscle,  or  even  larger;  but  grad- 
ually the  size  becomes  more  uniform.  Some  parts  of  the  embryonic 
connective  tissue,  especially  where  a  vascular  tissue  such  as  the  fat 
is  about  to  be  developed,  are  completely  studded  with  cells  like  these, 
occupied  by  a  number  of  colored  spheroids  and  forming  nests  of 
blood-corpuscles,  or  minute 'blood-islands.'  After  a  time  the  cells 
become  elongated  and  pointed  at  their  ends,  and  processes  grow  out 
to  join  prolongations  of  neighboring  blood-vessels  or  of  similar  cells. 
At  the  same  time  vacuoles  form  within  them,  and  becoming  enlarged 
coalesce  to  form  a  cavity  filled  with  fluid  in  which  the  reddish 
globules,  which  are  now  becoming  disc-shaped,  float.  Finally,  the 
cavity  extends  through  the  cell  processes  into  those  of  neighboring 
cells,  and  a  vascular  network  is  produced,  and  this  becomes  eventu- 
ally united  with  pre-existing  blood-vessels,  so  that  the  blood-corpus- 
cles which  have  been  formed  within  the  cells  in  the  manner  described 
get  into  the  general  circulation.  This  'intracellular  '  mode  of  devel- 
opment of  red  blood-corpuscles  ceases  in  most  animals  before  birth, 
although  in  those  which,  like  the  rat,  are  born  very  immature  it 
may  be  continued  for  a  few  days  after  birth.  Subsequently,  although 
new  vessels  are  found  in  the  same  way,  blood-corpuscles  are  not  pro- 


BLOOD-VESSELS    AND    BLOOD.  223 

duoed  within  them,  and  it  becomes  necessary  to  seek  for  some  other 
source  of  origin  of  the  red-blood  discs,  both  during  the  remainder  of 
the  period  of  growth,  and  also  during  adult  life,  for  it  is  certain  that 
the  blood-corpuscles  are  not  exempted  from  the  continual  expenditure 
and  fresh  supply  which  affect  all  the  other  tissues  of  the  body." 

Very  early  in  embryonic  life  the  liver,  as  first  pointed  out  by  Kol- 
liker,  and  more  fully  demonstrated  by  Neumann,  74. 1 ,  becomes  the 
principal  seat  of  blood  formation.  The  secondary  vascular  anlages 
are  very  prominent  in  the  foetal  liver  and  in  sheep  embryos  of  four 
centimetres  and  more  in  length.  P.  Kuborn,  90.1,  has  traced  the 
development  of  red  plastids  from  the  protoplasm  onty,  as  described 
by  Schafer.  A  similar  result  is  reached  by  R.  Nicolaidos,  91.1, 
from  studying  the  production  of  red  plastids  in  the  mesentery  of 
young  guinea-pigs,  see  also  Wissosky,  77. 1.  The  process  of  plastid 
development  is  easily  followed  in  the  mesentery  of  the  human  fcetus. 

It  seems  to  me  probable  that  research  will  ultimately  establish  the 
origin  of  red  plastids  in  the  adult  also,  as  intracellular  protoplasmatic 
bodies  entirely  distinct  from  the  nuclei,  and  in  no  way  to  be  homolo- 
gized  with  cells.  Kultschitzki,  however  (see  Hofmann-Schwalbe's 
Jahresber.,  1883,  58-59),  asserts  that  in  the  lymph-glands  of  the  rab- 
bit the  red  plastids  arise  within  cells  by  metamorphosis  of  the  nuclei ; 
to  nuclei  Balfour  traced,  he  supposed,  the  red  cells  of  birds,  com- 
pare p.  215,  foot-note. 

Origin  of  the  Blood -Plates.— C.  Mondino  and  L.  Sala,  88.1, 
affirm  that  the  blood-plates  multiply  by  division,  and  being  nucleated 
in  the  non-mammalian  vertebrates,  according  to  these  authors,  they 
divide  karyokinetically ;  while  in  mammals  the  plates  have  no  nu- 
cleus, but  the  larger  plates  have  chromatine  granules,  which,  how- 
ever, divide  as  do  the  plates.  They  state  that  the  plates  are  present 
in  mammalian  blood  as  soon  as  it  begins  to  circulate.  In  the  French 
resume  of  their  work  (Arch.  Ital.  BioL,  XII.,  304),  they  state  that 
Fusari  has  confirmed  their  observations  in  an  article  in  the  Riforma 
medico,  13  Agosta,  1889.  I  question  most  decidedly  the  trustworthi- 
ness of  these  statements,  for  the  author's  figures  suggest  at  once 
that  they  have  mistaken  distorted  blood-globules  for  blood-plates. 
No  other  observations  on  fcetal  blood-plates  are  known  to  me.  It 
should  be  added  that  L.  Lilienfeld,  92. 1,  has  advanced  the  hypothe- 
sis that  the  plates  are  derived  from  leucoc}~te  nuclei,  while  Howell, 
90. 1,  suggests  that  they  are  the  extruded  nuclei  of  red  cells' 

Morphology  of  the  Blood-Corpuscles. — The  following  con- 
ceptions have  been  advocated  by  Minot,  122.  The  preceding  sections 
show  that  the  vertebrate  blood-corpuscles  are  of  three  kinds :  1,  Red 
cells;  2,  White  cells;  3,  Plastids.  The  red  and  white  cells  occur 
in  all  (?)  vertebrates;  the  plastids  are  confined  to  the  mammals. 
The  red  cells  present  three  chief  modifications ;  whether  the  primitive 
form  occurs  in  any  living  adult  vertebrate  I  do  not  know ;  the  second 
form  is  persistent  in  the  Ichthyopsida,  the  third  form  in  the  Saurop- 
sida.  According  to  this  we  must  distinguish : 

A.  ONE-CELLED  BLOOD,  i.  e.,  first  stage  by  all  vertebrates ;  the  blood 

contains  only  red  cells  with  little  protoplasm. 

B.  TWO-CELLED  BLOOD,  having  red  and  white  cells.     The  red  cells 

have  either  a  large,  coarsely  granular  nucleus  (Ichthyopsida) 


224:  THE    EMBRYO. 

or  a  smaller,  darkly  staining  nucleus  (Sauropsida,  mammalian 

embryos) . 
C.  PLASTID  BLOOD,  without  red  cells,  but  with  white  cells  and  red 

plastids ;  occurs  only  in  adult  mammals. 

Mammalian  blood  in  its  development  passes  through  these  stages, 
as  well  as  through  the  two  phases  of  stage  .£>,  all  in  their  natural 
sequence ;  the  ontogenetic  order  follows  the  phylogenetic.  It  seems 
not  improbable  that  an  animal  may  yet  be  found  with  blood  inter- 
mediate between  B  and  C  in  the  adult  stage. 

II.     ORIGIN  OF  THE  HEART. 

The  heart,  as  has  been  stated,  is  developed  independently  of  the 
blood  and  blood-vessels ;  it  contains  at  first  a  clear  fiuid,  and  begins 
beating  before  the  blood-vessels  from  the  area  vasculosa  have  joined 
it.  The  primitive  form  of  the  heart  is  a  straight  median  tube  on 
the  ventral  side  of  the  cervical  region ;  the  cephalic  end  of  the  tube 
is  connected  with  the  arterial  system  of  the  embryo,  while  the  caudal 
end  is  primitively  connected  with  the  venous  system  of  the  yolk. 
These  relations  may  be  traced  in  all  vertebrates,  hence  the  heart 
arises  as  the  active  organ  of  communication  between  the  yolk  or 
primitive  food  supply  and  the  embryo. 

Primitive  Mode  of  Development  of  the  Heart. — In  re- 
gard to  the  development  of  the  heart  we  have  to  distinguish  the 
mode  still  preserved  in  the  primitive  vertebrates  (marsipobranchs, 
ganoids,  and  amphibians),  elasmobranchs,  and  in  some  but  not  all 
teleosts  (Mclntosh  and  Prince,  90.1,  775),  from  the  mode  in  the 
amniota.  In  the  first  mode  the  heart  arises  in  the  median  line ;  in 
the  second  mode  the  heart  arises  from  two  lateral  anlages,  which 
subsequently  unite  in  the  median  line.  The  difference  is  not  a  fun- 
damental one,  but  is  correlated,  as  first  pointed  out  by  Balfour,  with 
the  earlier  or  later  separation  of  the  cephalic  end  of  the  embryo  from 
the  yolk ;  when  that  separation  is  retarded  the  heart  is  differentiated 
before  the  neck  of  the  embryo  is  folded  off  from  the  yolk,  compare 
Chapter  XIII. ;  this  delay  occurs  in  varying  degrees  in  all  amniota. 
The  following  account  of  the  origin  of  the  heart  in  Amphibia  is 

based    on    C.    Rabl, 

Coe  69    En  86.1,  who  cites  the 

earlier  authorities. 
The  head  of  the  em- 
bryo early  becomes 
free  and  projects  so 

,i~\~,-    o/A\    r\  /^ 

r^h< _ ..  _ 

also.     The  mesoderm 

FIG.  128.  — Salamandra  Maculosa ;  Larva,  very  Young ;  Transverse  OV+C»T-»  i\  a    -Priv-wr  a  -nrl      r»-n 

section,  to  show  the  Formation  of  the  Coelom  in  the  Heart  Region.  6XIY 

Coe,  Ccelom;  En,  entoderm;  EC,  ectoderm;   mes,   mesothelium.  each       Side      between 

After  C.  Rabl.  ,     -, 

ectoderm  and  ento- 
derm, and  has  a  ccelomatic  cavity  on  each  side,  Fig.  128,  Coe.  The 
two  wings  of  mesoderm  do  not,  however,  meet  on  the  median  ventral 
line,  being  separated  by  a  ridge,  En,  of  entoderm  by  which  the  inner 
germ-layer  comes  into  immediate  contact  with  the  ectoderm,  EC. 


<^sPooS@sor  far  that  the  neck  is 


ORIGIN   OF   THE    HEART. 


225 


Whether  this  ridge  is  preserved  to  form  the  endothelium  of  the  heart 
or  is  resorbed  into  the  general  entoderm  is  not  positively  known. 
In  a  later  stage,  Fig.  129,  the  two  mesodermic  wings  have  met  in 


mes 


Coe 


EC 


msth. 


Ht 


FIG.  1£».  — Salamandra  Maculosa ;  Larva  with  Branchial  Arches.    Coe,  Coelom ;  msth,  mesothelium ; 
///.  I'li.lothflial  li.-art;  EC,  ectoderm;  mes,  inesoderm;  Ent,  entoderm.     After  C.  Rabl. 

the  median  line  below  the  intestinal  canal ;  the  ccelom  has  expanded ; 
between  the  mesothelium  of  each  side  in  the  median  line  is  a  small 
mass  of  cells,  Ht,  which  soon  shows  a  central  lumen,  which  be- 
comes the  cavity  of  the  heart,  while  the  cells  around  give  rise  to  the 
future  endothelium ;  the  endothelium  is  still  in  contact  with  the  en- 
toderm. Below  the  heart  the  mesothelia  are  in  actual  contact,  form- 
ing a  double  wall,  which  soon  breaks  through,  so  that  the  ccelom  on 
each  side  opens  into  the  other,  or,  in  other  words,  there  is  now  a  sin- 
gle pericardial  cavity.  The  heart  has  become  a  two-layered  tube ; 
the  inner  layer  consists  of  endothelium,  the  origin  of  which  is  dis- 
cussed in  a  separate  paragraph  below;  the  outer  layer  consists  of 
mesothelium,  vliich  gives  rise  to  the  muscular  wall  of  the  heart. 
Later  the  mesothelium  closes  over  the  dorsal  side  of  the  endothelium, 
thus  finally  separating  it  from  the  entoderm.  Still  later  the  tubular 
heart  loses  its  suspension  from  the  dorsal  side  of  the  pericardial  cav- 
ity and  is  attached  only  at  its  anterior  or  cephalic  and  posterior  or 
caudal  extremities,  and  hence  is  free  to  bend  and  twist  within  the 
pericardial  cavity  in  the  manner  necessary  for  the  evolution  of  the 
heart's  adult  form. 

Amniote  Mode  of  Development  of  the  Heart. — Observa- 
tions on  the  heart  are  to  be  found  in  many  of  the  older  writers  on 
embryology,  notably  in  Von  Baer,  Prevost  et  Lebert,  Remak,  Bisch- 
off ,  and  Coste,  but  until  the  introduction  of  section  cutting  the  details 
of  the  process  could  not  be  observed.  The  foundations  of  our  present 
knowledge  were  laid  by  W.  His,  68.1,  83-85,  and  the  subject  was 
further  elucidated  by  Kolliker's  invaluable  observations  on  the  chick 
and  rabbit,  recorded  in  his  "  Entwickelungsgeschichte ; "  Gasser, 
77.3,  has  published  an  admirable  description  with  figures  of  the  de- 
velopment in  the  chick ;  there  are  besides  numerous  references  to  the 
heart  scattered  in  recent  literature;  see,  for  instance,  Hensen,  76.1; 
Heape,  86.2;  Selenka,  86.1,  et  al. 

In  the  amniota  the  cephalic  coelom  very  early  dilates  to  a  much 
greater  degree  th°.ii  the  coelom  elsewhere,  thus  developing  on  each 
side  the  so-called  Pa rietalhohle  of  German  writers,  for  which  I  have 
15 


220 


THE   EMBRYO. 


proposed  the  name  of  amnio-cardial  vesicle.  In  the  chick  the  early 
and  extreme  dilatation  of  this  cavity  is  well  known,  and  is  intimately 
correlated  both  with  the  closure  of  the  archenteron  to  form  the  cer- 
vical entodermic  canal  ( Vorderdarm) ,  and  also  with  the  development 
of  the  heart  and  the  origin  of  the  amnion.  In  the  chick  the  dilata- 
tion forces  the  splanchnopleure  (splanchnic  mesoblast  and  entoderm) 
downward  on  each  side ;  then  bends  the  splanchnopleure  in  under 
the  embryo  until  the  two  membranes  meet  in  the  median  line  and 
fuse ;  their  fusion  shuts  off  the  Vorderdarm  from  the  yolk  and  leaves 
it  as  a  flattened  canal,  Fig.  129 A,  Ph;  for  further  details  see  Chapter 
XII.  The  layer  of  mesothelium  bounding  the  ccelom  is  everywhere 
distinct ;  the  mesenchyma  is  well  developed  all  about  the  medullary 
canal  and  notochord,  Fig.  129 A,  but  is  almost  entirely  absent  from 
the  walls  of  amnio-cardial  vesicles,  until  we  reach  the  distal  vascular 
.area,  consequently  when  the  vesicles  expand  the  mesothelium  is 
brought  close  against  that  portion  of  the  entoderm  which  is  destined  to 
form  the  Yorderdarm ;  where  the  contact  takes  place  there  appear  be- 
tween the  entoderm  and  mesothelium  a  few  very  irregularlygrouped 
mesenchymal  cells,  Fig.  129 A,  Endo;  these  are  theanlage  of  the 
endothelial  lining  of  the  heart,  or  Endothelherz  of  German  embryol- 
ogists.  The  mesothelium  of  each  side  meets  its  fellow  in  the  median 
ventral  line,  forming  a  thin  partition  or  ventral  mesocardium,  Fig. 
129 A,  which  subsequently  breaks  through;  from  the  ventral  wall  of 


FIG  129A.— Embryo  Chick  (Minot  Coll.  No.  AJ,  Section  384)  ;  Section  through  the  Anlage  of  the 
Heart.  Md,  Medullary  groove;  EC,  ectoderm ;' mes,  mesenchyma;  Am.  ves,  amniotic  vesicle; 
Ph,  pharynx ;  msth,  mesothelium ;  Endo,  cells  to  form  the  endothelium  of  the  heart. 

the  Vorderdarm,  Ph,  the  mesothelium  bulges  out  as  a  much-thick- 
ened layer,  msth,  which  develops  into  the  muscular  wall  of  the  heart, 
while  between  this  wall  and  the  entoderm  of  the  Vorderdarm  lie  the 
mesenchymal  cells.  Development  proceeds  by  the  mesothelial  fold 
becoming  more  protuberant  on  each  side,  and  the  mesenchymal  cells 
assuming  the  endothelial  character,  coming  to  bound  several  irregular 
cavities  on  each  side,  Fig.  130,  En. hi;  these  cavities  soon  fuse  into 
two  main  cavities  running  longitudinally;  as  the  two  cavities  enlarge 
they  meet  in  the  median  line  and  remain  separated  at  first  by  a  wall 
of  two  layers  of  endothelium;  this  wall  soon  breaks  through  and 


ORIGIN   OF   THE   HEART.  227 

there  results  a  single  median  tube  of  endothelium  connected,  by  long 
processes  of  cells,  across  quite  a  wide  space  with  the  mesothelium. 
Excellent  figures  of  all  these  changes  are  given  by  Gasser,  77.3. 
The  heart  is  now  a  double  tube,  connected  by  the  mesothelium  with 
the  tissues  above  and  below ;  but  soon  the  connection  on  the  ventral 
side  is  severed,  and  a  little  later  that  on  the  dorsal,  but  the  attach- 
ments are  retained  as  in  amphibia  at  both  ends  of  the  tube.  A  sec- 
tion through  the  end  of  the  heart  is  shown  in  Fig.  130;  the  ventral 


m.ht 


— -~^^\     ^ 

pro.  am.         ^a^f^gT 

FIG.  ISO.— Chick  Embryo  (Minot  Coll.  No.  AL,  section  119).  Mi,  Wall  of  medullary  tube; 
iit-fi,  notochord;  math,  mesothelium;  P/t,  pharynx;  en.ht,  endothelial  heart;  m. ht,  muscular 
heart. 

mesocardium  is  entirely  lost ;  the  dorsal  is  preserved,  as  also  at  the 
opposite  end  of  the  heart,  though  not  in  its  middle ;  the  thick  meso- 
thelial  wall  or  muscular  heart  is  widely  removed  from  the  thin  inner 
endothelial  heart  (Endothelherz). 

From  the  preceding  account  it  appears  that,  owing  to  the  devel- 
opment of  the  heart  beginning  before  the  Vorderdarm  closes,  the 
heart  is  distinctly  double  in  origin,  though  all  trace  of  the  duplex 
condition  is  quickly  lost.  In  mammals  the  double  stage  lasts  longer, 
the  Vorderdarm  being  closed  still  later. 

Our  knowledge  of  the  origin  of  the  heart  in  mammals  rests  chiefly 
on  the  observations  of  Kolliker  upon  rabbits;  this  paragraph  is 
therefore  based  on  the  description  given  in  Kolliker 's  "Grundriss," 
p.  96,  120.  Traces  of  the  heart  can  be  recognized  in  embryos  with 
five  protovertebrataB,  and  the  two  anlages  are  well  advanced  in  em- 
bryos with  eight  to  ten  segments,  and  in  surface  views,  Fig.  114, 
may  be  seen  at  either  side  of  the  head,  bending  anteriorly  toward  the 
median  line,  and  each  connected  posteriorly  with  the  developing 
omphalo-mesaraic  vein  of  the  same  side;  one  can  also  distinguish 
the  parietal  coelomatic  cavity  about  the  heart.  A  transverse  section 
through  the  region  of  the  heart  presents  a  very  uniform  picture  in 
all  mammals  thus  far  studied ;  compare  Fig.  95  of  the  opossum  with 
Fig.  114  of  the  rabbit.  The  parietal  ccelom  or  amnio-cardial  vesicle 


228  THE    EMBRYO. 

is  small  as  compared  with  that  of  the  chick,  Fig.  117,  and  lies  quite 
distant  from  the  median  line ;  the  splanchnic  mesothelium  forms  a 
large  fold,  which  projects  into  and  nearly  fills  up  the  ccelomatic 
cavity;  this  fold  forms,  as  in  the  chick,  one-half  of  the  muscular 
heart ;  in  the  interior  of  this  fold  lies  the  endothelial  heart,  which 
sends  out  processes  by  which  it  is  connected  with  the  surrounding 
mesothelium.  By  the  bending  down  of  the  layers  and  the  expansion 
of  the  ccelom  the  Yorderdarm  is  shut  off  and  the  two  lateral  heart 
anlages  are  brought  together  in  the  median  line  below  the  Vorder- 
darm,  and  there  fuse  into  a  single  structure ;  the  fusion  takes  place 
in  such  a  manner  that  the  two  mesothelial  folds  unite  by  their  edges 
to  form  a  single  thick  tubular  wall  around  the  double  endothelial 
heart;  it  is  not  long,  however,  before  the  two  endothelial  tubes  also 
fuse  into  one.  As  in  the  chick  the  two  mesothelia,  when  the  median 
heart  arises,  form  a  membrane  (mesocardium) ,  by  which  the  heart 
is  attached  to  the  tissues  above  and  below ;  both  mesocardial  mem- 
branes break  through,  putting  the  two  ccelomatic  cavities  into  com- 
munication and  leaving  the  tubular  heart  suspended  by  its  ends. 

In  amniota  the  heart  arises  from  a  double  anlage,  which  by  the 
bending  down  of  the  splanchnopleure  of  the  Vorderdarm  becomes  a 
single  median  anlage,  as  in  amphibians;  C.  K.  Hoffmann,  84.3,  has 
asserted  that  in  snakes  the  heart  arises  from  one  of  the  lateral 
anlages,  but  Junglow,  89.1,  has  rendered  it  probable  that  this  is 
merely  a  blunder  of  observation.  The  median  heart  is  at  first  a 
nearly  straight  tube  attached  by  each  end  to  the  wall  of  the  pericar- 
dial  coelom,  and  connected  in  front  with  the  aortaB  and  behind  with 
the  omphalo-mesaraic  veins ;  the  tube  is  double,  consisting  of  a  thin 
inner  endothelial  wall  of  mesenchymal  origin  separated  by  a  consid- 
erable space  from  the  outer  thicker  mesothelial  layer,  from  which  the 
muscular  tissue  of  the  heart  arises. 

Origin  of  the  Endothelium  of  the  Heart. — This  is  still 
unsettled.  As  we  have  seen,  the  endothelium  has  upon  its  first  ap- 
pearance nothing  of  an  endothelial  character,  but  resembles  instead 
the  cells  of  the  mesenchyma  at  the  time;  in  amphibia  they  are  large 
and  rounded  and  charged  with  yolk  granules;  in  amniota  they  are 
more  like  embryonic  connect^  e-tissue  cells.  These  cells  always  appeal- 
between  the  entoderm  of  the  cervical  archenteron  (Vorderdarm  of 
Von  Baer)  and  the  mesoderm  bounding  the  coelom,  and  when  they 
first  appear  there  are  no  other  cells  near  them  between  the  mesothe- 
lium and  entoderm,  compare  Figs.  128  and  129.  Whence  do  these 
cells  come?  I  consider  it  probable  that  they  are  the  forward  extension 
of  the  vascular  anlages  of  the  omphalo-mesaraic  veins  and  that  just 
as  the  endothelial  aorta5  are  formed  by  the  ingrowth  of  loose  strings 
of  cells  so  are  the  two  veins,  and  these  uniting  in  the  median  line 
form  the  endothelial  heart.  This  view  is  hypothetical.  A  variety 
of  other  conflicting  views  have  been  advanced,  of  which  the  follow- 
ing may  be  noted.  Balfour,  "  Elements, "  85,  89,  thinks  the  cells  come 
from  the  neighboring  mesoblast,  as  Oellacher  had  previously  consid- 
ered was  probable  in  teleosts,  73. 1,  84.  Goette  has  maintained  that 
in  Petromyzon,  90.1,  teleosts  and  amphibians,  75.1,  the  cells  come 
directly  from  the  entoderm,  and  C.  K.  Hoffmann,  92.1,  maintains 
the  origin  of  the  heart  to  be  entodermal  in  elasmobranchs.  Kabl, 


ORIGIN    OF   THE    HEART. 

86.1,  expresses  himself  very  cautiously,  but  inclines  to  the  view 
that  the  cells  come  from  the  entoderm,  and  in  regard  to  the  sharks 
he  is  uncertain,  89.2,  225.  J.  Riickert,  88.2,  believes  that  the 
cells  which  become  the  endothelium  are  thrown  off  in  elasmobranchs 
from  both  the  entoderm  and  mesoderm  at  the  points  where  the  cells 
first  appear.  Finally,  F.  Schwink,  90.1,  asserts  that  in  amphibia 
the  cells  are  derived  neither  from  the  neighboring  entoderm  nor 
mesoderm,  but  that  they  grow  in  from  the  mass  of  yolk-cells. 
Schwink's  observations  seem  very  careful,  and  may  turn  out  to  con- 
firm the  hypothesis  of  the  origin  of  the  endothelial  heart  from  the 
omphalo-mesaraic  veins  uniting. 

Origin  of  the  Vascular  System.— O.  Butschli,  83.3,  has  ad- 
vanced an  hypothesis  of  the  phylogenetic  origin  of  the  heart  and 
blood-vessels  which  has  much  plausibility.  He  suggests  that  the 
heart  is  a  remnant  of  the  primitive  or  segmentation  cavity  of  the 
embryo,  and  is  not  derived  from  the  secondary  or  permanent  body 
cavity  (schizocoele  or  entercoele)1.  He  endeavors  to  reconcile  this 
view  with  the  accounts  of  the  development  of  the  heart  in  vertebrates, 
maintaining  that  it  probably  arises  as  a  fissure  in  the  mesoderm, 
remaining  as  a  permanent  part  from  the  temporary  primitive  cavity. 
More  support  for  the  hypothesis  is  found  in  arthropods ;  for  it  has 
been  observed  in  several  forms  that  the  two  edges  of  the  mesoderm 
approach  one  another  in  the  median  dorsal  line,  leaving  a  space  be- 
tween them  which  belongs  to  the  primitive  cavity.  This  space 
becomes  the  heart.  Sometimes  it  is  cut  off  before,  sometimes  after, 
the  mesoderm  is  split  into  segments.  These  observations  were  upon 
the  bee  (Butschli),  Geophilus  (Metschinkoff),  and  Branchipus 
(Glaus).  An  investigation  to  answer  the  problem  propounded  by 
Butschli  would,  it  may  be  safely  said,  prove  fruitful  and  interesting. 
For  further  speculations  in  this  direction  see  Schimkevitsch,  85.1. 

As  to  the  evolution  of  the  vascular  system  the  course  of  develop- 
ment in  the  embryo  indicates,  it  seems  to  me,  that  the  immediate 
ancestors  of  vertebrates  had  no  capillary  vessels,  but  only  a  few  large 
afferent  and  efferent  trunks  with  a  few  anastomoses,  as  is  now  found 
in  many  annelids.  With  the  acquisition  of  the  large  yolk  the  devel- 
opment of  accessory  blood-channels  over  the  surface  of  the  yolk  pre- 
sumably followed  to  secure,  more  efficient!}",  nutrition  for  the  embryo. 
These  first  channels  were,  if  we  may  rely  on  the  ontogenetic  indi- 
cations, grooves  on  the  surface  of  the  yolk  bounded  on  one  side 
by  mesenchymal  cells,  by  the  further  differentiation  of  which  the 
grooves  become  endothelial  tubes ;  in  this  manner  we  can  account 
for  the  blood-vessels  appearing  first  in  the  extra-embryonic  area. 
Since  the  blood-cells  are  developed  from  the  walls  of  the  vessels,  it  is 
possible  that  the  walls  may  have  acquired  hemoglobin,  and  the  cells 
then  have  been  set  free  by  a  further  evolution,  but  it  is  perhaps 
equally  possible  that  the  isolation  of  the  blood-cells  from  their  matrix 
(the  vascular  wall)  may  have  preceded  the  acquisition  of  haemoglobin, 


CHAPTER   XI. 


Md 


Nch 


ORIGIN  OF  THE  UROGENITAL  SYSTEM. 

THE  outlines  of  vertebrate  morphology  were  given,  in  the  main, 
correctly  by  the  older  anatomists,  except  as  regards  the  urogenital 
system.  In  1875  Carl  Semper  announced  the  discovery  that  the 
excretory  tubules  of  elasmobranchs  have  a  funnel-shaped  opening 
in  the  abdominal  cavity — a  fact  discovered  by  Balfour,  78.3,  at 
about  the  same  time.  Both  authors  recognized  that  this  discovery 
was  profoundly  significant,  but  it  is  chiefly  to  Semper  that  we  owe 
the  reform  of  conceptions  in  this  field.  It  is  unnecessary  to  attempt 
a  historical  review ;  the  reader  will  find  in  Max  Furbringer's  admi- 
rable monograph,  7 8. 1,  a  thorough,  critical,  and  trustworthy  revision 
of  all  that  had  been  done  up  to  that  time.  For  notices  of  the  subse- 
quent literature  see  Riickert,  88.1,  Van  Wijhe,  89.1,  and  H.  H. 
Field,  91.1.  R.  Semon's  valuable  memoir,  91.1,  became  accessible 
to  me  too  late  to  enable  me  to  remodel  this  chapter  as  his  results 
render  necessary. 

Fundamental  Parts  of  the  Urogenital  System. — For  a 
general  explanatory  description  we  may  consider  the  fundamental 

parts  to  be  four  on  each  side  of  the 
vertebrate  embryo,  compare  Fig. 
131.  The  four  parts  are  two  lon- 
gitudinal ducts:  the  pronephric 
or  Wolffian  duct,  W.  D,  and  the 
Mullerian  duct  or  oviduct,  M.  D; 
and  two  ridges  on  the  dorsal  side 
of  the  body-cavity,  Coe,  into 
which  they  protrude ;  each  ridge 
is  covered  by  mesothelium  resting 
on  mesenchyma.  The  smaller 
ridge,  Gen,  is  called  the  genital, 
since  it  is  transformed  into  the 
genital  glands ;  it  lies  nearest  the 
median  line;  its  cephalic  end 
is  probably  identical  with  the 
so-called  glomus  of  the  prone- 
phros.  The  larger  ridge,  Ex,  is 
called  the  Wolffian  or  nephridial 
ridge;  it  contains  the  transverse 
excretory  tubules  (segmental  tu- 
bules, nephridia)  which  are  de- 
veloped from  the  nephrotomes, 
the  expansion  of  which  probably 
causes  the  bulging  of  the  mesothelium,  which  results  in  the  forma- 
tion of  the  Wolffian  ridge.  The  nephridia  open  into  the  pronephric 


Ao 


Gen 


FIG.  131.— Diagrammatic  Cross-Section  of  a 
Vertebrate  to  show  the  Fundamental  Relations 
of  the  Urogenital  System.  Md,  Medullary 
tube;  Nch,  notochord ;  Ao,  aorta;  Gen,  genital 
ridge;  W.  D,  Wolffian  duct;  M.  D,  Miiller's 
duct ;  Ex,  excretory  or  Wolffian  ridge ;  Msth, 
mesothelium ;  Coe,coelom ;  Som,  somatopleure : 
Ach,  archenteron. 


ORIGIN   OF   THE   UROGENITAL   SYSTEM. 


231 


duct.  The  cephalic  end  of  the  nephridial  or  Wolffian  ridge  give  rise 
to  the  pronephros,  while  the  remainder  of  the  ridge  is  for  the  chief 
part  at  least  converted  into  the  Wolffian  body  (primitive  kidney, 
mesonephros,  Urniere). 

Head-kidney  or  Pronephros. — The  head-kidney  being  the 
first  part  of  the  urogenital  system  to  be  differentiated  in  the  verte- 
brate embryo,  must  be  regarded  as  the  phylogenetically  oldest  part. 
It  is  found  in  the  embryos  of  (probably)  all  vertebrates,  but  disap- 
pears before  adult-life  in  selachians,  some  teleosts,  and  all  amniota. 

The  head -kidney  is  always  situated  in  the  segments  immediately 
behind  the  heart,  and  is  a  paired  organ  with  a  longitudinal  duct, 
which  finally  opens  into  the  cloaca  or  hind  end  of  the  alimentary 
tract;  the  duct  has  great  morphological  importance ;  its  development 
is  described  in  the  next  section.  The  head-kidney  consists  of  from 
one  to  five  or  more  transverse  tubules  which  are  differentiated  from 
the  nephrotomes  and  have  on  the  one  hand  an  opening  into  the  ven- 
tral coslom  or  abdominal  cavity,  and  on  the  other  into  the  longitu- 
dinal duct.  Each  tubule  consists  of  epithelium  and  when  well 
developed  takes  a  conviilated  course.  The  number  of  these  trans- 
verse tubules  is  said  to  be  greatest  in  Myxine ;  in  Petromyzon  there 
are  four  or  five,  in  Torpedo  six,  Pristiurus  four,  Coscilia  four,  Ah- 
ura  three,  Urodela  two ;  but  in  teleosts  and  cartilaginous  ganoids 
one  only.  The  head-kidney  often  protrudes  somewhat  into  the  body- 
cavity,  and  the  part  of  the  body-cavity  into  which  it  protrudes  may 
become,  as  in  teleosts  and  the  lamprey,  shut  off  from  the  remaining 
coelom.  There  is  also  developed  a  so-called  glomus,  which  is  a  fold 
of  the  mesothelium  arising  near  the  base  of  the  mesentery,  and  con- 


nch 


FIG.  132.— Rana  Temporaria.  Tadpole  of  12  mm.  Cross-Section  through  the  pronephros. 
nch,  Notochord;  w,  muscles;  /,  funnel-shaped  opening  of  tubule  or  second  nephrotome;  V. 
blood-vessels;  EC,  ectoderm;  f,  tubule  of  pronephros;  gl,  glomus;  Lu,  lung.  After  M.  Fiirbrin- 
ger.  x  90  diams. 

taining  numerous  blood-vessels.     The  structure  of  the  organ  is  well 
illustrated  by  Fig.  132. 

The  development  of  the  head-kidney  varies  considerably  in  the 
different  classes  of  vertebrates,  so  that  we  are  still  uncertain  as  to 
what  are  the  essential  and  typical  features  of  its  development.  The 


232  THE    EMBRYO. 

confusion  is  probably  due  to  the  fact  that  it  is  only  recently  that  we 
have  gained  the  knowledge  that  between  the  myotome  and  the  lateral 
plate  comes,  in  every  segment,  the  nephrotome,  to  which  the  origin 
of  the  transverse  excretory  tubule,  both  of  the  head-kidney  and  of 
the  Wolffian  body  (mesonephros)  has  been  traced  in  a  number  of 
cases.  Since  we  have  known  that  the  essential  question  is,  Do  the 
pronephric  tubules  arise  from  the  nephrotomes?  sufficient  investi- 
gations have  not  been  undertaken.  But  it  has  been  shown  in  several 
cases  that  the  nephrotomes  do  produce  the  tubules.  The  typical 
mode  of  development  both  for  the  pronephros  and  mesonephros  is,  I 
think,  probably  as  follows :  The  nephrotomes  typically  contain  a 
ccelomatic  cavity ;  when  they  separate  from  the  myotome  the  myo- 
tomic  end  of  the  nephrotomic  cavity  becomes  closed,  but  the  other 
end  remains  open  and  becomes  the  permanent  nephrotome  opening 
of  the  nephric  tubules  (Segmentalorgane  of  Semper) ;  the  nephrotome 
now  lengthens  out  and  unites  secondarily  with  the  pronephric  or 
segmental  duct.  Until  further  research  of  a  far  more  thorough 
character  than  anything  we  yet  have  shall  decide  the  question,  this 
hypothetical  account  is  the  best  that  can  be  presented. 

In  the  cylostomes,  teleosts,  and  amphibia  the  pronephros  is  said 
to  arise  from  the  mesothelium  of  the  ventral  ccelom ;  but  as  this  takes 
place  so  that  the  mesothelium  is  close  to  the  myotome,  it  is  more 
than  possible  that  we  have  to  do  really  with  nephrotomic  tissue. 
If  Goette's  account  of  the  process  in  Petromyzon,  90.1,  54,  55,  be 
correct,  then  it  may  be  that  in  the  lamprey  the  nephrotomic  anlage 
separates  from  the  myotomes,  and  while  still  connected  with  the 
lateral  plates  undergoes  segmental  division.  In  the  lamprey  (Goette, 
/.  c.),  teleosts  (Mclntosh  and  Prince,  90.1,  783-785),  and  amphib- 
ians (Furbringer,  78.1),  the  mesothelium,  which  produces  the 
tubules,  produces  the  longitudinal  duct  also,  but  in  view  of  what  is 
stated  of  other  vertebrates  this  has  been  questioned.  Our  knowledge 
of  the  head-kidney  in  amphibia  has  been  very  much  extended  by  the 
recent  researches  of  Mollier,  90.1,  Marshall  and  Bles,  90.1,  K. 
Semon,  91.1,  and  H.  H.  Field,  91.1.  My  lack  of  personal  famil- 
iarity with  the  amphibian  pronephros  makes  me  unwilling  to  attempt 
a  critical  summary  of  their  researches. 

The  pronephros  of  elasmobranchs  begins  to  develop  in  (Pristiurus) 
embryos  with  twenty-seven  segments ;  the  three  foremost  segments 
are  subsequently  included  in  the  head,  so  the  fourth  is  the  first  seg- 
ment of  the  rump  (Van  Wijhe,  89. 1,  473).  In  the  first  four  (Pris- 
turius)  or  six  (Torpedo)  of  the  rump  segments  the  somatic  mesothe- 
lium (wrongly  termed  somatopleure  by  Van  Wijhe)  and  nephrotome 
becomes  thickened;  these  thickenings  come  into  contact  with  one 
another,  and,  according  to  Riickert,  88.1,  with  the  ectoderm;  they 
subsequently  acquire  a  lumen ;  thus  each  nephrotome  has  an  exten- 
sion of  its  cavity,  and  becomes  a  canal  with  an  opening  into  the 
main  coelom,  and  extending  in  a  curved  line  outward  and  backward 
toward  the  ectoderm.  These  four  to  six  canals  unite  with  the  longi- 
tudinal duct  which  arises  from  the  ectoderm.  Our  knowledge  is 
based  chiefly  on  Riickert's  very  detailed  investigations,  88. 1. 

In  amniota,*the  head-kidney  was  first  described  by  A.  Sedgwick, 

*  For  a  fuller  review  and  discussion,  see  H.  H.  Field,  91.1,  272-281. 


ORIGIN"    OF   THK    UKOOEXITAL   SYSTEM.  233 

81.1,  and  has  been  studied  also  by  Renson,  83.1,  Mihalkovics, 
85. 1,  Janosik,  85. 1,  Wiedersheim,  90.3,  and  others.  A  few  scat- 
tered observations  are  inserted  by  Hoffmann  in  his  "Reptilien,"  p. 
2047-2063  of  Bronn's  "  Thierreich."  Braun,  77.4,  has  shown  that  in 
reptiles  the  nephrotomes  become  much  enlarged  and  appear  as 
rounded  vesicles  of  epithelium ;  the  anterior  three  or  four  of  these 
vesicles  retain  the  open  communication  of  their  cavities  with  the 
ventral  coelom,  and  these  vesicles  co»respond  to  the  pronephric  an- 
lage.  Most  of  what  little  we  know  of  their  history  is  due  to 
Mihalkovics,  85.1,  41-47,  55-6G.  Each  of  the  pronephric  segmental 
vesicles  acquires  a  communication  by  means  of  a  fine  fissure  with 
the  longitudinal  duct,  which  has  meanwhile  formed  between  the 
nephrotomes  and  the  ectoderm.  The  opening  of  the  vesicle  into  the 
ventral  coelom  (pleuro-peritoneal  cavity)  is  now  closed,  and  the 
nephrotome  is  completely  separated  from  the  lateral  plates  or  meso- 
thelium  of  the  body-cavity  proper.  The  next  change  (Lacerta 
embryos  of  3.5-4.0  mm.)  is  effected  by  the  lateral  wall  of  the  vesicle 
sinking  in,  thus  converting  the  round  vesicle  into  a  tube  bent  into 
an  S-shape  and  having  its  general  course  at  right  angles  to  the  body- 
axis.  It  is  uncertain  whether  Mihalkovics  has  described  true  pro- 
nephric tubules  or  merely  anterior  tubules  of  the  Wolffian  body.  If 
the  structures  are  pronephric  there  ought  to  be  some  trace  of  a  peri- 
toneal glomerulus  near  the  opening  of  the  tubule,  which  there  is  not. 
On  the  other  hand,  if  they  are  Wolffian  tubules  there  should  be  a 
glomertilus  formed  from  the  tubule  itself,  and  this  seems  to  be  the 
case,  (seo  Mihalkovics,  I.  c.,  Taf.  I.,  Fig.  9).  R. Wiedersheim,  90.3, 
states  that  the  head-kidney  is  very  well  developed  in  crocodile  em- 
bryos (10-12  mm.)  and  occupies  the  anterior  end  of  the  urogenital 
ridge;  it  merges  into  the  Wolffian  body;  the  right  pronephros  has 
sixteen,  the  left  thirteen,  funnel-shaped  nephrotomes;  the  glomus 
is  the  anterior  continuation  of  the  urogenital  ridge,  and  is  imperfectly 
segmentally  subdivided ;  the  tailward  end  of  the  pronephros  is  shut 
off  from  the  main  ccelom  by  a  prolongation  of  the  embryonic  dia- 
phragm (septum  transversum) .  In  birds  (Mihalkovics,  85.1,  58) 
the  nephrotomes  of  the  fourth  to  seventh  segments  form  the  pro- 
nephric tubules,  which  open  into  the  body-cavity,  and  taking  a  trans- 
verse S-like  course  empty  into  the  lateral  duct ;  on  the  mesenterial 
side  of  the  peritoneal  opening  of  the  tubule  a  glomerulus  is  formed ; 
the  relations  can  be  seen  in  chicks  of  three  to  four  days,  or  better  in 
ducks  of  the  same  incubation ;  the  pronephric  tubules  disappear  the 
fourth  day,  but  the  glomeruli  enlarge  somewhat  and  do  not  disap- 
pear until  the  seventh  day.  Balfour  and  Sedgwick,  78.1,  have 
advanced  a  different  view ;  they  state  that  the  Miillerian  duct,  the 
development  of  which  is  described,  p.  244,  has  three  anterior  open- 
ings at  first,  and  these  they  homologize  with  the  head-kidney ;  but 
Sedgwick,  81.1,  has  withdrawn  his  opinion.  Renson,  83.1,  Koll- 
mann,  82.2,  and  Mihalkovics,  85.1,  moreover,  deny  the  existence 
of  the  three  openings.  In  mammals  the  head-kidney  has  been  stud- 
ied by  Renson,  83. 1,  606,  who  states  that  in  rabbits  of  ten  days  the 
S-shaped  tubules  with  openings  into  the  coelom  are  present  and  empty 
into  the  primitive  longitudinal  duct;  they  disappear  very  soon; 
nothing  corresponding  to  the  pronephric  glomus  has  been  recorded. 


234  THE    EMBRYO. 

Our  knowledge  of  the  pronephros  is  unsatisfactory.  The  fullest 
review  of  the  literature  known  to  me  is  given  by  H.  H.  Field,  91.1. 

The  Pronephric  Duct. — The  primitive  longitudinal  duct  of  the 
urogenital  system  is  known  by  various  names ;  the  most  important 
are  pronephric  or  segmental  duct,  ( Vornierengang) ,  and  Wolffian 
duct,  but  it  is  doubtful  how  far  the  Wolffian  duct  of  the  amniota 
can  be  homologized  with  the  pronephric  duct  of  anamnia,  compare 
below.  Both  ducts  bear  the  same  relation  to  the  pronephros  and 
Wolffian  bodies,  but  differ  in  their  connection  with  the  Miillerian 
duct,  which  in  selachians  seems  to  arise  from  the  pronephric  duct 
and  in  amniota  arises  independently  of  the  Wolffian  duct.  For  the 
purposes  of  this  paragraph  it  is  assumed  that  the  pronephric  and 
Wolffian  ducts  are  identical,  and  the  term  pronephric  is  used  for  both. 

The  pronephric  duct  is  the  first  longitudinal  duct  of  the  urogenital 
system  to  appear.  AVhen  first  differentiated  it  always  lies  between 
the  nephrotomes  and  the  ectoderm,  Fig.  92,  W,  and  always  close 
against  the  mesodermic  tissue.  It  can  be  first  seen  (Kolliker, 
"  Grundriss,"  411)  in  chicks  during  the  second  half  of  the  first  day,  in 
rabbits  about  the  end  of  the  eighth  day  in  the  region  of  the  fourth 
and  fifth  segments ;  it  lengthens  out  very  rapidly,  so  that  in  the  chick 
the  end  of  the  second  day  it  extends  nearly  or  quite  to  the  last  seg- 
ment formed.  The  intimate  association  of  the  duct  with  the  meso- 
derm  led  to  the  general  belief  that  it  arose  from  cells  of  the  interme- 
diate mass  (nephrotomes)  or  from  the  lateral  plates  (splanchnocoelic 
mesothelium) .  This  opinion  was  shared  until  recently  by  many 
good  observers — see  the  citations  of  authorities  by  Fiirbringer,  78. 1, 
and  Mihalkovics,  85. 1,  47-52.  The  first  to  call  it  in  question  seri- 
ously was  Hensen  (Virchow's  Arch.,  XXXVII.,  81,  foot-note),  who 
in  1866,  definitely  asserted  the  origin  of  the  Wolffian  duct,  as  W. 
His,  65.2,  had  previously  suggested,  from  the  ectoderm,  which 
overlies  the  duct  when  it  appears.  The  matter  then  was  forgotten 
until  attention  was  recalled  to  it  by  the  very  exact  demonstration  by 
Count  Spee,  84.1,  that  the  duct  is  developed  in  the  guinea-pig  in 
connection  with  the  ectoderm.  Spee's  discovery  has  since  been  con- 
firmed by  Flemming,  86.1,  for  the  rabbit,  by  Bonnet,  87.1,  for 
the  sheep.  But  Fleischmann  and  Martin,  88.1,  were  unable  to 
confirm  it.  No  satisfactory  evidence  of  the  ectodermal  origin  in 
birds  has  come  yet,  although  G.  Brook,  87.1,  has  affirmed  it;  but 
for  reptiles  we  have  good  evidence  through  Perenyi,  87. 1,  Ostromoff, 
88.1,  and  Mitsukuri,  88.1,  while  Strahl,  86.1,  failed  to  find  any. 
The  best  evidence  of  all  is  that  furnished  for  elasmobranchs  by  J. 
Beard,  87.1,  Van  Wijhe,  86.1,  89.1,  and  J.  Riickert,  88.1;  the 
investigations  of  the  last  two  authors  appear  quite  conclusive.  On 
the  other  hand,  it  must  be  mentioned  that  H.  V.  Wilson,  91.1,  247, 
expressly  denies  the  accuracy  of  Brook's  statements,  and  that  H.  H. 
Field,  91.1,  reasserts  that  in  amphibians  the  duct  has  no  connection 
with  the  ectoderm. 

The  authors  who  defend  the  ectodermal  origin  of  the  duct  essenti- 
ally agree  with  one  another;  they  find  the  tailward  end  of  the  duct 
has  a  solid  cord  of  cells  which  ends  by  fusing  with  the  ectoderm, 
and  where  the  fusion  takes  place  the  cells  of  the  outer  layer  are  often 
in  karyokinesis,  as  if  the  cells  were  proliferating  to  be  added  to  the 


ORIGIN   OF   THE   UROGENITAL   SYSTEM.  235 

duct.  In  the  amniota  the  connection  exists  for  a  very  short  distance 
only,  and  may  be  easily  overlooked ;  but  the  length  of  the  fusion  is 
variable,  especially  so  in  Torpedo,  for  Riickert  states  he  found  it 
there  extending  anywhere  from  eleven  to  twenty-five  segments,  and 
even  differing  in  extent  on  the  two  sides  of  the  same  embryo.  In  a 
series  of  transverse  sections — for  a  good  figure  see  J.  Ruckert,  88. 1, 
Fig.  35 — we  see  running  from  back  headward — first,  behind  the  end 
of  the  duct,  the  thin  ectoderm  consisting  of  a  single  layer  of  cells ; 
wnrnd)  the  thickening  of  the  ectoderm  (see  Flemming,  His'  Archiv, 
1886,  Taf.  XL,  Fig.  7);  third,  the  inner  layer  of  cells  separated 
from  the  outer ;  this  separation  may  take  place  in  various  ways,  for 
the  cells  to  form  the  duct  may  make  a  flat  plate  or  a  round  cord,  or 
there  may  even  be  a  groove  in  the  ectoderm,  and  when  the  groove 
closes  it  is  separated  as  a  canal ;  fourth,  a  cord  of  cells  lying  within 
the  ectoderm ;  the  cord  is  round  in  section,  soon  develops  a  central 
lumen,  and  its  cells  become  distinctly  epithelial. 

In  view  of  the  remarakble  unanimity  of  the  descriptions  by  vari- 
ous observers,  I  think  it  probable  that  it  will  be  found  ultimately 
that  the  pronephric  duct  is  developed  from  the  ectoderm  in  all 
vertebrates. 

The  backward  growth  of  the  duct  is  accomplished  by  the  addition 
of  cells  from  the  ectoderm  to  its  caudal  end,  and,  when  it  reaches 
its  hindmost  extension  (Pristiurus  embryos  of  about  eighty  segments, 
with  five  open  gill-clefts)  it  passes  beyond  the  limit  of  the  mesoderm ; 
and  hence,  according  to  Van  Wijhe's  observation  on  elasmobranchs, 
89. 1,  486,  comes  into  direct  contact  with  the  entoderm  or  wall  of  the 
cloaca  (hind  end  of  the  archenteron) ;  it  then  fuses  with  the  entoderm 
and  separates  from  the  ectoderm,  after  which  it  develops  a  lumen ; 
thus  the  duct  comes  to  open  into  the  cloaca.  The  cloacal  opening  is 
invariably  present  in  all  vertebrates  during  a  certain  embryonic 
period  at  least ;  it  is  the  permanent  condition  in  anamnia. 

The  amniote  Wolffian  duct  is  round  in  cross-section  when  first 
formed,  but  soon  becomes  elliptical.  By  the  changes  in  position, 
effected  by  the  further  development  of  the  embryo,  the  duct  is 
brought  to  run  directly  below  the  cardinal  vein,  and  as  it  enlarges 
still  farther  its  dorsal  epithelium  becomes  flattened  against  the  vein, 
and  the  cross-section  of  the  duct  becomes  a  triangle  with  the  apex 
down ;  still  later  mesenchyma  and  Wolffian  tubules  grow  between 
the  duct  and  the  vein,  which  finally  becomes  widely  separated  as  in 
Fig.  137.  The  triangular  section  of  the  duct  is  retained  for  some 
time,  but  the  elliptical  section  is  gradually  resumed. 

The  Wolffian  Body.— The  Wolffian  body  (corps  de  Wolff,  Ur- 
niere,  mesonephros,  primitive  kidney)  is  the  chief  occupant  of  the 
embryonic  Wolffian  ridge ;  in  anamnia  it  is  the  chief  renal  organ 
throughout  life;  in  amniota  it  disappears  during  embryonic  life,  being 
replaced  by  the  true  kidney,  except  that  a  small  part  of  the  cephalic 
end  is  retained,  as  described  in  Chapter  XXIII.,  and  enters  into 
special  relations  with  the  sexual  organs. 

In  its  primitive  form  the  Wolffian  body  appears  to  have  consisted 
of  a  series  of  transverse  tubules  (Wolffian  or  segmental  tubules) 
emptying  into  the  Wolffian  or  pronephric  duct.  As  was  shown  by 
Semper 's  investigations  011  Plagiostomes,  75.2,  there  is  primitively 


236 


THE   EMBRYO. 


one  tubule  for  each  segment  of  the  body,  and  each  tubule  begins  with 
a  funnel-shaped  opening  into  the  peritoneal  cavity  and  takes  a  con- 
voluted transverse  course  to  the  laterally  situated  Wolffian  duct. 
Sedgwick  observed  that  the  tubules  do  not  arise  as  evaginations  of 

the  mesothelium   of  the 
-^:--^,      s  main  ccelom  or  splanch- 

/  noccele,    which   was  the 

view  held  by  Semper  and 
Balfour,  but  from  the  in- 
termediate cell  mass,  and 
that  the  cavity  of  the 
tubule  is  the  original 
coelomatic  cavity  of  the 
nephrotome  by  which  the 
ccelom  of  the  myotome 
communicates  with  the 
splanchnoccele.  Subse- 
quently the  nephrotome 
is  severed  from  the  myo- 
tome, and  by  elongation 
becomes  a  Wolffian  tu- 
bule ;  the  connection  with 

^ne   SplanCmlOCOele    IS    TQ- 

tained  to  form  the  funnel 
rm       j  • 
1  lie  U1S- 

r»r»ai-Hrm  r>f  ilio  r\aT-t«  ^ar> 
PUk 

be  understood  from  the 
accompanying  figure.  Fig.  133  gives  a  caudal  view  of  a  Wolffian 
tubule  of  an  Acanthias  embryo  of  28.2  mm.  The  tubules  begin  at 
N,  immediately  below  the  cardinal  vein,  VC,  and  runs  obliquely  — 
1,  upward  and  outward  to  the  glomerulus,  2,  and  then  makes  several 
convolutions,  3-9,  until  it  terminates  in  the  longitudinal  Wolffian 
duct,  W.  d.  At  2  the  tubule  is  distended  to  make  room  for  the 
glomerulus,  which  projects  into  it. 

The  development  of  the  Wolffian  body  commences  in  Salamandra 
(Fiirbringer,  78.  1),  with  the  formation  of  a  series  of  solid  cords  de- 
rived from  the  nephrotomes;  in  the  cephalic  end  of  the  body,  the 
cords  remain  connected  with  the  splanchnoccelic  mesothelium,  but 
in  the  remaining  segments  the  cords  have  no  connection  with  the 
peritoneal  epithelium.  A.  Sedgwick  states  that  the  cords  are  all 
without  union  with  the  peritoneum  in  the  frog.  The  connection 
with  the  peritoneum,  when  present,  is  soon  lost.  The  cords  develop  a 
cavity,  which  acquires  a  vesicular  form  ;  the  vesicle  becomes  flattened 
and  S-shaped  ;  the  medial  end  develops  into  a  Malpighian  corpuscle 
as  in  the  amniota  (see  below),  while  the  lateral  end  remains  narrower, 
joins  the  Wolffian  duct,  and  afterward  lengthens  out  to  form  the 
coiled  tubule;  at  the  junction  of  the  tubule  proper  with  the  corpuscle 
a  canal  grows  out  in  a  ventral  direction  which  meets  the  peritoneal 
epithelium  and  then  develops  a  funnel-shaped  opening  (nephrostome) 
into  the  body-cavity;  the  epithelium  of  the  funnel  becomes  ciliated. 
The  anterior  tubules  are  rudimentary,  the  first  fully  developed 
Wolffian  tubules  being  in  the  sixth  segment  behind  the  pronephros. 


FIG.  133.—  Nephridium  (or  Wolffian  Tubule)  of  an  Acan- 
thias  Embryo  of  28.2  mm.  ,  seen  from  the  caudal  side;  re- 
constructed  from  the  sections.  Gen,  Genital  ridge  ;  N,  coe-  •> 

lomatic  opening  of  the  nephridium  ;  M.d.,  Mullerian  duct  ;    Or  nepnrOStome. 
W.d,  Wolffian  duct;    VG,  vena  cardinalis;    1-9,   successive 
portions  of  the  nephridium  ;  at  2  is  seen  the  glomerulus. 


ORIGIN    OF    THE    UROUEXITAL,   SYSTEM.  33? 

The  tubules  are  more  numerous  than  the  segments — hence  the  nephro- 
tomes  must  divide  in  some  way,  but  just  how  is  unknown.  The 
tubules  subsequently  become  branched,  and  each  branch  develops  a 
Malpighian  corpuscle  and  a  nephrostome;  in  the  adult  the  Wolffian 
body  or  so-called  kidney  is  studded  over  with  numerous  funnels  as 
discovered  by  Spengel,  73.3.  How  the  secondary  branches  develop 
is  still  to  be  ascertained.  In  amphibians,  then,  we  have  two  essential 
differences  from  the  elasmobranchs — namely,  the  nephrostomes  are  not 
the  retained  openings  of  the  nephrotomes,  but  new  formations,  and 
the  number  of  tubules  is  greater  than  one  for  each  segment ;  this 
increase  in  number  implies  a  very  precocious  subdivision  or  budding 
of  the  nephrotomes,  and  is  a  secondary  feature;  for  there  is,  of 
course,  only  one  nephrotome  on  each  side  in  every  segment. 

In  all  amniota  the  nephrotomes  all  become  completely  separated 
from  both  the  myotomes  and  peritoneum  throughout  the  region  of 
the  Wolffian  body,  except  that  possibly  in  a  few  anterior  segments 
the  connection  with  the  peritoneum  is  retained,  as  is  suggested  by 
Sedgwick's  observations  (Foster  and  Balfour's  "Embryology,"  191) 
and  Kolliker's  ('*  Grundriss,"  p.  413).  Referring  to  the  chick  of  the 
third  day  Balfour  thus  describes  Sedgwick's  results :  "  In  front  of 
about  the  sixteenth  segment  special  parts  of  the  intermediate  cell 
mass  remain  attached  to  the  peritoneal  epithelium  on  this  layer,  be- 
coming differentiated,  there  being  several  such  parts  to  each  segment. 
The  parts  of  the  intermediate  cell-mass  attached  to  the  peritoneal 
epithelium  becomes  converted  into  S-shaped  cords,  which  soon  unite 
with  the  Wolffian  duct  and  constitute  the  primitive  Wolffian  tubules. 
Into  the  commencement  of  each  of  these  cords  the  lumen  of  the 
body-cavity  is  for  a  short  distance  prolonged,  so  that  this  part  con- 
stitutes a  rudimentary  peritoneal  funnel  leading  from  the  body-cavity 
into  the  lumen  of  the" Wolffian  tubule."  (Foster  and  Balfour's  "  Ele- 
ments," second  edition,  101). 

The  following  account  of  the  development  of  the  Wolffian  body  in 
amniota  is  based  upon  Mihalkovics,  85.1.  The  tissue  of  the 
nephrotome  is  at  first  quite  loose  and  not  obviously  epithelial;  it 
becomes  severed  in  each  segment  from  both  myotome  and  peritoneum ; 
the  cells  assume  a  radial  arrangement  and  a  cavity  appears  in  the 
centre ;  the  cavity  enlarges  and  forms  a  vesicle  with  epithelial  walls ; 
these  vesicles  were  called  "  Urnierenblaschen  "  by  Remak,  "  Segmen- 
talblaschen"  by  M.  Braun,  77.4,  133.  In  the  lizards  (Braun,  /.  c.) 
the  number  of  vesicles  corresponds  with  the  number  of  segments,  but 
in  birds  (Mihalkovics)  the  vesicles  are  more  numerous  than  the  seg- 
ments ;  this  may  be  due  to  the  nephrotomes  separating  from  the  myo- 
tomes and  then  expanding  less  than  the  muscular  plates ;  Van  Wijhe 
has  shown,  89. 1,  that  the  number  of  Wolffian  tubules  is  apparently 
increased  in  shark  embryos  by  this  process.  The  lateral  wall  of  the 
vesicle  very  soon  comes  into  contact  with  the  Wolffian  duct,  and  the 
epithelia  of  the  two  structures  fuse  and  shortly  their  cavities  open 
through.  The  dorsal  wall  of  the  vesicle  now  sinks  in,  and  the  con- 
necting piece  toward  the  duct  lengthens  out;  the  tubule  thus  acquires 
an  S-shaped  course,  Fig.  134;  it  runs  inward  from  the  duct,  then 
downward  and  outward,  and  finally  downward  and  inward  again, 
ending  in  the  ladle-shaped  blind  end,  which  is  the  anlage  of  the 


238 


THE    EMBRYO. 


Malpighian  corpuscle ;  the  dorsal  epithelium  of  the  anlage  is  con- 
siderably thickened,  or  rather  has  retained  its  original  thickness, 
while  the  ventral  epithelial  layer  thins  out  rapidly.  It  is  important 


£+£  < 
I^M^V  «#»*"•;'..£"£ 

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J^fe^f****1 


msth 


FIG.  134.— Section  through  a  Wolfflan  Tubule  of  a  Chick  with  Primitive  Segments,  mas,  Meso- 
derm;  F,  vena  cardinalis ;  Ao,  aorta;  W.D,  Wolfflan  duct;  t,  Wolffian  tubule;  gt,  vessel  of 
glomerulus :  msth,  mesothelium. 

to  note  that  in  amniota  the  part  of  the  tubule  running  from  the 
glomerulus  to  the  nephrostome  is  never  developed  as  in  anamnia,  all 
connection  with  the  peritoneum  being  lost  very  early  indeed.  The 
further  development  proceeds  by  the  differentiation  of  the  Malpighian 
corpuscle  and  the  lengthening  and  coiling  of  the  tubule  proper.  The 

differentiation  of  the  corpuscle  takes 
place  by  the  thinning  out  of  the  epi- 
thelium of  the  ladle-shaped  blind  end 
of  the  tubule ;  the  two  layers  of  epi- 
thelium, Fig.  135,  lie  close  together, 
thus  reducing  the  cavity  of  the  cor- 
puscle to  a  narrow  fissure ;  both  layers 
are  convex  toward  the  ventral  side; 
the  concavity  of  the  upper  layer  is 
filled  with  mesenchymal  tissue,  the 
cells  of  which  are  at  first  loosely 
together,  but  soon  become  densely 
crowded ;  into  the  dense  mesenchyma 
vessels  from  the  neighboring  aorta 
penetrate  and  form  a  capillary  plexus ;  the  rounded  vascular  mass 
covered  by  epithelium  constitutes  the  glomerulus  proper,  while  the 
lower  layer  of  epithelium  forms  the  capsule  of  Bowman,  Fig.  138, 


W.D 


FIG.  135.— Wolfflan  Tubule  of  a  Sheep 
Embryo  of  9  mms.  msth,  Mesothelium ; 
v.c.  vena  carninalis;  t,  Wolfflan  tubule; 
W.D,  Wolfflan  duct;  yl,  glomerulus. 
After  G.  Mihalkovics.  X  70  diams. 


ORIGIN   OF   THE    UROGEXITAL   SYSTEM.  239 

The  tubule  lengthens  rapidly  and  is  characterized  by  a  cylinder 
epithelium  somewhat  higher  than  that  of  the  Wolffian  duct, 
W.  d;  it  retains  for  some  time  its  simple  S-shape,  although  the 
curves  of  the  S  become  more  and  more  exaggerated ;  the  Wolffian 
duct,  W.  d,  at  this  time  appears  triangular  in  cross-section ;  the  base 
of  the  triangle  is  dorsal,  being  appressed  against  the  overlying  cardi- 
nal vein.  The  growth  of  the  tubules  and  of  the  mesenchyma  around 
them  causes  a  rapid  and  increasing  protrusion  into  the  body-cavity, 
resulting  in  the  formation  of  the  Wolffian  ridge,  see  Fig.  136,  and 
Fig.  137. 

The  transverse  course  of  the  tubules,  their  dilated  medial  ends  and 
narrower  lateral  ends  opening  into  the  Wolffian  duct,  can  be  readily 
seen  in  a  fresh  embryo  dissected  so  as  to  expose  the  dorsal  wall  of 
the  abdomen.  The  appearances  thus  obtained  were  familiar  to  the 
older  embryologists  and  have  been  repeatedly  figured. 

Mihalkovics  thus  classifies  the  various  views  as  to  the  origin  of 
the  Wolffian  tubules:  A,  They  are  evagination  of  the  Wolffian 
duck,  Remak,  50.1,  p.  xxvii;  Waldeyer,  70.1,  119;  B,  They  are 
evaginations  either  as  canals  or  solid  cords  of  the  splanchnoccelic 
mesothelium  (lateral  plates  or  peritoneum),  Semper,  76.2;  Spengel, 
76.3;  Braun,  77.4  ;  A.  Kolliker,  "  Entwicklungsges. ;"  Fiirbringer, 
78. 1,  and  others;  (7,  They  arise  from  the  intermediate  cell-mass  or 
nephrotome ;  this  view  was  approached  by  many  of  the  older  writers, 
especially  in  Germany,  who  designated  as  Urnierenblastem  the 
tissue,  which  we  know  now  as  the  nephrotome.  The  correct  view 
was  first  brought  forward  in  1880  by  Adam  Sedgwick,  80.1,  2, 
who  clearh-  recognized  the  significance  of  the  intermediate  cell-mass. 

Multiplication  of  the  Wolffian  Tubules. — After  the  first 
set  of  tubules  is  developed,  secondary  additional  tubules  appear. 
The  origin  of  these  is  not  certainly  known.  Mihalkovics,  85. 1,  82, 
follows  Bornhaupt  and  Balfour  ("  Comp.  Embryology")  in  tracing 
their  formation  to  a  new  differentiation  of  the  mesoderm  of  the  Wolf- 
fian ridge ;  but  the  details  of  the  process  are  not  given  by  him,  so 
that  this  view  is  merely  an  opinion.  Fiirbringer,  78.1,  thinks  the 
secundary  tubules  are  developed,  as  he  supposes  the  first  to  be,  as 
evaginations  of  the  peritoneum,  but  the  evidence  is  drawn  from  the 
Amphibia,  and  seems  to  me  questionable  even  for  them.  Braun, 
77,4,  144,  follows  Spengel,  76.3,  in  assuming  that  the  Malpighian 
corpuscles  divide  and  that  the  division  extends  along  the  tubule, 
thus  accounting  for  the  collecting  tubules  (Sammelrohrchen)  with 
branches  each  ending  in  a  Malpighian  corpuscle.  Still  another 
method  is  suggested  by  Balfour's  observation  of  buds  growing  out 
from  the  segmental  vesicles  or  Malpighian  corpuscles,  and  this  ex- 
planation has  been  formally  adopted,  C.  K.  Hoffmann,  for  reptiles 
(Reptilien  of  Bronn's  "Thierreich,"  p.  2057) ;  Hoffmann  asserts  that 
a  bud  grows  out  from  a  primary  corpuscle  and  forms  a  blind  tube, 
which  lengthens  and  twists  until  its  blind  end  joins  the  Wolffian 
duct;  Malpighian  corpuscles  with  two  ducts  are,  he  says,  by  no 
means  uncommon ;  later  the  corpuscles  divide  and  each  tubule  then 
has  its  own  corpuscle.  I  question  the  accuracy  of  this  account. 

However  effected  it  seems  certain  that  there  is  an  increase  in  all 
amniota  of  the  number  of  tubules  opening  into  the  Wolffian  duct, 


240 


THE   EMBRYO. 


01. 

M. 


and  also  that  some  at  least  of  the  tubules  become  branching ;  it  is 
probable  that  every  tubule  ends  with  a  glomerulus.  In  the  chick 
new  corpuscles  and  tubules  appear  during  the  third  day  in  the  tenth 
to  twentieth  segments  and  usually  on  the  dorsal  side  of  the  primary 
tubules  (Mihalkovics,  85.1,  83) ;  they  have  at  first  the  form  of  seg- 
mental  vesicles  (Urnierenbldschen) ,  and  become  converted  into 
Wolffian  tubules  by  the  same  series  of  changes  as  the  primary  vesi- 
cles; the  more  advanced  stages  are  always  found  head  ward,  the 
differentiation  progressing  from  in  front  tailward  as  with  other 

organs.  Tertiary  vesicles  (and  tubules) 
arise  either  above,  below,  or  between  the 
primary  and  secondary  tubules,  and  in 
sections  (chicks  five  to  eight  days)  one 
may  see  at  once  two  or  even  three  tubules 
opening  into  the  Wolffian  duct.  Still 
further  tubules  are  formed  in  a  similar 
manner;  these  do  not  open  into  the 
Wolffian  duct,  but  into  one  of  the  three 
sets  of  tubules  already  formed.  The 
total  number  of  tubules  formed  in  each 
segment  is  at  least  five  or  six,  probably 
more,  in  a  chick  of  seven  or  eight  days 
(Mihalkovics,  85.1,  88).  In  mammals 
it  is  very  rare  to  see  more  than  one  tu- 
bule opening  into  the  Wolffian  duct  in 
one  section. 

Structure  of  the  Mature  Wolffian 
Body.* — The  Wolffian  body  reaches  its 
maximum  development  in  the  chick  of 
seven  to  eight  days,  in  rabbits  of  18-20 
mm.,  in  sheep  and  cow  embryos  of  25-30 
mm.,  and  in  human  embryos  of  the  sev- 
enth week.  The  Wolffian  bodies  occupy 
nearly  the  entire  length  of  the  Wolffian 
ridges,  but  do  not  extend  into  the  cepha- 
lic or  caudal  ends  of  the  ridges ;  they  ap- 
pear, therefore,  as  two  longitudkial  pro- 
tuberant masses  on  either  side  of  the 
mesentery ;  they  are  suspended  from  the 
dorsal  surface  of  the  body-cavity  and 
stretch  from  near  the  rudimentary  dia- 
phragm or  septum  transversum  imme- 
diately behind  the  heart  into  the  pelvic 
regi  on .  The  Wolffian  body  tapers  toward 
each  end,  and  on  its  convex  lateral  sur- 
face can  be  distinguished  the  Wolffian 
duct,  and  later  the  Miillerian  duct  also ; 

compare  Fig.  136.  The  organ  consists  of  a  number  of  relatively 
wide,  branching,  and  contorted  epithelial  tubules,  the  general  course 
of  which  is  transverse  to  the  axis  of  the  body.  One  end  of  the  tubule 

*  The  best  descriptions  of  the  Wolffian  bodies  known  to  me  are  those  by  Waldeyer,  70.1,  118, 
referring  to  the  chick,  and  Mihalkovics,  85.1,  92,  referring  to  mammals. 


l-Um. 


p.!. 


FIG.  136.—  Coste's  Embryo  of  Thirty- 
five  Days.  Ol,  Olfactory  pits;  M, 
mouth;  Per,  pericardium;  al,  anteri- 
or limb;  Ven,  stomach;  W.  b,  Wolf- 
fian body;  Um,  umbilical  cord;  Al, 
allantois-stalk ;  e,  tail;  p.  I,  poste- 
rior limb;  Om.,  omphalo-mesaraic 
veins;  In,  In',  intestine;  Lu,  lung; 
Ht,  heart.  After  Coste. 


ORIGIN   OF   THE   UROGENITAL   SYSTEM. 


241 


opens  into  the  Wolffiaii  duct,  the  other  terminates  blindly  in  a  Mal- 
pighian  corpuscle,  which  lies  toward  the  medial  and  ventral  surface 
of  the  organ,  Fig.  137.  The  tubules  are  not  of  uniform  structure; 
the  portion  which  joins  the  Wolffian  duct  has  a  low  clear-celled  epi- 
thelium, Fig.  138,  while  the  rest  of  the  tubule  is  wider  in  diameter, 
Fig.  137,  and  has  a  higher  cylinder  epithelmm  with  more  granular 
cells ;  it  is  customary  to  distinguish  the  two  parts  as  the  collecting 
and  excretory  divisions,  but  we  possess  no  certain  knowledge  as  to 
the  functions  of  the  tubular  epithelium. 

The  secretory  portion  opens  widely  into  the  lateral  side  of  the 
Malpighian  corpuscle;  its  epithelium  changes  quite  abruptly  into 
the  thin  epithelium  lining  the  cavity  of  the  corpuscle,  and  which  is 


*••••« 

s*s*  -* 

[»»t>Af*«   S9    a 

^<^/<H 
?f  00  ^>  «>„ 


\V:<L 


«:& 

\r^ 


vn 


mp.c 


FIG. 
Days. 


117.—  Transverse  Section  of  the  WolCRan  Body  or  Primitive  Kidney  of  a  Rabbit  of  Thirteen 
Coe,  Coelom;  W.  rf,  Wolffian  duct;  t  t,   Wolffian   tubules;  F,  cardi 


mesothelium  ;  mp.  c,  Malpighian  corpuscle.     X  116  diams. 


cardinal   vein;  Ao,  aorca; 


known  as  the  capsule  of  Bowman,  Fig.  138;  the  epithelium  is  of 
course  reflected  over  the  surface  of  the  glomerulus,  which  it  completely 
covers  ;  the  epithelium  of  the  glomerulus  is  thicker  than  that  of  the 
capsule,  and  is  characterized  by  crowded  spherical  nuclei,  very  gran- 
ular in  appearance.  The  glomerulus  itself,  Fig.  138,  is  very  irreg- 
ular in  shape,  being  imperfectly  divided  into  lobes  and  lobules  ;  its 
interior  is  occupied  chiefly  with  the  wide  capillaries  of  the  vascular 
network,  between  which  is  a  small  amount  of  embryonic  connective 
tissue.  In  not  quite  mature  Wolffian  bodies  the  distinction  between 
the  dorso-lateral  tubular  and  medio-  ventral  corpuscular  zone  is  very 
evident,  especially  in  transverse  sections,  Fig.  137. 

The  first  curve  of  the  S-shaped  tubule,   or  that  portion  which 
empties    into   the   Wolffian   duct,   is   probably   converted   into   the 
collecting   tubule,  the   second   curve   of  the   S   into   the  excretory 
16 


242 


THE    EMBRYO. 


tubule.  In  a  sheep  embryo  of  25  mm.,  according  to  Mihalkovics, 
85. 1,  88-89,  the  collecting  tubule  ascends  from  the  duct  on  the  lat- 
eral side  of  the  body,  then  bends  toward  the  median  line,  descends 
through  the  midst  of  the  organ,  passing  just  laterally  of  the  Mal- 
pighian  corpuscles,  and  turning  upward  again  passes  into  the  much 
contorted  convoluted  tubule,  which  after  many  turnings  opens  into 
the  Malpighian  corpuscle.  The  course  of  the  tubules  may  also  be 
studied  by  isolating  after  maceration  with  hydrochloric  acid,  as  first 
practised  by  Dursy. 

The  accompanying  figures  137  and  138,  illustrate  the  structure  and 
relations  of  the  mesonephros  in  the  rabbit  of  thirteen  days,  some- 


Ye 


FIG.  138.  —Longitudinal  Vertical  Section  of  the  Wolffian  Body  of  a  Rabbit  Embryo  of  Thirteen 
Days.  Ve,  Cardinal  vein,  with  its  endothelium;  it,  Wolffian  tubules ;  Mp,  Malpighian  glom- 
erulus;  cap,  cavity  of  Bowman's  capsule.  X  116  diams. 

what  before  the  organ  has  reached  its  highest  development.  The 
transverse  section,  Fig.  137,  shows  the  Wolffian  body  hanging  ob- 
liquely downward  into  the  body-cavity,  Coe;  it  is  close  to  the  me- 
dian aorta,  Ao;  overlying  it  is  the  cardinal  vein,  F,  and  on  its 
median  side  is  the  much  smaller  genital  ridge.  The  Wolffian 
duct,  W.  d.,  lies  about  the  middle  of  the  lateral  side,  close  to  the 
surface,  and  causes  a  slight  bulging  of  the  mesothelium,  msth,  at 
that  point.  The  lateral  zone  occupied  by  tubules  is  very  distinct 
from  that  occupied  by  the  glomeruli,  mp.c.  The  collecting  tubules 
are  readily  distinguished  by  their  thinner  epithelium  from  the  ex- 
cretory tubules.  The  longitudinal  section,  Fig.  138,  shows  that 


ORIGIN    OF    THE    UROGENITAL    SYSTEM.  ->43 

the  cardinal  vein  receives  numerous  branches  from  the  organ,  and 
illustrates  more  fully  the  structure  of  the  Malpighian  corpuscles; 
the  glomeruli  are  usually  attached  to  the  dorsal  side  of  the  capsule, 
and  the  epithelium  of  the  capsule  is  somewhat  thicker  on  the  ventral 
side.  Particularly  noteworthy  is  the  small  amount  of  connec- 
tive tissue.  The  fully  developed  Wolffian  body  of  amniota  has  the 
tubules  more  closely  crowded  together,  and  by  its  expansion  obliter- 
ates the  genital  ridge,  Fig.  137,  as  a  distinct  protuberance;  the 
Mullerian  duct  also  appears  running  parallel  with  the  Wolffian  duct; 
finally  the  shape  of  the  body  is  changed  because  the  expansion  takes 
place  chiefly  in  the  region  between  the  Wolffian  duct,  W.  d,  and  the 
cardinal  vein,  F.,  thus  causing  the  surface  along  which  the  duct 
runs  to  face  ventrally. 

Historical  Xote. — The  following  data  are  taken  from  Mihalko- 
vics,  85. 1,  93.  The  Wolffian  bodies  were  discovered  by  Casper  Fr. 
Wolff,  "  Theoria  general,"  in  1759.  They  received  their  present  name 
from  H.  Rathke,  20. 1,  in  1820,  but  Rathke  termed  the  same  organs 
in  mammals  Oken'sche  Korper.  In  1824  Jacobson  (K.  danske 
Videnskab.  Selsk.,  Kjobenhavn)  introduced  the  name  Primordial- 
niere,  and  discovered  that  in  birds  the  bodies  secreted  uric  acid.  The 
bodies  were  recognized  in  man  by  J.  Fr.  Meckel  ("  Beitr.  z.  vergl. 
Anat."  I.,  71-72)  and  Johannes  Miiller,  30.1.  The  older  writers 
held  them  to  be  either  beginnings  of  the  kidneys,  or  spermiducts,  or 
horns  of  the  uterus,  etc.  Rathke,  /.c.,  by  discovering  the  origin  of 
the  true  kidney,  led  the  way  to  true  conceptions.  The  glomeruli 
were  discovered  by  Johannes  Miiller,  30.1.  The  next  important 
advances  were  made  by  Bornhaupt,  67.1.  Semper,  75.2,  and  Bal- 
four,  78.3,  founded  our  present  morphological  notions  of  the  organs, 
and  Sedgwick,  80.1,  80.2,  Van  Wijhe,  and  others  have  elucidated 
the  genetic  relation  of  the  tubules  to  the  nephrotomes.  Mihalkovics* 
fine  monograph,  85. 1,  is  the  most  important  recent  publication. 

Resorption  of  the  Wolffian  Bodies. — The  cephalic  end  of 
the  Wolffian  body  is  retained  in  the  adult  and  enters  into  special 
relations  with  the  sexual  organs  to  be  described  later.  The  remainder 
of  the  organ  is  resorbed,  leaving  only  a  few  insignificant  remnants. 
The  resorptioii  begins  immediately  after  the  bodies  have  attained 
their  full  development,  in  the  chick  by  the  ninth  day,  in  rabbits 
of  18-20  mm.,  in  human  embryos  by  the  eighth  week;  in  man  the 
relations  are  nearly  those  of  the  adult  by  the  sixteenth  week.  In 
lizards  the  bodies  are  functionally  active,  at  least  during  the  first 
year  of  life,  so  that  as  we  ascend  the  vertebrate  scale  we  find  the 
functional  period  of  the  mesonephros  gradually  diminishing. 

The  resorption  proceeds  as  follows :  The  formation  of  new  tubules 
and  corpuscles  ceases,  the  wide  blood-vessels  become  smaller  and 
their  space  is  taken  by  interstitial  connective  tissue.  In  the  secretory 
tubules  the  cells  lose  their  characteristics,  becoming  indifferent 
cylinder  cells,  their  protoplasm  staining  deeper  than  before.  The 
canals  remain  in  this  condition  until  the  sixteenth  or  seventeenth 
day  in  the  chick  and  in  rabbit  embryos  until  they  are  3.5-4.0  cm. 
long,  when  the  epithelium  of  the  tubules  begins  to  degenerate ;  the 
tubules  shrink;  fine  fat  granules  appear  in  the  cells;  the  cell  bound- 
aries become  first  indistinct  and  are  then  lost;  the  cells  break  down 


244  THE   EMBRYO. 

to  a  fatty  detritus,  which  fills  the  tubules,  which  thus  become  solid 
cords  of  more  or  less  imperfect  cells.  Meanwhile  the  Malpighian 
corpuscles  are  also  degenerating;  their  vessels  contract  and  the 
shrunken  glomeruli  no  longer  fill  the  Bowman's  capsules;  gradually 
the  corpuscles  shrivel  up. 

The  diminution  of  the  mesonephros  is  accompanied  by  an  enlarge- 
ment of  the  sexual  ridge,  so  that  the  proportion  in  size  of  the  two  struct- 
ures is  reversed,  and  instead  of  the  sexual  anlage  forming  a  small 
strip  on  the  medial  side  of  the  much  larger  W  olffian  body,  the  body 
forms  a  diminishing  protuberance  along  the  base  of  the  enlarged 
sexual  anlage  (genital  ridge,  i.  e.  testis  or  ovary) .  The  base  of  the 
genital  ridge,  by  which  it  is  suspended  from  the  dorsal  wall  of  the 
splanchnoccele,  by  being  narrowed,  is  converted  into  the  suspensory 
membrane  (mesovarium  or  mesorchium)  of  the  genital  ridge ;  it  is 
from  the  lateral  side  of  this  membrane  (Gekrose)  that  the  contracted 
Wolffian  body  projects,  Fig.  139.  As  it  is  converted  into  connective 

Ov  nisch        W.b 


\: 


rasth 


FIG.  139.  —Section  through  the  Testis  of  a  Human  Embryo  of  Sixty-three  to  Sixty-eight  Days 
(Minot  Coll.  138).  Ov,  Testis  with  sexual  cords;  msch,  mesorchium;  W.b,  Wolfflan  body.  Msth, 
mesothelium,  represented  by  a  dark  line. 

tissue,  and  as  the  Wolffian,  W.  d.,  and  Mullerian  ducts,  M.  d.,  run- 
ning in  it  lengthwise  are  retained,  the  mass  of  the  Wolffian  body 
becomes  merely  the  wing  connecting  the  two  ducts  with  the  base  of 
the  genital  ridge;  this  wing  is  known  in  the  adult  male  as  the  liga- 
inentum  epididymis,  in  the  female  as  the  ala  vespertilionis;  the 
detailed  history  of  these  changes  is  given  in  Chapter  XXIII. 

The  Mullerian  Duct. — This  duct,  which  is  the  primitive  female 
duct,  arises  in  all  amniota  from  the  mesothelium  of  the  Wolffian 
body  close  to  the  Wolffian  duct,  see  Fig.  141,  M.D.  It  appears  rela- 
tively late :  in  chicks  about  the  fifth  day,  in  rabbits  about  the  six- 
teenth or  somewhat  before  the  Wolffian  body  is  mature;  it  is  devel- 
oped throughout  its  entire  length  as  soon  as  the  Wolffian  body 
attains  its  full  size.  Mihalkovics,  85.1,  285,  has  described  its 
relations  at  this  stage  for  pig  embryos,  5-6  cm.  long;  on  the 


ORKJEX    OF    THE    URO<TKNITAL    S  VST  KM.  X'45 

lateral  side  of  the  Wolffian  body  there  is  a  longitudinal  whitish 
band,  due  to  a  thickening  of  the  mesothelium ;  this  band  is  called 
the  Tiibenfalte  by  M.  Braun,  the  Tubenleiste  by  Mihalkovics,  be- 
cause it  is  along  this  band  that  the  Miillerian  duct  is  differentiated 
and  from  a  part  of  that  duct  the  tuba  Fallopii  is  developed.  The 
epithelial  band  stretches  on  to  the  rudimentary  diaphragm  (septum 
trans versum) ;  this  part  of  the  band  has  been  described  by  Kolliker 
("Entwickelungsges.,"  959)  astheZwerchfellbandder  Urniere;  the 
band  also  stretches  caudally  along  the  Wolffian  ridge  beyond  the 
Wolffian  body  proper.  The  "  Tubenleiste"  quickly  becomes  more 
prominent,  and  its  interior  is  filled  with  connective  tissue  (mesen- 
chyma)  in  the  midst  of  which  runs  the  Miillerian  duct  close  to  the 
Wolffian  duct.  The  Miillerian  duct,  Fig.  141,  M.D.,  is  merely  a  small 
tube  of  epithelium ;  if  we  follow  it  forward  it  is  seen  to  bend  down, 
join  the  mesothelium  and  open  into  the  splanchnocoele  close  to  the 
anterior  end  of  the  genital  ridge  and  to  the  septum  transversum ;  this 
ostiuni  abdoni  in<il<>  becomes  the  fimbriate opening  (Morsus  diaboli) , 
of  the  Fallopian  tube ;  it  is  sometimes  called  the  Miillerian  funnel. 

That  the  Miillerian  duct  arises  from  the  mesothelium  of  the 
Wolffian  body  was  first  maintained  in  18G5  by  Dursy,  65.1,  and 
confirmed  by  W.  Waldeyer,  70.1,  124-126,  who  conceived  that  in 
the  chick  a  band  of  epithelium  is  invaginated;  Bornhaupt,  67. 1,  57, 
had,  however,  previously  asserted  that  it  is  only  at  the  anterior  end 
that  the  mesothelium  is  invaginated,  and  that  the  duct  grows  back- 
ward, and  this  view  has  been  generally  adopted,  see  Gasser,  74.1, 
Sernoff,  74. 1,  Fiirbringer,  78. 1,  Braun,  77.4,  Kolliker,  "  Entwicke- 
lungsges.," 978,  Mihalkovics,  85.1,  etc.  Sedgwick  and  Balfour  (see 
Foster  and  Balfour,  "Embryology,"  215)  have  modified  this  view  by 
asserting  that  there  are  three  successive  involutions  of  the  Wolffian 
mesothelium ;  but  it  is  probable  that  these  are  accidental  variations 
owing  to  the  irregular  thickening  of  the  mesothelium. 

The  development  of  the  "  Tubenleiste"  precedes  the  appearance  of 
the  Miillerian  duct;  it  is  produced  by  a  gradual  thickening  of  the 
mesothelium  along  a  narrow  band  running  lengthwise  of  the  Wolffian 
body ;  in  lizards  this  band  is  on  the  ventral  side  of  the  bod}* ;  in 
birds  and  mammals  on  the  lateral  side ;  in  the  latter  the  "  Leiste" 
becomes  more  prominent  by  the  increase  of  connective  tissue  in  it. 
After  the  duct  is  developed  the  epithelium  of  the  "  Tubenleiste"  again 
flattens  out.  Nothing  is  known  as  to  the  morphological  significance 
of  this  peculiar  mesothelial  structure. 

The  actual  development  of  the  Miillerian  duct  in  amniota  may  be 
described  as  follows :  The  mesothelium  on  the  ventral  (reptilia)  or 
lateral  (birds  and  mammals)  surface  of  the  Wolffian  body  is  thick- 
ened to  form  the  Tubetifnltc.  A  triangular  area  at  the  cephalic  end 
of  the  thickened  longitudinal  band  is  invaginated ;  when  the  invag- 
ination  is  complete  we  find  an  oblique  funnel  widely  open  to  the 
splanchnocoele,  and  with  its  apex  lying  inside  the  mesothe- 
lium ;  the  connection  of  the  Miillerian  funnel  with  the  mesothelium 
can  be  readily  seen  in  sections.  In  birds  the  invagination  is 
somewhat  irregular,  so  that  there  may  be  more  or  less  marked 
modifications  of  from  one  to  three  or  even  four  invaginations ;  no 
special  significance  attaches  to  this  peculiarity,  although  Balfour  has 


246  THE    EMBRYO. 

sought  to  homologize  the  anterior  end  of  the  Mullerian  duct  in  am- 
niota  with  the  pronephros;  but  this  homology  is  untenable  in  my 
judgment.  The  point  of  the  Mullerian  funnel  is  closed  and  tapering ; 
it  grows  rapidly  backward,  elongating  as  a  solid  cord,  which  becomes 
a  canal  by  the  gradual  backward  extension  of  the  lumen  of  the  fun- 
nel into  the  cord ;  in  its  growth  the  cord  follows  along  underneath 
the  thick  mesothelium  of  the  "Tubenleiste,"  and  on  the  ventral  side 
of  the  Wolffian  duct.  It  continues  (probably  solely  by  the  prolifera- 
tion of  its  own  cells)  its  growth  backward  beyond  the  Wolffian  duct 
through  the  caudal  extremity  of  the  Wolffian  ridge  to  the  cloaca, 
with  the  entodermal  lining  of  which  it  ultimately  fuses,  so  that  the 
completed  duct  opens  into  the  cloaca.  While  it  is  developing,  the 
duct  continues  to  enlarge  and  is  therefore  for  a  time  wider  headward 
than  tailward.  In  front  it  appears  in  cross-sections  as  an  epithelial 
ring  with  a  considerable  lumen ;  the  further  back  we  go  the  smaller 
the  cross-section  becomes.  The  solid  growing  point  is  found  in  close 
contact  with  the  epithelium  of  the  Wolffian  duct ;  this  fact  has  led 
Balfour  and  Sedgwick,  79.1,  to  maintain  that  the  Mullerian  duct 
grows  by  cells  added  to  its  end  from  the  Wolffian  duct,  but  Mihal- 
kovics,  85.1,  298-299,  has  shown  that  this  assumption  is  erroneous. 
As  to  the  time  when  the  duct  appears — the  Mullerian  funnel  is 
developed  in  lizard  embryos  of  14-16  mm.,  or  eighteen  to  nineteen 
days  after  the  eggs  are  laid  (M.  Braun,  77.4,  182),  in  snake  em- 
bryos of  15-18  mm.  (Mihalkovics,  85. 1,  290),  in  ducks  the  fifth  day, 
in  chicks  the  end  of  the  fourth  day. 

In  elasmobranchs  the  Mullerian  and  Wolffian  ducts  are  united  in 
one,  as  first  shown  by  Semper,  75.2,  and  consequently  the  former 
appears  to  be  split  off  from  the  ventral  side  of  the  Wolffian  or  seg- 
mental  duct.  Semper 's  observations  have  since  been  amply  confirmed 
by  several  observers,  Balfour,  Van  Wijhe,  Riickert,  and  others. 
Spengel,  76.3,  13,  has  asserted  that  the  duct  arises  in  the  same  way 
in  certain  amphibians.  Since  this  discovery  there  has  been  a  strong 
tendency  to  accept  the  theory  first  advanced  by  Gegenbaur  in  his 
"  Handbuch  der  vergleichenden  Anatomie,"  that  there  was  primi- 
tively a  single  urogenital  duct,  which  split  into  two.  This  theory  is 
open  to  obvious  objections ;  the  facts  upon  which  it  rests  are  derived 
chiefly  from  the  embryology  of  elasmobranchs,  a  type  far  removed 
from  the  direct  line  of  vertebrate  evolution,  and  presenting  many 
secondary  modifications;  the  origin  of  the  Mullerian  duct  in  elas- 
mobranchs has  not  been  shown  to  agree  with  that  in  any  other  type, 
and  is  known  to  differ  from  it  essentially  in  the  only  type  in  which 
the  development  of  the  Mullerian  duct  has  been  accurately  worked 
out ;  and  finally,  even  in  elasmobranchs  the  Mullerian  funnel  arises 
from  the  splanchnocoelic  mesothelium.  Now  as  we  see  that  in  all 
vertebrates  the  Mullerian  duct  lies  close  to  the  Wolffian,  and  as  the 
former  is  known  to  arise  in  part  or  wholly  from  the  mesothelium, 
while  the  latter  arises  from  the  ectoderm,  we  must,  I  think,  assume 
that  the  two  ducts  were  primitively  distinct  and  that  their  temporary 
union  in  elasmobranch  embryos  is  a  secondary  modification,  which 
recurs,  perhaps,  in  no  other  vertebrate.  The  view  here  advocated 
has  been  suggested  by  Jungersen,  89.1,  196,  a  pupil  of  Semper's, 
and  is  favored  by  R.  Wiedersheim,  90.3,  343. 


,  ORIGIN    OP    THE    UROGENITAL   SYSTEM.  247 

The  Genital  Fold. — The  genital  fold  is  a  small  longitudinal 
ridge  which  appears  on  the  dorsal  wall  of  the  splanchnocoele  of  the 
embryo,  where  it  is  situated  between  the  Wolffian  body  and  the  base 
of  the  mesentery,  Fig.  140,  Gen.  In  the  elasmobranchs  there  is  an 
early  development  of  connective  tissue  in  the  ridge,  which  causes  it 
to  project  considerably ;  in  the  primitive  vertebrates  this  is  not  the 
case  so  far  as  we  can  judge  from  the  development  in  Petromyzon 


.•      .- 


FIG.  140.— Transverse  Section  through  an  Advanced  Embryo  of  a  Shark,  Scymnus  lichia;  from 
the  abdominal  region  (the  dots  represent  nuclei).  Sp,  Spinal  process  of  the  vertebra;  Ar, 
arachnoid  space ;  Md,  medulla  or  spinal  cord ;  n.  a,  neural  arches  of  the  vertebra ;  s,  inner 
sheath  of  the  notochord;  s',  outer  sheath  of  the  notochord;  C7i,  notochord;  t.  p,  transverse 
process  of  the  vertebra ;  v.  car,  cardinal  vein ;  Ao,  dorsal  aorta ;  mes,  mesentery ;  Gen,  genital 
fold:  W.  d,  Wolffian  duct;  W,  Wolffian  bodies  with  tubules;  c,  young  cartilage ;  Jfsc,  develop- 
ing muscles. 

and  amphibians,  but  the  ridge  is  produced  chiefly  by  a  thickening 
of  the  peritoneal  epithelium ;  this  thickened  band  of  mesothelium 
was  first  shown  by  Waldeyer,  70.1,  to  give  rise  to  the  egg-cells, 
and  has  since  been  shown  to  share  in  the  development  of  the  sper- 
matozoa, hence  it  is  called  the  germinal  epithelium  (Keimepithel) , 
and  must  be  regarded  as  the  primitive  and  essential  part  of  the  gen- 
ital glands.  In  amniota  the  "Wolffian  body  enlarges  so  early  and 


248 


THE    EMBRYO. 


M.D, 


G.ep. 


mes. 


rapidly  that  it  carries  the  genital  fold  along  with  itself,  so  that  the 
latter  becomes  merely  a  band  on  the  medial  side  of  the  Wolffian 
body,  Fig.  141.  But  in  all  anmiota  the  genital  fold  is  first  clearly 

marked  out  by  the  differen- 

'  tiation   of   its  mesothelium 

from  that  of  the  peritoneum 
proper.  In  reptiles  this  dif- 
ferentiation takes  place  even 
before  the  Wolffian  tubules 
have  united  with  the  Wolf- 
fian duct,  but  in  mammals 
it  is  deferred  until  the  Wolf- 
fian body  is  quite  large; 
hence  in  the  mammals  the 
genital  ridge  seems  to  be 
derived  from  the  Wolffian 
body ;  but  this  must  be  re- 
garded as  a  secondary  mod- 
ification. 

The  genital  ridge  extends 
nearly  or  quite  the  entire 
length  of  the  abdominal 
cavity;  its  cephalic  end  is 
probably  the  anlage  of  the 
glomus  of  the  pronephros; 
its  caudal  end  remains  in- 

FIG.  141.— Section  of  the  Urogenital  Fold  of  a  Chick  different,     forming    the    SO- 

Embryo   of  the  Fourth  Day.      W D      Wolffian   duct-  n     i           i 

M.D.<  Muller's  duct;  Coe,  coBlom;  TT,  Wolfflan  tubule1  Called    gubemaculum ;     the 

<?Z,  glomerulus;  G.ep,  genital  epithelium;  Of,  primitive  rpQt  nf  thp  rirlo-p    fhoi  ie    ifa 

ova;  mes,  mesentery.    After  W.  Waldeyer.  iage)  tnat  !S,  ItS 

middle  region,  which  occu- 
pies the  greater  part  of  its  extent,  is  the  anlage  proper  of  the  genital 
gland. 

We  can  distinguish  four  stages  in  the  development  of  the  gen- 
ital ridge:  1,  the  production  of  mesenchyma  from  the  mesothelium; 
2,  the  development  of  the  medullary  cords ;  3,  the  appearance  of  the 
true  primitive  ova;  4,  differentiation  of  the  sexual  glands.  Let  us 
study  these  stages  in  order. 

1.  Production  of  the  Genital  Mesenchyma. — The  genital 
ridge  is  a  product  of  that  portion  of  the  splanchnoccelic  mesothelium 
which  lies  between  the  nephrotome  or  Wolffian  tubule,  and  the  base 
of  the  future  embryo ;  in  very  young  amniote  embryos,  Fig.  161,  it 
is  that  part  of  the  mesothelium  nearest  the  dorsal  aorta.  This  ger- 
minal epithelium  very  early  begins  throwing  off  mesenchymal  cells 
from  its  inner  surface;  the  process  has  yet  to  be  studied  carefully;  I 
find  that  it  is  going  on  both  in  the  germinal  epithelium  and  in  the 
neighboring  mesothelium  in  chicks  at  the  time  when  the  Wolffian 
tubules  have  just  joined  the  Wolffian  duct,  as  can  be  clearly  seen  in 
Fig.  134;  the  mesenchyma,  mes,  extends  through  the  genital  region 
and  passes  without  demarcation  into  the  mesothelium,  msth.  The 
continued  production  of  mesenchyma  has  been  specially  emphasized 
for  all  classes  of  amniota  by  Mihalkovics,  and  that  the  germinal 
epithelium  contributes  to  the  ovarian  stroma  (in  other  words,  forms 


OKIGIX    OF    THE    URO(*KXITA  L    SVSTKM.  249 

mesenchyma),  was  discovered  as  long  ago  as  ISO:)  by  Borsenkow 
(Wiirzburger  Naturwiss.  Zeitschr.,  IV.),  and  has  been  maintained 
by  Egli,  76.1,  Balfour,  78.2,  K.  Schulin,  81.1,  and  others.  The 
significance  of  the  fact,  which  was  obscure  hitherto,  is  clear  enough 
with  our  present  knowledge  of  the  genesis  of  the  mesenchyma. 

The  mesenchymal  proliferation  of  the  genital  mesothelium  is  ac- 
companied also  by  the  appearance  of  enlarged  clear  cells  with 
enlarged  clear,  distinctly  nucleolated  nuclei.  These  cells  from  their 
appearance  have  been  termed  by  most  writers  primordial  ova  ( Ureier) 
from  their  assumed  identity  with  the  primordial  ova  of  later  stages. 
Mihalkovics,  85.1,  has  pointed  out  that  they  entirely  disappear  be- 
fore the  development  of  the  ova  begins,  and  that  they  cannot  be 
identified  with  a  kind  of  cells  which  arise  much  later ;  he  proposes 
to  call  them  sexual  cells  (Geschlechtszellen) ,  but  the  name  is  not 
felicitous;  before  giving  them  any  special  name  it  will  be  well  to 
learn  more  about  them.  I  think  they  may  have  to  do  with  the 
growth  of  the  layer,  for  such  enlargement  occurs  sometimes  when  a 
simple  tissue  is  growing  rapidly.  The  cells  in  question  have  been 
seen  in  nearly  all  classes  of  vertebrates ;  for  sharks  see  C.  Rabl,  89.2, 
Taf.  X.,  Figs.  7  and  8,  Van  Wijhe,  89.1,  and  Riickert,  88.1;  for 
Amphibia  see  Goette,  75.1;  for  reptiles  see  M.  Braun,  77.4,  145; 
for  birds  and  mammals  see  Mihalkovics,  85. 1. 

2.  Development  of  the  Medullary  Cords. — There  appears 
very  early  in  the  mesenchyma  of  the  genital  fold  peculiar  cords  of 
closely  compacted  cells  which  stretch  up  above  the  level  of  the  fold 
alongside  the  cardinal  vein.  The  sexual  cords  are  destined  to  gen- 
erate the  ova  or  spermatozoa  according  to  the  sex  of  the  embryo. 
Our  knowledge  of  the  cords  is  still  unsatisfactory  and  confused.  So 
far  as  I  know  the  homologies  of  these  cords  have  not  been  thoroughly 
studied  in  any  of  the  anamnia,  hence  the  following  account  is  based 
on  the  study  of  aniniota.  They  were  first  distinctly  recognized  by 
Bornhaupt,  67.1;  they  were  carefully  studied  by  M.  Braun,  77.4, 
in  reptiles,  who  terms  them  Segment  ctlstriinge  the  name  medullary 
cords  (Markstrdnge)  was  proposed  by  Kolliker  ("  Entw.-Ges.,"  970) ; 
Schmiegelow,  82. 1,  describes  them  in  birds;  in  mammals  they  have 
been  studied  by  Balfour,  78.2,  Ed.  Van  Beneden,  80.2,  M.  Nuss- 
baum,  80. 1,  and  especially  by  Harz,  83. 1,  whose  paper  is  excellent. 
Mihalkovics,  85.1,  gives  a  monographic  treatment  of  the  cords, 
calling  them  Sexualstrttngc.  W.  Nagel,  89.3,  323,  has  shown  that 
the  true  sexual  cords  are  concerned  in  the  production  of  the  ova  and 
spermatozoa,  and  must  be  carefully  distinguished  from  the  remains 
of  the  Wolffian  tubules,  which  can  be  observed,  especially  in  later 
stages,  in  the  genital  ridge. 

The  cords  appear  in  Lacerta  embryos  of  12-14  mm.,  in  chicks  of 
the  sixth  day,  in  sheep  of  10-12  mm.,  and  have  been  seen  in  human 
embryos  of  15  mm.,  or  about  five  weeks.  In  the  lizard  they  are 
very  large,  and  appear  one  in  each  segment  as  first  shown  by  M. 
Braun,  who  accordingly  has  named  them  segmental  cords;  each 
cord  nearly  fills  the  interior  of  the  genital  fold,  then  stretches  up- 
ward close  to  a  Malpighian  corpuscle  of  the  Wolffian  body  and  rises 
above  to  the  level  of  the  cardinal  vein.  Braun  held  that  the  cord 
arises  from  the  epithelium  of  the  Malpighian  corpuscle,  but  Mihal- 


250  THE    EMBRYO. 

kovics  maintains  that  the  two  structures  are  always  separated.  In 
the  chick  the  cords  can  be  seen  the  sixth  day,  but  they  are  smaller 
and  more  numerous  than  in  the  lizard;  at  first  their  borders  are 
vague  and  they  have  no  definite  demarcation  from  the  stroma  of 
the  ovary.  For  a  description  of  these  cords  in  the  male  chick  of 
seven  days  see  E.  Schmiegelow,  82.1,  161;  he  terms  them  "An- 
lagen  der  Samencanalchen."  In  the  sheep  (10-11  mm.)  the  cords 
first  appear  without  demarcation  from  the  ovarian  mesenchyma;  in 
embryos  of  the  sheep  and  rabbit  of  12-14  mm.  the  cords  are  more 
distinct.  Of  the  appearance  of  the  fully  formed  mammalian 
medullary  cords  at  the  stage  preceding  the  appearance  of  the 
primitive  ova  I  know  no  exact  description.  They  are  three  or  four 
cells  wide,  with  distinct  outlines ;  are  twisted  and  branching ;  stain 
somewhat  deeper  than  the  stroma  proper  of  the  genital  ridge,  of 
which  they  occupy  a  large  part. 

3.  Appearance  of  the  Primitive  Ova. — Since  Semper 's  re- 
searches, 75.2,  it  has  come  to  be  more  and  more  generally  admitted 
that  the  development  of  the  genital  glands  leads  in  both  sexes  through 
an  early  stage  characterized  by  the  appearance  of  primitive  ova 
(Ureier,  Primordialeier,  ovoblast).  The  primitive  ova  are  merely 
enlarged  cells  of  the  germinal  epithelium  (or  of  the  so-called  medul- 
lary cords).  They  are  readily  recognized  by  being  more  trans- 
parent than  the  remaining  cells  and  by  having  a  larger  and  more 
transparent  nucleus,  which  a  little  later  has  a  well-marked  nucleolus. 
These  cells  have  long  been  known  in  the  ovary,  where  they  can  be  read- 
ily followed  along  in  their  development  into  egg-cells  and  mature  ova, 
but  in  the  male  their  history  has  still  to  be  worked  out  satisfactorily. 
Most  authors  have  assumed  that  in  the  higher  amniota  the  primitive 
ova  break  up  in  the  male  and  disappear  very  early,  without  wander- 
ing from  their  first  lair,  but  Semper,  75.2,  found  that  they  are 
included  in  the  embryonic  seminiferous  tubules  of  the  sharks;  in 
amniota  no  such  process  has  been  observed,  but  Mihalkovics  has 
found  in  human  embryos  of  14-16  cm.,  that  the  tubules  contain 
cells  resembling  primitive  ova,  but  in  earlier  stages  there  are  no 
such  cells.  In  fact  we  must  admit  that  the  history  of  the  male 
primitive  ova  is  practically  unknown,  consequently  I  give  now 
merely  the  description  of  the  primitive  ova  as  they  appear  in  the  ger- 
minal epithelium,  deferring  the  further  history  of  the  genital  glands 
to  Chapter  XXIII. 

The  primitive  ova  are  modified  cells  of  the  germinal  epithelium. 
In  elasmobranchs  the  epithelium  is  at  first  a  layer  of  cuboidal  cells 
of  uniform  character,  with  small  darkly  stained  nuclei ;  as  soon  as 
the  genital  ridge  begins  to  protrude  (Acanthias  of  19  mm.),  the 
epithelium  becomes  thicker  and  here  and  there  single  ones  of  its 
cells  become  much  larger,  and  their  nuclei  become  much  larger, 
spherical,  granular,  and  stain  more  lightly;  the  thickening  of  the 
epithelium  continues  as  the  fold  becomes  more  prominent,  but  is 
confined  to  the  medial  side  of  the  gland  for  a  certain  period;  the 
thickening  is  accompanied  by  an  increase  in  the  number  of  clear 
cells,  which  tend  to  lie  in  groups  of  three  or  four,  see  Semper,  75.2, 
335-345,  and  Balfour,  " Comp.  Embryol."  I.,  55-58.  In  reptiles, 
Braun,  77.4,  145,  the  history  is  essentially  the  same,  except  that 


ORIGIN    OF   THE   UROGENITAL   SYSTEM.  251 

the  genital  fold  contains  little  mesenchyma,  and  therefore  the  epi- 
thelium (Lacerta  embryos  12-14  mm.)  constitutes  the  greater  part 
of  the  fold;  the  primitive  ova  resembles  those  of  the  elasmo- 
branchs  and  are  similarly  imbedded  in  unmodified  mesothelial 
cells.  So  far  as  known  to  me,  there  are  no  satisfactory  descriptions 
or  figures  of  the  germinal  epithelium  with  primitive  ova  at  the  in- 
different stage  of  the  genital  fold  of  any  bird  or  mammal  whatsoever; 
it  is  to  be  hoped  that  this  omission  will  soon  be  made  good.  There 
are  brief  references  to  the  indifferent  stage  of  the  higher  vertebrates 
in  all  the  current  text-books. 

4.  Differentiation  of  the  Sexual  Glands. — The  conversion 
of  the  fully  developed  genital  fold  into  the  sexual  glands  is  described 
in  Chapter  XXIII. 

Evolution  of  the  TJrogenital  System. — The  embryology  of 
vertebrates  renders  it  evident  that  we  have,  as  stated  in  the  intro- 
duction of  this  chapter,  four  parts  as  the  primitive  constituents  of 
the  urogenital  system  on  each  side:  1,  the  genital  ridge;  2,  the 
Wolffian  ridge;  3,  the  Wolffian  duct;  4,  the  Miillerian  duct.  A 
few  words  in  regard  to  the  probable  evolution  of  each  of  these  is  in 
place. 

1.  The  genital  ridge  is  marked  out  essentially  as  a  specialized 
area  of  the  splanchnocoelic  mesothelium,  in  which  the  primordial 
ova  are  developed.     The  protuberance  of  the  ridge  is  increased  by 
the  stroma  or  mesenchyma  developed  below  the  mesothelium,  but 
this  is  presumably  a  secondary  modification.     We  may,  therefore, 
regard  the  genital  ridge  as  having  been  primarily  a  mesothelial  band 
on  the  dorsal  side  of  the  body-cavity,  between  the  root  of  the  mesen- 
tery and  the  opening   (nephrostome)   of  the  nephrotome  into  the 
splanchnocoele.     This  brings  us  not  far  from  the  condition  in  am- 
phioxus  and  annelids,  for  in  both  types  the  genital  products  arise 
from  the  mesothelium. 

2.  The  Wolffian  ridge  is  the  protuberance  produced  by  the  in- 
crease in  size  and  number  of  the  Wolffian  tubules,  and  its  inclusion 
of  the  genital  ridge  is  a  secondary  result  of  its  enlargement  in  the 
higher  vertebrata.     Moreover,  it  does  not  at  first  protrude  at  all,  so 
that  we  have  to  consider  not  the  existence  of  an  actual  ridge  but  the 
presence  of  Wolffian  tubules  as  the  essential  feature.     The  tubules,  as 
we  have  seen,  are  developed  from  the  nephrotomes,  and  when  the 
nephrotomes  have  a  distinct  cavity  that  cavity  is  preserved  to  make 
the  lumen  of  the  tubule.     In  all  true  vertebrates,  however,  the  ttfbule 
or  nephrotome  has  lost  its  connection  with  the  myotome,  but  retains 
(in  the  anamiiia)  the  opening  (nephrostome)  into  the  splanchnoccele. 
The  conection  with  the  Wolffian  duct  is  secondary,  and  the  manner 
in  which  it  was  acquired  cannot  be  satisfactorily  accounted  for  at 
present. 

Out  of  the  Wolffian  ridge  there  arise  three  primary  organs,  the 
pronephros  (head  kidney),  the  Wolffian  body  (anamniote  or  primi- 
tive kidney),  and  the  true  kidney  of  the  amniota  (metanephros).  Of 
these  the  last  is  not  a  primitive  vertebrate  organ,  since  it  is  found 
only  in  the  higher  forms ;  its  development  as  an  outgrowth  of  the 
Wolffian  duct  is  described  in  Chapter  XXIII. ;  nothing  is  known  as 
to  its  probable  evolution.  As  regards  the  pronephros  and  Wolffian 


252  THE   EMBRYO. 

body  or  mesonephros  the  first  question  is  whether  they  are  parts  of 
the  same  series  of  primitive  organs  or  distinct  organs.  That  they 
are  separate  organs  has  been  maintained  by  several  recent  writers, 
notably  by  Riickert,  88. 1,  and  Van  Wijhe,  89. 1.  The  former  lays 
stress  upon  the  lateral  outgrowths  of  the  nephrotomes  of  the  prone- 
phros,  but  this  is  of  little  importance,  especially  as  the  nephrotomes 
of  the  mesonephros  expand  (as  in  amniota)  and  their  side  walls  fuse 
with  the  proiiephric  duct;  hence  I  cannot  admit  that  there  is  any 
essential  difference.  Van  Wijhe,  /.  c.,  505-506,  states:  1,  that  the 
pronephros  develops  earlier  than  the  mesonephros,  but  in  all  organs 
the  cephalic  portions  are  more  advanced  in  the  embryo  than  the 
caudal,  and  we  cannot  on  this  ground  any  more  divide  the  series 
of  nephrotomes  than  we  can  the  series  of  nerves  or  myotomes :  2, 
the  pronephric  and  mesonephric  tubules  differ  slightly  in  origin,  but 
he  overlooks  that  they  are  both  derived  from  the  nephrotomes;  3, 
the  pronephric  duct  unites  with  the  pronephric  tubules  as  soon  as  it 
appears,  but  the  union  with  the  Wolffian  tubules  is  retarded ;  this 
point  is  insignificant,  for  variations  in  the  time  of  development  of 
organs  are  specially  characteristic  of  embryos,  and,  moreover,  Mihal- 
kovics  has  made  it  probable  that  there  is  a  gradual  transition ;  4, 
the  presence  of  the  glomus  is  characteristic  of  the  pronephros,  but 
since  the  glomus  is  apparently  only  the  anterior  end  of  the  genital 
ridge  and  as  the  genital  ridge  stands  in  the  same  relation  as  the 
glomus  to  the  nephrostomes  this  difference  can  be  assigned  a  second- 
ary value  only.  It  appears  then  that  none  of  the  arguments  in  favor 
of  an  essential  distinction  have  force.  On  the  other  hand,  the  facts 
—that  all  the  tubules  are  developed  from  the  nephrotomes,  that  the 
nephrotomes  in  all  cases  unite  by  their  lateral  walls  with  the  prone- 
phric duct,  that  pronephric  and  Wolffian  tubules  never  both  occur 
in  the  same  segment,  that  there  is  in  some  and  probably  in  all  cases 
a  gradual  transition  from  the  pronephric  to  the  Wolffian  body- 
seem  to  me  ample  to  establish  the  conclusion  that  the  two  organs 
are  parts  of  a  single  series.  The  pronephros  is  distinguished  princi- 
pally by  the  glomus  or  vascularization  of  the  genital  fold  in  its 
neighborhood,  and  it  becomes  separated  from  the  Wolffian  body 
proper  by  the  abortion  of  several  tubules  between  the  pronephros  and 
mesonephros.  Why  this  separation  occurred  we  do  not  know. 

We  have,  if  the  view  just  defended  be  correct,  to  consider  that 
the  excretory  organs  were  primitively  a  uniform  series  of  nephro- 
tomic  tubules  (Semper's  segmental  organs),  each  beginning  with  an 
opening  (nephrostome)  into  the  splanchnoccele  without  connection 
with  the  myotomes,  but  opening  laterally  into  the  Wolffian  duct. 
Semper  has  sought  to  homologize  these  tubules  with  the  segmental 
organs  of  annelids;  the  latter  are  excretory  tubules  opening  into  the 
coelom,  one  on  each  side  for  each  segment,  and  emptying  externally 
through  the  ectoderm.  Now  if  we  assume  that  the  line  of  these 
external  openings  became  a  groove  and  then  a  canal,  we  may  pass  at 
once  from  the  annelidan  to  the  vertebrate  type,  as  suggested  by 
Haddoii,  85. 1,  and  "  Practical  Embryol.,"  250,  and  at  the  same  time 
account  for  the  ectodermal  origin  of  the  Wolffian  duct  as  a  survival 
of  a  stage  intermediate  between  the  annelidan  and  vertebrate  types. 
The  Semper-Haddon  hypothesis  has  much  in  its  favor,  and  I  should 


ORIGIN    OF   THE    UROGENITAL   SYSTEM.  253 

be  strongly  inclined  to  adopt  it,  were  it  not  for  the  recent  investiga- 
tions of  Boveri,  90.1,  which  suggest  that  quite  a  different  line  of 
homologies,  to  be  traced  through  Amphioxus,  may  be  the  true  one. 

3.  The  Wolffian  duct,  as  we  have  seen,  is  probably  of  ectodermal 
origin.     As  indicated  at  the  close  of  the  preceding  paragraph,  there 
are  two  hypotheses  to  account  for  this  assumed   origin,  namely, 
Haddon's,  85.1,  and  Boveri's,  90.1.     Haddon,  accepting  Semper's 
hypothesis  that  the  tubules  are  homologous  with  annelidaii  segmental 
organs,  assumes  that  they  opened  exteriorly  and  that  the  line  along 
which  they  opened  became  a  groove  and  then  a  canal.     Th.  Boveri 
asumes  that  the  genital  chamber  of  Amphioxus  corresponds  to  part 
of  the  nephrotome,  and  that  ducts,  which  lead  in  the  branchial  region 
of  Amphioxus  according  to  Boveri's  discovery  from  the  body  cavity 
into  the  peribranchial  chamber,  represent  the  excretory  portion  proper 
of  the  nephrotome ;  thus  he  is  led  to  homologize  the  peribranchial 
chamber  with  the  Wolffian  duct  in  that  both  are  of  ectodermal  origin 
and  both  receive  the  nephrotomic  tubules ;  he,  however,  regards  the 
duct  as  representing  merely  a  specialized  part  of  the  amphioxan 
peribranchial  chamber.     Boveri's  brilliant  comparisons  are  certainly 
very  seductive,  but  his  observations  on  Amphioxus  do  not  suffice  as 
yet  to  carry  his  conclusion  above  the  hypothetical  stage.     There  is 
one  difficulty  which  he  has  overlooked,  namely,  that  by  his  hypothe- 
sis the  sexual  gland  is  further  from  the  splanchnoccele  and  nearer 
the  myotome  than  is  the  excretory  tubule,  whereas  in  true  vertebrates 
the  relations  are  the  reverse  of  this,  so  that  the  sexual  glands  in  the 
two  types  would  not  be  homologous,  if   Boveri's  hypothesis  were 
correct. 

4.  The  Miillerian  duct,  as  stated  in  the  section  on  its  develop- 
ment, is  in  my  opinion  probably  not  derived  from  the  Wolffian  duct 
as  maintained  by  Gegenbaur,  Semper,  and  their  followers,  but  is  a 
new  organ  developed  from  the  mesothelium  within  the  vertebrate 
type.     Its  evolution,  however,  is,  properly  speaking,  merely  a  mat- 
ter of  speculation  at  present. 


CHAPTER   XII. 
EARLY   DEVELOPMENT  OF  THE  ARCHENTERON. 

THE  archenteron  is,  strictly  speaking,  the  entire  cavity  lined  by 
the  entoderm,  a  fact  which  conies  out  very  clearly  in  the  primitive 
type  of  vertebrate  development  as  preserved  in  Petromyzon,  ganoids, 
and  amphibians.  In  these  forms  the  yolk  is  the  ventral  floor  of  the 
archenteron  and  consists  of  a  thick  mass  of  cells,  all  charged  with 
yolk  granules  (deutoplasm) ,  see  Fig.  70,  Vi.  In  all  amniota,  on 
the  contrary,  the  first  step  is  the  division  of  the  primitive  archen- 
teron into  the  primitive  alimentary  canal  of  the  embryo  and  the 
yolk-sac.  Before  this  occurs  the  mesoderm  has  appeared  and  the 
coelom  is  present  in  it,  so  that  splanchnic  mesoderm  is  already 
differentiated  and  united  with  the  entoderm  to  form  the  splanchno- 
pleure ;  henceforth  the  two  germ-layers  develop  in  close  correlation 
with  one  another,  and  the  history  of  the  archenteron  becomes  the 
history  of  the  splanchnopleure,  except  that  in  the  cervical  region  the 
entodermal  canal  is  developed  without  the  coelom  appearing. 

We  have  to  consider  the  following  stages:  1,  Separation  of  the 
archenteron  proper  from  the  yolk-sac ;  2,  origin  of  the  allantois ;  3, 
origin  of  the  pharynx. 

Entodermal  Cells. — In  the  primitive  type  of  vertebrate  devel- 
opment (marsipobranchs,  ganoids,  and  amphibia)  the  archenteric 
cavity  is  of  small  diameter  and  is  bounded,  Fig.  102,  Ent,  by  cells 
which  differ  from  those  of  the  surrounding  yolks  very  slightly  at 
first,  although  they  early  assume  an  epithelioid  arrangement.  In 
the  amniota,  on  the  other  hand,  the  archenteric  cavity  undergoes 
great  lateral  expansion  at  a  very  early  stage ;  this  has  been  accom- 
panied by  certain  modifications  in  the  entoderm,  which  becomes 
divided  into  a  cellular  part  on  the  dorsal  side  of  the  cavity  and  a 
multinucleate  vitelline  part  on  the  under  side ;  moreover  the  cellular 
part  becomes  divided  into  two  regions ;  one,  known  as  that  of  the 
area  pellucida,  includes  and  surrounds  the  meroblastic  embryo 
proper,  and  is  characterized  by  the  cells  becoming  very  much  flattened 
and  expanded,  compare  Figs.  144  and  161,  and  the  other,  known  as 
that  of  the  area  opaca,  surrounds  the  first  and  is  characterized  by 
having  high  cylinder  cells,  with  more  or  less  of  remnants  of  yolk 
grains  in  them.  The  cylinder  cells  of  the  opaca  entoderm  pass  to- 
ward the  embryo  into  the  thin  cells  of  the  area  pellucida,  and  in  the 
opposite  direction  when  they  reach  the  periphery  of  the  expanded 
archenteron  they  pass  into  the  yolk ;  the  transition  constitutes  the 
so-called  germinal  wall.  As  development  progresses  we  see  that  the 
region  of  the  opaca  belongs  to  the  yolk-sac  and  not  to  the  embryo, 
hence  the  cylinder  cells  represent  part  of  the  entodermal  lining  of 
the  yolk-sac,  and  in  the  higher  mammals  the  whole  yolk-sac  is  lined 
by  cylinder  cells,  which  represent  both  the  opaca  cells  and  the  yolk 


EARLY    DEVELOPMENT    OF   THE    ARCHEXTERON. 


255 


Som 


vi 


Coo 


mass  of  Sauropsicla,  there  being  no  non-cellular  yolk  in  placental 
mammals.     For  further  details  see  Chapter  XVI.,  on  the  yolk-sac. 

As  soon  as  the  embryonic  archenteron  begins  to  separate  off  in 
amniote  embryos  the  thin  entoderm  grows  thicker,  until  in  young 
embryos  it  resembles  a  cylinder  epithelium,  but  in  the  area  pellucida 
around  the  embryo  the  entoderm  remains  thin,  the  thickening  being 
strictly  confined  to  the  embryonic  portion. 

1.  Separation  of  the  Archenteron  proper  from  the  Yolk- 
sac. — The  following  diagrams,  Figs.  142  and  143,  may  help  to  ren- 
der the  process  clear  as  it  occurs  in  the  Sauropsida ;  both  figures  are 
supposed  to  represent  a  hen's  ovum  with  the  embryo  iti  situ  aii<l 
cut  transversely ;  the  embryo  is  much  too  large  in  proportion,  and 
the  coelom  is  much  too  wide.  In  Fig.  142  the  archenteron,  Ach, 
is  a  wide  cavity  bounded  by  the 
cellular  entoderm  above,  and  the 
yolk-mass  below ;  the  cellular  en- 
toderm in  the  axial  region  corre- 
sponding to  the  area  pellucida  is 
thin,  and  thickens  at  each  side 
corresponding  to  the  area  opaca. 
The  mesoderm  is  divided  into  its 
two  layers  by  the  ccelomatic  space, 
Coe,  but  it  is  to  be  noted  that  in 
reality  the  extension  of  the  ccelom 
around  the  yolk  is  gradual,  and  at 
this  stage  of  the  embryo  is  never 
completed;  the  splanchnic  meso- 
blast  is  thus  laid  against  the  cellu- 
lar entoderm  on  the  dorsal  side  of 
the  archenteric  cavity,  forming 
the  splanchnopleure,  Spl;  that  por- 
tion of  the  splanchnopleure  which 
lies  within  the  area  pellucida,  and 
is  therefore  lined  by  the  thin  re- 
gion of  the  entoderm,  is  alone  des- 


FIG.  142.  —  Diagrammatic  Section  of  the 
Yellow  of  a  Hen's  Egg  at  an  Early  Stage  to 
show  the  Relations  or  the  Archenterou,  Ach, 


to  the  Yolk-sac.  Som,  Somatopleure ;  Spl, 
splanchnopleure;  In,  intestinal  portion  of 
archenteron:  r/,  vitelline  portion  of  archen- 
teron ;  Coe,  coelom ;  Yolk,  yolk  mass. 


tined  to  form  the  intestinal  canal  or  embryonic  archenteron.  This 
is  effected,  as  indicated  by  Fig.  20,  p.  35,  by  a  simple  folding-down 
of  the  splanchnopleure,  by  which  the  archenteron  is  divided  into  an 
upper  embryonic  portion,  In,  the  anlage  of  the  intestinal  canal, 
and  a  lower  portion,  the  walls  of  which  are  constituted  chiefly  by 
the  enormous  mass  of  yolk.  In  the  next  stage,  Fig.  19,  the  separa- 
tion of  the  yolk-sac,  Yolk,  from  the  embryonic  archenteron  is  still 
more  marked  than  before;  there  remains,  however,  a  narrow  con- 
nection between  the  intestinal  canal,  In,  and  the  yolk-sac,  Yolk, 
making,  as  it  were,  a  hollow  pedicle ;  the  pedicle  is  kiiQwn  as  the 
yolk-stalk  or  vitelline  stalk. 

If  we  view  the  embryo  in  longitudinal  section  we  find  the  relations 
are  similar  and  that  the  head  and  the  tail  end  of  the  embryo  become 
free  from  the  yolk,  and  as  the  embryo  grows  its  head  and  tail  project 
more  and  more.  This  is  often  described  as  a  separation  of  the  em- 
bryo by  the  folding-iii  of  the  germ-layers,  but  this  traditional 
description  is  incorrect,  for  the  separation  of  the  embryo  is  really 


THE   EMBRYO. 


due  to  the  expansion  of  the  embryo,  not  to  the  constriction  of  its 
connection  with  the  yolk,  compare  Chapter  XIII.  The  accompany- 
ing diagrams,  Fig.  143,  A,  B,  and  C,  show  at  a  glance  how  the 


FIG.  143.— Diagrams  to  Illustrate  the  Separation  of  the  Embryo  from  the  Yolk, 
/i,  head ;  Ach,  archenteron ;  ec,  ectoderm ;  Yolk,  yolk-mass. 


bl,  Blastopore; 


original  width  of  the  communication  is  retained,  while  the  intestinal 
canal,  or  embryonic  archenteron,  extends  forward  and  backward. 
In  A,  the  archenteron  is  open  to  the  yolk  throughout  its  entire  ex- 
tent ;  in  B,  the  head  has  begun  to  be  free,  and  with  it  the  archenteric 
cavity  has  begun  to  extend  forward  and  forms  a  distinct  cephalic  por- 
tion, which  is  entirely  within  the  embryo  and  is  not  open  directly  to  the 
yolk;  in  C,  the  tail  has  also  grown  forth  from  the  yolk  and  the  arch- 
enteron with  it,  so  that  now  we  have  a  caudal  embryonic  archenteron. 
By  further  development  the  embryo  enlarges  more  and  more,  but  the 
opening  into  the  yolk-sac  remains  nearly  the  same  absolute  size.  I 
know  of  no  exact  data  as  to  the  dimensions.  The  proportion  between 
the  yolk-stalk  and  the  embryo  steadily  changes,  the  stalk  becoming 
relatively  smaller. 

I  believe  that  the  entire  separation  of  the  archenteron  from  the 
yolk-sac  is  due  to  the  primitive  connection  being  retained  with  little 
or  no  change  of  size  while  the  embryo  is  growing,  and  that  the 
bending  of  the  layers,  instead  of  being  the  cause  of  the  closure  of  the 
archenteron,  is  the  result  of  the  arrested  growth  of  the  splanchno- 
pleure  where  it  passes  from  the  embryo  proper  to  the  yolk-sac. 

The  development  in  mammals  proceeds  in  the  same  manner  as 
here  described  for  the  Sauropsida,  but  the  appearances  are  modified 
principally  in  two  ways :  first,  by  the  loss  of  the  yolk  material,  and 
second,  by  the  retarded  development  of  the  mesoderm.  The  loss  of 
yolk  leaves  the  yolk-sac  a  hollow  body  lined  by  well-developed  epi- 
thelium (entoderm),  a  condition  reached  by  the  Sauropsida  only  after 
most  of  the  yolk  matter  (deutoplasm)  has  been  resorbed,  see  Chapter 
XVI.  The  delayed  expansion  of  the  mesoderm  is  very  marked  in 
the  rabbit,  and  probably  in  other  mammals ;  in  the  rabbit  the  em- 
bryo becomes  separated  from  the  yolk-sac  and  far  advanced  in  devel- 
opment, while  the  mesoderm  extends  only  over  the  embryonic 
hemisphere  of  the  ovum ;  hence  in  the  rabbit  the  yolk-sac  is  complete 


EARLY    DEVELOPMENT    OP    THE    ARCHEXTK1J«  »N. 


257 


only  over  half  its  surface;  the  complete  sac  of  the  entoderm  is  pres- 
ent, but  only  the  embryonic  half  of  the  entodermal  sac  is  covered  by 
mesoderm  and  separated  from  the  ectoderm  by  a  coelom ;  the  inferior 
half  of  the  sac  has  its  entoderm  lying  directly  against  the  ectoderm, 
and  this  half,  moreover,  degenerates  and  entirely  disappears,  as  dis- 
covered by  Duval,  90. 1. 

In  man  the  mesoderm  extends  completely  around  the  embryo  at  a 
very  early  stage,  and  so  also  does  the  ccelom ;  in  the  earliest  known 
human  embryos  both  of  these  processes  had  been  finished  and  the 
yolk-sac  lay  entirely  free,  the  fundamental  morphological  relations 
agreeing  with  the  diagram,  Fig.  166.  For  details  see  the  descrip- 
tions of  young  human  embryos  in  Chapter  XIII. 

The  actual  appearance  of  a  rabbit  embryo  as  seen  in  cross  sections 
is  indicated  by  Fig.  144,  while  a  similar  stage  in  the  chick  is  shown 


FIG.  144.— Cross-Section  of  a  Rabbit  Emrbyo  of  Eight  Days  and  Two  Hours.  Seg,  Primitive  seg- 
nit-nr :  Am.  amnion;  Md.  medullary  tube ;  Ao,  aorta;  G'oe,  splanchnocoele ;  Spl,  splanchnopleure ; 
Ch,  uotochord.  x  144  diams. 

by  Fig.  161;  in  both  the  embryo  only  is  represented,  and  the  space 
below  the  splanchnopleure,  &p/,  is  the  part  of  the  archenteric  cavity 
underlaid  by  yolk  in  the  natural  condition. 

The  portion  of  the  embryonic  archenteron,  Fig.  143,  C,  in  front  of 
the  yolk-sac,  gives  rise  to  the  pharynx,  oesophagus,  stomach,  an- 
terior part  of  the  intestine  proper,  and  all  their  appendages.  The 
portion  behind  the  yolk-stalk  produces  most  of  the  small,  and  the 
whole  of  the  large  intestine  and  its  appendages.  The  first  develop- 
ment of  the  anterior  division  is  the  differentiation  of  the  pharynx, 
of  the  posterior  the  formation  of  the  allantois ;  as  the  latter  organ 
always  appears  earlier  than  the  former  we  must  consider  it  first. 

Origin  of  the  Allantois. — The  first  indication  of  the  allantois 
in  all  amniota  is  a  considerable  accumulation  of  mesenchyma  around 
and  below  the  posterior  extremity  of  the  embryonic  archenteron. 
This  accumulation  is  probably  a  remnant  of  the  primitive  streak. 
A  longitudinal  section,  Fig.  145,  shows  the  relations  clearly,  as 
17 


258  THE    EMBRYO. 

found  in  the  sheep  at  about  sixteen  days ;  the  corresponding  stage 
occurs  in  the  chick  at  the  end  of  the  second  day ;  the  three  germ- 
layers  are  all  fused  in  the  primitive  streak,  pr.  s;  the  anal  mem- 
brane, a.m,  is  well  marked  by  the  absence  of  mesoderm ;  the  amnion, 

Am,    arises    close    behind ;     the 
n  archenteric  cavity  forms  a  wide 
diverticulum     behind    the    anal 
prs     membrane,   and   this    diverticu- 
lum, All,  is  the  commencement 
of  the  allantois ;  it  is  lined  by  the 
entoderm,  En,  and  has  an  exter- 
nal layer  of  thickened  mesoderm, 
mes.     The    development  of  the 

FIG.  145.— Longitudinal  Section  of  the  Posterior       n       j.    •       •       xi  r~'    i      v          i 

End  of  a  Sheep  Embryo  of  Sixteen  Days.     After  allantOlS    111    the    ChlCK    has    been 

RBonnet.   A mn,  Amnion;  a. m,  anal  membrane;  cfnrh'prl    in    rlpftnl    hv    TT.      H-aec^ 

pr.  s,  primitive   streak;   En,   entoderm;    Ach,  \  ®Y    **•    UaSSei , 

archenteric  cavity,  or  archenteron ;   All,  allan-  74.1.  and   by    x.  VOn   Dobrvnill, 

71.1.  It  begins  before  the  anal 

membrane  is  formed  as  a  small  pouch  extending  upward  into  the 
hind  end  of  the  primitive  streak;  the  tip  of  this  pouch  lies  just 
behind  the  bottom  of  the  furrow,  which  marks  off  the  caudal  ex- 
tremity of  the  embryo ;  the  bottom  of  this  furrow  is  the  site  of  the 
anal  plate;  the  pouch  gradually  enlarges  and  assumes  the  dipper 
shape,  very  much  as  in  the  sheep,  Fig.  145,  All;  in  the  chick,  how- 
ever, this  stage  is  reached  relatively  later  than  in  mammals,  for  in 
the  chick  we  find  the  tail  already  far  advanced,  so  that  it  not  only 
projects  freely  but  has  begun  to  curl  over  downward  so  as  to  bring 
the  allantois  and  anus  on  to  the  ventral  side  of  the  body  as  well  as 
to  cause  the  formation  of  the  enddarm,  which  is  a  short  extension  of 
the  archenteron  into  the  caudal  extremity.  The  whole  series  of 
metamorphoses  is  admirably  illustrated  by  Gasser,  /.  c.,  Taf.  I.  In 
mammals  the  formation  of  the  tail  is  somewhat  retarded,  but  in 
them  also  it  results  in  curling  over  and  so  bringing  both  the  allan- 
tois and  the  anal  plate  on  to  the  ventral  side,  with  the  further  result 
that  the  allantois  now  comes  to  lie  headward  of  the  anal  plate, 
although  before  the  curling  over  it  lay  behind  it. 

It  is  important  to  note  that  the  amnion  arises  between  the  anal 
plate  and  the  allantois,  and,  as  shown  in  Fig.  145,  fuses  with  the 
wall  of  the  allantois. 

The  allantois  is  characterized  by  the  rapid  development  of  its 
mesoderm,  which  seems  to  be  derived  from  the  middle  cells  of  the 
hind  end  of  the  primitive  streak.  The  amount  of  the  allantoic  meso- 
derm is  subject  to  much  variation  during  the  early  stages  of  the 
organ,  being  much  greater  in  mammals  than  in  birds,  so  far  as  ob- 
served. The  mesoderm  is  particularly  conspicuous  in  rodents ;  in 
the  rabbit  it  makes  a  distinct  mound,  compare  Fig.  196;  in  the 
guinea-pig  (E.  Selenka,  84.1,  Taf.  XL)  it  acquires  an  excessive 
size,  becoming  larger  than  all  the  rest  of  the  embryo ;  in  Mus  the 
precocious  development  is  almost  equally  marked ;  it  is  into  this  mass 
of  mesoderm  that  the  allantoic  diverticulum  of  the  archenteron 
grows.  In  the  opossum  (Selenka,  87.1,  139)  the  amount  of  meso- 
derm is  more  nearly  as  in  the  rabbit.  The  mesoderm  is  also  charac- 
terized in  rodents,  and  perhaps  in  other  mammals,  by  its  precocious 


EARLY  DEVELOPMENT  OF  THE  ARCHENTERON.        259 

vascularization,  which  has  been  expressly  emphasized  for  the  rabbit 
by  0.  Rabl,  89.2,  152,  Taf.  IX.,  Fig.  14;  the  vessels  give  the  tissue 
a  spongy  character.  The  protuberance  caused  by  the  allantoic  meso- 
derm  is  termed  Allantoishocker  by  recent  German  writers,  the 
AJlantoiswulst  by  Kolliker. 

The  earliest  stages  of  the  human  allantois  are  not  known.  There 
has  been  some  discussion  as  to  whether  there  is  a  free  allantois,  but 
no  proof  that  such  a  stage  occurs  has  been  brought.  The  matter  is 
discussed  in  the  chapter  on  the  youngest  known  human  embryos, 
and  in  that  on  the  umbilical  cord,  compare  also  Fr.  Keibel,  91.4. 

Primitive  Anus. — The  terminal  portion  of  the  intestinal  canal 
receives  in  early  stages  the  urogenital  ducts,  a  condition  which  is 
permanent  in  the  Sauropsida ;  the  portion  of  the  archenteron  com- 
mon to  these  ducts  is  known  as  the  cloaca.  The  ectoderm  in  amniota 
forms  very  early  a  small  anal  invagination  (proctodseum)  which 
grows  in  toward  the  cloaca  until  the  ectoderm  and  entoderm  come 
into  contact;  the  membrane  formed  by  the  two  epithelia  breaks 
through  and  the  cloaca  thereby  acquires  an  opening  to  the  exterior ; 
this  opening  subsequently  divides  into  two:  1,  the  urogenital  open- 
ing ;  2,  the  permanent  anus ;  in  distinction  from  the  latter  the  clo- 
acal  opening  may  be  called  the  primitive  anus. 

In  amniota  the  anal  membrane  arises  in  the  anterior  region  of  the 
primitive  streak  some  distance  behind  the  neurenteric  or  chorda 
canal.  It  has  been  studied  in  birds  by  Bornhaupt,  67.1,  and  more 
fully  by  Gasser,  80. 1.  It  has  been  noticed  in  Lacerta  by  H.  Strahl, 
86.2,  166,  who  states  that  it  appears  in  that  type  at  a  much  later 
stage  than  in  birds  or  mammals.  In  mammals  it  was  mentioned 
first,  I  think,  by  Kolliker,  83.1,  and  has  been  since  then  studied  by 
H.  Strahl,  86.2,  F.  Keibel,  88.2,  410,  R.  Bonnet,  89.1,  90,  Ket- 
terer,  90.2,  Tourneux,  90.3,  and  especially  by  C.  Giacomini,  88.1, 
most  of  all  these  observations  having  been  made  on  the  rabbit.  In 
rabbit  embryos  with  five  pairs  of  myotomes,  the  anal  mem- 
brane can  be  distinctly  recognized  near  the  rear  of  the  primi- 
tive streak,  compare  Strahl,  1.  c.,  Taf.  IV.,  Fig.  6;  it  begins  as  a 
slight  depression  of  the  ectoderm ;  behind  it  are  situated  the  amniotic 
fold  and  the  allantois ;  the  depression  rapidly  deepens,  pushing  away 
the  mesodermic  cells  until  the  ectoderm  comes  into  contact  with  the 
entoderm,  which  at  this  spot  becomes,  meanwhile,  thickened  into  a 
cylinder  epithelium ;  when  the  contact  takes  place  a  slight  entodermic 
depression  appears.  The  two  layers  soon  become  indistinguishable, 
and  by  the  proliferation  of  their  cells  produce  a  cord  of  cells ;  a  simi- 
lar cord  has  been  found  in  the  sheep  by  R.  Bonnet,  88. 1,  and  in  the 
guinea-pig  by  F.  Keibel,  88.2;  the  latter  states  that  the  cord  is  con- 
nected only  with  the  ectoderm ;  the  cord  is  completely  surrounded 
by  typical  primitive-streak  tissue;  according  to  Giacomini,  88.1, 
287j  the  cord  develops  very  soon  a  transient  lumen,  which  he  calls 
the  "anal  canal."  While  during  the  further  development  of  the 
embryo  the  caudal  extremity  is  rolled  over  ventralward,  the  cord 
changes  in  character,  becoming  a  membrane,  and  at  the  same  time 
it  is  brought  on  to  the  ventral  side  of  the  body  and  comes  to  lie  be- 
hind, instead  of  in  front,  of  the  amnion  as  it  did  before  the  rolling 
up  of  the  embryo.  The  change  just  referred  to  consists  in  rendering 


260  THE    EMBRYO. 

the  two  epithelia  distinct  again  and  converting  each  into  a  single 
cell-layer,  making  a  double  epithelial  membrane  from  which  the 
mesoderm  is  entirely  excluded,  and  which  has  been  appropriately 
named  the  anal  membrane  by  Strahl.  The  membrane  lies  at  the 
bottom  of  a  shallow  pit,  which  is  commonly  viewed  as  an  ectodermal 
invagination,  and  has  been  called  the  Afterdarm  by  German,  procto- 
da3um  by  some  English  writers.  It  is  to  be  regarded  as  the  rudi- 
mentary homologue  of  the  well-developed  invagination  of  annelids 
and  other  invertebrates,  which  forms  in  them  a  considerable  portion 
of  the  digestive  canal ;  the  anal  invagination  results  in  invertebrates 
in  the  formation  of  the  so-called  Hinterdarm  (hind-gut  of  Foster 
and  Balfour) ,  which  must  not  be  confused  with  the  vertebrate  Hin- 
terdarm, which  is  derived  from  the  archenteron. 

The  rupture  of  the  anal  membrane  is  said  to  occur  in  the  rabbit  about 
the  twelfth  (Kolliker,  "Grundriss,"  359)  or  thirteenth  day  (Strahl, 
86.2,  165).  I  know  of  no  exact  description  of  the  process  in  mam- 
mals. In  the  chick  the  epithelial  cord  arises  and  becomes  perfo- 
rated, according  to  Gasser,  without  passing  into  the  stage  of  anal 
membrane  observed  in  mammals ;  irregular  cavities  appear  in  the 
cord  (Gasser,  /.  c.,  Taf.  XIII.,  Figs.  6a,  7 a) ;  these  cavities  enlarge 
and  fuse,  the  cells  of  the  cord  or  plate  meanwhile  undergoing  degen- 
erative changes ;  the  rupture  is  completed  about  the  fifteenth  day  of 
incubation.  The  anal  ectodermal  invagination  is  somewhat  more 
marked  than  in  mammals  and  gives  rise  on  its  dorsal  side  to  a  con- 
siderable diverticulum,  the  bursa  Fabricii,  which  is  found  in  birds 
but  not  in  mammals  or  reptiles. 

The  anus  of  the  lower  vertebrates  arises,  as  has  already  been 
shown,  in  intimate  relation  with  the  blastopore.  This  fact  was  first 
discovered  by  Max  Schultze,  56.1,  in  Petromyzon,  ascertained  in 
alytes  by  Gasser,  82.3,  in  the  newt  by  Alice  Johnson,  84.1,  in 
Rana  by  Durham,  86. 1.  The  nature  of  this  relation  was  first  eluci- 
dated by  Schanz,  87.1,  and  has  since  been  worked  out  for  various 
amphibia,  as  described,  p.  189. 

Th.3  Enddarm. — The  prolongation  of  the  archenteron  into  the 
tail  of  amniote  embryos  is  generally  known 'as  the  Enddarm,  the 
German  name  most  in  use;  it  is  also  called  Schwanzdarm,  tail-gut, 
and  post-anal  gut.  It  results  from  the  differentiation  and  rolling 
over  of  the  tail.  The  tail  is  produced  by  the  growth  of  the  tissue  of 
the  primitive  streak  between  the  anal  membrane  and  the  blastopore 
or  neurenteric  canal,  compare  Chapter  XIII. ;  the  growth  occurs  in 
such  a  way  that  the  tissue  curls  downward,  and  folds  off  the  region 
of  the  archenteron  underlying  the  primitive  streak,  and  the  disposi- 
tion becomes  as  shown  in  Fig.  102  of  Kolliker's  "Grundriss,"  2te 
Aufl. ,  the  enddarm  extending  into  the  tail  behind  the  ventralry  situ- 
ated anal  membrane. 

I  consider  the  enddarm  co  be  distinct  from  the  neurenteric  canal, 
with  which  Balfour  ("Cpmp.  Embryol.,"  II.,  193,  Fig.  124)  brings 
it  into  relation.  O.  Hertwig  apparently  agrees  with  Balfour^  since 
he  copies  the  latter's  diagram  ("Entwickelungsges.,"  3te  Aufl.,  Fig. 
126).  It  seems  to  be  confined  to  early  embryonic  life,  but  there  are 
a  few  data  as  to  its  ultimate  fate.  Prenant,  91.2,  231,  studying  the 
rabbit  found  the  post-anal  gut  to  be  a  short  wide  pouch  before  the  tail 


EARLY  DEVELOPMENT  OF  THE  AKCHENTEROX.        2<U 

develops ;  as  the  tail  develops,  the  gut  extends  into  it  and  becomes 
long  and  narrow,  and  its  posterior  extremity  merges  with  the  fused 
anlages  of  the  medullary  tube  and  notochord.  In  still  older  em- 
bryos it  degenerates. 

Origin  of  the  Vorderdarm. — As  is  well  known  the  first  part  of 
the  embryo  in  vertebrates  to  project  from  the  yolk  is  the  head  end. 
In  the  same  measure  as  the  head  and  neck  become  free  the  portion 
of  the  archenteron  which  pertains  to  them  becomes  closed  below  and 
shut  off  from  the  yolk.  A  longitudinal  section  of  a  chick  in  which 
the  head  has  just  become  free  is  shown  in  Fig.  146.  In  consequence 
of  the  head  end,  H,  having  grown  forward  above  the  proamnion, 
pro.  a,  which  overlies  the  extra  embryonic  archenteric  cavity,  it  has 
become  free  on  all  sides,  and  at  the  same  time  the  archenteron  has 
been  carried  forward  with  the  head,  making  the  so-called  Vorder- 
darm, Vd,  of  German  authors.  The  term  fore-gut  has  been  proposed 
by  Foster  and  Balfour  as  an  equivalent  English  term,  but  has  not 
come  into  general  use,  so  I  have  prefered  to  use  the  German  term. 
Vorderdarm  is  also  used  in  invertebrate  embryology,  but  in  a  dif- 
ferent sense,  for  it  designates  the  oral  invagination  of  the  ectoderm, 


FIG.  146.— Longitudinal  Median  Station  «>f  Voting  Chick  Embryo.  H,  Head;  Fd,  Torderdarm; 
men,  meso  lerm :  /o,  fovea  cardiaca;  p,  pericardia!  cavity;  pro.  a,  proamnion;  Ach,  archen- 
teric cavity;  Pr.  *-,  primitive  streak. 

whereas  the  vertebrate  vorderdarm  is  the  cephalic  portion  of  the 
archenteron. 

Even  at  the  stage  of  Fig.  140,  the  vorderdarm  has  begun  to  be 
differentiated  into  an  anterior  division  and  a  posterior,  which  at  this 
time  are  distinguished  chiefly  by  the  ccelom,  p,  being  present  only 
in  the  mesoderm  below  the  posterior  division.  The  anterior  division 
forms  the  pharynx  proper.  The  distinction  between  the  two  parts 
of  the  vorderdarm  has  long  been  recognized  (see,  for  example,  Goette's 
observations  on  Bombinator,  75.1,  221),  but  its  morphological  sig- 
nificance has  been  overlooked.  The  vorderdarm  is  a  short  canal 
under  the  anterior  end  of  the  medullary  groove ;  it  ends  blindly  in 
front,  but  opens  widely  behind  into  the  general  archenteric  cavity ; 
this  opening  is  termed  the  fovea  cardiaca  (rordere  Darmpforte  of 
Kolliker) ,  having  been  so  named  by  C.  F.  Wolff.  The  fovea  is  easily 
seen,  when  the  chick  embryo  is  removed  from  the  yolk  in  the  usual 
manner,  and  viewed  from  the  under  side ;  its  curving  edge  marks 
the  end  of  the  closed  archenteron  behind  which  the  archenteric  cavity 
of  the  embryo  opens  directly  into  the  yolk-sac.  In  transverse  sec- 
tions, Fig.  147,  the  vorderdarm  appears  widely  expanded  sideways, 
but  compressed  dorso- vent  rally,  and  also  bent,  the  concavity  being 
upward;  it  is,  of  course,  completely  lined  by  entoderm,  the  cells  of 
which  form  a  very  thin  layer  on  the  dorsal  side  and  a  much  thicker 
layer  on  the  ventral  side ;  moreover,  on  the  ventral  side  the  entoderm 
is  thickened  toward  the  median  line.  These  features  are  highly 
characteristic,  but  their  significance  is  quite  unknown.  Are  they 
ancestral  in  origin? 


202 


THE    EMBRYO. 


In  the  explanations  usually  given,  the  development  of  the  vorder- 
darm  is  not  attributed  to  the  forward  growth  of  the  head,  but  to  the 
down-folding  of  the  splanchnopleures.  Indeed  if  sections  of  succes- 
sive stages  be  compared  the  idea  appears  justified  ,  for  at  first  the 
cephalic  archenteroii  opens  widely  into  the  yolk-sac,  then  as  the  head 
of  the  embryo  begins  to  rise  up  and  project  forward  from  the  yolk  it 
seems  as  if  the  sides  of  the  head  were  being  tucked  under ;  but  if  it 


md. 


EC 


FIG.  147'. — Transverse  Section  of  the  Head  of  a  Chick  Embryo  with  seven  Segments,  nch,  Noto- 
chord;  Gl,  ganglion;  md,  medullary  wall;  mes,  mesoderm :  P/i,  pharynx;  £"c,  ectoderm;  JEn, 
entoderm ;  Am.  V. ,  amniotic  vesicle. 

be  remembered  that  the  head  is  growing  and  that  the  opening  be- 
tween the  archenteron  proper  and  the  yolk  enlarges  very  little,  it 
will  be  clear  that  the  growth  of  the  head  is  the  real  cause  of  the 
formation  of  the  vorderdarrn. 

In  mammals  the  process  is  the  same  as  in  birds,  but  the  vorder- 
darm  is  less  expanded  laterally  and  less  compressed  dorso-ventrally 
than  in  the  chick,  hence  the  appearance  in  cross-section  is  somewhat 
different.  In  the  opossum,  however,  there  is  a  marked  resemblance 
to  the  avian  type  in  the  shape  of  the  vorderdarm,  see  Selenka,  86. 1, 
Taf.  XXII.,  Figs.  9-10,  and  it  is  probable  that  more  careful  study 
will  show  that  the  mammalian  vorderdarm  passes  through  the  flat- 
tened form  before  assuming  its  more  familiar  shape. 

The  Oral  Plate. — The  fact  that  the  anterior  end  of  the  vorder- 
darm lies  against  the  ectoderm  has  long  been  known  for  advanced 
embryos.  The  two  germ-layers,  entoderm^and  ectoderm  are  soldered 
together  with  no  mesoderm  between  them,  thus  forming  a  double 
epithelial  plate  (as  shown  in  Figs.  10G  and  170,  o.pZ),  which  separates 
the  buccal  from  the  archenteric  cavity.  The  plate,  which  may  be 
called  the  oral  plate  (membrana  fauces,  Rachenhaut,  Mundrachen- 
haut) ,  by  its  subsequent  rupture  brings  the  mouth  into  communica- 
tion with  the  pharynx. 

Fr.  Carius,  88.1,  22,  has  shown  that  the  oral  plate  is  present  in 
the  rabbit  at  a  very  early  stage,  the  spot  where  the  entoderm  and 
ectoderm  come  into  contact  being  distinguishable  before  the  head  is 
separated  from  the  yolk.  This  spot  lies  just  in  front  of  the  interior 
end  of  the  medullary  groove  and  of  the  chorda,  the  end  of  which 


EARLY  DEVELOPMENT  OF  THE  ARCHENTERON.        263 

fuses  with  the  entoderm  of  the  membrane.  As  the  head  of  the  em- 
bryo grows  forward  and  bends  downward  toward  the  yolk  the  oral 
plate  is  rolled  over  so  as  to  lie  on  the  ventral  side  of  the  embryo,  and 
to  constitute  part  of  the  ventral  floor  of  the  vorderdarm  as  shown  in 
Fig.  100. 

Origin  of  the  Pharynx.  —From  what  has  been  said  in  the  pre- 
ceding section  it  appears  that  the  vorderdarm  very  early  divides 
into  an  anterior  part  without  any  splanchnocoele  in  the  surrounding 
mesoderm  and  a  posterior  part,  underneath  which  lies  the  pericardial 
division  of  the  ccelom.  The  anterior  division  becomes  the  pharynx 
proper  and  is  remarkable  for  its  rapid  enlargement  during  the  earliest 
embryonic  periods  of  amniota ;  the  large  size  of  the  pharynx  is  char- 
acteristic of  the  lower  vertebrates,  hence  we  have  in  the  pharynx 
another  illustration  of  the  appearance  in  the  embryo  of  a  higher  form 
of  features  characteristic  of  the  adult  lower  forms.  The  posterior  or 
epicardial  division  of  the  vorderdarm  undergoes  differentiation 
later  than  the  pharynx,  but  ultimately  gives  rise  to  the  oasophagus 
and  stomach ;  as  the  lungs  arise  near  the  junction  of  the  two  divi- 
sions, it  is  riot  quite  certain,  at  present,  whether  they  make  part  of 
the  anterior  or  posterior  division. 

The  pharynx  then  is  the  anterior  portion  of  the  vorderdarm,  and 
is  further  characterized  by  never  having  a  continuous  coelomatic 
cavity  developed  in  the  mesoderm  surrounding  it. 

The  relations  of  the  pharyngeal  entoderm  to  the  ectoderm  are  ex- 
tremely important  to  the  morphologist,  since  they  result  in  the 
formation,  1,  of  the  oral  plate  and  consequently  of  the  mouth  cavity; 
2,  of  the  gill-clefts,  which  in  their  turn  determine  to  a  large  extent 
the  complex  morphology  of  the  head. 

The  Branchial  Clefts,  or  gill-clefts,  are  permanent  structures  in 
the  fishes  and  tailed  amphibia,  larval  structures  in  anoura,  and 
embryonic  structures  in  amniota.  They  arise  as  a  series  of  paired 
pouches  from  the  sides  of  the  pharynx.  They  are  called  Schlund-, 
Kiemen-  or  Visceral-spalten  in  German;  fentes  branchiales  in 
French. 

The  number  of  gill-clefts  varies  in  the  different  classes  of  verte- 
brates. In  mammals  and  birds  there  are  four;  in  reptiles,  tailed  am- 
phibians, and  most  fishes,  five ;  among  the  selachians,  however,  the 
number  is  variable,  there  being  often  six  and  in  the  Notidanidse  eight, 
it  is  said.  In  the  lamprey  there  are  eight  during  larval  life,  but  the 
first  aborts  when  the  larva  (Ammoccetes)  changes  into  the  adult 
(Petromyzon) .  In  Amphioxus  the  pharynx  has  eighty  to  one  hun- 
dred openings  and  even  more.  These  facts  have  led  to  the  general 
conclusion  that  within  the  vertebrate  series  the  number  of  gill-clefts 
has  been  gradually  reduced — a  hypothesis  of  great  importance,  from 
its  bearing  upon  the  solution  of  the  morphology  of  the  head. 

In  all  birds  and  mammals  there  are  four  pairs  of  gill  pouches 
developed,  all  in  essentially  the  same  manner.  The  anterior  pair 
appears  first,  the  others  in  succession  behind  it.  The  entoderm  of 
the  pharynx  forms  a  small  outgrowth  on  each  side,  making  a  pouch, 
which  expands  until  it  reaches  the  ectoderm.  Soon  a  second  pair  of 
outgrowths  appear  behind  the  first,  and  a  third  and  a  fourth.  For 
a  long  time  it  was  believed  that  the  membrane  formed  by  the  eiito- 


264 


THE    EMBRYO. 


derm  and  ectoderm  at  the  end  of  each  pouch  ruptured  and  converted 
each  pouch  into  an  actual  cleft  or  opening  by  which  a  free  passage 
was  established  through  the  side  of  the  neck  into  the  pharynx,  as 
occurs  in  all  Ichthyopsida.  W.  His  pointed  out,  81.1,  319,  that 
this  was  open  to  question,  and  later  showed  that  the  membrane  is 
not  ruptured  in  birds  and  mammals — a  conclusion  which  has  since 
been  confirmed  by  Born,  83.1,  275,  Kolliker,  "Grundriss,"  p.  77, 
and  Piersol,  and  which  is,  I  think,  probably  correct,  for  those  who 
have  called  it  in  question  (De  Meuron,  Kastschenko,  and  Liessner) 
seem  to  me  to  have  offered  insufficient  evidence.  Piersol,  88.1,  162, 
studied  the  question  with  great  care  in  the  rabbit,  and  finds  no  sat- 
isfactory evidence  of  the  closing  membrane  being  ruptured  in  any  of 
the  branchial  clefts  at  any  time. 

The  shape  of  the  pharynx  and  its  four  pairs  of  branchial  pouches 
has  been  carefully  studied  in  the  rabbit  by  G.  A.  Piersol,  88.1,  by 
means  of  models  of  the  cavity  at  various  ages,  constructed  in  wax 
by  Born's  method.  Two  views  of  the  model  or  cast  of  the  pharyn- 
geal  cavity  at  eleven  days  are  given  in  Fig.  148.  As  the  oral  plate 
is  already  ruptured  at  this  age,  the  buccal  and  pharyngeal  cavities 


FIG.  148.— Two  Views  of  a  Wax  Model  of  the  Cavity  of  the  Pharynx  of  a  Rabbit  Embryo  of 
Eleven  Days.  A,  Showing  the  lateral  and  ventral  surface ;  B,  showing  dorsal  and  lateral  surface. 
After  Piersol. 

have  fused,  and  the  models  show  also  the  oral  evagination  of  the 
hypophysis,  hy.  The  figures  sufficiently  indicate  the  complex  configu- 
ration of  the  pouches  with  their  wing-like  expansions  and  ascending 
dorsal  points,  as  well  as  the  progressive  diminution  in  size  from  the 
first  to  the  fourth  pouch. 

It  must  be  borne  in  mind  that  while  the  gill-slits  are  developing 
the  head  is  growing,  and  therefore  lengthening,  so  that  the  pharyn- 
geal portion  of  the  vorderdarm  elongates.  ,  At  the  time  the  first  gill- 
cleft  is  formed  there  is  not  room  for  the  remaining  clefts,  but  the 
growth  of  the  pharynx  provides  the  needed  room  soon.  Thus  in  the 
chick  there  is  at  first  only  a  very  small  distance  between  the  region 
of  the  pericardium  (and  heart)  and  the  anterior  extremity  .of  the 
embryo  (see  Fig.  146),  but  by  the  end  of  the  third  day  there  is  a  con- 
siderable interval  between  the  anterior  end  of  the  heart  and  the 
actual  head.  This  interval  constitutes  the  embryonic  neck,  and  cor- 


EARLY  DEVELOPMENT  OP  THE  ARCHENTEROX.        2G5 

responds  to  the  pharyngeal  region,  and  is  characterized  by  two  prin- 
cipal features:  1,  the  absence  of  a  splanchnocoele ;  2,  the  presence 
of  the  gill  pouches. 

As  soon  as  the  pharyngeal  evaginations  reach  the  ectoderm  they 
become  attached  to  it,  first  on  the  dorsal  side  and  then  downward 
until  the  attachment  is  completed  throughout  the  whole  area  of  con- 
tact (A.  Goette,  75.1,  222).  It  seems  now  as  if  the  ectoderm  were 
actually  held  down  where  resting  upon  the  entoderm,  for  we  see  as 
the  next  phase  that  the  germ-layers  grow  freely  in  front  and  behind 
each  gill  pouch,  thus  producing  columns,  which  are  placed  at  the 
side  of  the  pharynx  and  are  separated  from  one  another  by  the  gill- 
clefts.  As  there  are  four  gill-clefts  it  follows  that  there  are  five 
columns.  These  columns  are  known  as  the  branchial  arches,  also 
as  the  gill  or  visceral  arches  (Kiemenbogen,  Visceralbogen,  arcs 
branchiaux) .  Each  arch  is  marked  out  by  projecting  into  the  pha- 
rynx and  upon  the  outside,  and  consequently  soon  after  the  gill 
pouches  are  developed  the  arches  become  easily  distinguishable  upon 
the  exterior,  and  the  depressions  between  them  show  the  positions 
of  the  pouches.  The  depressions  become  part  of  the  gill-clefts  when 
the  membrane  (ectoderm  and  entoderm)  breaks  through;  hence, 
when  the  clefts  become,  as  in  the  lower  vertebrates,  open  passages, 
their  lining  is  partly  of  entodermic,  partly  of  ectodermic  origin,  but 
as  the  epithelia  fuse  perfectly,  the  line  of  demarcation  cannot  be  dis- 
tinguished in  the  open  clefts. 

As  to  the  time  at  which  the  gill-clefts  appear,  we  need  more  exact 
information.  C.  Rabl,  89. S,  21G,  gives  the  following  data  for  sela- 
chian embryos  (Pristiurus) : 

Embryos  with  18  myotomes  show  the  first  gill  pouch. 

23-24  the  second  pouch  beginning. 

26-27  the  second  pouch  well  formed. 

31-32  the  third  pouch  well  advanced. 

38-40  the  fourth  pouch  beginning. 

45-40  the  fourth  pouch  completed,  and  the 

second  breaking  through. 
54-59  the  fifth  pouch  begun,  and  the  first 

and  third  breaking  through. 

GG-G8  the  first,  second,  and  third  pouches 

are  clefts,  the  fourth  is  breaking 
through. 

74  the  sixth  pouch  is  forming,  the  first 

four  are  open,  the  fifth  opening. 

In  the  chick  the  gill-clefts  begin  to  appear  with  third  day,  the 
fourth  being  present  at  the  end  of  that  day.  In  the  rabbit  the  first 
pouch  is  seen  the  ninth  day,  and  the  fourth  the  tenth  day.  In  man 
the  pouches  are  developed  during  the  beginning  of  the  third  week. 

The  pharynx  expands  rapidly  in  all  directions  during  the  develop- 
ment of  the  branchial  clefts,  and  there  is  a  corresponding  enlargement 
of  the  cervical  region,  whereby  the  form  of  the  embryo  is  affected. 
The  external  features  resulting  from  the  development  of  the  pharynx 
are  described  in  Chapter  XIII. ,  to  which  the  reader  is  referred.  It 
may,  however,  help  to  make  the  fundamental  relations  of  the  pha- 
rynx clear,  to  insert  here  the  figure  of  a  longitudinal  horizontal  sec- 


266 


THE   EMBRYO. 


tion  of  a  dog-fish  embryo.  The  pharynx  is  a  very  wide  cavity,  P/?, 
the  sides  of  which  are  bounded  by  the  five  gill-arches ;  the  gill-clefts 
behind  each  of  the  arches  are  already  open  through ;  the  space  in 
front  of  the  first  arch,  J,  is  part  of  the  opening  of  the  mouth,  which 
came  into  communication  with  the  pharynx  at  a  much  earlier  stage 
than  that  -represented  in  the  figure.  The  size  of  the  pharynx  forms 

a  striking  contrast  with  that  of  the  intes- 
tinal canal,  In;  each  branchial  arch  con- 
sists of  a  mass  of  connective  tissue  bounded 
by  a  layer  of  epithelium  derived  partly 
from  the  entoderm  of  the  pharynx,  partly 
from  the  ectoderm. 

The  shapes  and  positions  of  the  gill- 
slits  are  remarkably  uniform  in  all  verte- 


br3 


Li 


In 


•  , 


FIG.  149. — Acanthias  Embryo  of  17  rrm. 
Horizontal  section  of  the  anterior  half.  Mb, 
Mid-brain;  he,  Balfour^s  mandibular  cavity; 
Ph,  pharynx;  I.,  II.,  III.,  IV.,  V.,  gill-' 
arches;  Ht,  heart;  Fe,  vein;  Jn,  intestine. 
Lt,  liver. 


FIG.  150.— Chicken  Embryo  of  Sixty-eight 
Hours.  Ar,  Vitelline  artery;  F,  vitelline 
vein;  S,  segment;  Ao,  aorta;  br3,  third 
branchial  cleft;  Ot,  otocyst;  Hb,  hind  brain; 
3/6,  mid-brain;  L.  lens;  H, hemisphere ;  //f, 
heart. 


brates.  They  are  elongated  dorso-ventrally  and  narrow  in  the  di- 
rection of  the  longitudinal  axis  of  the  embryo,  Fig.  150.  The  first 
is  the  largest  and  the  remaining  ones  gradually  diminish  from  in 
front  backward.  Viewed  from  the  outside  they  are  seen  not  to  be 
strictly  parallel,  but  to  converge  somewhat  toward  the  ventral  side, 
the  angle  between  the  first  and  second  clefts  being  the  largest.  It 


EARLY  DEVELOPMENT  OF  THE  ARCHENTEROX. 


267 


N 
MX. 


* 


Mid-brain;  N,  nasa 

P*t;  Mr,maxilla;  3f, 

mouth:  Md,  mandi- 


stalk- 


is  also  noteworthy  that  the  lower  edges  of  the  clefts  recede  further 

and  further  from  the  median  ventral  line  from  the  first  to  the  last 

cleft,  Fig.   151;   the  first  clefts  nearly  meet  on  the 

ventral  side,  while  the  fourth  and  fifth  clefts  are  far 

apart.     The  observation  of  this  peculiarity  has  led 

to  the  supposition  that  the  mouth  may  have  been 

evolved  by  the  meeting  of  two  gill-clefts  which  have 

fused  into  one  opening  on  the  median  line;  this  hy- 

pothesis  is  discussed  in  the  section  on  the  evolution 

of  the  mouth. 

The  Branchial  Arches.  —  These  are  structures 
of  great  morphological  importance,  which  undergo 
modifications  of  increasing  complexity  as  we  ascend 
the  vertebrate  series.  They  are  also  termed  gill- 
arches  and  visceral  arches  (Kiemenbogen,  Visceral- 
bogen).  In  their  earliest  form  they  are  merely  the 
columns  of  tissue  bordering  the  gill-clefts;  in  a 
horizontal  section  of  the  pharynx  of  an  embryo  they 
are  cut  transversely  and  are  then  seen  to  consist 
merely  of  a  core  of  mesenchyma,  surrounded  by  a 
layer  of  cylinder  epithelium,  derived  in  part  from 
the  ectoderm,  in  part  from  the  entoderm,  as  explained 
above.  In  those  cases  where,  as  in  the  amniota, 

i  f,  i 

the  gill-cletts  do  not  become  open,  or  course  the  ecto- 

derm  from  one  arch  passes  across  to  the  next,  and  the 

entoderm  likewise,  but  not  the  mesoderm,  compare 

Fig.  258.    As  previously  stated  the  inner  and  outer  layers  together 

form  a  membrane  (Verschlussplatte)  ,  which  closes  the  gill-cleft. 

In  more  advanced  stages  additional  parts  are  gradually  differenti- 
ated in  each  gill-arch.  Typically  there  are  four  principal  structures 
developed,  an  aortic  vessel,  a  downgrowth  of  the  myotome  overlying 
the  dorsal  end  of  the  arch,  two  branches  of  nerves,  and  a  rod  of  car- 
tilage —  and  they  appear  in  the  order  named.  The  aortic  vessels  arise 
very  early  and  establish  a  direct  communication  between  the  ventral 

and  dorsal  aortaB,  and  are  called  the 
aortic  arches.  Their  arrangement 
and  metamorphoses  are  discussed  in 
Chapter  XXIV.  Fig.  152  shows  the 
aortic  arch,  A,  in  a  cross  section  of 
a  gill-arch.  The  parts  have  a  t}rpical 
primitive  arrangement  from  which 
all  modifications  are  derived.  The 
details  are  discussed  in  subsequent 

Chapters. 

Viewed  externally  the  gill-arches 

AT-      r   n         •  i-       '..• 

present  the  following  peculiarities  in 

amniote  embryos  at  the  stage  when 
the  gill-arches  have  their  maximum 
typical  development.  The  first  arch  divides  the  mouth  from  the 
first  branchial  cleft,  and  has  its  lower  end  enlarged  and  somewhat 
knob-like  ;  the  second  arch  has  a  similar  knob,  but  a  little  smaller  ; 
at  first  the  four  knobs  are  quite  distinct,  but  they  scon  fuse  and 


FIG.  153.—  Cross  Section  of   a   Branchial 

Arch  of  an  Advanced  shark  Embryo. 

Pristiurus.  /,  Branchial  filament;  A,  aortic 
arch;  msth,  mesothelium.  N,  nerve;  cart, 

lTterTbohrnmmunicatingvein;  "*  ^ 


268  THE    EMBRYO. 

become  more  or  less  indistinct  ;  the  third  and  fourth  arches,  on  the 
contrary,  simply  thin  out  and  melt  into  the  general  ventral  surface. 
The  anterior  (cranial)  border  of  the  mouth,  after  the  buccal  cavity 
has  formed,  is  also  thickened  and  its  upper  end  joins  the  dorsal  end 
of  the  first  branchial  arch  and  hence  is  sometimes  called  the  maxil- 
lary process  (Oberkiefer  fort  sat  z)  of  the  first  arch.  Additional  data 
and  figures  of  the  external  appearances  are  given  in  Chapter  XXVI. 
Seessel's  Pocket.  —  This  term  is  applied  to  a  small  diverticulum 
which  appears  in  birds  and  mammals  on  the  dorsal  side  of  the  phar- 
ynx. It  was  first  described  by  Seessel,  78.1,  and  has  been  noted 
since  by  various  observers,  Piersol,  88.1,  et  aL 

Origin  of  the  Liver.  —  The  liver  in  the  primitive  type  of  devel- 
opment, as  preserved  in  Petromyzon  and  amphibia,  appears  exceed- 
ingly early,  Fig.  153  (compare  also  A.  Goette's  figures  75.1,  Taf. 
II.,  Figs.  34—38).  It  is  a  diverticulum  of  the  archenteron,  Fig.  15o, 
Li,  near  its  anterior  extremity,  and  projecting  on  the  ventral  side 
downward  into  the  mass  of  yolk-cells.  The  short  stretch  of  the 
archenteron  in  front  of  the  hepatic  evagination  is  the  homologue  of 
the  vorderdarm,  which  shows,  however,  in  this  type  of  development 

no  trace,  as  yet,  of  its  sub- 

Md  nch         sequent  division   into  pha- 

ryngeal  and  epicardial  re- 
gions. When,  however, 
the  heart  appears  the  two 
regions  of  the  vorderdarm 
become  distinguishable,  and 
the  liver  diverticulum  is 
seen  to  lie  immediately  be- 
hind the  posterior  or  venous 
extremity  of  the  heart.  It 
is  probable  from  these  facts 
that  the  liver  is  an  older 
organ  in  the  ancestral  his- 


FIG.  153.—  Longitudinal  Section  of  an  Embryo  of  Pe-  f/^rv  r»f  vprtohratoQ  than 

tromyzon  Planer  i,  Four  Days   Old,  Reared  at  Naples.  '  ^  C 

Md,  Medullary  tube;  EC,  ectoderm;  bl,  blastopore;  nch,  pharynx    Or   even  the  heart. 

notochord;o.pZ,  oral  plate;  i/,  liver.     After  Kupffer.  ^  Jgituation    of    the 


causes  it  to  lie  close  to  the  veins,  which  are  subsequently  developed 
to  pass  from  the  yolk  to  the  heart  ;  these  veins  are  especially  devel- 
oped in  amniota  and  are  known  as  the  omphalo-mesaraic  veins. 
The  further  development,  to  be  described  later,  brings  the  liver  into 
peculiar  intimate  relations  with  the  venous  circulation. 

In  elasmobranchs  (Balfour,  "'Works,"  I.,  455)  the  liver  arises  dur- 
ing stage  I  (i.  e.,  three  gill-pouches  begun,  but  the  first  not  open  yet) 
as  a  ventral  outgrowth  at  the  hind  end  of  the  vorderdarm  and  immedi- 
ately in  front  of  the  union  of  the  yolk-sac  with  the  archenteron,  or 
in  other  words  just  in  front  of  the  yolk-duct  or  umbilical  canal,  thus 
bringing  the  liver  into  proximity  with  the  vitelline  veins  entering 
the  heart.  As  the  gill-pouches  are  present  the  pharynx  is  already 
differentiated,  and,  therefore,  the  liver  arises  relatively  later  than  in 
Petromyzon  and  the  amphibians.  u  Almost  as  soon  as  it  is  formed 
this  outgrowth  develops  two  lateral  diverticula,  opening  into  a 
median  canal.  The  two  diverticula  are  the  rudimentary  lobes  of  the 


EAELY  DEVELOPMENT  OF  THE  ARCHENTEKOX.        209 

liver,  and  the  median  duct  is  the  rudiment  of  the  common  bile  duct 
(ductus  choledochus)  and  gall  bladder.  By  stage  K  the  hepatic 
diverticula  have  begun  to  bud  out  a  number  of  small  hollow  knobs." 

In  teleosts  the  liver  arises  quite  late,  e.  g.,  in  trout  the  twenty  - 
fifth  day — as  a  solid  outgrowth  from  the  archenteron  close  behind 
the  heart — thus  offering  one  of  the  many  instances  of  a  solid  growth 
in  the  embryo  replacing  a  hollow  growth.  (Mclntosh  and  Prince, 
90. 1,  774,  give  their  own  and  cite  some  previous  observations.) 

In  amniota  the  anlage  of  the  liver  arises  in  the  same  position  as 
in  the  anamnia,  but  has  the  peculiarity  of  showing  its  bifurcation 
almost,  if  not  quite,  from  the  start,  at  least  in  birds  and  mammals. 
The  two  forks  embrace  between  them  the  omphalo-mesaraic  or 
vitelline  veins  just  before  they  empty  into  the  sinus  venosus.  In 
the  chick  the  anlage  appears  between  the  fifty-fifth  and  sixtieth 
hour  (Foster  and  Balfour,  "Elements,"  178,  179),  the  right  fork  being 
in  all  cases  of  greater  length  but  less  diameter  than  the  left.  In  the 
rabbit  (Uskow,  83.2,  220)  the  anlage  appears  during  the  tenth  day, 
and  on  the  eleventh  sends  out  branches;  according  to  Kolliker 
(•'Grundriss,"  372)  only  the  left  branch  appears  on  the  tenth  day, 
the  right  on  the  day  following.  In  man  the  anlage  is  well  marked 
in  embryos  of  three  millimetres  (His,  81.1,  Taf.  XL,  fig.  7-8,  also 
"  Anat.  Menschl.  Embry.,"  Heft  III.,  1G-17).  Hishasshown,  81.1, 
o22-->23,  that  the  liver  anlage  is  a  long  strip  on  the  ventral  side  of 
the  vorderdarm,  and  that  when  the  vorderdarm  is  separated  off 
from  the  yolk-sac  the  most  ventral  part  of  the  entoderm  of  the  vor- 
derdarm already  sh<  >ws  traces  of  the  hepatic  differentiation.  In  front 
of  and  above  the  heart  the  vorderdarm  is  completely  shut  off  from 
the  rest  of  the  archenteron  (cavity  of  the  future  yolk-sac) ,  but  imme- 
diately behind  the  heart  the  entoderm,  as  it  passes  from  the  vorder- 
darm around  the  edge  of  the  fovea  cardiaca,  and  so  out  on  to  the 
extra-embryonic  region,  is  caught,  so  to  speak,  and  forms  the  anlage 
of  the  liver,  so  that  the  liver  is  initiated  not  so  much  by  a  local 
growth  of  the  entoderm  as  by  retention  of  the  downward  extension 
of  the  layer,  which  results  from  the  manner  by  which  the  embryo  is 
separated  from  the  yolk.  The  point  is  important  as  an  illustration 
of  the  comparatively  simple  mechanical  factors  of  development. 

Relation  of  the  Liver  to  the  Septum  Trans versum. — The 
tissue  through  which  the  vitelline  veins  pass  to  enter  the  heart  forms 
a  transverse  partition,  which  divides  the  peri  card  ial  ccelom  from  the 
abdominal  coelom.  This  partition  is  the  rudiment  of  the  diaphragm, 
and  has  been  named  the  septum  transversum  by  W.  His.  It  lies 
just  behind  the  heart,  and  forms  the  ventral  edge  of  the  fovea  cardi- 
aca, or  opening  of  the  vorderdarm  into  the  general  archenteron ;  it 
is  overlaid  in  the  median  line  by  the  hind  end  of  the  vorderdarm, 
and  consequently  the  anlage  of  the  liver  is  situated  in  the  dorsal 
median  portion  of  the  septum.  As  the  great  veins  also  pass  through 
the  septum  to  reach  the  heart,  the  hepatic  anlage  comes  into  imme- 
diate contact  with  the  veins ;  in  their  further  development  the  veins 
and  entodermal  liver  are  closely  connected,  with  the  result  of  com- 
plex modifications  in  both  parts. 

Comparison  of  Mammalian  and  Amphibian  Archenteron. 
— For  the  convenience  of  students  I  have  inserted  the  accompanying 


370 


THE    EMBRYO. 


diagrams,  Fig.  154,  A  and  B.  They  are  extremely  conventionalized 
and  may  be  considered  especially  inaccurate  in  that  they  fail  to  show 
the  way  in  which  the  head  (and  with  it  the  vorderdarm)  projects 
forward,  and  in  that  the  heart  and  liver  are  omitted.  Emb  is  the 
axis  of  the  embryo  represented  in  nature  by  the  medullary  tube  and 
notochord ;  bl  is  the  blastopore  or  neurenteric  canal,  behind  which 

the  anal  opening  or  anal 


Emb, 


plate  should  be  added 
were  the  diagram  to  be 
completed.  All  is  the 
infra-blastoporic  diverti- 
culum  or  allantois;  Ent 
is  the  cavity  of  the  arch- 
enteron — the  letters  being 
placed  where  the  archen- 
teron  of  the  embryo  prop- 
er passes  into  that  of  the 
yolk-sac;  br  indicates  the 
four  gill-slits.  The  yolk- 
sac,  T%  is  represented  as 
enveloped  in  mesoderm, 
indicated  by  a  shaded 
layer  and  lined  by  ento- 
derm  which  is  indicated 
by  a  broad  black  line;  it 
must  be  remembered  that 
in  amphibians,  A,  the 
cavity  is  really  filled 
with  yolk-cells,  which  are 
represented  in  mammals, 
B,  only  by  a  layer  of  epi- 
thelial cells.  Ch  is  the 
chorion,  consisting  of  a 
layer  of  ectoderm  indi- 
cated by  the  outside  black 
line,  and  a  layer  of  meso- 
derm, indicated  by  shad- 
ing. Between  the  chorion 
and  the  yolk-sac  lies  a 
space  which  is  the  extra- 
embryonic  coelom.  In 
amphibia  this  part  of  the 
coelom  develops  gradually;  in  man  it  is  developed  very  early  com- 
pletely around  the  yolk-sac ;  in  rabbits  it  never  extends  more  than 
half-way  round,  and  other  variations  occur  in  other  mammals ;  to 
suggest  these  differences  in  mammals  the  lower  half  of  the  yolk-sac 
in  B  is  drawn  with  a  dotted  line  only ;  vt.  is  the  vena  terminalis. 

These  diagrams  suffice  to  show  that  the  closest  homologies  exist 
between  the  two  types,  however  much  the  actual  proportions  may 
differ.  The  primitive  homologies  of  the  archenteron  hold  true  of  all 
vertebrates. 


FIG.  154. — Diagrams  to  Indicate  the  Fundamental  Rela- 
tions of  the  Archenteron.  A,  in  amphibia;  B,  in  mammals. 
For  explanation  of  the  letters  see  tne  text. 


CHAPTER   XIII. 
THE  GERMINAL   AREA,  THE    EMBRYO  AND  ITS    APPENDAGES. 

I.     THE  GERMINAL  AREA. 

Definition. — The  germinal  area  (area  germinativa,  area  embry- 
oii  alis,  Keimhof,  airegerminative]  is  that  portion  of  the  meroblastic 
vertebrate  ovum  in  the  centre  of  which  the  embryo  is  differentiated. 
It  therefore  comprises  both  the  embryo  proper  and  the  region  imme- 
diately surrounding  it.  It  exists  in  all  amniota,  but  of  course  in  the 
higher  mammals,  owing  to  the  loss  of  yolk  in  the  ovum,  the  primitive 
relations  are  less  clear  than  in  Sauropsida.  The  area  is  further  char- 
acterized by  various  gradually  developed  peculiarities,  three  of  which 
deserve  special  mention.  To  take  them  in  the  order  of  their  appear- 
ance, the  three  peculiarities  are,  first ,  the  extension  of  the  archenteric 
cavity  under  nearly  the  whole  of  the  area ;  second,  the  extension  of 
the  coelom  over  nearly  the  whole  of  the  area ;  third,  the  development 
of  blood-vessels  and  blood  beginning  peripherally  in  the  splanchnic 
leaf  of  the  mesoderm  and  extending  gradually  into  the  embryo. 

1.  Extension  of  the  Archenteric  Cavity. — As  shown  in  the  pre- 
vious chapter,  only  a  small  part  of  the  archenteron  of  amniota  is 
taken  up  into  the  embryo,  and  the  rest  of  the  cavity  remains  as  the 
cavity  of  the  yolk-sac,  and  therefore  the  entoderm  of  the  area  belongs, 
for  the  most  part,  to  the  future  yolk-sac.     As  pointed  out  in  the  sec- 
tion on  the  entodermal  cells  in  the  preceding  chapter,  it  is  only  on 
the  upper  side  of  the  expanded  archenteron  that  the  entoderm  be- 
comes d  istinctly  cellular ;  on  the  lower  side  the  yolk  is  multinucleate, 
but  not  divided  into  discrete  cells ;  at  the  edge  of  the  expanded  cavity 
the  upper  cellular  layer  passes  gradually  into  the  yolk  and  the  region 
of  the  transition  is  known  as  the  germinal  wall,  the  structure  of 
which  is  discussed  in  the  chapter  on  the  yolk-sac.     As  previously 
pointed  out,  the  cells  very  early  assume  two  forms,  becoming  thin 
and  flattened  in  the  central  region  of  the  area,  and  remaining  as  long 
cylinder  cells  in  the  peripheral  zone ;  this  difference  results  in  a  greater 
transparency  in  the  central  zone,  which  has  accordingly  received  the 
name- of  area  pellucida,  while  the  peripheral  zone,  owing  to  its  rela- 
tively great  opacity,  has  been  named  the  area  opaca.      Another 
result  of  the  extension  of  the  archenteron  is  that  all  the  layers  above 
it  can  be  easily  removed  from  the  rest  of  the  ovum,  keeping  their 
natural  connections  otherwise  intact;  they  form  when  thus  removed 
a  thin  membrane,   which,   following  the  terminology  of  the  older 
embryologists,  we  commonly  speak  of  as  the  blastoderm ;  compare 
the  section  on  the  meroblastic  embryo,  p.  128. 

2.  The  extension  of  the  ccelom  of  course  divides  the  mesoderm 
into  an  upper  (somatic)  and  lower  (splanchnic)  layer.     But  the  divi- 
sion does  not  take  place  in  certain  definite  regions,  which  are,  1,  the 


THK    KMBKYO. 


primitive  streak;  2,  the  axis  of  the  embryo;  3,  the  proamniotic 
area,  in  which  for  a  long  period  there  is  no  mesoderm  in  amniota. 
It  might  also  be  added  that  as  the  mesoderm  is  excluded  from  the 
oral  and  anal  membranes  there  is  no  ccelom  in  them.  Throughout 
the  rest  of  the  germinal  area  the  ccelom  gradually  extends,  but  for  a 
long  time  it  fails  to  reach,  and  in  certain  animals  never  reaches,  the 
periphery  of  the  constantly  expanding  mesoderm.  The  history  of 
the  embryonic  ccelom  is  given  in  special  chapters,  the  history  of  the 
extra-embryonic  ccelom  is  indicated  in  the  section  of  this  chapter 
upon  the  origin  of  the  amnion. 

3.  The  appearance  of  the  blood-vessels  and  blood  has  been  con- 
sidered in  Chaper  X. ;  it  leads  to  the  differentiation  of  the  area  vas- 
culosa  (Gefasshof,  aire  vasculaire),  which  is  the  region  of  the 
extra-embryonic  circulation.  As  soon  as  the  embryonic  area  con- 
tains a  distinct  vascular  network,  there  appears  a  peripheral  vessel 
which  marks  the  boundary  of  the  area  vasculosa,  and  is  called  the 
sinus  terminalis.  The  vasculosa  does  not  reach  to  the  outer  bound- 
ary of  the  germinal  area,  so  that  the  region  of  the  blood-vessels  is 
inclosed  in  a  ring  which  is  known  as  the  area  vitellina. 

Topography. — The  first  differentiation  in  the  germinal  area, 
which  can  be  clearly  recognized  by  the  naked  eye,  is  the  appearance 

of  the  area  pellucida,  which 
pro.  am 


a.c.v 


is  shortly  followed  by  that  of 
the  primitive  streak,  Fig. 
78,  p.  131.  Further  prog- 
ress results  in  the  gradual 
differentiation  of  the  embryo, 
the  steady  expansion  of  the 
germinal  area  over  the  yolk, 
in  the  sharper  demarcation 
of  the  area  pellucida,  which 
becomes  pear-shaped,  and  in 
the  appearance  of  the  blood- 
vessels. Fig.  155  represents 
the  embryonic  area  of  a  hen's 
ovum  after  about  thirty 
hours'  incubation.  The  em- 
bryo is  well  advanced  in  de- 
velopment, for  although  the 
primitive  streak,  pr,  still  re- 
mains in  part  and  the  medul- 
lary groove,  Md,  is  still  open 
behind,  the  brain  is  already 
marked  out  and  the  head  has 
become  partly  free;  along- 
side the  medulla  lie  nine  pairs 
of  segments  (proto-vertebraa, 
auct.) ;  around  the  embryo  one  easily  recognizes  the  pear-shaped  area 
pellucida,  A.p,  and  the  darker  area  opaca,  Ao,  by  which  it  is  inclosed ; 
the  area  vasculosa  stands  out  conspicuously  and  is  bounded  by  the 
already  distinguishable  sinus  terminalis,  st;  around  and  under- 
neath the  head  is  the  translucent  proamniotic  area,  pro.  am,  from 


pr 


FIG.  155. — Chickeu  Embryo  and  Gum  Area,  after 
Twenty-seven  Hours'  Incubation,  fov,  Fovea  carclia- 
ca;  pro.  am,  proamniotic  area;  u.c.  •?>,  ainnio-cardial 
vesicle;  st,  sinus  terminalis;  pr,  primitive  groove; 
Ao,  area  opaca;  Ap,  area  pellucida.  After  Duval. 


THE    GERMINAL   AREA. 


273 


which  the  mesoderm  is  altogether  absent,  and  which  therefore  cannot 
contain  any  blood-vessels,  nor  are  there  at  this  stage  any  vessels  in 
front  of  the  proamnion. 

In  the  ovum  of  the  mammalia  there  occurs  a  modification  of  the 
ectoderm,  where  that  layer  is  attached  to  the  walls  of  the  maternal 
uterus.  The  region  over  which  the  attachment  takes  place  gives 
rise  in  the  higher  mammals  to  the  placenta.  Hence  the  area  of 
modified  ectoderm  may  be  called  the  placental  area.  It  has  been,  as 
yet,  very  little  studied.  As  it  is  not  possible  at  present  to  speak  in 
general  terms  of  the  embryonic  area  of  mammals,  I  confine  myself 
to  a  description  of  the  area  in  the  much-studied  rabbit,  following 


paa- 


OL.V. 


FIG.  156.— Embryonic  Area  of  a  Rabbit  of  Eleven  Days,with  the  Placental  Area  Partly  Torn  Off. 
After  Van  Beneden  and  Julin.  pr.  a,  Proainnion;  a.a,  amniotic  area,  approximately  identical 
with  the  area  pellucida;  a.  r.,  area  vaseulosa;  v.pl,  area  placentalis;  v.  t,  sinus  terminalis. 

Van  Beneden  and  Julin,  84.1.  The  germinative  area,  Fig.  156, 
is  nearly  circular,  and  at  the  stage  figured  shows  the  following 
peculiarities.  The  nearly  straight  embryo  lies  in  the  centre  and 
exhibits  plainly  the  central  nervous  system  and  the  proto- vertebra ; 
around  the  head  of  the  embryo  is  a  clear  space,  pr.  a.,  the  pro- 
amniotic  area,  over  which  no  mesoderm  is  developed ;  around  the 
sides  and  hind  end  of  the  embryo  is  another  light  place  which  con- 
tains mesoderm,  but  is  distinguished  by  the  retarded  vascularization ; 
this  is  the  amniotic  area,  a.a.,  and  is  converted  by  a  process  of  up- 
folding  into  the  amnion,  which  covers  the  posterior  portion  of  the 
rabbit  embryo.  The  remainder  of  the  germinal  disc  constitutes  the 
area  vaseulosa,  a.  v.,  with  the  terminal  sinus,  blood-islands,  etc. 
The  area  consists  of  two  membranes,  the  upper,  the  somatopleure, 
18 


274 


THE   EMBRYO. 


the  lower,  the  splanchnopleure ;  a  large  portion  of  the  former  behind 
the  embryo  has  been  torn  off,  a.  pi.;  this  defect  is  due  to  the  fact  that 
over  this  region  villosities  have  appeared,  and  a  close  connection 
established  between  this  region  and  the  uterine  wall ;  it  is  by  this 
means  that  the  ovum  is  attached ;  hence,  when  the  embryo  is  re- 
moved from  the  uterus,  this  area  of  the  splanchnopleure  (chorion) 
remains  adherent  to  the  uterus.  As  development  proceeds,  the 
allantois  grows  up  against  this  area,  over  which  the  differentiation 
of  the  placenta  takes  place ;  hence  the  name,  area  placentalis. 

Area  Vasculosa. — Soon  after  the  capillary  network  of  the  area 
opaca  and  pellucida  has  penetrated  the  embryo,  certain  lines  of  the 
network  begin  to  widen,  and  soon  distinctly  assume  the  size  and 
functions  of  main  trunks;  some  of  these  unite  with  the  posterior 


Orn.A. 


FIG.  157.— Diagram  of  the  Circulation  in  a  Chick  at  the  End  of  the  Third  Day,  as  seen  from  the 
Under  or  Ventral  Side.  The  embryo,  with  the  exception  of  the  heart,  Ht. ,  is  dotted;  Arc,  aortic 
arches;  D.C.,  ductus  Cuvieri;  Jug.,  jugular  vein;  card.,  cardinal  vein.  The  remaining  letters 
are  explained  in  the  text.  The  veins  are  black;  the  arteries  cross-lined. 

venous  end  of  the  heart,  which  has  meanwhile  been  formed  in  the 
embryo,  and  others  become  connected  with  the  anterior  or  aortic 
end ;  even  before  this  the  heart  has  begun  to  beat,  so  that,  as  soon  as 
all  connections  are  made,  the  primitive  circulation  starts  up.  The 
arrangement  of  the  vessels  is  not  the  same  in  birds  and  mammals, 
although  commonly  so  stated.  The  disposition  in  birds  is  indicated 
by  the  diagram  shown  in  Fig.  157,  in  which,  it  should  be  remem- 
bered, the  embryo  and  the  capillary  network  are  drawn  many  times 
too  large  in  proportion  to  the  area  vasculosa.  The  area  is  bounded 
by  a  broad  circular  vessel,  the  sinus  terminalis,  S.T.,  which  consti- 
tutes a  portion  of  the  venous  system  in  birds,  for  in  front  of  the  head 
of  the  embryo  the  sinus  leaves  a  gap,  and  is  reflected  back  along  the 
sides  of  the  body  of  the  embryo  to  make  two  large  veins,  which,  after 
uniting  with  other  venous  channels  coming  from  various  parts  of 


THE    GERMINAL   AREA. 


275 


the  area  vasculosa  on  each  side,  enter  the  embryo  as  two  large  trunks, 
Om.  r.,  known  as  the  omphalo-mesaraic  veins;  these  two  veins 
unite  in  a  median  vessel,  the  sinus  venosus,  S.V.,  which  runs 
straight  forward  and  enters  the  posterior  end  of  the  heart.  The 
sinus  venosus  also  receives  the  veins  from  the  body  of  the  embryo, 
namely,  the  jugulars,  Jug.,  and  cardinals,  card.;  the  former  from  in 
front  unite  each  with  the  cardinal  of  the  same  side,  making  a  short 
transverse  trunk,  known  as  the  duct  us  Cuvieri,  D.C.;  the  two 
ducts  empty  into  the  sinus  venosus.  The  entire  venous  current  is 
thus  brought  to  the  heart  in  a  united  stream ;  it  passes  out  through 
the  aorta ;  the  greater  part  ascends  the  aortic  arches  and  passes  back 
as  shown  in  the  figure,  Ao.,  and  divides  at  the  posterior  fork  of  the 
aorta,  the  bulk  of  the  two  currents  passing  out  through  omphalic 
arteries,  Oni.A.,  and  thence  to  the  capillaries  of  the  area  vasculosa 
and  so  on  to  the  venous  trunks  again.  As  shown  in  the  figure, 
which  presents  the  under  side  of  the  area,  the  left  omphalo-mesaraic 
vein  preponderates,  and  in  the  latter  stages  this  difference  becomes 
more  marked  until  finally  the  right  stem  is  very  inconsiderable  in 
comparison  with  the  great  left  vein.  The  time  at  which  the  dis- 
parity commences  is  extremely  variable,  as  is  also  the  degree  of 
inequality  between  the  two  veins. 

The  following  description  probably  represents  what  was  the  prim- 
itive condition  of  vessels   in    the  mammalian   area  vasculosa.     It 


FIG.  158.  —Area  Vasculosa  and  Embryo  of  a  Rabbit.     After  Van  Beneden  and  Julin. 

applies  to  an  early  stage  in  the  rabbit,  which  has  been  figured  by 
Bischoff,  42.1,  Tab.  XIV.,  Fig.  60,  whose  figure  is  copied  in  Kolli- 
ker's  "Grundriss,"  Fig.  90,  p.  109.  An  essentially  similar  arrange- 


276  THE    EMBRYO. 

ment  of  the  vessels  exists  also  at  a  corresponding  stage  in  the  dog, 
Bischoff,  45.1,  Taf.  VII.,  Fig.  37,  C.  The  veins  are  much  more 
symmetrical  than  in  the  chick,  and  have  the  same  general  plan ;  the 
sinus  terminalis  belongs  to  the  venous  system,  so  that  the  connection 
with  the  arterial  circulation,  found  later,  is  secondary ;  the  aorta  of 
the  embryo  is  double,  and  gives  off  on  each  side  (segmentally  ar- 
ranged?) transverse  branches,  one  of  which  develops  into  the  large 
trunk  shown  in  Fig.  158;  the  network  of  small  vessels  forms  two 
layers,  of  which  the  upper  is  connected  with  the  arteries,  the  lower 
with  the  veins.  The  change  from  the  earlier  condition  to  the  later 
has  still  to  be  followed. 

Selenka  has  figured  the  vascular  area  of  an  opossum,  86.1,  Taf. 
XXIII.,  Fig.  3,  in  a  condition  which  suggests  at  once  a  transition 
from  between  that  just  described  and  that  described  in  the  next 
paragraph;  the  figure  shows  the  veins  without  direct  connection 
with  the  sinus,  while  the  aorta,  though  it  gives  off  numerous 
small  branches,  has  extended  tailward  of  the  embryo  and  joined  the 
sinus. 

According  to  Van  Beneden's  recent  researches  on  the  rabbit  the 
arrangement  of  the  main  vessels  in  the  area  vasculosa  at  a  later  stage 
is  quite  different.  The  sinus  terminalis  forms  a  complete  ring,  Fig. 
158,  and  is  connected  with  the  arterial  system  by  a  single  trunk,  which 
corresponds  to  the  left  omphalic  artery  of  the  bird.  For  some  time  the 
connection  between  the  embryonic  arteries  and  the  area,  vasculosa  is 
entirely  through  capillaries,  and  the  arterial  trunk  on  the  vascular 
area  does  not  appear  in  the  rabbit  for  several  days.  There  are  two 
veins,  one  arising  from  each  side  of  the  body  and  passing  out  on  to 
the  area  vasculosa  over  the  back  of  the  embryo ;  they  are  the  two 
large  upper  vessels  in  the  figure. 

Growth  of  the  Vascular  Area. — As  the  blood-vessels  appear 
at  first  only  in  the  splanchnic  mesoderm,  the  vascular  area  belongs 
to  the  splanchnopleure,  or,  in  other  words,  is  part  of  the  wall  of  the 
yolk-sac ;  hence  the  circulation  of  the  area  is  often  spoken  of  as  the 
vitelline  circulation.  The  growth  of  the  vascular  area  is  therefore 
part  of  the  history  of  the  yolk-sac,  and  is  considered  now  from  con- 
venience merely.  The  expansion  of  the  vascular  area  is  due  to  the 
growth  and  differentiation  of  the  mesoderm,  and  in  those  mammals 
in  which,  as  in  the  rabbit,  the  mesoderm  extends  only  part  way  over 
the  yolk,  the  vascular  area  cannot  spread  over  the  whole  blastodermic 
vesicle ;  but  in  those  mammals  in  which,  as  in  man,  the  mesoderm 
grows  completely  around  the  yolk,  the  vascular  area  may  also  extend 
completely  around  the  yolk,  with  the  consequence  of  the  disappear- 
ance of  the  sinus  terminalis.  In  the  earliest  known  stages  of  man, 
the  yolk-sac  was  found  completely  vascularized. 

The  gradual  spread  of  the  area  vasculosa  over  the  yolk  may  be 
readily  followed  in  the  hen's  egg.  It  is  due,  as  just  stated,  to  the 
growth  and  differentiation  of  the  mesoderm.  The  size  of  the  vascu- 
lar area  is  very  variable,  but  the  following  table  represents  the 
approximate  sizes,  for  several  ages,  as  measured  on  blastoderms 
removed  from  the  yolk,  flattened  and  hardened;  the  total  circumfer- 
ence of  the  hen's  yolk  is  about  90  mm.  The  area  vasculosa  of  the 
chick  measures — 


THE    FORM    OF    THE    KMBRYO.  277 

At  2  days  about  9  mm.  in  transverse  diameter. 

«     25    u  u      15    « 

a     o          «  «     -i  q    u  ..  «  « 

u      o    ~     «  «      90     <t  ..  «  .c 

O .  f )  iC  tO 

a      f          a  t(     9p    u  ..  «  it 

"    4.5  "          "    30  "        "          "  " 

U        /•»  U  a        <  r\      ».  (.  u  (4 

It  is  not  until  the  seventeenth  day  of  incubation  that  the  yolk  is 
completely  overgrown  by  the  vascular  area,  Duval  "  Atlas,"  Fig.  651. 

II.     THE  FORM  OF  THE  EMBRYO. 

It  has  been  pointed  out  already  that  among  vertebrates  there  are 
two  principal  types  of  embryonic  form :  one,  which  is  the  more  prim- 
itive, characterized  by  the  yolk-mass  being  included  in  the  body  of 
the  embryo ;  the  other  is  secondary  and  characterized  by  the  separa- 
tion of  the  embryo  and  the  yolk. 

The  primitive  type  of  vertebrate  embryo  is  found  in  the 
lampreys,  ganoids,  and  amphibians ;  the  ventral  side  of  the  embryo 
is  very  much  distended  to  allow  room  for  the  yolk,  which  consists, 
after  the  segmentation  is  completed,  of  a  mass  of  cells,  which  lie  for 
the  most  part  below  the  archenteric  cavity,  as  cross-sections  show  at 
once.  As  the  development  progresses,  the  embryo  lengthens  out,  but 
the  swelling  caused  by  the  yolk  persists  for  a  long  period,  the  yolk 
material  being  only  gradually  resorbed  by  the  embryo ;  the  swelling  is 
readily  recognized,  even  up  to  larval  stages. 

The  secondary  type  of  vertebrate  embryo  is  found  in 
elasmobranchs  and  amniota.  In  elasmobranchs,  when  the  embryo 
appears  it  occupies  only  a  small  part  of  the  ovum,  which  is  very  large 
and  contains  much  yolk.  Soon  after  the  appearance  of  the  medul- 
lary groove,  the  head  of  the  embryo  begins  to  grow  forward  entirely 
free  from  and  above  the  yolk ;  and  by  the  time  the  medullary  groove 
is  converted  into  the  medullary  canal  the  tail  begins  to  grow  back- 
ward in  a  similar  manner  independently  of  the  yolk ;  hence,  only  the 
central  region  of  the  embryo  remains  connected  with  the  yolk.  As 
the  growth  of  the  embryo  continues,  while  the  area  of  its  body  at- 
tached to  the  yolk  increases  very  little  in  size,  it  follows  that  the 
connection  becomes  relatively  smaller,  until  it  becomes  merely  a 
narrow  stalk  as  compared  either  with  the  embryo  or  the  mass  of 
yolk.  The  traditional  and  often-repeated  description  of  the  separa- 
tion of  the  embryo  from  the  yolk  attributes  the  separation  to  a  fold- 
ing off  of  the  embryo  by  the  germ-layers  being  tucked  in  under  the 
embryonic  anlage,  but  it  seems  to  me  that  the  process  is  only  appar- 
ent, and  that  it  is  by  its  own  growth,  as  above  described,  that  the 
embryo  becomes  partly  separated  from  the  yolk ;  and  I  hold  the  same 
view  as  regards  the  amniota. 

The  yolk  is  covered  by  the  extra-embryonic  extensions  of  the  meso- 
derm  and  octoderm,  the  yolk  proper  being,  of  course,  entoderm.  If 
the  mesoderm  develops  a  coelomatic  fissure  around  the  yolk,  we 
have  the  non-embryonic  parts  of  the  ovum  converted  into  a  double 
sac;  an  outer  sac  formed  by  the  united  ectoderm  and  mesoderm 
(somatopleure) ,  and  an  inner  sac  of  mesoderm  filled  with  the  yolk- 


THE    EMBRYO. 

mass  (vitelline  entoderm) ,  the  two  representing  the  splanchiiopleure. 
The  outer  sac  in  all  vertebrates  may  be  called  the  chorion,  the  name 
by  which  it  is  known  in  mammalia ;  the  inner  sac  is  the  yolk-sac  or 
umbilical  vesicle. 

In  amniota,  the  separation  of  the  embryo  from  the  yolk  takes  place 
in  the  same  general  manner  as  just  described  for  elasmobranchs,  but 
there  are  additional  complications  due  to  the  development  of  the  am- 
nion  and  allantois  taking  place  very  early — see  the  following  division 
of  this  chapter. 

Form  of  the  Amniote  Embryo. — It  is  not  proposed  to  give 
here  a  comparative  account  of  the  forms  of  amniote  embryos  at  suc- 
cessive stages,  but  merely  to  briefly  indicate  the  characteristics  of 
the  stage  in  which  all  the  principal  anlages  of  the  primary  organs 
are  present,  but  not  specialized.  The  stage  may  be  taken  to  be  that 
of  the  hen's  ovum  at  fifty  to  sixty  hours  of  incubation,  Fig.  150.  The 
blastoderm  reaches  at  this  time  over  nearly  half  of  the  yolk,  the 
extreme  margin  of  the  opaque  area  being  near  the  equator,  but  the 
vascular  area  is  much  smaller,  being  only  about  20mm.  in  diameter; 
still  smaller  is  the  pear-shaped  area  pellucida,  in  the  centre  of 
which  lies  the  rapidly  growing  embryo.  At  this  period  the  vascular 
area  may  be  said  to  be  in  the  stage  of  its  most  complete  development ; 
for  though  it  will  afterward  become  larger,  it  will  at  the  same  time 
become  less  definite  and  relatively  less  important.  The  arterial  system 
already  has  its  main  trunks,  Fig.  157.  A.v.,  and  the  main  stems  of  the 
omphalo-mesaraic  veins,  om.  V,  are  differentiated.  As  regards  the 
embryo  the  most  striking  features  are  the  advanced  development  of 
the  head  and  the  slight  differentiation  of  the  tail.  The  head  has 
grown  forward  so  as  to  be  entirely  free  from  the  yolk,  and  is  turned 
so  that  its  left  side  rests  upon  the  yolk,  and  as  the  tail  end  of  the 
embryo  still  rests  symmetrically  upon  the  yolk,  it  follows  that  the 
intermediate  portion  of  the  body  is  twisted.  This  warping  or  tor- 
sion of  the  embryo,  in  order  that  the  side  of  the  flattened  head  may 
rest  upon  the  yolk,  occurs  in  Sauropsida  and  to  a  slight  extent  in  pla- 
cental  mammals,  but  not  among  any  of  the  Ichthyopsida.  We  must, 
therefore,  regard  it  as  a  special  feature  of  the  amniote  embryo,  which 
has  been  lost  in  the  placental  mammals,  probably  as  a  result  of  the  loss 
of  food  yolk  in  the  ovum.  The  head  is  remarkable  for  the  advanced 
differentiation  of  its  parts;  the  anlages  of  the  eye,  Fig.  150,  L,  and 
ear,  Ot,  are  present;  four  branchial  pouches  are  developed,  br*;  the 
heart  is  large  and  already  bent,  Ht;  the  medullary  tube  is  very  much 
dilated  and  distinctly  divided  into  its  three  primary  vesicles,  H, 
Mb,  Hb.  The  head  is  also  bent  at  the  region  of  the  mid-brain,  M b, 
so  as  to  form  almost  a  right  angle  with  the  axis  of  the  hind-brain, 
Hb,  and  neck.  This  head-bend  or  cervical  flexure  is  highly  charac- 
teristic of  all  vertebrates ;  it  is  beautifully  shown  in  elasmobranch 
embryos,  and  can  be  easily  recognized  in  all  classes.  It  is  a 
bend  in  the  median  plane  of  the  embryo  by  which  the  end  of  the 
head  is  brought  over  toward  the  heart,  Ht.  Following  along  back- 
ward we  encounter  the  first  distinct  segments  just  behind  the  oto- 
cyst,  Ot,  and  can  follow  them  some  distance  behind  the  vitelline 
arteries,  until  they  merge  into  the  undivided  segmental  zone,  Ar; 
the  limit  of  the  body  of  the  embryo  is  already  indicated  by  the 


THE   FORM   OF  THE   EMBRYO.  279 

parietal  zone,  but  the  zone  will  be  encroached  upon  by  the  vas- 
cular area,  and  the  whole  zone  of  this  stage  is  not  destined  to  be 
included  in  the  body  of  the  embryo. 

In  a  sheep  embryo,*  although  the  fundamental  characteristics  are 
the  same,  there  are  many  minor  differences  both  from  the  chicken 
and  the  rabbit.  The  most  striking  peculiarities  of  the  embryo  are 
due  to  the  foetal  appendages,  the  development  of  which  presents 
special  modifications  ki  ruminants,  as  more  fully  described  in  the 
next  division  of  this  chapter;  the  yolk-sac  is  long  and  narrow,  and  is 
connected  by  a  broad  twisted  yolk-stalk  with  the  embryo ;  the  allan- 
tois  has  already  become  a  very  large  transversely  expanded  vesicle ; 
the  amnion  invests  the  embryo  closely  and  gives  off  a  long  cord 
(Amnionstrang) ,  by  which  it  is  still  attached  to  the  chorion.  The 
embryo,  5  mm.  in  length,  is  curving  throughout  its  lengthy  the 
head-bend  is  developed,  and  consequently  the  end  of  the  head  lies 
near  the  heart ;  the  torsion  of  the  whole  embryo  is  very  marked,  the 
dorsal  side  of  the  fore- brain  facing  us,  of  the  neck  being  turned  away 
from  us,  of  the  tail  facing  us  again ;  the  embryo  makes  nearly  one 
complete  spiral  turn.  The  head  is  small,  laterally  compressed,  and 
less  advanced  than  in  the  chick  described  above,  for  the  anlage  of 
the  eye  is  only  just  begun ;  that  of  the  ear  is  not  differentiated,  and 
the  first  two  visceral  arches  are  present,  while  the  third  is  only  just 
beginning.  The  medullary  groove  is  still  open  in  the  region  of  the 
forebrain,  and  widely  open  at  its  tail  end,  but  closed  throughout  the 
rest  of  its  length ;  there  are  fourteen  segments ;  none  of  the  vessels 
yet  contain  any  red  blood. 

Typical  Embryo  in  Cross-Section. — For  this  purpose  I  select 
a  dog-fish  embryo.  The  following  description  is  intended  especially 
for  the  convenience  of  students.  The  body  is  bounded  by  a  single 
layer  of  ectodermal  cells,  EC,  the  anlage  of  the  future  epidermis ; 
the  central  nervous  system,  Md,  appears  as  a  tube,  with  very  much 
thickened  cellular  walls ;  it  lies  on  the  dorsal  side  of  the  embryo,  and 
although  developed  from  the  outer  germ-layer,  has  no  connection 
with  the  ectoderm;  below  the  nervous  system  lies  the  very  large 
notochord,  nch,  which  contains  a  loose  network  in  its  centre,  and  a 
denser  peripheral  layer  of  cells;  it  is  invested  by  a  thin  hyaline 
structureless  sheath;  the  notochord  as  we  ascend  the  vertebrate 
series  diminishes  in  size;  at  corresponding  stages  in  amphibians  it 
is  decidedly  smaller  in  proportion  to  the  medullary  tube  than  in 
sharks — in  birds  its  diameter  is  not  more  than  a  fifth — in  mammals 
not  more  than  a  twelfth  of  the  diameter  of  the  medullary  tube.  Be- 
low the  notochord  comes  the  dorsal  aorta,  Ao,  on  either  side  of  which, 
a  little  lower  in  position,  may  be  seen  a  cardinal  vein,  c.  V,  while 
between  the  notochord  and  aorta  is  a  small  string  of  cells  known  as 
the  subnotochordal  rod  or  hypochorda,  a  structure  which  has  not  yet 
been  observed  in  any  of  the  amniota.  The  body-cavity  proper,  or 
splanchnoccele,  Coe,  is  a  wide  space,  bounded  externally  by  the  body 
walls,  /Sow,  and  containing  the  intestinal  canal,  In,  which  has  been 
developed  from  the  splanchnopleures,  and  which  is  suspended  from 
the  dorsal  wall  by  the  membranous  mesentery ;  the  cavity  of  the  in- 
testine is  lined  by  entoderm,  En,  and  takes  a  spiral  course  which  is 

*See  Bonnet,  89.1,  Fig.  13. 


280 


THE   EMBRYO. 


characteristic  of  the  elasmobranchs,  but 
classes ;  the  abdominal  cavity  is  lined  by 


Md 


FIG.  159.— Transverse  Section  of  the  Rump  of  a  Dog-Fish 
Embryo,  14  mm.  long.  EC,  Ectoderm;  Md,  medullary 
tube;  My,  myotome;  nch,  notochord;  Mus,  muscle;  Ao, 
aorta;  c.  V,  cardinal  vein;  s.s,  segmental  tubule;  S.  D, 
segmental  or  Wolfflan  duct ;  Coe,  ccelom ;  Mst,  mesentery 
(the  reference  line  has  been  omitted) ;  Som,  sornatopleure ; 
En,  entoderm;  si.  V,  sub-intestinal  vein. 


is  not  encountered  in  other 
the  epithelial  mesoderm  or 
mesothelium.  The  prim- 
itive longitudinal  urogeni- 
tal  duct  appears  in  cross 
section  just  above  the 
splanchnoccele,  Coe,  while 
near  it  on  one  side  can  be 
seen  the  opening  of  one 
of  the  transverse  Wolffian 
or  segmental  tubules,  st, 
which  has  been  developed 
from  the  nephrotomic  por- 
tion of  the  primitive  seg- 
ment ;  if  the  tubule  is  fol- 
lowed out  its  other  end  is 
found  to  open  into  the 
Wolffian  duct ;  in  amniota 
the  opening  into  the  body- 
cavity  is  lost  at  a  much 
earlier  stage.  The  myo- 
tome, My,  which  also  is 
developed  from  the  prim- 
itive segment,  is  a  double 
plate,  its  two  walls  being 
so  closely  appressed  that 
the  cavity  between  them  is 
completely  obliterated ;  the 
inner  wall  is  partly  con- 
verted into  muscular  tis- 
sue. The  mesenchyma, 
mes,  has  grown  more  than 
any  other  tissue,  and  con- 
stitutes in  bulk  the  greater 
part  of  the  embryo;  it  is 
destined  before  adult  life 
is  attained,  to  be  differen- 
tiated into  a  large  variety 
of  tissues. 


III.    ORIGIN  OF  THE  FCETAL  APPENDAGES. 

Under  this  head  we  have  to  consider  the  origins  of  the  chorion, 
yolk-sac,  allantois,  proamnion  and  amnion,  but  as  we  have  already 
considered  the  development  of  the  yolk-sac.,  p.  255,  the  allantois,  p. 
257,  and  the  proamnion,  p.  150,  we  shall  recur  to  them  now  inciden- 
tally only,  and  concern  ourselves  principally  with  chorion  and  amnion. 

Extension  of  the  Extra-Embryonic  Coelom. — The  distance 
to  which  the  coelom  can  extend  around  the  ovum  depends  upon  the 
extension  of  the  mesoderm,  for  of  course  the  cavity  cannot  go  farther 
than  the  layer  within  which  it  is  developed.  Now,  as  we  have  seen, 
the  mesoderm  expands  gradually  and  a  little  more  slowly  than  the 


ORIGIN    OF    THE    FOSTAL,    APPENDAGES.  281 

germinal  area.  This  gradual  expansion  occurs  in  all  vertebrates. 
In  the  primitive  type  (Petromyzon  and  amphibians)  the  mesoderm 
and  the  coelom  both  grow  completely  around  the  yolk ;  and  this  was 
undoubtedly  the  primitive  condition,  but  in  the  lower  amniota  the 
growth  of  the  mesoderm  has  to  be  much  greater  in  order  to  cover 
the  enormous  yolk  mass ;  hence  in  amniota  the  spread  of  the  meso- 
derm is  slow  and  long  continued,  and  the  embryo  advances  far  in 
its  development  before  the  yolk  is  inclosed.  In  mammals  the  expan- 
sion of  the  mesoderm  over  the  yolk-sac  is  also  slow,  and  in  rabbits 
(and  probably  in  their  allies)  the  mesoderm  never  extends  over  the 
whole  yolk-sac,  but  in  man,  on  the  contrary,  the  ccelom  as  well  as 
the  mesoderm  are  developed  completely  around  the  yolk-sac  very 
early.  No  explanation  of  these  differences  among  mammalia  can  be 
offered  at  present. 

In  the  lampreys  and  amphibians  the  appearance  of  the  coelom 
around  the  yolk  merely  completes  the  separation  of  the  body- wall  or 
somatopleure  of  the  embryo.  In  the  amniota  it  also  separates  the 
somatopleure  from  the  splanchnic  mesoderm  around  the  yolk,  but 
owing  to  the  division  of  the  developing  ovum  into  embryo  proper 
and  yolk-sac,  only  a  small  part  of  the  somatopleure  shares  in  the  for- 
mation of  the  embryo,  while  the  rest  acts  as  a  covering  membrane 
of  the  yolk.  This  membrane  in  the  mammalia  is  universally  known 
as  the  chorion,  and  I  shall  apply  this  name  to  it  hereafter  for  all 
vertebrates. 

Primitive  Chorion.  —The  chorion  has  been  defined  by  Minot 
(Buck's  "Handb.,"IL,  143  )  to  be  the  whole  of  that  portion  of 
the  extra-embryonic  somatopleure  ichich  is  not  concerned  in  the 
formation  of  the  amnion.  The  term  primitive  chorion  may  be 
employed  for  the  whole  extra-embryonic  somatopleure  before  the 
differentiation  of  the  amnion  from  it,  and  the  term  chorion  or  true 
chorion  be  still  used,  as  defined,  for  what  remains  of  the  membrane 
after  the  separation  of  the  amnion. 

The  somatopleure  consists  of  two  layers — the  ectoderm  and  somatic 
mesoderm.  The  ectoderm  consists  of  a  single  layer  of  epithelial 
cells.  The  mesoderm  consists  of  a  layer  of  mesothelium  next  the 
coelom,  and  a  thicker  layer  of  mesenchyma  between  the  mesothelium 
and  ectoderm.  The  exact  appearances  of  these  layers  are  described 
with  the  aid  of  figures  in  the  special  chapters  on  the  amnion  and 
chorion. 

Origin  of  the  Amnion. — The  amnion  is  developed  out  of  that 
part  of  the  extra-embryonic  somatopleure  which  immediately  sur- 
rounds the  embryo  and  the  proamniotic  area,  or  in  other  words,  the 
amniotic  region  of  the  germinal  area  is  part  of  the  area  pellucida,  and 
perhaps  includes  the  whole  of  the  pellucida.  The  amnion  owes  its 
development  to  the  expansion  of  the  ccelom.  In  the  Sauropsida  the 
process  is  about  the  same  in  all  forms,  but  in  mammals  there  are 
several  modifications  of  the  development  known ;  hence  we  consider 
first  the  sauropsidan,  then  the  mammalian  types. 

In  the  Sauropsida  the  formation  of  the  amnion  begins  with  the 
appearance  of  the  amnio-cardial  vesicles,  p.  198,  which  form  con- 
spicuous dilatations  011  either  side  of  the  neck,  Fig.  117;  the  vesicles 
steadily  enlarge  and  spread  laterally  and  forward  so  as  to  inclose  the 


282 


THE   EMBRYO. 


proamniotic  area,  and  finally  fuse  in  front  of  it.  The  dilatation  takes 
place  in  such  a  manner  that  the  splanchnopleure  is  bent  down 
slightly,  while  the  somatopleure  is  bent  upward  to  an  extreme  de- 
gree, forming  a  sort  of  dome.  Transverse  sections  of  a  chick  at  this 
stage,  at  the  level  of  the  heart,  show,  Fig.  117,  the  amnio-cardial 
vesicle  of  each  side  fused  with  its  fellow  in  the  median  line  below 
the  heart,  Hi;  the  somatopleure,  Som,  of  the  embryo  makes  a  sharp 
turn  outward  and  upward,  Am,  and  then  bends  away  again,  Cho, 
from  the  embryo  and  finally  joins  the  splanchnopleure  of  the  yolk, 
Spl.  As  the  upbending  of  the  somatopleure  goes  on  around  the  entire 
head  of  the  embryo,  it  follows  that  the  cephalic  end  of  the  embryo 
lies  in  a  depression,  the  sides  of  which  are  formed  by  a  part,  Am,  of 
the  extra-embryonic  somatopleure.  While  this  is  going  on,  the  head 
of  the  embryo  bends  over,  and  the  whole  head  gradually  rolls  over 
ventralward  and  thus  is  forced  into  the  yolk,  but  since  the  proamni- 


Cho 


FIG.  160.  —Section  through  the  Rump  of  a  Rabbit  Embryo  of  Eight  Days  and  Three  Hours.  Md, 
Medullary  tube;  Seg,  primitive  segment;  C/io,  chorion;  Am,  aninion ;  Sow,  somatopleure  of 
embryo;  Coe,  coelom:  -SpJ,  splanchnopleure;  Ent,  entoderm;  C%,  notochord;  Ao,  aorta. 

otic  area  lies  just  here,  it  is  invaginated  along  with  the  head,  and 
consequently  the  head  seems  covered  by  a  proamniotic  membrane, 
which  is  known  as  the  cephalic  cap  (Kopfkappe,  capuchon  cepha- 
lique).  This  cap  is  very  noticeable  in  young  chicks,  for  the  head  is 
hidden  in  it,  while  the  rest  of  the  embryo  is  uncovered.  The  actual 
relations  are  still  further  complicated  by  the  singular  fact  that  the 
edge  of  the  cap  is  extended  backward  by  the  growth  of  the  ectoderm 
alone,  as  shown  by  Duval;  the  backward  growth  of  the  ectoderm 
occurs  also  in  turtle  embryos,  and  to  a  much  greater  distance  than 
in  birds  before  the  mesoderm  follows  it  (K.  Mitsukuri,  90.1). 
Sooner  or  later  the  mesoderm  penetrates  the  3ctodermal  fold,  and  the 
ccelom  appears  in  it  as  a  forward  extension  of  the  cavities  of  the 
amnio-cardial  vesicles. 

The  cephalic  end  of  the  embryo  now  soon  becomes  completely  cov- 
ered over  by  the  extra-embryonic  somatopleure ;  this  is  due  to  the 
expansion  of  the  ccelom  on  all  sides. 

The  changes  in  the  extra-embryonic  somatopleure  around  the  pos- 


ORIGIN    OF    THE    FCETAL    APPENDAGES. 


283 


terior  half  of  the  embryo,  are  similar  in  a  general  way  to  those 
around  the  anterior  half,  but  the  dilatation  of  the  coelom  is  confined 
to  the  extra-embryonic  region,  hence  the  pictures  obtained  from  cross 
r<ections  of  the  two  parts  of  the  embryo  present  certain  essential 
differences.  Fig.  161  is  a  section  through  the  rump;  here  we  see 
that  the  dilatation 
of  the  coalom  causes 
the  somatopleure  to 
form  a  longitudinal 
fold  along  each  side 
of  the  embryo ;  each 
fold,  passing  back- 
ward, joins  its  fel- 
luw  behind  the  em- 
bryo, so  that  they 
may  be  described 
conjointly  as  the 
tail-fold  (Sckwanz- 
k'oppe,  capuchon 
caudale).  The  tail- 
fold  is  developed 
considerably  later 
than  the  head-fold, 
but  as  one  grows 
forward  and  the 
other  grows  back, 
they  finally  meet 
and  constitute  the 
complete  amniotic 
fold  around  the  en- 
tire embryo.  The 
tail -fold  gradually 
closes  over  the  em- 
bryo; the  proc<vs 
may  be  understood 
from  the  accom- 
panying figures. 
Fig.  100  represents 
a  cross-section  of  a 
rabbit  embryo.  The 
somatopleure,  Soin, 
of  the  embryo  bends 
over  as  the  amnion, 
Am,  so  as  to  cover 
the  embryo,  above 
which  it  again 
bends  outward  as  the  chorion,  Clio;  we  can  already  distinguish  the 
embryonic,  amniotic,  and  chorionic  portions  of  the  somatopleure 
from  one  another ;  where  the  amniotic  portion  joins  the  chorionic, 
the  edge  is  prolonged  by  a  thickening  of  the  ectoderm,  which  re- 
minds us  of  the  similar  thickening  at  the  edge  of  the  cephalic  cap; 
the  two  edges  have  almost  met  over  the  back  of  the  embryo;  the 


ft 


284  THE    EMBRYO. 

asymmetry  of  the  folds  exists  in  all  amniota  and,  as  shown  in 
the  figure,  is  very  marked  in  the  rabbit,  but  is  much  less  marked 
in  the  Sauropsida.  In  the  next  stage,  Fig.  161,  the  folds  have 
actually  met ;  their  edges  grow  together  by  their  ectodermal  thick- 
ening ;  for  some  time  the  thickened  ectoderm  persists  and  offers  in 
sections  a  characteristic  feature ;  after  a  time  the  mesoderm  grows 
across,  and  the  ectoderm  of  the  amnion  is  entirely  separated  from 
that  of  the  chorion ;  still  later  the  cavity  of  the  chorion  also  pene- 
trates and  completes  the  final  separation  of  the  amnion  from  the 
chorion,  Fig.  19.  The  process  of  separation  is  essentially  the  same 
in  the  case  of  the  cephalic  amnion.  The  separation  of  the  amnion 
from  the  chorion  progresses  most  rapidly  at  the  head  end ;  at  the  tail 
end  it  begins  later  and  progresses  forward ;  hence  the  portion  of  the 
amnion  over  the  middle  of  the  rump  is  the  last  to  be  formed,  as  can 
at  once  be  seen  if  the  fresh  ovum  be  examined. 

In  surface  views  the  gradual  closure  of  the  amniotic  folds  over  the 
embryo  can  be  beautifully  followed;  for  example,  in  the  hen's  ovum 
incubated  about  sixty  hours,  we  find  the  anterior  half  of  the  embryo 
entirely  hidden  by  the  cephalic  cap,  while  the  posterior  third  of  the 
rump  is  also  covered  by  the  tail-fold,  and  at  the  sides  of  the  rump  the 
amniotic  folds  have  partially  arched  over  the  embryo.  These  ar- 
rangements leave  a  small  longitudinal  oval  opening  through  which 
we  can  look  down  upon  the  embryo.  The  opening  gradually  dimin- 
ishes as  the  edges  of  the  folds  advance,  and  is  finally  closed  by  the 
meeting  of  the  edges  from  all  sides.  As  the  edges  have  their  ecto- 
derm thickened,  their  final  meeting  is  marked  by  a  local  thickening 
of  the  ectoderm,  which  persists  for  some  time  after  the  actual  closure. 
In  ruminants  the  connection  between  the  amnion  and  chorion  at  the 
point  of  final  closure  is  retained  for  a  long  time  by  means  of  tissue, 
which  grows  out  into  a  long  thread,  the  so-called  amniotic  cord 
( Amnionst rang) .  A  somewhat  similar  structure  occurs  in  the 
opossum,  Selenka,  87.1,  Taf.  XXV.,  Fig.  2. 

After  the  amniotic  folds  have  closed,  the  embryo  is  surrounded  by 
two  membranes,  both  derived  from  the  extra-embryonic  somato- 
pleure.  Of  these  the  outer  is  the  true  chorion,  Fig.  19,  C7io,  the 
inner  the  amnion ;  from  the  manner  of  their  formation  the  former 
has  its  ectodermal  layer  external,  the  latter  its  ectoderm  internal  or 
facing  the  embryo.  The  amnion,  Fig.  19,  Am,  is  the  direct  pro- 
longation of  the  somatopleure  of  the  embryo ;  the  space  between  the 
amnion  and  the  embryo  is  called  the  amniotic  cavity;  it  is  lined 
throughout  by  ectoderm. 

In  mammals  the  development  of  the  amnion  was  presumably  at 
first  like  that  in  the  Sauropsida,  for  not  only  do  we  find  many  traces 
of  it  still  preserved,  but  also  Selenka,  86.1,  130,  has  shown  that  in 
the  opossum  the  sauropsidan  stage  is  passed  through,  although  some- 
what modified  by  the  excessive  development  of  the  proamnion.  The 
increased  importance  of  the  proamnion  can  be  seen  also  in  the  rabbit 
(Van  Beneden  and  Julin) ,  and  is  possibly  characteristic  of  mammals 
as  a  class.  In  the  two  animals  mentioned,  more  than  half  of  the  embryo 
is  covered  by  the  proamnion  at  the  time  the  amnion  closes,  and  hence 
the  amnio-cardial  vesicles  cannot  attain  the  size  or  importance  they 
have  in  birds,  and  they  are  unable,  in  the  opossum,  to  extend  around 


ORIGIN   OF   THE   FCETAL,   APPENDAGES. 


285 


the  proamniotic  area ;  hence  in  front  of  that  area  there  is  no  coelom 
developed,  the  three  germ-layers  remaining  in  close  contact  and 
forming,  as  it  were,  a  single  membrane;  in  the  rabbit  the  ccelom 
does  appear  in  front,  as  in  birds. 

In  ruminants  the  amnion  appears  very  early,  the  folds  being  well 
advanced  before  the  medullary  groove  appears.  The  formation  of 
the  amnion  is  induced  by  the  precocious  development  of  the  extra- 
embryonic  coelom,  which,  as  Bonnet's  researches  on  the  sheep,  89. 1, 
have  proven,  extend  very  early  around  the  embryo  in  a  wide  oval 
ring,  which,  by  raising  the  somatopleure,  forms  an  annular  amniotic 
fold,  before  the  embryo  can  be  said  to  be  differentiated ;  these  folds 
close  over  the  anlage  of  the  embryo,  and  by  their  union  produce  the 
two  foetal  membranes,  amnion  and  chorion,  in  the  same  manner  as 
in  birds ;  as  already  mentioned,  there  is  formed  at  the  point  of  closure 
a  long  cord  of  tissue  (funiculus  amnii) ,  by  which  the  two  mem- 
branes remain  united  for  a  considerable  period. 

In  the  rodents  with  so-called  inversion  of  the  germ-layers  (e.  </., 
guinea-pigs,  rats,  mice,  etc.),  the  development  of  the  amnion  is  ex- 
tremely modified  from  the  original  type.  The  cavity  of  the  Trager, 
Fig.  87,  a,  becomes  in  part 
the  cavity  of  the  amnion.  The  ..  .  A 

*•«•».««  "I  Am.  wiiJ-i/i..  •*•    *- 

manner  in  which  this  takes 
place  and  the  way  in  which 
the  process  may  be  deduced 
from  the  primitive  mode  of 
development  are  both  well  il- 
lustrated by  Selenka's  dia- 
grams, 84.1,  Taf.  Xyi. 

The  hunt  an  amnion  in  the 
earliest  stages  yet  known  has 
been  found  completely  closed 
over  the  embryo,  so  that  noth- 
ing is  known  as  to  its  devel- 
opment by  direct  observation. 
The  earliest  known  disposition 
was  first  described  by  W.  His, 
whose  account  has  been  con- 
firmed by  subsequent  observ- 
ers. The  embryo  is  from  2.5 
to  3.0  mm.  long;  its  relations 
to  the  rest  of  the  ovum  are  in- 
dicated by  the  diagram,  Fig. 
162,  B ;  it  rests  upon  the  large 
yolk-sac,  F,  and  is  connected 
by  a  short  stalk,  Al,  with  the 
chorion,  Ch.  The  amnion 
arises  under  the  head  at  the 
junction  of  the  embryo  and  ^ 
yolk-sac,  and  from  the  sides  of 

the  embryo  and  from  the  allantoic-stalk,  and  arching  over  the 
dorsal  side  of  the  embryo  completely  incloses  it.  To  explain  this 
disposition  His  has  advanced  the  following  hypothesis  as  to  the 


FIG.  162.— Diagrams  to  illustrate  His1  theory  of 
the  Origin  of  the  Human  Amnion :  A.  First  stage ;  B, 
second  stage.  Am,  Amnion;  Al,  allantoic-stalk  or 
Bauchstiel ;  C7t,  chorion,  the  villi  of  which  are  drawn 
smaller  and  more  numerous  than  in  nature ;  F,  yolk- 


286  THE    EMBRYO. 

course  of  development.  The  embryo  arises  upon  the  surface  of  the 
blastodermic  vesicle  in  the  usual  manner ;  its  somatopleure  passes 
over  into  the  primitive  chorion,  which  is,  at  an  extremely  early 
age,  completely  separated  from  the  yolk-sac;  the  chorion  now 
forms  a  fold,  as  shown  in  Fig.  162,  A,  which  arches  backward  over 
the  head  of  the  embryo;  while  the  tail  end  of  the  embryo,  retaining 
its  direct  connection  with  the  chorion,  becomes  the  allantoidean 
stalk,  Al.  The  head-fold,  of  which  the  inner  leaf  is  the  amnion, 
Am,  the  outer  leaf  a  part  of  the  true  chorion,  grows  backward  over 
the  embryo  as  indicated  by  the  dotted  line,  Am',  until  it  finally 
reaches  the  allantois-stalk,  Al,  and  thus  completely  covers  in  the 
embryo.  This  hypothesis  is  probably  correct,  but  it  is  possible  that 
the  amnion  is  preceded  by  a  true  proamnion,  which  becomes  obliter- 
ated very  early  by  the  precocious  development  of  the  mesoderm  and 
the  coelom  in  the  human  ovum.  If  Graf  Spee's  plausible  sugges- 
tion, 89. 1.  170,  that  there  is  a  so-called  inversion  of  the  germ-layers 
in  the  human  embryo  be  verified,  then  we  shall  probably  find  that 
the  human  amnion  is  developed  according  to  the  rodent  type  men- 
tioned above. 

The  True  Chorion  is  that  portion  of  the  extra-embryonic  soma- 
topleure which  remains  around  the  ovum  after  the  separation  of  the 
amnion ;  it  consists  of  an  outer  layer  of  ectoderm  and  an  inner  layer 
of  mesoderm;  the  cavity  within  it  is  part  of  the  coelom.  By  the 
closure  of  the  amniotic  folds  the  chorion  becomes  a  membrane  sur- 
rounding all  the  other  parts  of  the  ovum,  and  makes  a  complete  bag, 
which  is  termed  the  chorionic  vesicle.  The  chorion  is  the  outermost 
of  the  foetal  envelopes.  It  is  sometimes  termed  the  serous  membrane 
or  envelope  (membrana  serosa,  serose  Hulle) ,  especially  in  writings 
on  sauropsidan  embryology.  Its  relations  may  be  rendered  clear  by 
the  help  of  diagrams,  Figs.  20  and  19. 

IV.  KNOWN  HUMAN  OVA  OF  THE  SECOND  AND  THIRD  WEEKS. 

As  no  synopsis  has  ever  been  made  of  our  knowledge  of  the  early 
stages  of  man,  I  have  attempted  to  collate  all  the  descriptions  of 
embryos  not  over  three  weeks.  A  summary  of  the  descriptions  is 
given,  p.  308. 

Classification  by  Stages. — Any  attempt  to  divide  embryos  into 
stages  must  necessarily  establish  artificial  groups,  for  in  nature  there 
is  no  demarcation.  Division  into  stages  is  for  convenience,  and 
ought,  therefore,  to  be  made  by  natural  and  obvious  characteristics. 
After  much  deliberation  I  have  chosen  eight  stages,  which  seem  to 
me  natural  and  convenient,  and  I  have  classified  the  thirty-eight 
embryos  reviewed  in  the  preceding  pages,  placing  them  according 
to  my  best  judgment  in  their  respective  stages ;  when  the  assignment 
is  doubtful  I  have  indicated  it  by  an  interrogation  mark. 
First  Stage:  Appearance  of  the  primitive  streak. 

1.  Reichert's. 

2.  Breus'. 

3.  Wharton  Jones'. 

4.  Ahlfeld's. 

5.  Beigel  and  Lowe's. 


KNOWN    HUMAN    OVA. 

1 .   Kollmaim's  a. 

8.  "  b. 

9.  Schwabe's. 

Second  Stage:  Appearance  of  the  medullary  plate. 

10.  W.  His'XLIV.  (Bff). 

11.  Keibel's. 

12.  Spee's. 

Third  Stage:  Appearance  of  the  medullary  groove. 

13.  W.  His'  E. 

14.  Allen  Thomson's  No.  I. 

15.  W.  His'  SR. 

16.  Allen  Thomson's  No.  II. 

Fourth  Stage:  Formation  of  the  heart  and  medullary  canal. 

17.  Spee's  second  embryo. 

18.  Kollmann's  embryo  of  2.2  mm. 

19.  Von  Baer's  youngest  ovum. 
Fifth  Stage:  First  external  gill-cleft. 

None. 
Sixth  Stage:  Two  external  gill-clefts. 

20.  Minot's  No.  195. 

21.  No.  143. 

22.  W.  His'  LXVIII.  (Lg). 

23.  "       "     LXVI.  (Sch.  I.). 

24.  "       "     L. 
24A.  Janosik's. 

25.  Coste's. 

26.  Schroeder  van  der  Kolk's. 
?  27.  Hennig's. 

??  (9.  Schwabe's.) 
?  28.  Remy's. 

Seventh  Stage:  Three  external  gill-clefts. 
28A.  Chiarugi's. 

29.  W.  His',  Rf. 

30.  "  M. 

31.  "  BB. 

32.  "  Lr. 

33.  Allen  Thompson's  No.  III. 
?  35.  Ecker's. 

?  36.   Hecker's. 
?    5.   Beigel's  (abnormal). 
Eighth  Stage:  Four  external  gill-clefts. 
?  34.  Von  Baer's. 
?  37.  Johannes  Miiller's. 
38.  R.  Wagner's. 

Descriptions  of  the  Known  Ova.  1. — Reichert's  ovum,  73.1, 
was  thought  by  him  to  be  twelve  or  thirteen  days  old,  and  probably 
correctly  so,  as  it  was  obtained  at  a  post-mortem  examination  of  a 
young  German  girl  under  circumstances  which  render  the  estimate 
of  the  age  quite  trustworthy.  The  ovum  itself  was  very  imperfectly 
examined  by  Reichert,  whose  very  lengthy  memoir  deals  largely  with 
cognate  subjects  and  contains  much  speculative  matter.  The  actual 


288 


THE    EMBRYO. 


FIG.  163.  —  Reichert's  Ovum. 
Two  views  engraved  from  the 
original  plate 


description  of  the  ovum  is  brief  (pp.  25-28)  ;  but  as  far  as  he  went 
Rei chert  worked  with  exemplary  accuracy,  which  gives  value  to  his 
research.  The  ovum  in  question  was  a  flattened  sphere  with  a  short 
diameter  of  3.3  mm.,  and  an  equatorial  diameter  of  5.5  mm. ;  smooth 
around  both  poles,  and  with  a  marginal  or  equatorial  zone  of  villi 

separating  the  two  smooth  areas.  The 
smaller  and  flatter  of  these  two  areas  faced 
the  uterine  wall  and  bore  on  its  inner  sur- 
face (i.e.,  within  the  ovum)  a  small  accu- 
mulation of  rounded  cells.  The  opposite 
area  was  more  convex.  The  villi  were 
short  (0.2mm.)  thick  cylinders  with  round- 
ed ends  and  no  branches.  The  walls  of  the 
vesicle  consisted  only  of  epithelium,  which 
also  formed  the  simple  hollow  villi.  The 
contents  of  the  vesicle  were:  1,  The  inner 
cell-mass  lying,  as  before  mentioned,  at 
one  pole ;  2,  A  network  of  threads,  appar- 
ently the  result  of  coagulation  of  the  con- 
tained fluid,  for  no  nuclei  were  found  in  it. 
Kollmann,  79. 1,  294,  thinks  that  Reichert's 
ovum  must  have  had  really  two  layers 
forming  the  vesicular  walls — an  inner  one 
mesoderm  (young  connective  tissue)  and  an 
outer  one  of  true  epithelium ;  further,  that 
the  true  epithelium  had  been  lost,  and  that 
only  the  connective  tissue  remained,  which  Rei  chert  mistook  for 
epithelium.  This  supposition  is,  I  think,  not  probable.  Reichert's 
ovum  is  presumably  younger  than  any  other  hitherto  described,  and 
may  have  been  in  the  stage  before  the  mesoderm  had  grown  over  the 
chorion.  The  villi  are  described  as  hollow  ~by  Reichert — a  statement 
not  compatible  with  the  supposition  that  he  mistook  a  solid  core  of 
mesoderm  for  the  hollow  shell  of  the  ectoderm;  we  know  now  that 
young  villi  usually  contain  no  mesoderm  at  first. 

2.  Breus'  ovum,  77. 1,  must  be  considered  further  advanced  than 
Reichert's,  although  the  author  fixes  its  age  as  presumably  ten  days. 
The  total  diameter  of  the  ovum  including  the  villi  was  only  5  mm., 
and  as  the  villi  were  about  1  mm.  long,  the  diameter  of  the  chorionic 
vesicle  must  have  been  about  3  mm.  The  villi,  some  branched,  but 
mostly  without  branches,  were  thick  set,  but  left  one  spot  bald,  agree- 
ing in  this  with  Jones'  ovum  (see  below).  The  chorion  was  smooth 
on  its  inner  surface,  and  consisted  of  (1)  an  outer  epithelial  layer, 
and  (2)  an  inner  connective-tissue  layer  which  sent  out  extensions 
partly  filling  the  villi.  The  ovum  contained  a  thready  mass  which 
Breus  thinks  was  probably  a  product  of  coagulation,  and  an  inner 
cell-mass  about  1  mm.  long  and  0.5  mm.  wide.  The  presence  of  villi 
and  the  existence  of  the  mesodermic  layer  of  the  chorion,  contrasted 
with  the  absence  of  any  embryonic  structure,  led  Breus  to  consider 
his  ovum  abnormal.  But  it  is  rather  the  contrary  conclusion  we 
must  draw,  since  all  our  knowledge  points  to  the  deduction  that,  as 
compared  with  the  embryo,  the  development  of  the  chorion  is  very 
precocious  in  mammalia.  I  deem  it,  therefore,  probable  that  Breus' 


KNOWN    HTMAN    OVA.  t'S'.J 

ovum  was  normal,  and  that  the  inner  cell-mass  he  describes  was  in 
reality  the  embryo,  compare  Keibel's  ovum. 

3.  Wharton  Jones,   37.1,  long  ago  described  briefly  a  human 
ovum,  the  chorion  of  which  measures  in  his  figure  (said  to  be  nat- 
ural size)  6  by  4  mm.     The  following  is  all  that  can  be  gathered 
from  Jones'  description :  The  ovum  was  already  covered  by  the  de- 
cidua,  and  bore  shaggy  villi  on  the  side  toward  the  uterus,  while 
the  other  side  was  bald.     "  The  whole  cavity  of  the  chorion  was  filled 
with  a  fine  gelatinous  cellular  tissue,  imbedded  in  which,  toward 
one  extremity  of  the  ovum,  was  a  small  round  body;  it  was  evidently 
the  vesicular  blastoderma.     On  being  taken  and  examined  under  the 
microscope,  it  presented  the  same  friable,  globular  structure  found 
in  the  vesicular  blastoderma  of  the  rabbit  in  the  preceding  observa- 
tion.    There  was  no  vitellary  membrane  to  be  seen. "     To  judge  from 
the  minute  figure  given,  the  villi  were  already  branched ;  in  Rei- 
chert's  ovum  they  were  still  simple. 

4.  Ahlf eld's  ovum,   78.1,  represents  perhaps  the  same  age  as 
Jones',  but  he  does  not  give  its  diameter,  which  appears  from  inci- 
dental references  to  have  been  about  5  mm.     The  author's  descrip- 
tion is  not  exhaustive  by  any  means,  but  he  mentions  two  points  of 
great  interest:   first,  the  presence  of  a  layer  of  connective  tissue 
(mesoderm)    underneath   the  chorionic  epithelium,   and  extending 
into  but  only  partially  filling  the  villi  of  the  chorion;  second,  the 
character  of  the  villi,  which  are  slightly  branched  and  are  constricted 
at  the  base,  only  their  tips  touched  the  surface  of  the  decidua  (reflexa 
and  serotina) .     He  also  states  that  the  epithelium  of  the  villi  pre- 
cedes in  its  growth  the  connective  tissue.     This  ovum  was  supposed 
to  be  fourteen  to  sixteen  days  old  (?).     Owing  to  an  accident,  no 
observations  of  its  internal  contents  were  made. 

5.  6.  Beigel's  ovum,  78.1,  of  which  he  maintains  that  it  is  the 
third  smallest  known,  is,  if  we  may  judge  from  his  plate,  certainly 
abnormal  to  an  extreme  degree.     I  hold  it  to  be  a  malformed  ovum 
of  the  fifth  or  sixth  week.     The  ovum  described  by  Beigel  and  Lowe, 
77.1,  is  of  an  even  more  questionable  character.     Moreover,  their 
account  is  considered  by  Breus  and  Ahlfeld  to  be  very  inaccurate. 
It  is  noteworthy  that  Beigel  and  Lowe  have  also  noticed  the  early  pres- 
ence of  the  mesoderm  under  the  chorionic  epithelium.     Lowe," 79. 1, 
defends  himself  against  Ahlf eld's  attack,  and  insists  with  justice 
upon  the  presence  of  connective  tissue  on  the  inside  of  the  chorion  in 
ova  of  the  second  and  third  week. 

7,  8.  Kollmann's  memoir,  79. 1,  is  by  far  the  most  valuable  which 
had  appeared  up  to  the  time  of  its  publication  upon  the  structure  of 
very  young  human  ova.  He  describes  two  ova,  a  and  6,  both  pre- 
served in  the  anatomical  collection  at  Basle.  Ovum  a  had  been 
placed  in  glycerin  and  water,  which  preserved  the  form  of  the  speci- 
men but  ruined  it  histologically ;  nothing  was  made  out  as  to  the 
contents  of  the  chorionic  vesicle.  The  vesicle  itself  measured  5.5  by 
4.5  mm.,  and  therefore  was  slightly  flattened.  This  measure  does 
not  include  the  villi,  which  were  from  1  to  1.2  mm.  long,  and  re- 
peatedly branched.  Ovum  6,  5.5  mm.  in  diameter,  was  well  preserved 
in  alcohol;  the  villi  were  somewhat  branched;  the  contents  of  the 
ovum  were  lost.  On  the  other  hand,  the  uterus  belonging  to  this 
19 


290  THE    EMBRYO. 

ovum  was  also  preserved,  and  forms  the  basis  of  a  very  valuable 
description  of  the  uterus  in  early  pregnancy,  to  which  I  hope  to 
recur  on  another  occasion.  Kollmann's  two  ova  are  both  much  more 
advanced  than  those  of  Reichert,  Breus,  and  Jones,  as  is  shown  by 
their  greater  size  and  the  branching  of  the  villi.  It  is  a  matter  of 
profound  regret  that  only  the  chorion  was  left,  but,  fortunately, 
Kollmann  has  taken  good  advantage  of  his  opportunity.  His  paper 
also  gives  an  excellent  critical  analysis  of  nearly  all  the  previous 
literature.  He  points  out  that  the  two  primtive  layers  of  the  chorion 
are  probably  normally  present  at  this  stage.  The  chorion  of  his  ova, 
he  says,  consists  of  "  einer  Lage  jugendlichen,  embryonalen  Bin- 
degewebes,  das  zahlreiche  Rund-  und  Spindelzellen  enthalt,  und  das 
bedeckt  wird  von  einer  einfachen  Lage  platter  Zellen"  (p.  293).  He 
then  passes  the  literature  in  review,  and  insists  strongly  upon  the 
fact  that  the  two  layers  have  been  distinguished  in  nearly  all  the 
very  young  human  ova  known  except  Reichert's.  Kollmann,  there- 
fore, as  was  mentioned  above,  questions,  I  think  without  sufficient 
foundation,  the  accuracy  of  Reichert's  account.  Concerning  the 
connective-tissue  layer  Kollmann  says  but  little.  As  regards  the 
epithelium,  he  points  out  that  the  nuclei  occupy  a  basal  position  so 
that  the  outer  parts  of  the  cells  form  a  granular  stratum,  which  some 
authors  have  considered  a  distinct  membrane.  The  author  supposes 
this  granular  stratum  to  become  the  cuticula  described  in  later  stages. 
Jassinsky,  67. 1,  is  the  chief  defender  of  the  existence  of  a  cuticula, 
which,  however,  he  designates  under  the  extraordinary  name  of 
tunica  propria,  extraordinary  because  the  term  is  properly  applied  to 
the  layer  of  connective  tissue  immediately  upon  which  an  epithelium 
rests.  It  is  probable  in  the  light  of  our  present  knowledge  that 
Kollmann  saw  the  outer  darker  layer  found  in  Spee's  ovum,  see  below, 
and  in  many  others  a  little  older.  This  outer  layer  is  nucleated,  but 
the  nuclei  might  be  overlooked.  Finally  Kollmann  adds  (p.  297^.) 
observations  on  the  growth  of  the  villi  in  ova  of  the  fourth  week. 
The  outgrowth  of  branches  is  very  rapid,  and  occurs  with  every  de- 
gree of  participation  of  the  connective  tissue.  There  are  two 
extremes:  1.  A  bud  consisting  wholly  of  epithelium,  which  may 
stretch  out  into  a  process  with  a  long  thin  pellicle  and  a  thickened 
end,  the  whole  remaining  until  it  has  become  quite  large  without 
any  connective  tissue.  2.  A  thick  bud  with  a  well-developed  core  of 
connective  tissue ;  such  a  bud  probably  grows  out  as  a  nearly  cylin- 
drical branch.  Between  these  two  extremes  every  intermediate  state 
can  be  found.  The  various  forms  of  growing  branches  may  lie  close 
together.  Probably  this  complex  mode  of  growth  persists  in  older 
villi,  which  would  explain  the  multiplicity  of  forms  in  the  villous 
branches. 

9.  Schwabe,  79.1,  has  described  an  ovum  which  he  considers 
thirteen  to  fifteen  days  old,  but  he  is  certainly  mistaken,  since  both 
the  data  he  gives  as  to  the  age  and  his  account  of  the  embryo 
shows  that  it  is  more  advanced  and  belongs  distinctly  in  the  third 
week.  In  connection  with  Kollmann's  observations  we  must  notice 
those  of  Orth,  77.1,  who  has  shown  that  at  all  ages,  even  at  full 
term,  the  villi  of  the  chorion  in  the  placenta  have  epithelial  buds, 
which  are  at  first  hollow  and  are  afterward  filled  up  with  a  vascular- 


KNOWX    HUMAN    OVA.  291 

ized  ingrowth  of  connective  tissue.  Apropos  of  this  observation 
Orth  discusses  Boll's  theory  of  growth,  making  the  point  that  in  this 
case  the  shaping  of  the  parts  depends  primarily  upon  the  growth  of 
epithelium.  Boll  had  maintained,  as  a  general  principle,  that  in  the 
development  of  organs  the  shaping  is  dependent  on  the  co-operation 
of  the  epithelial  and  connective  tissues. 

10.  His'  embryo,  XLIV.  (Bff),  described  in  his  "  Anat.  mensch- 
licher  Embryonen,"  Heft  II.,  pp.  32  and  87,  belonged  to  a  chorioiiic 
vesicle  measuring  7  by  8  mm. ;  the  vesicle  was  somewhat  flattened, 
and  on  one  part  had   fewer  villi  than  elsewhere;   the  villi  were 
branched.     Closely  attached  to  the  inner  surface  was  a  small  body 
1.4  mm.  long  in  its  greatest  diameter;  the  body  consisted  apparently 
of  a  yolk-sac  and  closed  amnion ;  of  the  embryo  no  further  descrip- 
tion has  yet  been  published. 

11.  The  ovum  described  by  Keibel,  90.1,  consisted  of  a  some- 
what flattened  chorionic  vesicle  more  than  half  covered  with  little 
villi  and  containing  a  somewhat  distorted  embryo.      The   vesicle 
measured  8. 5  by  7. 75  by  6.0  mm.     The  villi  were  arranged  in  a  band 
or  zone  leaving  the  two  flattened  poles  of  the  ovum  smooth  as  in 
Reichert's  ovum ;  the  smooth  areas  were  of  very  unequal  size,  at  the 
edge  of  the  smaller  one  the  embryo  was  attached  by  means  of  its 
allantois-stalk  to  the  inner  surface  of  the  chorion.     The  embryo, 
about  1  mm.  long,  was  found  twisted  at  its  hind  end,  which  was 
continued  as  an  allantois-stalk  attached  to  the  chorion ;  the  stalk  was 
nearly  or  quite  as  large  as  the  embryo  proper ;  the  yolk  was  broadly 
attached  along  nearly  the  whole  length  of  the  embryo,  and  opposite 
the  embryo  the  yolk-sac  was  attached  to  the  chorion  as  if  the  coelom 
had  not  completely  developed.     Sections  showed  that  there  was  no 
medullary  groove  yet  formed,  but  the  amnion  was  already  closed 
over  the  embryo.     Keibel  places  his  embryo  as  intermediate  between 
His'  embryo,  XLIV.,  and  Spec's  embryo.     KeibePs  ovum  resem- 
bled externally  those  of  Reichert  and  Wharton  Jones,  and  as  it  con- 
tained an  embryo,  he  suggests  that  it  is  probable  that  the  ova  of 
Reichert  and  Jones  also  contained  an  embryo  without  medullary 
groove,  but  with  an  allantoic-stalk  nearly  as  large  as  the  embryo. 
But  it  seems  tome  that  since  Keibel's  ovum  is  nearly  twice  as  large, 
it  cannot  be  of  the  same  stage ;  the  presence  of  the  equatorial  zone  of 
villi  is  explainable  as  an  instance  of  retarded  development.     The  ex- 
cessive variability  of  embryos  is  well  known. 

12.  Spee's  embryo,  89.1,  was  contained  in  a  chorionic  vesicle 
measuring,  including  the  villi,  8.5  by  10  by  6.5  mm.     The  tips  of 
the  villi  were  attached  to  the  surface  of  the  decidual  capsule.     The 
embryo  was  attached  by  a  very  short  allantoic-stalk  to  the  chorion, 
and  was  closely  invested  by  the  amnion ;  the  attachment  of  the  yolk- 
sac  occupied  nearly  the  entire  length  of  the  embryo,  for  the  head-end 
had  scarcely  begun  to  project;  the  embryo  was  1.54  mm.  long;  its 
dorsal  surface  was  occupied  by  the  very  broad  medullary  plate  of 
thickened  ectoderm;  as  seen  from  above  the  plate  seemed  somewhat 
constricted  in  the  middle  of  the  embryo,  owing  to  the  arching  of  the 
body  at  that  region ;  the  centre  of  the  plate  showed  a  narrow  longi- 
tudinal furrow,  Fig.  104,  /;  at  the  caudal  end  this  furrow  widened 
out  and  disappeared;   just  behind  it  was  the  open  and  relatively 


202 


THE    EMBRYO. 


ct 


FIG.  164.— Cross-Section  of  Spee's  Embryo, 
tion  in  text. 


large  neurenterie  canal  behind  which  was  the  short  remnant  of  the 
primitive  streak.  The  embryo  was  cut  into  transverse  sections,  of 
which  there  were  about  180,  counting  the  allantois-stalk  (Bauchstiel) ; 
section  81,  counted  from  the  head,  is  represented  in  Fig.  164;  the 

ectoderm,  ek,  is  very  much 
thickened  to  constitute  the 
medullary  plate,  Md ;  the 
narrow  central  longitudinal 
furrow,  /,  mentioned  above 
is  very  noticeable;  outside 
of  the  embryo  the  ectoderm 
is  reflected  on  to  the  amnion, 
ct,  over  the  back  of  the  em- 
bryo. The  entoderm,  en,  is 
a  thin  layer  of  cells  in  the 
centre  of  which  the  noto- 
chordal  band  can  be  distin- 
guished ;  in  sections  nearer 
Expiana-  the  neurenterie  canal  the 
band  is  better  marked,  being 
there  much  thicker  than  the  remaining  entoderm.  The  mesoderm, 
me,  is  a  distinct  layer,  although,  as  other  sections  show,  it  is  fused 
in  the  median  line  of  the  primitive  streak  behind  the  neurenterie 
canal  with  both  ectoderm  and  entoderm. 
The  embryonic  coelom  has  only  just  begun 
to  appear  as  a  small  fissure,  p,  but  the  extra 
embryonic  coelom  is  completed,  so  that  out- 
side the  embryo  the  mesoderm  is  completely 
divided  into  a  somatic  leaf,  ct,  which  helps 
form  the  amnion  and  chorion,  and  a 
splanchnic  leaf,  df,  which  forms  one  layer 
of  the  wall,  of  the  yolk-sac.  The  sections 
through  the  head-end  show  that  the  head 
had  grown  forward  far  enough  to  lead  the 
separation  of  the  very  short  vorderdarm; 
sections  through  the  allantois-stalk  showed 
that  the  allantoic  diverticulum  extended  as 
a  small  canal  through  the  great  accumula- 
tion of  mesoderm;  throughout  the  rest  of 
its  extent  the  archenteron  is  nowhere  differ- 
entiated from  the  yolk-sac.  Fig.  165  is  a 
section  passing  through  the  neurenterie 
canal,  which  leads  through  the  centre  of  the 
medullary  plate  into  the  wide  yolk-sac ;  the 
part  of  the  sac  farthest  from  the  embryo  has 
its  mesoderm  thickened  and  vascularized, 
the  vessels  containing  young  blood-cells 
often  in  some  stage  of  division.  The  cho- 
rion of  Spee's  embryo  had  a  layer  of  meso- 
derm, with  cells  of  a  well-marked  mesenchy- 
mal  type,  and  an  outer  layer  of  ectoderm  consisting  of  a  thinner 
outer  layer  darkly  stained,  without  distinct  cell  boundaries,  but  with 


b.- 


FIG.  165. —Section  Passing 
through  the  Blastopore  of  Spec's 
Embryo,  am,  Amnion ;  ek,  ecto- 
derm; ct,  amniotic  mesoderm; 
g,  meeting  point  of  somatopleure 
and  splanchnopleure ;  df,  meso- 
derm of  yolk-sac ;  666,  blood 
vessels;  en,  entoderm;  n,  blas- 
topore;  d,  cavity  of  yolk-sac. 
After  Graf  Spee. 


KNOWN   HUMAN   OVA. 

small  nuclei  and  an  inner  lighter  layer  of  distinct  cells  with  larger 
nuclei  ;  the  ectoderm  appeared  somewhat  as  if  ciliated.  Unfortun- 
ately Spee  gives  no  account  of  the  villi  beyond  a  few  words  to  say 
that  they  resembled  those  of  later  stages. 

13.  We  come  now  to  the  embryos  with  a  well-developed  medul- 
lary groove  ;  the  number  of  these  is  four.     Their  probable  age  is  about 
fourteen  days.     The  least  advanced  is  His'  embryo  E  ("  Anat.  mensch. 
Embryonen,  "  I.  ,  Heft  I.,  p.  145)  ,  of  which  only  His'  sketches  are  avail- 
able, the  attempt  to  microtome  the  specimen  not  having  been  fortu- 
nate.    The  ovum  was  presumably  normal;  it  measured  8.5  by  5.5 
mm.,  and  was  entirely  covered  by  short  branching  villi.     For  the 
convenience  of  the  reader  I  have 

constructed  from  the  author's 
sketches  and  descriptions  the  ac- 
companying diagram.  His  states 
that  the  chorionic  vesicle  bore  at 
one  point  a  thick  stalk,  Al,  which 
ran  to  the  posterior  end  of  the 
embryo;  the  length  of  the  embryo 
from  the  anterior  extremity  to 
the  base  of  the  stalk  was  2.G  mm. 
The  head-end  of  the  embryo  was 
somewhat  thickened,  and  appar- 
ently showed  the  medullary  groove 

Still  Open.       The  Small,  round  yolk- 

sac  had  a  broad  connection  with 

the  Ventral  Surface  Ot  the  embryo, 

The  amnion  sprang  from  theall^n- 

•fr>i^   nnrl    •nacGorl    r»v~m»    flit*    Vie»arl  r\f    number   and   shape.     Emb,  Embryo;   Al,  sup- 

ns  ana  passea  o\er  tne  neaa  or  poseastalkof  theaiiantois. 
the  embryo.     The   disposition  of 

the  caudal  extremity  was  not  made  out.  There  were  no  limbs,  gill- 
clefts,  nor  organs  of  any  kind  discernible  —  not  even  a  protuberance 
between  the  head  and  yolk-sac,  such  as  marks  the  position  of  the 
heart  in  older  embryos. 

14.  Allen  Thomson,  39.  1,  published  an  excellent  article  on  young 
human  ova  in  1830.     He  gives  a  very  good  critical  review  of  what 
previous  authors  had  written,  and  describes  himself  three  embryos, 
which  have  become  classical,  for  the  figures  and  descriptions  given 
of  them  by  Thomson  have  been  copied  again  and  again.     They  are 
especially  known  by  the  reproductions  in  Kolliker's  "  Embryologies,  " 
and  in  Quain's  "Anatomy."     Two  of  these  embryos  (numbered  I. 
and  II.  by  Thomson)  belong  in  the  group  we  are  now  considering. 
I  cannot,  however,  admit  at  present  that  either  of  them  is  certainly 
fully  normal,  though  perhaps  they  are  only  slightly  malformed.     In 
number  I.,  (see  Kolliker's  "  Grundriss,"   1884,  Fig.  112,  and  "Ent- 
wickelungsgeschichte,"  1879,  Fig.  225)  the  yolk-sac  was  abnormally 
dilated  and  the  characteristics  of  the  embryo  were  not  ascertained. 
His  ("Anat.  Emb."  Heft  II.,  pp.  35-3(5)  has  shown  that  the  embryo 
proper  was  not  observed,  and  that  what  Thomson  called  the  embryo 
was  really  only  the  amnion,  springing  from  the  allantois-stalk  and 
passing  over  the  embryo.     Kolliker  questions  the  accuracy  of  this  in- 
terpretation, but  upon  what  ground  is  not  evident,  for,  so  far  as  I  can 


NJG.  -Diagram  of  His'  Embryo  E: 


294  THE   EMBRYO. 

see,  it  accords  perfectly  with  our  present  knowledge.  The  embryo 
in  question  was  presumably  little  advanced  beyond  His'  embryo  E, 
Fig.  165,  but  had  an  abnormally  hypertrophied  yolk-sac.  As  no 
sufficient  description  of  the  embryo  exists,  and  as  it  is  quite  certain 
that  the  specimen  was  more  or  less  abnormal,  it  cannot  be  longer 
regarded  as  a  fair  representative  of  a  young  ovum. 

15.  The  third  embryo  of  this  group,  His'  SR  (I.,  Heft  I.,  140-144) 
measured  2.2  mm.  in  length,  and  was  probably  fourteen  days  old. 
The  chorion  was  9  by  8  mm.  in  diameter.     It  shows  considerable 
advance  of  development  beyond  the  three  embryos  above  considered. 
The  neck  of  the  yolk-sac  is  already  somewhat  contracted,  or,  in  other 
words,  the  connection  between  the  embryo  and  the  yolk-sac  is  no 
longer  so  broad  and  long  as  it  was.     The  head  is  considerably  en- 
larged ;  between  it  and  the  anterior  wall  of  the  yolk-sac  is  a  large 
thickening  corresponding  to  the  heart.     From  the  under  side  of  the 
caudal  extremity  runs  off  the  stalk  of  the  allantois,  which  is  still 
short  and  thick;   the  amnion  lies  quite  close  to  the  embryo;   the 
medullary  ridges  are  still  separated  by  an  open,  though  deep,  and 
relatively  narrow   groove;    myo tomes    (protovertebraB,   auct.*)    are 
present,  but  their  number  was  not  ascertained.     When  the  embryo 
is  viewed  in  profile,  the  middle  of  the  back  shows  a  marked  concavity 
which  has  been  noticed  in  other  older  embryos,  and  is  probably  an 
artificial  distortion.     We  shall  have  to  return  to  this  matter.     Small 
openings  were  visible  on  the  inner  surface  of  the  chorion.     These  I 
take  to  be  the  openings  to  the  still  hollow  villi,  such  as  have  been 
seen  in  both  younger  and  older  ova.     His  attempted  to  obtain  sec- 
tions of  his  specimen,  but  when  cut  the  sections  fell  into  fragments. 

16.  Much   more  valuable  is  the  account  of  Thomson's   second 
ovum,  which  he  had  better  opportunities  of  studying.     The  original 
description  has  been  supplemented  by  His,  "  Anat.  Embry., "  II.,  p.  34, 
who  examined  Thomson's  original  drawings,  and  called  attention  to 
an  important  error  in  the  engraving  in  Thomson's  plate.     Kolliker, 
however,  still  reproduces  the  incorrect  figure  in  the  second  edition  of 
his  "Grundriss,"  Fig.  114.     An  erroneous  figure  is  also  reproduced 
in  Ecker's  "Icones,"  Taf.  XXV.,  Fig.  3.      The  chorionic  vesicle 
measured  0.60  by  0.45  of  an  inch,  and  was  covered  with  branching 

villi.  The  contained  embryo  was  very  small ; 
according  to  Kolliker,  only  2.5  mm.  The 
embryo  rested  upon  the  round  yolk-sac  of  2.2 
mm.  The  embryo  consisted  of  two  thick 
longitudinal  ridges,  Fig.  167,  A,  which 
curved  round  in  front  so  as  to  become  con- 
tinuous with  one  another,  and  were  broken 
off  posteriorly — an  important  fact  noted  by 
FIG.  ir,7. -Thomson's  second  His  (cf.  sup.) .  These  ridges  are  presumably 
B™mbryAo  Sim^US*1 a  '  the  medullary  folds.  At  the  hind-end  of  the 

embryo  was  a  tear,  making  a  hole  into  the 

hollow  yolk-sac.  As  His  suggests,  this  is  probably  where  the 
allantois  was  inserted  and  broken  off.  No  amnion  was  observed, 


*  It  must  be  remembered  that  the  term  protovertebrse  is  an  entire  misnomer,  and  is  inherited 
from  the  time  when  the  primitive  muscular  segments  (myotomes)  were  mistaken  for  the  com- 
mencements of  the  vertebrae. 


KNOWN   HUMAN   OVA.  295 

and  the  nature  of  the  connection  of  the  embryo  with  the  chorion  was 
not  ascertained.  What  we  learn  from  this  embryo  is  something 
more  definite  than  is  afforded  by  His'  observations  as  to  the  size 
and  disposition  of  the  medullary  ridges  and  the  hollowness  of  the 
yolk-sac.  The  apparent  hypertrophy  of  the  chorion  enforces  caution 
as  to  accepting  the  embryo  as  normal ;  but  it  is  not  rare  to  find  in 
abortions  a  small  typical  embryo  with  an  enormously  dilated  chorion, 
so  that  it  is  not  impossible  that  the  embryo  in  the  present  case  was 
quite  normal. 

17.  Spee    has   briefly   described  a    second    ovum,    but  his  ac- 
count is  not  now  accessible  to  me.     According  to  the  notes  given  by 
Fr.   Keibel,   90.1,   261,  the   chorionic   vesicle  measured  15x14x10 
mm.,  the  yolk-sac  3.5  mm.     The  embryo  had  seven  myotomes,  and 
its  age  in  maximo  was  thirteen  days. 

18.  J.  Kollmaim,  89. 1,  108-12^  describes  an  embryo  of  about  2.2 
mm. ;  the  yolk-sac  was  attached  to  the  embryo,  Fig.  1(58,  for  a  dis- 


Yks" 


FIG.  168. — Human  Embryo  of  Thirteen  to  Fourteen  Days.  Am,  Amnion ;  S.  7,  seventh  segi 
fd,  medullary  groove,  still  open;  Ht,  heart;  Yk.s,  yolk-sac;  Al,  allantois-stalk.  Art 
[ollmann. 


'ment  ; 
fter  J. 

Kol  '    ~ 


tance  of  1.5  mm.,  leaving  the  head  to  project  0.58  mm.,  the  tail  to 
project  0.3  mm.  The  head  is  already  somewhat  enlarged  and 
slightly  bent  over  ventralward;  it  forms  at  least  a  third  of  the 
whole  embryo ;  there  were  thirteen  *  primitive  segments  which 
marked  themselves  externally ;  the  segmented  region  of  the  body  is 
bent  so  that  its  dorsal  outline  is  concave ;  the  medullary  groove  is 
open  throughout  the  anterior  two-thirds  of  its  length,  but  the  caudal 
third  is  closed ;  the  tail  is  slightly  curled  over,  and  is  connected  on 
its  under  side  with  a  thick,  short  allantois-stalk,  or  Bauchstiel,  by 
which  the  embryo  is  attached  to  the  chorion ;  there  are  no  visceral 
or  branchial  arches,  although  the  gill  pouches  may  have  begun  form- 
ing in  the  pharynx ;  no  anlage  of  the  eye  or  ear  could  be  distin- 
guished; the  oral  invagination  has  formed,  but  the  oral  plate  (Rach- 
enhaut)  is  still  intact;  the  heart  is  not  straight  but  an  already  much 
bent  tube,  which  receives  at  its  hind  end  the  two  veins  from  the 
yolk-sac,  which  consists  of  vascularized  mesoderm  and  an  entodermal 
lining.  The  amnion  was  a  thin,  transparent  membrane  springing 
from  the  body  of  the  embryo  close  around  the  yolk-sac,  and  envelop- 
ing the  embryo  very  closely.  The  chorion  formed  a  vesicle  covered 

*  The  figure  shows  fifteen  segments. 


200 


THE    EMBRYO. 


externally  by  branching  villi ;  its  diameter  including  the  villi  was  18. 
cm.  Although  the  data  were  not  very  satisfactory,  Kollmann  esti- 
mated the  age  of  this  specimen  to  be  thirteen  to  fourteen  days. 

19.  The  description  of  the  ovum  of  thirteen  to  fourteen  days  by 
Yon  Baer,  88.1,  was  drawn  up  over  sixty  years  ago.     The  ovum 
measured  a  little  over  three  lines,  and  was  covered  with  villi ;  the 
embryo  was  about  two-thirds  of  a  line  long ;  Von  Baer  appears  to 
have  recognized  the  amnion  and  yolk-sac  and  to  have  seen  the  allan- 
toic-stalk  (his  Harnsack) ,  though  he  did  not  observe  its  connection 
with  the  chorion ;  as  he  states  that  the  back  was  already  formed,  it 
is  probable  that  the  medullary  groove  was  closed.     It  is  with  much 
hesitation  that  I  place  the  embryo  here  in  the  series. 

There  has  been,  so  far  as  I  am  aware,  no  human  embryo  with 
one  gill-cleft  described,  unless,  indeed,  Coste's  embryo  described  be- 
low was  such.  But  sev- 
eral with  two  clefts 
marked  externally  have 
been  described,  most  of 
them  by  His.  Those  of 
them  which  can  be  as- 
sumed to  be 
normal  present 
a  remarkable 
bend  in  the 
back  or  dorsal 
flexure,  by 

which  their  shape  is  so  much 
altered  from  that  of  the  slightly 
younger  stage,  and  so  unlike 
that  of  the  next  older  stage, 
that  the  embryos  with  the  dor- 
sal flexure  might  be  considered 
abnormal  had  we  not  positive 
reasons  to  the  contrary.  In- 
deed it  seems  probable  that 
embryos  in  this  stage  may 
have  been,  because  assumed  to 
be  abnormal,  discarded.  His' 
embryo  L,  described  below, 
and  perhaps  Coste's,  p.  300, 
both  probably  belong  in  this  stage  and  were  artificially  straightened 
out.  Nothing  similar  to  the  dorsal  flexure  of  the  human  embryo 
has  been  observed  in  any  other  vertebrate,  though  it  may  occur  in 
apes  and  monkeys. 

20,  21.  Two  specimens  in  my  collection  are  in  this  stage.     The 
younger  of  these  is  represented  in  Fig.  169,  and  is  very  near  the 
embryo  designated  as  Lg  by  His ;  just  behind  the  heart  the  whole 
body  bends  downward  and  then  bends  abruptly  upward,  so  that  the 
caudal  end  of  the  embryo  runs  nearly  at  right  angles  to  the  pharyn- 
geal  region ;  from  the  under  side  of  the  tail  end  runs  off  the  thick 
allantoic-stalk  by  which  the  embryo  was  attached  to  the  chorion. 
The  other  features  observed  are  shown  in  the  figures.      Sections 


FIG.  169.— Embryo  of  the  Beginning  of  Third 
Week  (Minot  Coll.,  No.  195).  All,  Allantois;  Am, 
amnion;  br.  branchial  region;  H.  fore-brain:  f/r, 
heart;  Ffc,  yolk. 


KNOWN    HUMAN    OVA. 


297 


OP. 


o.pl 


HI 


Ot 


showed  that  the  specimen  was  imperfectly  preserved,  and  I  cannot 
be  sure  that  it  was  entirely  normal  in  shape,  though  it  differs  but 
little  from  the  certainly  normal  embryos  of  His.  My  second  speci- 
men (Coll.  No.  143)  is  a  little  older,  I  think,  but  as  it  is  somewhat 
distorted,  it  is  hardly  worth  figuring  and  describing  separately. 

22,  23.  Far  better  preserved  are  the  two  embryos  of  His,  which 
he  has  studied  with  such  splendid  thoroughness.  He  designates 
them  as  Lg  (or  LXVIII.)  and  Sch.  1,  (or  LXVL),  Fig.  17,  p.  39. 
They  resemble  one  another  very  closely,  the  most  marked  differences 
being  that  in  Sch  the  heart  is  more  exposed  and  the  neck  of  the  yolk- 
sac  more  constricted  than  in  Lg.  Lg  measured  2.15  mm. ;  Sch,  2.20 
mm.  The  differences  noted  indicate  that  the  latter  is  slightly  more 
advanced.  The  following  description  applies  especially  to  Lg.  In 
external  form  the  embryo  is  very  similar  to  Minot's  Fig.  1G9,  but  no 
trace  of  a  third  gill-cleft  was  visible  externally,  and  the  amnion  was 
attached  along  nearly  the  entire  length  of  the  allantois-stalk  (His* 
Bauchstiel) .  The  anatomy  can  be  understood  from  the  accompany- 
ing Fig.  170.  The  head  bend-being  well  marked,  the  central  nervous 
system  makes  at  the  mid-brain,  a  bend 
at  nearly  a  right  angle,  so  that  the  fore- 
brain  is  brought  very  near  the  heart, 
which  lies  in  the  large  pericardial  sac, 
which  protrudes  conspicuously  between 
the  head  of  the  embyro  and  the  yolk- 
sac.  Between  the  head  and  the  peri- 
cardial sac  is  situated  the  oral  invagi- 
nation  or  future  mouth  cavity,  separated 
from  the  vorderdarm  by  an  intact  oral 
Plate  (Rachenhaut)  o.pl.  As  regards 
the  archenteron  we  find  the  vorderdarm 
above  the  heart,  Ht,  with  two  gill 
pouches  formed  at  its  head-end  and  its 
lower  end  widened ;  out  of  this  wider 
part  the  lungs  and  the  stomach  are  to 
be  differentiated  in  later  stages;  the 
vorderdarm  is  compressed  dorso-ven- 
trally  but  widely  expanded  transversely ; 
the  middle  portion  of  the  archenteron 
opens  widely  into  the  yolk-sac ;  where 
the  vorderdarm  joins  this  middle  divi- 
sion is  found  the  outgrowth  of  the  liver, 
Li,  extending  toward  the  heart;  in  the 
posterior  region  of  the  embryo  the  arch- 
enteron has  also  become  distinct  from 
the  yolk-sac  and  ends  with  a  dilatation 
(His'  bur  so]  in  the  tail  of  the  embryo ;  from  the  under  side  of  the  bursa 
runs  out  the  allantoic  diverticulum,  All,  which  extends  as  a  narrow 
tube  of  entoderm  through  the  allantoic  stalk  to  the  level  of  the  chorion 
where  it  ends  blindly.  The  central  nervous  system  forms  in  bulk  a 
very  large  part  of  the  embryo;  from  the  fore-brain  the  optic  vesicles, 
Op,  have  grown  out ;  the  mid-brain  is  only  slightly  dilated ;  the  hind- 
train  is  as  long  as  the  mid-  and  fore-brain  together,  and  is  nearly  as 


FIG.  170.— Human  Embryo  of  2.15 
mm ;  Anatomy  Reconstructed  from  the 
Sections.  Op.  Optic  vesicle ;  o.pl,  oral 
plate;  Ht.  endothelial  heart;  Li,  liver; 
On,  omphalo  -  mesaraic  vein  ;  Yfc, 
yolk-sac:  All,  allantoic  diverticulum 
of  archenteron ;  Of.otocyot;  Ao,  aorta; 
w.i'.,  umbilical  vein.  Af ter  W.  His. 


298  THE   EMBRYO. 

long  as  the  vorderdarm,  which  it  overlies ;  near  the  centre  of  the  hind- 
brain  lies  the  open  ectodermal  invagination,  Of,  destined  to  form  the 
auditory  vesicle  or  otocyst ;  the  remainder  of  the  medullary  canal 
corresponds  to  the  future  spinal  cord  and  gradually  tapers  tailward ; 
alongside  it  His  was  able  to  distinguish  in  Lg  twenty-nine  myotomes. 
The  heart,  Ht ,  is  very  largely  and  asymmetrically  bent ;  the  heart  at 
this  stage  and  for  some  time  later  may  be  described  as  consisting  of  two 
tubes,  a  small  inner  one,  Ht ,  formed  of  endothelial  cells,  and  a  larger 
outer  one  formed  chiefly  of  contractile  elements,  which  are  gradually 
differentiated  into  the  striated  muscles  of  the  adult  heart.  The  way 
in  which  the  heart  is  bent  can  be  best  seen  in  front  views ;  the  great 
veins  enter  the  heart  in  the  median  line  just  above  the  liver;  the 
heart  tube  runs  toward  the  head  and  the  left  side,  making  the  auric- 
ular limbs ;  then  the  tube  bends  to  the  ventral  side  and  runs  obliquely 
backward  to  the  right  side,  making  the  ventricular  limb,  and  finally 
takes  a  curving  course  as  indicated  in  the  figures  to  the  median  line, 
and  ends  close  behind  the  mouth ;  this  third  part  is  the  aortic  limb. 
The  endothelial  heart  tube  is  continued  beyond  the  pericardial  cavity 
as  the  aorta,  which  soon  divides  into  two  branches  on  each  side, 
which  pass  up  around  the  pharynx,  one  branch  in  front  of  each  gill- 
cleft  ;  the  front  branch  curves  over,  and,  passing  tailward,  joins  the 
second  branch;  the  branches  which  pass  around  the  pharynx  are 
known  as  the  aortic  arches ;  the  united  vessels  run  toward  the  tail 
on  the  dorsal  side  of  the  pharnyx ;  they  are  called  the  dorsal  aorta3, 
and  by  uniting  in  the  median  line  form  the  single  dorsal  aorta,  which 
runs  away  back  nearly  to  the  tail  of  the  embryo,  where  it  forks,  and 
its  branches,  passing  one  on  each  side  of  the  intestinal  canal,  enter 
the  allantois-stalk  and  run  to  the  chorion,  where  they  branch  out. 
The  veins  of  the  embryo  are  tht  jugular,  which  comes  from  the  head 
and  meets  cardinal  vein  from  the  rump  about  at  the  level  of  the 
liver ;  these  two  veins  unite  as  a  short  stem,  which  runs  transversely 
toward  the  venous  end  of  the  heart  and  is  termed  the  ductus 
Cuvieri ;  the  ductus  is  joined,  as  in  adult  fishes,  by  the  omphalo- 
mesaraic  vein,  Om  coming  on  the  same  side  from  the  yolk-sac,  and 
the  umbilical  vein,  u.v,  coming  from  the  allantois;  the  four  united 
veins  meet  their  fellows  from  the  opposite  side  and  form  with  them 
the  median  sinus  reuniens,  which  communicates  directly  with  the 
heart ;  the  course  of  the  umbilical  vein  is  curious,  as  it  takes  a  short 
cut  from  the  allantois  through  the  somatopleure  along  the  base  of 
the  amnion  to  the  heart ;  how  this  course  is  possible  can  be  under- 
stood by  comparing  figures  17  and  166. 

24.  We  pass  now  to  His'  embryo  L,  and  Coste's  youngest  em- 
bryo. It  must  be  seriously  doubted  whether  either  of  these  embryos 
represent  the  normal  shape.  The  former  had  two  gill-slits  and  parts 
of  it  were  torn  away,  so  that  we  may  surmise  that  it  had  had  the  dorsal 
flexure  but  was  artificially  straightened.  Concerning  Coste's  embryo 
see  the  next  paragraph.  His'  embryo  L  is  described  in  his  "An  at. 
menschl.  Embryonen,"  Heft  I.,  pp.  135-139.  It  measured  2.4  mm. 
in  length,  and  was  obtained  from  a  chorionic  vesicle  of  8  to  9  mm. 
diameter.  The  specimen  had  been  considerably  injured,  and  no  exact 
knowledge  could  be  obtained  in  regard  to  the  heart  or  the  disposition 
of  the  allantois  or  the  amnion.  Precisely  these  three  points  are 


KNOWN   HUMAN   OVA. 


elucidated  by  Coste,  while  His  has  worked  out  the  internal  anatomy 
of  his  specimen ;  in  short,  the  two  descriptions  complement  one  an- 
other in  a  remarkable  manner.  Nearly  all  that  His  ascertained  is 
represented  in  the  accompanying  illustrations,  Fig.  171.  A  gives  a 
side  view  showing  the  thickening  of  the  head-end  and  the  upward 


FIG.  171.— His1  Embryo  L,  2.4  mm.  long.  A,  Side  view;  B,  ventral  view;  C,  ventral  view, 
with  the  walls  of  the  body  and  intestine  seen  in  frontal  section ;  D.  dorsal  view,  showing  the 
central  nervous  system.  M,  Mouth;  MX,  inferior  maxilla  or  mandible;  2,  hyoid  arch;  Vd,  yor- 
derdarm;  f7,  splanehnopleura  of  the  yolk-sac;  2,  3,  and  4,  gill  arches;  Coe,  coelom  or  primi- 
tive body-cavity ;  Op,  optic  vesicle ;  Au,  auditory  vesicle  (otocyst) ;  a.  point  where  the  medul- 
lary groove  has  not  yet  closed. 

curving  of  the  tail,  and  the  two  gill-slits  in  the  cervical  region ;  the 
mouth,  M,  is  very  large ;  between  it  and  the  first  gill-slit  intervenes 
the  thick  ridge,  MX,  of  the  first  gill  arch  (branchial  or  visceral  arch, 
a  net.),  which  becomes  the  mandible;  between  the  two  slits  is  the 
second  or  hyoidean  arch,  in  connection  with  which  the  hyoid  bone 
afterward  arises.  A  large  body  cavity  is  present,  C,  Coe;  the  walls 
of  the  body  (somatopleures)  pass  over  along  an  extended  line  into  the 
amnion ;  the  connection  between  the  embryo  and  the  yolk-sac  is  al- 
ready much  restricted  compared  with  Coste's  embryo,  Fig.  172;  at 
the  side  of  the  head  a  line  and  shadow  mark  the  position  of  the  optic 
vesicle.  B  is  a  ventral  view ;  it  shows  the  large  wide  mouth,  Jf, 
which,  according  to  His,  was  apparently  in  communication  with 
the  intestinal  canal,  which  is  nothing  but  a  straight  tube  with  a 
great  pharyngeal  dilatation,  and  a  wide  open  union  with  the  yolk-sac ; 
the  median  light  band  shown  at  the  back  of  the  mouth  is  the  central 
nervous  system  shining  through  the  covering  tissue.  C  is  intended 
to  show  the  digestive  tract,  and  is  partly  a  horizontal  section.  Es- 
pecially to  be  noticed  is  the  enormous  size  of  the  pharynx  (the  region 
of  the  branchial  arches) ,  the  straight,  short  intestine,  and  on  each 
side  of  the  latter  the  distinct  body-cavity,  Coe;  there  are  indications 
of  four  visceral  arches,  MX,  2,  3,  and  4;  in  front  of  the  pharynx  is 


300  THE   EMBRYO. 

shown  the  ventral  surface  of  the  fore-brain  or  first  cerebral  vesicle, 
with  its  lateral  diverticula,  the  optic  vesicles.  D  is  a  dorsal  view 
of  the  brain  and  medullary  canal  which  is  still  open  at  a.  The  brain 
and  spinal  cord  are  already  differentiated  by  the  dilatation  of  the 
former.  The  brain  subdivides  very  early  in  all  vertebrate  embryos 
into  three  dilatations  or  primary  vesicles;  but  in  this  embryo  the  two 
anterior  dilatations  are  not  yet  clearly  separated  from  one  another, 
hence  there  is  only  one  widening  of  the  brain  in  front ;  the  front  end 
is  seen  to  bend  downward  and  give  off  the  conspicuous  optic  vesicles, 
Op,  which,  therefore,  arise  before  there  is  any  trace  of  the  cerebral 
hemisphere — an  important  fact ;  the  posterior  and  larger  dilatation 
is  the  primitive  medulla  oblongata ;  no  trace  of  the  cerebellum  has 
appeared.  The  whole  nervous  system  is  a  tube  the  walls  of  which 
are  of  nearly  uniform  thickness,  except  that  the  dorsal  wall  of  the 
third  vesicle  (the  cavity  of  which  becomes  the  fourth  ventricle  of  the 
adult)  is  very  thin.  This  thin  wall  is  persistent  in  the  adult  and 
never  develops  into  nervous  substance.  On  each  side  of  the  medulla 
lies  a  round  cyst,  the  auditory  sac,  Au,  the  beginning  of  the  adult 
membranous  labyrinth.  Three  other  points  not  shown  in  the  figures 
remain  to  be  noticed.  1.  *En  the  tissue  at  the  back  of  each  body- 
cavity,  Coey  was  found  a  single  longitudinal  epithelial  canal,  the 
Wolffian  duct,  the  first  part  of  the  urogenital  apparatus  to  be  devel- 
oped. 2.  Close  below  the  nervous  system  lay  a  median  rod  of  cells 
with  a  small  central  cavity ;  this  rod  is  the  notochord  or  chorda 
dorsalis,  the  primitive  embryonic  axis  around  which  the  vertebrae 
are  formed  later.  3.  All  the  tissues  are  still  embryonic — that  is, 
the  cells  are  not  yet  differentiated  into  tissues.  Unfortunately,  the 
number  and  disposition  of  the  myotomes  were  not  ascertained. 

24A.  Janosik,  87.1,  describes  an  embryo  with  two  gill  pouches 
and  three  aortic  arches,  giving  a  few  anatomical  details. 

25.  Coste's  embryo  has  been  beautifully  figured  in  his  great  work, 
47.1.  It  is  possible  that  it  really  belongs  to  an  older  stage  with 
the  dorsal  bend,  compare  Fig.  169,  and  that  it  was  stretched  out  by 
Coste ;  the  difficulty  of  assigning  it  its  place  is  due  to  the  entire 
uncertainty  as  to  its  actual  dimensions.  Coste's  private  collection 
is,  I  believe,  now  in  the  College  of  France,  but  upon  search  this 
particular  specimen  could  not  be  found,  so  that  His'  inquiries  to 
ascertain  its  actual  length  were  resultless.  Kolliker  states  that  it 
was  4.4  mm.  long,  but  his  authority  for  the  statement  is  not  given; 
the  measure  was  probably  taken  from  Coste's  figure,  "grandeur 
naturelle."  Since  embryos  of  this  length  are  far  more  developed 
than  Coste's,  it  is  probable  that  Coste's  data  as  to  the  magnification 
of  his  figures  are  inaccurate.  If  we  assume  the  embryo  to  have  been 
really  about  2.5  mm.,  ifc  will  then  agree,  except  as  to  the  great  length 
of  the  rump,  very  closely  with  what  we  know  otherwise  of  such  young 
embryos.  I  give  the  accompanying  figures,  which  are  careful  copies 
from  the  original  plates  published  by  Coste  (4  "  Espece  humaine, " 
PI.  II.),  whose  illustrations,  made  by  his  assistant,  Gerbe,  have  never 
been  surpassed  for  beauty  and  life-like  accuracy.  The  embryo  in 
question  was  inclosed  in  a  villous  chorion,  Fig.  172,  and  was  provided 
with  a  large  vitelline  sac,  Vi,  having  a  very  broad  connection  with 
the  embryo  and  covered  with  a  network  of  vessels,  in  which  was  a 


KNOWN   HUMAN   OVA. 


301 


fluid  not  yet  red.  A  thick  allantois-stalk,  Al,  can  be  seen  running 
from  the  under  side  of  the  embryo's  tail  to  the  chorion ;  from  the 
anterior  side  of  the  stalk  springs  the  amnion,  Am,  completely  inclos- 


~\^c. 


FIG.  172.— Ovum  Supposed  to  be  from  Fifteen  to  Eighteen  Days  Old ;  after  Coste.  The  chorion 
has  been  opened  and  spread  out  to  show  the  embryo  and  its  annexa.  Al,  Allantois;  Am,  amnion 
surrounding  the  embryo. 

ing  the  embryo.  It  is  important  to  notice  that  in  this,  as  in  still 
older  embryos,  the  disposition  of  the  amnion  is  essentially  the  same 
as  in  the  earliest  stages  (v.  sup.) ;  the  line  of  attachment  of  the  am- 
nion is  down  the  sides  of  the  allantois  and  around  the  embryo  about 


302 


THE    EMBRYO. 


on  a  line  with  the  top  of  the  yolk.  As  regards  the  embryo,  it  is 
drawn  slightly  canted  on  to  its  left  side ;  its  back  is  concave ;  the 
head-end  is  thickest  and  shows  three  gill-arches,  hence  there  were 
probably  two  branchial  clefts ;  behind  and  be- 
low the  gill-clefts  can  be  seen  the  heart,  already 
a  bent  tube,  shining  through ;  behind  the  arches 
again,  but  on  the  dorsal  side,  the  light-looking 
oesophagus  is  distinguishable ;  in  the  figure  a 
wedge-shaped  shadow  intervenes  between  the 
straight  oesophagus  and  the  bent  heart;  the 
heart  causes  a  conspicuous  bulging  of  the  body 
between  the  head  and  the  yolk-sac ;  the  caudal 
extremity  rj  thick  and  rounded,  and  curves 
upward.  Fig.  173  is  a  ventral  view  of  the  same 
embryo  after  most  of  the  yolk-sac  has  been  cut 
off;  its  walls,  Spl  (splanchnopleure),  are  seen 
to  pass  over  without  any  break  into  those  of  the 
mmtMti  intestinal  cavity.  In  the  central  line  the  chorda 
dorsalis,  s,  can  be  perceived  through  the  trans- 
lucent dorsal  wall  of  the  intestinal  cavity ;  it  is 
flanked  on  each  side  by  the  row  of  square  mus- 
cular segments  (myotomes).  We  see  the  large 
allantois,  Al,  behind,  and  in  front  the  tubular 
heart,  Ht,  with  a  decided  flexure  to  the  right 
of  the  embryo ;  the  anterior  end  of  the  heart 
makes  an  opposite  bend,  separating  off  a  limb, 
which  becomes  the  bulbus 
aortce.  The  chorion  con- 
of  two 


heart;  Spl, splanchnopleure, 
extending  beyond  the  em- 
bryo to  form  the  yolk-sac; 
S,  chorda  dorsalis  with  a 
row  of  myotomes  on  each 
side.  Al,  stalk  of  the  allan- 
tois. 


-EC* 


FIG.  173.  —  Embryo  Sup- 
posed to  be  from  Fifteen  to 
Eighteen  Days  Old;  after 
Coste.  Ventral  view;  the  One  of  which  paSS6S  COn-  <| 

tinuously  over  the  inner  ^ 
surface  of  the  chorion,  \ 
while  the  other  outer  mem-  | 
brane  alone  forms  the  hol- 
low villi,  Figs.  172  and  j 
176;  hence,  in  looking  at 

the  inside  of  the  chorion,  we  see  numerous 
round  openings  which  do  not  penetrate  the  in- 
ner membrane.  Fortunately  we  learn  from 
Kolliker  ("  Entwickelungsgeschichte, "  1879,  p. 
309)  who  had  an  opportunity  in  1861  to  exam- 
ine the  chorion,  that  the  outer  membrane  was  FIG.  174. -Fragment  ot  the 
epithelial  with  cells  of  the  same  character  as  in  £hori?iL,o£  ^g-  &  -Jligr^ 

,f  ..,     , .  *•     i  i  i       •       i       .IT   *          i     magnified.      EC,  Epithelial 

the  epithelium  of  older  vascularized  villi,    and  layer,  Mes,  connective-tis- 
that  the  inner  layer  consisted  of  developing 
connective  tissue,  and  carried  fine  blood-vessels. 
It  thus  appears  that  Coste  was  the  first  to  observe  the  role  of  the  epi- 
thelium in  the  growth  of  the  villi. 

26,  27,  28.  It  will  be  as  well  to  mention  here,  rather  than  later, 


lium- 


*  "Hierbei  ZeigtS  sich,  dass  die  Zotten  und  die  sie  tragencle  Haut  ganz  und  gar  ausepithelarti- 
gen  Zellen,  vonderselben  Beschaffenheit  wie  des  Epithelsder  sptiteren  gefasshaltigen  Chorion- 
zotten  bestehen." — Kolliker,  I.e. 


• 
KNOWN    HUMAN    OVA.  303 

three  descriptions  of  young  embryos,  Avhich  either  belong  in  this 
stage  or  are  a  little  older.  Of  these  descriptions  Remy's  alone  brings 
much  of  any  positive  information,  but  the  size  and  age  of  his  embryo 
can  only  be  guessed  at.  The  first  of  the  embryos  is  Schroder  van 
der  Kolk's  (51.1,  p.  106  jf.,  with  figures  on  PL  II.).  Kolk's  figures 
are  not  very  clear.  He  states  that  his  specimen  had  two  gill-clefts 
and  measured  1.8  mm.  in  length;  one  can  but  ask,  Was  it  not  really 
larger?  Kolk's  figure  suggests  that  the  specimen  was  doubled  up ; 
if  this  was  the  case,  the  embryo,  when  straightened  out,  would  agree 
fairly  well  with  His'  embryo  L,  above  described.  Professor  His, 
for  reasons  not  clear  to  me,  considers  Kolk's  specimen  as  somewhat 
older,  but  to  this  opinion  I  am  unwilling  to  accede.  The  second 
embryo  is  that  of  Hennig,  whose  description,  73.1,  leaves  very 
much,  and  whose  figures  leave  everything  to  be  desired.  From  this 
paper  we  can  gather  very  little,  except  confirmation  of  Coste's  state- 
ments in  regard  to,  (1)  the  disposition  of  the  amnion  and  its  connec- 
tion with  the  stalk  of  the  allantois ;  (2)  the  absence  of  a  yolk-stalk. 
Schwabe's,  79. 1,  embryo,  to  which  reference  has  already  been  made, 
and  which  he  assumes  to  be  thirteen  to  fifteen  days  old,  was  probably 
sixteen  to  twenty  days  old,  as  shown  botn  by  his  own  data  and  by 
the  description  of  the  ovum.  Very  likely  it  was  a  little  younger 
than  Coste's  embryo,  v.  sup.  There  were  a  well-developed  yolk-sac 
and  an  amnion  closely  investing  the  embryo,  which  was  connected 
with  the  chorion  by  a  short  allantoic  stem.  The  chorionic  villi  were 
considerably  branched  and  entirely  filled  with  mesoderm ;  their  tips 
had  little  thickenings  of  the  epithelium  by  which  they  were  attached  to 
the  decidua  •  this  was  the  only  connection  between  the  foetal  and 
maternal  tissues.  This  last  fact  is  an  interesting  confirmation  of  the 
observations  of  Ahlfeld  and  Langhans.  Remy's  embryo,  80. 1,  was 
also  a  young  one,  but  its  exact  age  is  not  stated,  nor  are  the  measures 
of  its  length  given  except  in  the  title,  where  it  is  called  "long  d'un 
centimetre."  From  the  stage  of  development,  and  from  the  state- 
ment in  the  text  that  the  chorionic  cavity  measured  20x10  mm.,  it 
seems  impossible  that  the  embryo  was  so  large;  we  should  rather 
expect  an  embryo  of  3  mm.  Remy's  figure  is  too  inexact  for  one 
to  make  out  the  form  of  the  embryo.  If  he  gives  the  length  cor- 
rectly, the  specimen  must  have  been  a  month  old.  As  to  its  struc- 
ure,  Remy  gives  the  following  details :  The  medullary  canal  was 
still  united  with  the  ectoderm  at  its  lower  end,  and  extensively  so 
over  the  fourth  ventricle,  which  was  entirely  closed.  The  heart 
already  had  muscular  striaa.  The  epidermis  had  two  layers  of  cells, 
the  outer  somewhat  flattened,  the  inner  cuboidal.  The  cutis  was 
not  differentiated.  The  epithelium  of  the  chorion  he  describes  as 
maternal — a  common  error.  He  also  distinguished  the  inner  mem- 
brane of  the  chorion,  the  allantoic.  He  has  also  seen,  apparently, 
what  is  known  as  Langhans'  cellular  layer,  but  has  taken  it  for  a  deep 
portion  of  the  epithelium,  which  he  accordingly  calls  many-layered. 
The  stage  with  three  gill-clefts  is  known  through  five  embryos, 
four  of  which  have  been  studied  by  His,  and  belong  to  the  end  of 
this  stage,  since  in  all,  except  one  (Rf),  of  which  we  have  no  detailed 
description,  the  fourth  gill-pouch  of  the  pharynx  was  partly  formed, 
and  in  all  there  were  five  aortic  arches.  The  fifth  embryo  is  de- 


304:  THE   EMBRYO. 

scribed  by  Chiarugi,  and  had  three  gill-clefts  and  three  aortic  arches; 
it  therefore  belongs  to  the  beginning  of  this  stage. 

28  A.  Chiarugi 's  embryo,  88. 1,  had  a  very  marked  dorsal  flexure 
(insenatura  dorsale);  its  greatest  length  was  2.6 'mm. ;  its  chori- 
onic  vesicle  measured  15x12x8  mm. ;  the  villi  were  much  longer  (1.5 
mm.)  than  upon  the  other.  The  embryo  had  three  gill-clefts  showing 
externally,  and  unlike  the  two  embryos  of  His,  BB,  Lr,  only  three 
internal  gill-pouches  and  three  aortic  arches ;  the  otocyst  was  closed 
but  still  connected  with  the  ectoderm ;  the  yolk-sac  had  a  broad  con- 
nection with  embryo,  and  measured  in  vertical  diameter  1.9  mm. ; 
in  transverse,  1.8  mm.;  in  antero-posterior,  l.G  mm.  These  points 
show  that  the  embryo  was  intermediate  between  His'  L  and  M. 
In  Chiarugi's  specimen  the  Wolffian  bodies  had  become  protuberant; 
the  cephalic  and  spinal  ganglia  were  present,  but  the  spinal  motor 
roots  were  not  developed ;  the  notochord  measured  30//  in  transverse, 
24/*  in  dorso- ventral  diameter,  and  its  caudal  termination  was  indis- 
tinct. Chiarugi  gives  a  full  and  admirable  description  of  all  the 
parts,  but  as  in  the  respects  not  specially  mentioned  above,  the 
structure  is  very  similar  to  that  of  other  embryos  with  three  gill- 
clefts,  further  details  may  be  omitted. 

29—32.  The  four  embryos  with  three  gill-clefts  described  by  His 
have  been  designated  by  him  as  Rf ;  M,  Fig.  175 — BB,  and  Lr,  Fig. 
16 — they  being  named  in  the  presumable  order  of  development.  M 

and  Lr  are  pro- 

Xgf  bably  the  most 

perfect ;  Rf  is 
somewhat  rolled 
up;  BB  has  a 
distinct  dorsal 
flexure,  but,  as 

.,%  His  himself  re- 

marks, this  was 
^   probably  due  to 
a  mecha  n  i  c  a  1 
I  strain  and  is  ar- 
tificial ;  hence 
we  may  assume 
that  in  all  em- 
s,-  bryos  of    this 

stage  the  dorsal 
^^ -"•"•"  n    *         ,        , . 

flexure  has  dis- 
appeared and 
the  back  has  be- 

FIG.  175.— His'  embryo  M. 

come    convex. 

The  four  embryos  are  described  and  figured  in  His'  "Anatomie 
menschlicher  Embryonen,"  Heft  I. -III."  Of  M  a  systematic  anatom- 
ical description  is  given  (Heft  I.,  166-134) ,  and  additional  details  con- 
cerning BB  and  Lr  are  scattered  through  Heft  III.  The  lengths  are : 
M,  2.6  mm. ;  BB,  3.2  mm. ;  Lr,  4.2  mm. ;  Rf  being  rolled  up  could 
not  be  measured  satisfactorily.  The  chorionic  vesicle  of  M  measured 
7.5x8.0  mm. ;  of  BB,  11x14  mm.  From  the  data  given  by  His,  the 
age  of  BB  may  be  estimated  at  probably  twenty  to  twenty-one  days. 


KNOWN    HUMAN   OVA. 


305 


X24- 


The  head  is  bent  down,  the  back  very  convex,  and  the  caudal  ex- 
tremity is  rolled  up  and  turned  teward  the  right — in  Lr,  however,  to 
the  left — while  the  head  is  twisted  slightly  toward  the  left ;  the  long 
axis  of  the  body,  therefore,  describes  a  large 
segment  of  a  spiral  revolution;  the  spiral 
form  is  more  marked  in  embryos  a  little 
older ;  it  is,  of  course,  produced  by  the  more 
rapid  growth  of  one  side;  in  view  of  the 
differences  between  right  and  left  in  the 
adult,  it  is  very  interesting  to  find  differences 
between  symmetrical  parts  showing  so  very 
early  in  the  heart  of  the  embryo  and  the 
twisting  of  the  body.  The  caudal  end  of  the 
body  has  grown  very  much;  the  allantois-  ch 
stalk  has  presumably  lengthened ;  the  neck 
of  the  yolk-sac  is  much  constricted ;  the  gill- 
clefts  can  be  distinguished  externally;  the 
otocyst,  Fig.  178,  ot,  has  become  somewhat 
pear-shaped.  The  neural  canal  is  completely 
closed;  the  mid-brain  and  fore-brain  have 
become  perfectly  distinct,  and  the  latter  has 
begun  to  form  the  hemispheres  in  front. 
The  mouth  is  large,  and  at  its  upper  corner 
the  protuberance  of  the  maxillary  process 
is  marked;  the  mandibular  process  is  very 
prominent.  Fig.  176,  a  geometrical  recon- 
struction from  the  sections,  shows  the  anatomy  of  the  entodermic 
canal.  The  pharynx,  bounded  on  each  side  by  four  branchial  arches, 
is  still  very  large  and  tapers  down  posteriorly ;  the  intestine  is  turned 

to  the  left  and  opens  into  the  broad  canal, 
Yks,  of  the  yolk-sac ;  just  in  front  of  the 
yolk-sac  there  is  a  small  ventral  diverticu- 
lum,  Li. ,  the  commencement  of  the  liver ; 
behind  the  yolk-sac  the  cylindrical  intestine 
runs  over  into  the  tail,  where  it  expands 
into  the  bursa  of  His,  and  gives  off  a  cylin- 
drical canal,  which  has  very  thick  connec- 
tive-tissue walls,  and  is  the  allantoic-stalk, 
Al,  which  carries  the  two  allantoic  veins 
and  the  two  large  allantoic  arteries,  Fig. 
178.  Fig.  177  gives  a  view  of  the  anterior 
wall  of  the  pharynx  of  BB ;  in  front  is  the 
large  opening  of  the  mouth,  Jf,  the  oral 
plate  between  the  mouth  cavity  and  the 
vorderdarm  having  disappeared ;  the  wide 


FIG.  176. —Digestive  Canal  of 
His'  Embryo  Lr,  4.2  mm.  long. 
Fig.    10,    p.   38,  and 


FIG.  177.— Anterior  Wall  of  the 
Pharynx  of  His'  Embryo  BB,  3.2 

X  conSta,o^Tahne£'rtiacrcaS  pharynx  shows  four  gill-pSuches,  ajid  at  its 

shown  by  dotted   lines;   5,  fifth    ' 
aortic   arch  ;    M,   mouth  ;      O, 

03sophagus  or  vorderdarm ;  Coc', 
coelom.     After  W.  His. 


lower  end  gradually  contracts  and  passes 
into  the  narrow  oesophagus.     The   aortic 
vessels  are  indicated  by  dotted  lines;  the 
cardiac  aorta  reaches  the  pharynx  between  the  bases  of  the  second  and 
third  gill-arches,  and  divides  into  two  branches  on  each  side ;  the  an- 
terior branch  forks  and  runs  through  the  first  and  second  arches ;  the 
20 


306 


THE    EMBRYO. 


posterior  branch  forks,  one  fork  going  to  the  third,  and  the  other 
after  again  forking  supplies  the  fourth  and  fifth  arches ;  this  arrange- 
ment of  the  aorta  is  typical.  Between  the  bases  of  the  first  and 
second  arches  is  a  small  protuberance  which  is  the  anlage  of  the 
tongue,  and  is  named  by  His  the  tuberculum  inpar.  The  body- 
cavity  of  the  abdomen  has  on  each  side  of  its  dorsal  surface  a  longi- 
tudinal ridge,  the  commencement  of  the  Wolfnan  body ;  the  ridge 
already  contains  traces  of  the  canals  of  the  Wolffian  body.  Of  spe- 
cial interest  is  the  arrangement  of  the  circulatory  apparatus,  Fig. 

178.     In  the  figure  the  arteries  are 
°P-       shaded  dark.     The  heart  is  an  S- 
shaped  tube,  the  venous  end  is  con- 


Op 


Car; 


FIG.  178.— W.  His'  Embryo  M.  op,  Optic  vesicle; 
A,  aorta;  Om,  omphalo-mesaraic  vein ;  Au,  arteries 
umbilicales;  All,  allantois;  Car,  cardinal  veins; 
Vh,  right  umbilical  vein ;  Ao,  dorsal  aorta ;  Jg,  jug- 
ular vein;  ot,  otocyst.  After  W.  His. 


FIG.  179.— Reconstruction  of  His1  Em- 
bryo BB,  3. 2  mm.  long,  to  show  the  Course 
of  the  Endothelial  Heart,  Ht,  and  aortic 
arches.  Op,  Optic  vesicle ;  Ht ,  heart ;  Li, 
liver:  1-4,  aortic  arches;  V,  allantoic 
vein;  Ait,  auricle.  After  W.  His. 


vex  toward  the  head,  the  arterial  end  convex  toward  the  tail;  when 
viewed  from  in  front  the  venous  portion  is  seen  on  the  left,  Fig.  179, 
the  arterial  portion  on  the  right  of  the  embryo.  The  heart  is  con- 
tinued forward  by  the  large  aorta,  Ao,  which  gives  off  five  branches 
on  each  side  of  the  neck ;  these  branches  unite  again  on  the  dorsal 
side  and  run  backward  to  unite  with  the  fellow-stem,  and  so  form 
the  single  median  dorsal  aorta,  Ao,  which  runs  way  back  and 
terminates  in  two  large  branches,  Fig.  178,  An,  which  curving  round 
pass  out  through  the  allantois-stalk.  The  five  branches  in  the  neck 
are  known  as  the  aortic  arches,  and  the  column  of  tissue  around  each 
branch  constitutes  a  so-called  branchial  or  visceral  arch;  between 
the  five  arches  are  four  spaces,  in  each  of  which  a  gill-cleft  is  ulti- 
mately formed.  The  reconstruction  of  Lr  in  a  side  view,  Fig.  180, 
affords  further  information  concerning  the  disposition  of  the  heart 
and  large  blood-vessels.  The  veins,  as  is  there  shown,  are,  1,  the 
jugular,  J,  and  cardinal,  car;  which  unite  and  form  a  single 


KNOWN   HUMAN   OVA. 


30^ 


DC 


transverse  stem,  the  ductus  Cuvieri,  D.C.;tliQ  cardinal  veins  receive 
chiefly  the  blood  from  the  Wolffian  bodies  and  atrophy  later  with 
those  bodies;  2,  the  large  umbilical  veins  which  pass  up,  Al.v. 
from  the  allantois  and  also  open  into  the  ducti  Cuvieri,  but  nearer 
the  heart  than  the  jugulars  and  cardinals;  3,  the  omphalo-mesaraic 
veins,  Om,  which  come  up  from  the  yolk-sac.  More  precise  details 
of  the  course  of  the  veins  through 
the  region  of  the  liver  will  be 
found  in  Chapter  XXIX.  The 
conformation  of  the  body- cavity 
(splanchnoccele)  can  be  better  con- 
sidered in  connection  with  the  his- 
tory of  the  septum  transversum, 
Chapter  XXII. 

33-36.  Of  other  embryos  about 
the  stage  of  those  described  in  the 
preceding  pages  several  are  known. 
His  has  referred  the  following  to 
this  stage : 

1.  Allen  Thomson's  ovum  III. 
(2),  39.1. 

2.  C.  E.  von  Baer's  described  in 
his     "  Entwickelungsgeschichte," 
Bd.   II.,  361-363,  Taf.  VI.,  Figs. 
15-19 ;  also  in  Von  Siebold's  Jour- 
nal   fiir     Geburtshiilfe     (1834), 
XIV.,  409. 

3.  Schroeder  van  der  Kolk's  (5), 
51.1. 

4.  Alexander  Ecker's  (9)  73.1. 

5.  Prof.  Hecker's  (vide  infra). 

6.  BeigePs  (vide  infra) . 

7.  Bruch's  (10). 

Of    +ViP«iA      TVi™n«rm'«i      prnhrvn 

DTJ  °>  ,                                        , , 

the  figure  Of  Which  reduced  in  Scale  ventricle;    Li,    liver;    oro,   omphalo-mesaraic 

£          J               TT-        /«   A  vein;  Al,  allantoic  diverticulum ;   Art,  allan- 

may     be     lOUna     in     XllS     (    AJiat.  toic  artery;  Al.  v,  allantoic  vein;  Am,  origin 

menschl.     Embryonen,"    Heft    II.,  $  thegamnlon;   D.  C,  ductus  Cuvieri.     After 

Fig.  18,  p.  32,  marked  A.  T.3),  is 

the  only  one  deserving  much  attention.  Thomson's  embrj-o  re- 
sembles His's  M  (see  below)  quite  closely,  not  only  in  general  form 
but  also  in  the  possession  of  distinct  gill-clefts  and  the  great  prom- 
inence of  the  heart.  Its  length  is  given  by  Thomson  at  one- 
eighth  of  an  inch,  about  3  mm.  Von  Baer's  embryo,  on  the 
contrary,  was  only  2  mm.  long ;  it  was  surrounded  by  an  amnion 
of  about  4.5  mm.  diameter,  which  is  abnormally  large;  Von  Baer 
observed  four  open  gill-slits ;  the  hind  end  of  the  body  was  partially 
atrophied,  which  accounts  for  the  short  length.  Van  der  Kolk's 
embryo,  as  I  have  already  stated,  I  refer  not  to  this  but  to  the  pre- 
vious stage,  perhaps  mistakenly,  but  I  think  not.  In  Ecker's  ovum 
the  chorion  measured  12  by  9  mm.,  and  the  embryo  only  2  mm;  the 
author's  description  is  very  meagre  and  his  figures  not  distinct; 
Ecker  expressly  compares  it  with  an  ovum  of  Wagner's,  figured  in 


FIG.  180.— Reconstruction  of   His'   Embryo, 

r,  (Fig.   16).     Of,  Otocyst;   J,    jugular;  car, 

carotid;  /,  first  aortic  arch;  Au, auricle;  Ven, 


308  THE   EMBRYO. 

Wagner's  "Icones  Physiologies, "  and  again  in  

Physiologic^, "  Taf.  XXV.,  Fig.  V. ;  but  the  comparison  apparently 
refers  only  to  the  chorion,  for  Wagner's  embryo  was  evidently  older, 
being  4.5  mm.  long  and  having  external  traces  of  limbs.  Hecker's 
ovum  (5)  I  know  only  through  Prof.  His'  reference,  which  leaves 
the  impression  that  Hecker's  description  is  so  unsatisfactory  as  to 
render  it  a  matter  of  surmise  exactly  what  stage  of  development  the 
specimen  had  reached.  In  regard  to  Beigel's  ovum  I  have  already 
expressed,  p.  289,  my  opinion  that  it  is  a  much  older  and  abnormal 
embryo;  I  do  not  differ  from  Prof.  His  as  to  the  slight  value  attaching 
to  Beigel's  description.  Bruch's  embryo  ( Abh.  Senck.  Ges.  VI. ,  Taf.  X. 
[40]  )  appears  to  me  from  his  description  and  plate  to  have  been  very 
abnormal.  Of  these  seven  embryos  Kolk's  and  Beigel's  do  not  belong 
to  this  stage;  Von  Baer's  and  Bruch's  were  abnormal;  Hecker's  is 
questionable,  Ecker's  somewhat  uncertain,  and  Thomson's  the 
only  satisfactory  one.  Of  Thomson's  only  the  general  appearance 
is  described,  but  that  confirms  what  we  learn  from  His'  observations 
on  this  stage. 

37,  38.  Of  embryos  with  four  gill-clef ts  we  possess  no  satisfactory 
descriptions,  unless,  indeed,  we  regard  His'  embryo  Lr,  described 
above,  as  belonging  to  this  stage,  since  the  fourth  pharyngeal  gill- 
pouch  is  found  in  it.  To  this  stage  may  perhaps  be  assigned  the 
embryo  described  by  Johannes  Miiller  ("  Physiologie, "  4te  Aufl.,Bd. 
II. ,  713,  Taf.,  and  in  Muller's  Archiv,  1834,  p.  8,  and  1836,  p. 
clxvii),  and  also  Wagner's  embryo  (Wagner's  k<  Icones  Physiol.," 
Taf.  VIII.,  Figs.  2  and  3,  also  in  Ecker's  "Icones  Physiol.,"  Taf. 
XXV.,  Fig.  5) ;  important  critical  remarks  on  these  two  embryos 
may  be  found  in  His'  "Anat.  menschlicher  Embryonen,"  Heft  1, 
162-163,  and  Heft  II.,  41-43).  Muller's  embryo  was  about  5.5  mm. 
alone,  Wagner's  4.5  mm.  They  both  had  a  marked  dorsal  flexure 
resembling  that  normally  present  in  embryos  with  two  gill-clefts ; 
but  this  flexure  was  probably  produced  artificially  by  a  strain  upon 
the  yolk-sac  pulling  the  back  down ;  the  flexure  is  to  be  regarded  as 
artificial,  because  in  embryos  which  were  certainly  normal  it  was 
not  found  in  the  next  younger  or  the  next  older  stage.  How  easily 
the  flexure  may  be  produced  is  shown  by  His'  observation  of  its 
occurring  in  his  embryo  W,  while  he  was  manipulating  it.  Neither 
of  the  two  embryos  under  consideration  are  described  or  figured  with 
sufficient  accuracy  of  detail  to  justify  a  fuller  description  of  them. 
As  Von  Baer  states  of  his  embryo,  34,  that  it  had  four  clefts,  it  must 
be  held  to  belong  in  this  stage  probably. 

Summary.  Known  Young  Human  Ova. — The  detailed  de- 
scriptions of  the  preceding  pages,  287  to  308,  are  summarized  in 
the  following  paragraphs. 

FIRST  STAGE:*  PRIMITIVE  STREAK.— No  human  ovum  has  been 
observed  to  have  a  primitive  streak,  but  there  are  several  which  are 
younger  than  the  embryos  with  open  medullary  groove,  and  there- 
fore presumably  are  in  this  stage ;  unfortunately  there  is  a  satisfac- 
tory description  of  the  embryonic  structures  in  no  one  of  them.  To 
this  group  have  been  assigned  the  embryos  1  to  9,  but  of  these 
Beigel's  (5)  is  certainly  abnormal,  and  Schwabe's  (9)  is  probably 

*  For  definitions  of  the  stages,  see  p.  286,  ante. 


KNOWN    HUMAN   OVA.  309 

both  abnormal  and  much  older.  From  the  preceding  review  of  the 
remaining  seven  ova  the  following  conclusions  may  be  drawn :  The 
human  ovum  by  the  twelfth  or  thirteenth  day  is  a  rounded,  somewhat 
flattened  sac  of  three  to  four  millimetres  in  diameter,  bearing  an 
equatorial  zone  of  short  unbranched  villi;  the  villi  are  probably 
formed  by  the  ectoderm  only ;  the  wall  of  the  sac  is  ectoderm,  whether 
underlaid  by  somatic  mesoderm  or  not  is  uncertain;  to  the  inner 
wall  of  the  sac  over  one  of  the  bare  poles  of  the  ovum  is  attached  a 
mass  of  cells,  constituting  the  anlage  of  the  embryo ;  as  to  the  ar- 
rangement of  these  cells  we  possess  110  knowledge. 

In  the  next  stage  the  villi  have  spread  over  the  germinal  area  and 
have  become  slightly  branched ;  the  villi  next  appear  over  the  oppo- 
site pole  of  the  ovum  and  rapidly  increase  their  length  and  ramifi- 
cations. The  germinal  area  faces  the  uterine  wall  (Jones'  ovum,  3). 
By  the  time  villi  are  present  over  the  whole  vesicle  there  is  probably 
always  a  layer  of  connective  tissue  underlying  the  epithelium 
(Breus  2,  Ahfeld  4,  Lowe  5,  etc.),  but  no  embryonic  structures  have 
been  recognized.  The  ova  of  twelve  to  fourteen  days  are  already 
completely  inclosed  by  the  decidua  (reflexa  and  serotina) ;  only  the 
tips  of  the  villi  adhere  to,  or  are  even  in  contact  with,  the  decidua; 
this  is  the  only  connection  between  the  maternal  and  fcetal  tissue, 
for  neither  does  the  uterine  mucosa  grow  in  between  the  villi,  nor  do 
the  villi  penetrate  the  cavities  of  the  uterine  glands.  The  epithelium 
of  the  chorion  and  villi  is  only  imperfectly  marked  with  boundaries 
for  the  single  cells ;  its  nuclei  all  occupy  a  basal  position,  leaving  a 
distinct  outer  layer,  often  mistaken  for  a  separate  structure.  The 
epithelium  forms  buds  which  become  branches  of  the  villi.  These 
buds  may  grow  out  to  a  considerable  size  without  connective  tissue 
(hollow  villi) ,  or  the  connective  tissue  may  penetrate  into  them  from 
the  start  (solid  villi).  The  human  ovum,  then,  is  remarkable  for 
its  precocious  development  of  the  chorion,  both  as  regards  the  villi 
and  the  connective  tissue  or  mesodermic  layer,  and  for  its  early 
complete  encapsulation  by  the  decidua.  All  these  events  (according 
to  the  scanty  observations  yet  made)  precede  the  appearance  of  the 
embryo.  It  is  also  noteworthy  that  the  villi  are  first  developed 
around  the  equator,  next  over  the  germinal  area  pole,  and  last  over 
the  area  of  the  opposite  pole. 

SECOND  STAGE:  MEDULLARY  PLATE. — To  this  stage  I  assign  the 
embryos,  His'  XLIY.  or  Bff  (10),  KeibePs,  11,  and  Spee's,  12,  and  I 
think  they  belong  in  the  order  named.  The  chorionic  vesicle  is 
rounded  and  somewhat  flattened ;  in  its  greatest  diameter  it  meas- 
ures 8-10  mm. ;  it  is  beset  with  short  branching  villi,  which  are 
present  over  the  entire  surface  except  in  one  case,  where  they  formed 
an  equatorial  band  as  in  Reichert's  ovum  1.  The  chorion  had  a 
distinct  ectodermal  and  a  distinct  mesodermal  layer;  the  former,  at 
least  in  Spee's  embryo,  had  two  strata  of  cells,  as  is  characteristic  of 
the  chorion.  To  the  inner  surface  of  the  chorion  was  attached  a 
thick  allantois-stalk  (Bauclistiel] ,  which,  curving  slightly,  passed 
over  without  any  demarcation  into  the  embryo,  which  in  Keibel's 
ovum  measured  about  1  mm,  in  Spee's  about  1.5  mm.  From  the 
sides  of  the  allantois-stalk  and  of  the  embryo  sprang  the  thin  am- 
nion,  which  was  completely  closed.  Along  nearly  the  entire  length 


310  THE    EMBRYO. 

of  the  ventral  surface  of  the  embryo  was  attached  the  yolk-sac,  which 
was  of  rounded  form  and  about  equal  in  diameter  to  the  length  of 
the  embryo;  in  Keibel's  ovum  the  yolk-sac  had  blood-vessels  con- 
taining nucleated  blood-corpuscles,  and  was  a  hollow  vesicle  whose 
thin  walls  were  composed  of  a  fine  lining  of  entoderm,  and  a  thicker 
sheet  of  mesoderm.  Spee  was  able  to  study  his  embryo  in  detail; 
it  had  a  well-marked  medullary  plate  with  a  median  furrow,  Fig. 
16-1;  at  the  posterior  end  of  the  plate  was  the  primitive  streak,  and 
at  the  anterior  end  of  the  primitive  streak  was  an  opening  (named 
by  Spee  the  neurenteric  canal)  leading  into  the  entodermal  cavity ; 
the  head  had  grown  forward  sufficiently  to  indicate  the  development 
of  the  vorderdarm ;  the  notochord  was  present,  as  a  median  band 
of  entodermal  cells,  running  forward  from  the  neurenteric  canal; 
the  allantoic  diverticulum  extended  as  a  narrow  tube  of  entoderm 
through  the  allantois-stalk  to  the  chorion ;  the  ccelom  had  not  ap- 
peared in  the  embryo  proper ;  the  anlage  of  the  heart  was  not  present. 

This  stage  is,  therefore,  characterized  by  the  size  of  the  chorionic 
vesicle,  8-10  mm.,  the  completed  development  of  the  extra-embryonic 
ccelom,  and  the  absence  of  the  embryonic  ccelom  and  heart  anlage ; 
by  the  presence  of  the  medullary  plate,  neurenteric  (or  blastoporic) 
canal,  notochordal  band  in  the  entoderm,  the  vascularized  yolk-sac, 
the  thick  allantois-stalk  with  the  tubular  allantoic  diverticulum. 
The  general  arrangement  can  be  understood  from  the  diagram,  Fig. 
1G6. 

THIRD  STAGE:  MEDULLARY  GROOVE. — The  development  of  both 
the  embryo  and  its  appendages  has  advanced.  Particularly  note- 
worthy are  the  large  size  of  the  medullary  ridges  and  the  precocious 
differentiation  of  the  chorion  and  amnion.  The  youngest  embryos 
of  this  group  are  in  the  neighborhood  of  2.2  mm.  in  length  (Thom- 
son gives  the  length  of  his  embryo  I.  as  2.5  mm.,  but  the  criticisms 
made  above  render  it  plain  that  this  measure  probably  refers  to  the 
length  of  the  amnion  plus  the  allantois-stalk) ;  the  embryo  not  seen 
by  Thomson  was  presumably  shorter.  The  embryo  has  a  broad 
attachment  to  the  yolk-sac,  which  in  diameter  nearly  equals  the 
length  of  the  embryo  and  is  already  furnished  with  blood-vessels. 
The  most  conspicuous  character  of  the  embryo  is  the  presence  of  two 
very  thick  dorsal  ridges — medullary  folds,  running  the  whole  length 
of  the  embryo  and  inclosing  the  medullary  groove,  central  nervous 
system  to  be,  between  them;  the  cephalic  extremity  is  somewhat 
thickened ;  from  the  ventral  side  of  the  caudal  extremity  springs  the 
short  and  thick  allantois-stalk,  the  opposite  end  of  which  is  inserted 
into  the  chorion.  The  amnion  completely  incloses  the  embryo,  and 
is  attached  on  the  one  hand  to  the  allantois-stalk,  on  the  other  to  the 
embryo  nearly  parallel  to  the  junction  of  the  embryo  and  the  yolk- 
sac.  The  next  change  involves  not  merely  the  growth  of  the  embryo, 
but  also  the  thickening  of  its  cephalic  end,  the  development  of  the 
great  heart  protuberance  between  the  yolk-sac  and  the  head,  the 
concave  flexion  of  the  back,  and  the  deepening  of  the  medullary 
groove,  which,  however,  still  remains  open.  The  chorion  forms  a 
relatively  large  vesicle,  its  average  diameter  being  about  8  mm.,  but 
the  four  specimens  vary  from  5.7  to  15  mm.  The  chorion  bears 
villi  over  its  whole  surface;  the  villi  are  considerably  branched. 


KXOWX    HUMAN   OVA.  311 

Probably  the  villi  are  formed  chiefly  if  not  solely  by  epithelium,  and 
probably,  also,  there  is  a  layer  of  connective  tissue,  very  likely  al- 
ready vascular,  which  lines  the  chorion,  but  does  not  extend  into 
the  villi.  There  are  many  still  unsolved  problems  as  to  the  develop- 
ment of  man.  It  will  be  observed  that  not  a  single  one  of  the  ova 
hitherto  noticed  has  been  adequately  investigated,  and  that  no  speci- 
mens have  yet  been  studied  at  all,  showing  the  first  appearance  of 
the  embryo,  the  origin  of  the  amnion  or  of  the  ailantois,  or  of  the 
yolk-sac ;  and  finally,  that  of  all  the  earliest  stages  our  knowledge  is 
extremely  imperfect.  It  is,  therefore,  much  to  be  hoped  that  all  who 
obtain  available  specimens  will  carefully  preserve  them  and  intrust 
them  to  a  competent  investigator.  From  the  above  considerations 
it  is  also  evident  that  the  summary  just  given  can  be  only  tentative. 

FOURTH  STAGE:  THE  HEART.— In  this  stage  the  embryo  is  prob- 
ably 2.2  to  2.5  mm.  long;  the  head  projects  in  front  of  the  yolk,  and 
on  the  under  side  of  the  cervical  region  the  heart  has  appeared;  the 
deep  neural  groove  is  partly  closed  to  form  the  medullary  canal,  but 
is  open  along  the  cephalic  region;  the  dorsal  outline  is  slightly  con- 
cave ;  the  myotomes  have  appeared,  the  number  varying ;  Spee  found 
seven,  Kollmann  thirteen ;  the  caudal  end  of  the  embryo  also  projects 
beyond  the  yolk,  but  less  than  does  the  head ;  the  auditory  invag- 
ination  is  probably  not  yet  formed ;  there  are  no  gill-clefts  showing 
externally. 

Concerning  the  chorionic  vesicle  at  this  stage,  satisfactory  data 
are  lacking. 

FIFTH  STAGE:  ONE  GILL-CLEFT. — No  human  embryo  with  only 
one  gill-cleft  showing  externally  is  known. 

SIXTH  STAGE:  T\vo  GILL-CLEFTS  AND  DORSAL  FLEXURE. — To 
this  stage  we  must  assign  not  only  my  two  specimens  referred  to 
above,  20  and  21,  and  His'  Lg,  22,  and  Sch  1,  23,  but  also  His'  L, 
•?  K  and  probably  Coste's,  although  in  neither  of  the  latter  does  the 
dorsal  flexure  appear.  It  is  possible  that  Schroeder  van  der  Kolk's 
ovum,  26,  Hennig's,  27,  Schwabe's,  9,  and  Remy's,  28,  also  belong 
in  this  stage,  but  for  reasons  given  above  in  detail  the  position  of 
these  four  is  very  doubtful,  that  of  Schwabe's  especially  so.  In  His' 
embryo  L,  and  in  Coste's,  the  dorsal  flexure  was  probably  obliterated 
artificially,  leaving  only  the  four  embryos,  20-23,  upon  which  the 
following  synopsis  is  based,  with  the  addition  of  some  anatomical 
facts  derived  from  Nos.  24  and  25. 

The  general  shape  of  the  embryo  and  its  remarkable  dorsal  flexure 
can  be  best  understood  from  Fig.  17.  The  head  bend  is  very 
marked  and  the  tail  end  of  the  embryo  is  also  bent  over  ventralward ; 
the  yolk-sac  extends  from  the  heart  backward  to  where  the  body  of 
the  embryo  turns  to  make  the  dorsal  flexure ;  the  heart  is  large  and 
very  protuberant ;  it  is  bent  so  that  we  can  clearly  distinguish  the 
auricular,  ventricular,  and  aortic  limbs,  and  it  consists  of  a  smaller 
inner  tube,  the  endothelial  heart,  or  endocardium  (which  is  con- 
tinuous at  one  end  with  the  walls  of  the  veins,  at  the  other  with  the 
walls  of  the  aorta) ,  and  of  an  outer  larger  tube,  the  muscular  heart 
or  myocardium ;  between  the  two  heart  tubes  is  a  considerable  space ; 
there  are  two  gill-clefts  and,  at  least  in  the  youngest  specimens, 
only  two  aortic  arches,  one  in  front  of  each  cleft  ;  between  the  head 


312  THE   EMBRYO. 

and  the  heart  the  oral  invagination  has  been  formed  but  is  still  sepa- 
rated by  the  oral  plate  (Rachenhaut)  from  the  vorderdarm ;  above 
the  gill-clefts  is  the  open  ectodermal  invagination  of  the  otocyst, 
which  in  His'  embryo  L,  24,  had  become  a  closed  vesicle.  The  cen- 
tral nervous  system  is  very  large  compared  with  the  whole  embryo ; 
the  brain  comprises  in  length  about  one-half  of  the  medullary  canal ; 
the  optic  vesicles  are  large,  and  the  optic  stalks  are  well  differenti- 
ated; the  head  bend  takes  place  in  the  region  of  the  mid-brain, 
which  is  imperfectly  separated  from  the  fore-brain ;  the  hind-brain  is 
about  equal  to  the  fore  and  mid  brains  together  in  length ;  there  were 
twenty-nine  myotomes  in  His'  embryo  Lg,  22.  The  vorderdarm-  is 
flattened  dorso-ventrally ;  the  liver  is  developing  in  the  septum  trans- 
versum ;  the  middle  portion  of  the  intestine  opens  into  the  yolk-sac, 
the  posterior  portion  is  closed  and  at  its  caudal  termination  is  dilated 
to  form  the  bursa  of  His,  and  curves  over  to  pass  as  the  narrow 
tubular  allantoic  diverticulurn  through  the  allantois-stalk  to  the  level 
of  the  chorion.  The  veins  show  the  typical  arrangement,  the  jugu- 
lars joined  by  the  cardinals  form  the  ducti  Cuvieri,  and  these  after 
receiving  the  omphalo-mesaraic  (or  vitelline)  and  the  umbilical  (or 
allantoic)  veins  unite  in  the  median  line  as  the  sinus  reunions ;  the 
course  of  the  allantoic  veins  is  peculiar  and  may  be  described  as  a 
short  cut  through  the  somatopleure  along  the  line  where  the  body 
wall  of  the  embryo  is  deflected  back  to  form  the  amnion. 

SEVENTH  STAGE:  THREE  GILL-CLEFTS. — All  the  accurately 
known  embryos,  except  one,  28A,  belonging  to  this  stage,  belong 
to  the  end  of  it,  and  one  of  them,  His'  Lr,  32,  is  so  far  advanced 
that  it  might  almost  be  classed  in  the  next  stage.  Five  good  em- 
bryos, 29-33,  are  to  be  placed  here,  and  four  others,  34,  35,  36,  and 
5  have  been  associated  with  them,  but  the  latter  are  all  doubtful 
cases ;  the  best  of  them  being  Von  Baer's,  34,  which  probably  should 
be  put  in  the  eighth  stage.  For  reasons  stated  in  the  section  on  the 
dorsal  flexure,  p.  313,  the  flexure  is  probably  normally  absent  in 
embryos  at  the  close  of  the  seventh  stage.  The  described  embryos 
vary  from  2.6  to  4.2  mm.  in  length;  His'  M,  30,  was  2.6  mm.  long, 
and  itschor ionic  vesicle  measured  11  by  14  mm,  His'  BB,  31,  was 
3.2  mm.  long,  and  its  chorionic  vesicle  measured  11  by  14  mm  :  the 
age  of  BB  was  probably  twenty  to  twenty-one  days.  The  back  of 
the  embryo  is  normally  (or  at  least  usually)  convex ;  the  head  is  bent 
to  one  side  (usually  to  the  right)  and  the  tail  to  the  other,  the  whole 
embryo  having  a  spiral  twist;  there  are  three  gill-clefts  showing 
externally ;  the  tail  end  has  grown  considerably  and  the  allantois- 
stalk  has  lengthened ;  the  yolk-stalk  (neck  of  the  yolk-sac)  is  both 
relatively  and  absolutely  smaller  than  in  the  previous  stage,  but  the 
embryo  is  larger.  The  heart  has  grown  very  much ;  in  the  older 
specimens  the  development  of  the  auricular  pouches  has  begun. 
The  otocyst  is  a  closed  pear-shaped  vesicle,  its  apex  pointing  toward 
the  dorsal  side.  The  mouth  cavity  has  deepened,  the  oral  plate  is 
ruptured;  above  the  mouth  the  maxillary  process  can  be  distin- 
guished. The  pharynx  is  wide,  compressed  dorso-ventrally,  and  has 
in  the  known  specimens  four  gill-pouches,  and  on  its  median  ventral 
floor  a  small  prominence,  His'  tuberculum  impar,  the  anlage  of  the 
tongue ;  the  diverticulum  of  the  liver  is  well  marked  in  the  youngest, 


KNOWN    HUMAN    OVA.  313 

and  enlarged  and  branching  in  the  oldest  specimens ;  the  Wolffian 
ridge  is  distinguishable  and  contains  Wolffiaii  tubules,  but  as  to  the 
number  and  form  of  these  we  possess  no  exact  information.  The 
medullary  canal  is  closed  throughout  its  length ;  the  mid  and  fore 
brains  have  become  clearly  separated  since  the  sixth  stage.  As 
regards  the  circulatory  system,  besides  the  appearance  of  the  auricles 
and  the  general  advance  of  the  heart,  we  have  to  note  that  the  great 
veins  passing  through  the  septum  transversum  have  begun  their 
transformations  into  $ie  hepatic  system,  and  that  the  aorta  has  five 
aortic  arches,  the  two  first  coming  from  one  branch,  the  remaining 
three  from  another  branch  on  each  side ;  no  embryos  are  known  with 
only  four  aortic  arches. 

EIGHTH  STAGE:  FOUR  GILL-CLEFTS. — The  three  embryos,  34, 
37,  38,  which  were  apparently  in  this  stage,  are  so  imperfectly  known 
that  there  is  practically  nothing  definite  to  say  in  regard  to  their 
anatomy.  Wagner's  specimen,  38,  measured  4.5  mm.;  Miiller's, 
.">;,  .">..")  mm. 

The  Dorsal  Flexure. — In  a  number  of  embryos  with  from  two 
to  four  gill-clefts  there  has  been  observed  a  deep  bend  in  the  rump, 
which  suggests  at  once  the  effect  of  a  pull  upon  the  yolk  having  pro- 
duced a  sharp  concavity  in  the  back,  compare  Fig.  169.  In  embryos 
with  two  gill-clefts  this  bend,  for  which  I  propose  the  term  dorsal 
fiexure  (Rtickenkrummung) ,  has  been  shown  by  His  to  be  normal. 
In  older  embryos  it  seems  to  be  abnormal,  for  in  one  with  three  clefts 
and  the  dorsal  flexure,  31,  the  tissues  in  the  region  of  the  bend  were 
lacerated,  and  in  a  still  older  specimen  (W  of  His)  the  bend  was 
artificially  produced  while  the  embryo  was  being  manipulated.  The 
facts  indicate  that  the  back  is  too  long  for  the  somatopleure  at  the 
side  of  the  body,  and  that  it  finds  room  at  the  stage  with  two  gill- 
clefts  by  becoming  concave ;  later  it  springs  into  a  new  position  of 
equilibrium  by  becoming  convex;  it  is  possible  that  the  change  from 
the  concave  to  the  convex  position  is  very  abrupt,  and  it  is  probable 
that  the  time  of  its  occurrence  is  very  variable,  so  that  we  may  find 
hereafter  embryos  in  the  seventh  and  eighth  stages,  which  are  per- 
fectly normal  though  still  having  the  dorsal  flexure. 


PART  IV. 

THE  FCETAL  APPENDAGES. 


CHAPTER  XIV. 
THE  HUMAN   CHORION. 

THE  human  chorion  has  been  the  object  of  greater  misconception 
than  perhaps  any  other  organ  of  the  body.  Even  at  the  present 
time  there  prevail  numerous  false  notions  concerning  it,  and  many 
of  these  errors  are  repeated  in  some  of  the  best  accredited  text-books. 
The  literature  of  the  subject  includes  a  majority  of  papers  of  little 
value,  and  often  remarkable  for  the  gross  crudity  of  the  observations 
they  record  and  for  the  ignorance  displayed  by  their  authors  of  other 
and  better  observations.  The  literature  also  includes  numerous  pa- 
pers by  investigators  of  exceptional  accuracy  and  intelligence,  such 
as  Coste,  Farre,  Turner,  Langhans,  Waldeyer,  etc.,  by  which  we 
are  enabled  to  give  a  fairly  complete  history  of  the  chorion.  This 
chapter  is  based  chiefly  on  the  account  given  in  my  paper  on  "  The 
Uterus  and  Embryo,"  98. 

General  Description. — The  chorion  has  already  been  defined 
as  the  whole  of  that  portion  of  the  extra-embryonic  somatopleure, 
which  is  not  concerned  in  the  formation  of  the  amnion.  The  human 
chorion,  as  stated  above,  p.  281,  is  remarkable  for  its  very  early 
complete  separation  from  the  yolk-sac  and  for  the  precocious  appear- 
ance of  its  villi.  As  shown  in  the  previous  chapter,  both  of  these 
developments  have  taken  place  in  all  the  very  young  human  ova 
hitherto  obtained,  even  in  Reichert's  ovum,  which  is  suppposed  to 
be  the  youngest  known,  see  p.  287.  At  about  twelve  days  the  cho- 
rionic  vesicle  is  a  closed  sac,  somewhat  flattened  and  3-4  mm.  in 
greatest  diameter ;  around  its  equator  there  is  a  broad  zone  of  simple 
villi.  About  a  day  later  the  vesicle  is  1  mm.  larger,  and  the  villi, 
which  are  beginning  to  branch,  have  developed  over  one  of  the  polar 
areas,  and  soon  after  develop  over  the  second  pole  also,  though  less 
thickly  than  elsewhere.  The  fourteenth  day  the  vesicle  measures 
about  6  mm. 

In  all  these  cases  the  vesicle  is  found  completely  encapsuled  in  the 
decidua,  and  the  decidua  reflexa  is  closed  over  it.  The  tips  of  the 
villi  alone  touch  the  decidual  surface,  to  which  they  are  lightly  at- 
tached, for  they  can  be  pulled  off  from  it  without  their  breaking, 
Spee,  89. 1,  160.  This  arrangement  leaves  a  space  which  is  bounded 
on  the  one  side  by  the  maternal  decidua,  on  the  other  by  the  foetal 
chorion,  and  which  is  crossed  by  the  foetal  villi;  it  is  commonly 
designated  as  the  intervillous  space. 

As  to  the  growth  of  the  chorionic  vesicle  I  have  failed  to  find  any 
extended  observations,  and  can  only  express  the  hope  that  the  omis- 
sion may  be  soon  supplied.  The  growth  at  first  is  very  rapid,  so  that 
during  the  second  month  there  is  always  a  considerable  space  around 
the  embryo  and  amnion,  but  after  the  second  month  the  space  is 


318  THE    FCETAL    APPENDAGES. 

relatively  diminished  by  the  growth  of  the  foetus.     W.  His  gives 
the  following  table  ("  Anat.  Menschl.  Embryonen,"  Heft  II.,  21). 

Diameter  of  Chorion,      <   1.5cm.;       Embryo,         2-4  mm. 
1.5-3.0     "  "   "  -1-10    " 

2.5-4.0     "  "  10-15    " 

3.5-5.0     "  "  15-20    " 

4.0-6.0     "  "  20-25    " 

The  contents  of  the  vesicle  are,  first,  the  embryo  with  its  allantois- 
stalk  and  yolk-sac,  and,  second,  the  chorionic  fluid;  concerning  the 
latter  I  know  of  no  exact  observations,  but  it  probably  resembles,  if 
indeed  it  be  not  identical  with,  the  amniotic  fluid,  compare  p.  337. 

The  history  of  the  villi  is  given  below  in  detail ;  in  this  paragraph 
we  need  refer  only  to  the  changes  in  the  villi,  by  which  the  mem- 
brane is  differentiated  into  the  chorion  Iceve  and  the  chorion  fron- 
dosum. I  consider  it  doubtful  whether  the  number  of  villi  increases 
at  all  after  a  comparatively  early  stage,  but  over  all  that  part  of  the 
chorion  which  overlies  the  decidua  serotina  (cf.  Chapter  I.)  the  villi 
continue  to  grow  both  in  size  and  in  the  number  of  their  branches 
for  a  long  time — perhaps  through  the  entire  period  of  pregnancy ; 
this  area  of  enlarged  villi  presents  a  shaggy  appearance  and  hence 
is  called  the  chorion  frondosum ;  it  participates  in  the  formation  of 
the  placenta ;  the  allantois-stalk  (or  later  the  umbilical  cord)  is  always 
inserted  into  the  chorion  frondosum.  Over  all  the  remainder  of  the 
chorion,  which  lies  against  the  decidua  reflexa,  the  villi  gradually 
atrophy  during  the  second  month,  so  that  this  region  becomes  smooth, 
and  hence  is  termed  the  chorion  Iseve. 

The  chorion  consists  histologically  of  an  external  layer  of  epithelial 
ectoderm  and  an  inner  thicker  layer  of  mesoderm ;  whether  the  mes- 
oderm  is  divisible  into  a  mesenchyma  and  mesothelium,  as  the  devel- 
opment of  the  chorion  out  of  the  somatopleure  leads  us  to  expect,  is 
uncertain,  but  if  there  is  an  interior  layer  of  epithelium  on  the  meso- 
dermal  surface  it  must  be  extremely  thin,  for  I  cannot  detect  it  in 
my  sections ;  the  bulk  of  the  mesoderm  is  undoubtedly  mesenchymal. 
The  ectoderm  in  the  earliest  stages  known  consists  of  two  clearly 
differentiated  layers,  a  thinner  outer  one  with  small  nuclei  and  with- 
out recognizable  cell  boundaries,  and  an  inner  one  consisting  of  dis- 
tinct cells  with  large  nuclei.  The  outer  layer  has  been  regarded  by 
some  authors  as  maternal  tissue — an  opinion  discussed  in  the  section 
on  the  histology,  p.  322. 

The  chorion  is  at  first  vascular  throughout  its  entire  extent,  re- 
ceiving its  blood  from  the  embryo  via  the  allantois-stalk  through 
two  arteries,  and  returning  it  by  the  same  route  through  two  veins, 
see  Chapter  XVII.  The  vessels  early  penetrate  the  villi,  but  as  the 
villi  disappear  from  the  chorion  Ia3ve  the  blood-vessels  also  abort 
there  and  remain  only  over  the  chorion  frondosum  (compare  Chapter 
XVII.),  to  maintain  the  circulation  of  the  foetal  placenta. 

Chorionic  Villi.  Development. — As  has  been  stated  both  in 
the  review  of  the  youngest  known  human  ova,  and  in  the  general 
description  of  the  chorion,  the  villi  arise  in  a  broad  zone  around  the 
equator  of  the  somewhat  flattened  chorionic  vesicle,  and  soon  after 
appear  over  both  polar  areas;  they  are  at  first  clumsy  cylinders 
which  may  grow  to  a  millimetre  in  length  before  they  begin  branch- 


THE    HUMAN   CHORION.  319 

ing.  They  arise,  as  shown  long  ago  by  the  observations  of  Coste, 
as  outgrowths  of  the  ectoderm  only,  Fig.  174;  the  hollo wness  of  the 
villi  and  their  clumsy  shape  are  to  be  especially  noted.  The  meso- 
derm  grows  into  them  subsequently.  The  openings  into  the  villi 
can  also  be  seen  in  iTig.  172,  scattered  over  the  surface  of  the  chorion. 

Branching. — The  branches  of  the  villi  grow  out  in  a  similar 
manner,  the  process  being  led,  as  it  were,  by  the  ectoderm.  Orth 
in  a  special  paper,  78. 1,  has  used  these  facts  to  argue  against  Boll's 
"Princip  des  Wachsthums."  Kollmann's  observations,  79.1,  297, 
on  the  growth  of  villi  during  the  fourth  week  are  particularly  in- 
structive. The  outgrowth  of  the  branches  is  very  rapid  and  occurs 
with  every  degree  of  participation  of  the  connective  tissue.  The  two 
extremes  are:  1,  a  bud  consisting  wholly  of  epithelium,  which  may 
become  a  process  with  a  long  thin  pedicle,  and  a  thickened  free  end 
remaining  entirely  without  mesoderm ;  2,  a  thick  bud  with  a  well- 
developed  core  of  connective  tissue,  and  having  a  nearly  cylindrical 
form.  Between  these  extremes  every  intermediate  state  can  be 
found.  Other  observers  have  noted  this  peculiar  manner  of  growth, 
which  I  have  found  still  going  on  in  the  placental  chorion  during  the 
fourth  month.  Robin,  54. 1,  appears  also  to  have  crudely  observed 
both  the  young  hollow  villi  and  the  solid  epithelial  buds.  The 
blood-vessels  he  traces  to  the  division  of  the  cavity  of  the  villi  into 
an  artery  and  a  vein ;  from  the  nature  of  things  he  offers  no  observa- 
tions in  support  of  this  assertion. 

Only  the  tips  of  the  villi  touch  the  surface  of  the  decidua  either  at 
first  or  subsequently,  except,  of  course,  over  the  chorion  laBve  during 
the  abortion  of  the  villi.  The  tips  of  the  villi  are  attached  to  the 
uterine  surface ;  they  penetrate  the  decidua  for  a  short  distance,  but 
even  in  the  placental  area  at  the  close  of  gestation  the  penetration 
is  slight  and  the  villi  make  their  way  only  into  the  surface  stratum 
of  the  decidua  serotina.  There  is  no  evidence  of  any  sort  that  the 
villi  penetrate  the  glands  at  any  period.  The  relation  of  the  villi  to 
the  decidua  has  now  been  so  accurately  ascertained  that  there  can 
be,  I  think,  no  longer  any  question  whatsoever  on  this  point.  The 
best  discussion  is  by  Langhans,  77.1,  p.  231  ff. 

The  shape  of  the  villi  varies  according  to  the  part  of  the  chorion 
and  the  age  of  the  embryo.  They  gradually  abort  over  the  chorion 
Ia3ve,  and  gradually  grow  over  the  chorion  f rondosum.  Let  us  begin 
with  the  placental  villi :  At  first  they  are  short,  thick-set  bodies  of 
irregular  shape,  as  shown  in  Fig.  174;  at  twelve  weeks  their  form 
is  extremely  characteristic,  Fig.  181 ;  the  main  stem  gives  off  nu- 
merous branches  at  more  or  less  acute  angles,  and  these  again  other 
branches,  until  at  last  the  terminal  twigs  are  reached ;  the  whole  of 
the  space  between  the  chorion  and  decidua  is  occupied  by  these  ram- 
ifications; the  branches  and  twigs,  as  the  illustration  shows,  are 
extremely  irregular  and  variable,  although  in  general  they  may  be 
described  as  club-shaped,  being  more  or  less  constricted  at  their  bases. 
The  branches  may  be  bigger  than  the  trunk  which  bears  them,  or 
of  any  less  size ;  some  of  the  smallest  are  merely  slender  outgrowths 
of  the  epithelial  covering  of  the  villus,  such  as  have  already  been 
alluded  to.  Gradually  there  is  a  change.  During  the  fifth  month 
we  find  the  irregularity,  though  still  very  marked,  decidedly  less 


320 


THE   FCETAL.  APPENDAGES. 


exaggerated,  Fig.  182;  the  branches  tend  to  go  off  at  more  nearly 
right  angles ;  one  finds  very  numerous  free  ends,  as  of  course  only  a 
small  proportion  of  the  branches  touch  the  decidual  surface;  the 


Fio.  181.— Isolated  Terminal  Branch  of  a  Villus 
from  the  Chorion  of  an  Embryo   of    Twelve 


FIG.  182.— Villous  Stem  from  a  Placenta  of 
the  Fifth  Month.     X  9  diams. 


branches,  too,  are  less  out  of  proportion  to  the  stems,  less  constricted 
at  their  bases,  or,  in  other  words,  less  remote  from  the  cylindrical 
form;  the  awkward  cucumber-shapes  of  the  twelfth  week  are  no 
longer  found  except  here  and  there.  The  change  continues  in  the 
same  direction;  that  is,  is  toward  greater  regularity  of  configura- 
tion. It  is  hardly  necessary  to  describe  the  intermediate  phases 

that  have  been  exam- 
ined, but  it  will  suf- 
fice to  describe  the 
form  at  full  term, 
Fig.  183,  when  the 
branches  are  long, 
slender,  and  1  e  s  s 
closely  set,  as  well  as 
less  subdivided,  than 
at  earlier  stages ;  they 
have  nodular  projec- 
tions like  branches 
arrested  at  the  begin- 
The  ning  of  their  develop- 
ment; there  are  nu- 
merous spots  upon  the 
surfaces  of  the  villi;  microscopical  examination  shows  that  these 
spots  &re  proliferation  islands,  as  we  may  call  them,  or  little  thick- 
enings of  the  ectoderm  with  crowded  nuclei.  It  appears  that  not  all 


FIG.  183.— Terminal  Villi  of  a  Placenta  at  Full  Term, 
little  spots  represent  the  proliferation  islands  of  the  covering 
epithelium. 


THE    HUMAN   CHORION. 


321 


FIG.  184.  —Section  of  the  Chorion  at  Three  Weeks,  a,  layer 
of  coagulum;  6,  mesoderm  of  chorion;  Ep,  epithelium,  also 
extending  over  the  villi,  Ft  and  Ft';  the  mesoderm,  6,  con- 
tains  a  number  of  blood-vessels,  nearly  all  in  transverse  sec- 
tion.  X  65  diams. 


the  villi  change  to  the  slender  form ;  for  some  villi,  having  still  the 
earlier,  thicker  form,  are  found  even  in  the  mature  placenta,  a  fact 
already  noticed  by  Jassinsky,  67.1,  346.  These  thick  villi  usually 
show  also  a  distinct  u  cellular  layer"  in  their  ectoderm,  a  peculiarity 
to  be  considered  below  again.  Seiler,  32. 1,  has  given  figures  of  the 
villi  at  various  ages,  but  fails  to  show  the  characteristic  forms. 
Langhans  has  observed 
the  alteration  in  the 
villi,  77.1,  199,  and 
even  justly  remarks  that 
many  of  the  villi  in  so- 
called  "  moulds"  are  not 
pathological,  as  they 
have  been  frequently 
considered,  but  normal 
young  villi.  The  differ- 
ences in  the  villi  accord- 
ing to  age  are  very  con- 
spicuous in  sections. 
The  sections  should,  of 
course,  be  made  so  that 
the  fragments  of  the 
villi  will  remain  in 
xihi;  imbedding  in  eel- 
Iodine  is  convenient  for 
this  purpose ;  if  this  end 
be  attained,  one  finds  below  the  chorionic  membrane  numerous  sec- 
tions of  villi;  if  the  specimen  be  a  young  chorion — first  to  third 
month — the  villi  are  large,  with  a  good  deal  of  room  between  them ; 
their  outlines  are  very  irregular  and  there  are  relatively  few  small 
branches,  Fig.  184.  The  older  the  specimen,  the  larger  the  propor- 
tion of  small  branches.  In  an  old  chorion — seventh  to  ninth  month 
— the  number  of  small  villi  of  nearly  uniform  size  is  very  striking 

(see  the  figure  of  a  section  through  a 
placenta  in  situ,  given  in  Fig.  213). 
The  abortion  of  the  villi  of  the 
chorion  la?ve  takes  place  by  an  arrest 
of  development  and  a  subsequent  slow 
degeneration  of  the  tissues,  which 
lose  all  recognizable  organization  in 

t^jk  l^^J  v"r\  ^e  protoplasm,  and  to  a  large  extent 

of  the  nuclei ;  at  the  same  time  they 
alter  their  shape,  Fig.  185,  becoming 
more  and  more  filamentous ;  by  the 
fourth  month  only  a  few  tapering 
threads,  with  very  few  branches,  re- 
main The  villi  disappear  almost  completely  from  the  laeve,  except 
near  the  edge  of  the  placenta,  where  they  are  to  be  found,  even  in  the 
after-birth,  imbedded  in  the  degenerated  epithelium  of  the  chorion 
and  the  upper  layers  of  the  decidua,  as  shown  by  Minot,  98,  the 
epithelium  and  decidua  being  so  fused  at  this  point  that  it  is  impos- 
sible to  determine  any  line  of  demarcation  between  them. 
21 


FIG.  185.— AbortiiiK  Villus  from  a  Chorion 
of  the  Second  Month. 


322  THE   FCETAL   APPENDAGES. 

Histology  of  the  Chorion. — The  chorion  being  a  portion  of 
the  somatopleure  consists,  of  course,  of  two  primary  layers,  the  meso- 
derm  and  ectoderm.  During  the  second  half  of  the  first  month,  the 
earliest  period  concerning  which  we  have  any  accurate  knowledge, 
the  mesoderm  is  already  a  vascular  layer  of  considerable  thickness 
(Figs.  184  and  188,  mes),  and  the  epithelium  (ectoderm)  has  two  layers 
of  cells  Fig.  188,  a  and  b;  of  which  the  outer  is  the  darker  in  speci- 
mens stained  with  osmic  acid,  carmine,  cochineal,  or  hsematoxylin, 
and  has  also  smaller  and  more  granular  nuclei.  The  same  distinc- 
tion exists  in  the  two-layered  stage  of  the  ectoderm  of  the  umbilical 
cord,  Fig.  208,  and  of  the  foetal  skin.  Hitherto  most  authors  have 
entirely  overlooked  the  inner  layer  at  early  stages.  It  was  first 
clearly  recognized  by  Langhans,  who  directed  attention  to  it  in  a 
special  memoir,  82.1,  he  having  already  described  its  later  history, 
77.1.  In  some  earlier  writers  are  allusions  to  the  layer.  Kast- 
schenko,  in  his  paper  on  the  chorionic  epithelium,  has  also  described 
it,  although  he  has  not  followed  its  history  very  far.  The  interpre- 
tation to  be  offered  seems  to  me  clearly  to  be  that  the  chorionic  epi- 
thelium advances  in  its  differentiation  to  a  stage  equivalent  to  the 
two-layered  stage  of  the  epidermis  and  there  stops ;  whatever  further 
change  occurs  is  degenerative. 

The  two  primitive  layers  of  the  chorionic  epithelium  have  been 
more  or  less  clearly  observed  at  later  stages  by  several  anatomists, 
and  have  been  variously  interpreted.  Ercolani  and  Turner  regard 
them  as  absolutely  distinct,  assigning  the  deep  layer  to  the  chorion 
as  its  true  and  only  epithelium,  and  the  outer  layer  to  the  uterus, 
thus  enabling  themselves  to  conceive  the  villi  as  covered  by  maternal 
as  well  as  a  foetal  epithelium,  so  that  maternal  blood  found  between 
the  villi  is  still  within  the  maternal  tissue.  After  accepting  the 
outer  layer  as  maternal,  the  question  as  to  its  origin  still  remained. 
Some  authors  affirmed  it  to  be  the  uterine  epithelium,  others  to  be 
the  lining  of  expanded  uterine  blood  sinuses.  So  far  as  I  am  aware, 
no  one  has  made  observations  to  show  by  the  developmental  history 
of  the  layer  that  one  or  the  other  of  the  last-mentioned  hypotheses 
is  correct.  When  we  consider  the  precision  and  exactitude  of  Kast- 
schenko's  observations,  which  actual  specimens  enable  one  to  verify, 
there  is  in  my  judgment  no  reason  left  for  differing  from  the  con- 
clusion that  both  layers  are  parts  of  the  foetal  ectoderm. 

Governed  by  the  difficulty  of  accounting  for  the  presence  of  ma- 
ternal blood  in  the  intravillous  spaces,  and  therefore  apparently 
outside  the  maternal  tissues,  several  investigators  have  been  led  to 
seek  for  at  least  an  endothelium  outside  the  chorionic  epithelium. 
Some  authors,  as,  for  instance,  W inkier,  have  asserted  the  existence 
of  such  an  endothelium,  but  after  a  prolonged  and  careful  search  I 
fail  to  find  anything  of  the  kind,  and  in  this  result  it  seems  to  me  the 
best  observers  are  agreed.  Waldeyer,  90.1,  33,  has  recently  again 
advocated  the  existence  of  an  endothelium,  but  from  his  description 
it  appears  to  me  that  his  supposed  endothelium  is  only  the  outer  layer 
of  the  ectoderm.  Keibel,  89.2,  reports  a  very  different  observation; 
in  a  young  ovum  (twenty-five  days?)  he  found  a  very  thin  endo- 
thelial  layer  outside  of  the  two  layers  of  ectoderm,  and  enclosing  the 
maternal  blood.  Now  in  the  rabbit  the  placental  villi  grow  down 


THE   HUMAN    CHORION.  W> 

into  the  uterine  mucosa ;  the  intervillous  maternal  tissue  disappears, 
leaving  only  the  maternal  capillaries,  which  become  enormously 
hypertrophied  and  take  up  the  entire  intervillous  room;  inconse- 
quence the  capillary  endothelium  immediately  covers  the  villi;  later 
this  endothelium  also  aborts,  leaving  the  blood  of  the  uterus  to  circu- 
late in  channels  bounded  by  the  chorionic  epithelium.  If  we  assume 
that  the  process  of  development  is  similar  in  man,  but  is  completed 
very  precociously,  we  can  understand  both  Keibel's  observation  and 
the  failure  to  detect  any  true  endothelium  in  later  stages.  For  a 
full  review  of  the  many  conflicting  opinions  concerning  the  covering 
of  the  villi,  see  W.  Waldeyer,  90.1,  33-47. 

Differentiation  of  the  Ectoderm. — The  epithelium  of  the 
chorion  becomes  differentiated  in  three  different  ways :  1,  upon  the 
chorion  froiidosum;  2,  upon  the  chorion  Iseve;  3,  upon  the  villi. 
For  a  correct  knowledge  of  the  remarkable  changes  which  the  epi- 
thelium undergoes,  particularly  in  the  placenta,  we  are  indebted  to 
the  unusually  exact  investigations  of  Langhans,  77.1,  and  82.1. 
This  author  left  two  points  of  importance  unsettled;  namely,  the 
origin  of  his  " Zellschicht,"  and  of  the  " canalisirtes  Fibrin." 
Kastschenko  has  traced  the  cellular  layer  (Zellschichf)  to  the  epithe- 
lium, as  already  stated ;  compare  pp.  4<;:}-4G9  of  his  memoir,  85.1. 
My  '.>\vn  observations  show,  I  think,  conclusively  that  the  canalized 
fibrin  arises  through  a  degenerative  metamorphosis  of  the  epithe- 
lium, which  begins  in  the  outer  layer  and  may  invade  the  inner 
layer  (Langhans'  Zellsch icht) .  Let  us  consider  separately  the  three 
series  of  modifications  of  the  chorionic  ectoderm. 

In  the  region  of  the  chorion  frondosum  the  inner  layer  of  the  ecto- 
derm (the  cellular  layer  of  Langhans)  becomes  irregularly  thickened 
in  patches,  which  present  every  possible  degree  of  variation  as  to 
number  and  as  to  their  breadth  and  thickness.  Although  at  first 
tlie  cellular  layer  is  more  or  less  continuous  and  composed  of  uni- 
form cells,  this  is  not  the  case  in  later  stages.  We  must  assume  that 
with  the  growth  of  the  membrane  the  epithelium  increases  in  area, 
but  remains  in  many  places  single-layered,  developing  no  Zell- 
xclticltt.  The  patches  of  cells  have  been  well  described  by  Lang- 
hans, 77.1,  and  Kastschenko,  85.1,  466,  and  are  represented  with 
lower  power  in  Fig.  189,  c,  and  with  a  higher  power  in  Fig.  186,  c. 
They  vary  much  in  appearance ;  the  cells  are  more  distinct  in  the 
small  patches,  but  are  less  individual  in  the  large  patches,  owing  to 
the  spread  of  the  process  of  degeneration  into  the  layer,  Fig.  186,  c. 
The  cell  bodies  are  lightly  stained,  and  the  granular  nuclei  are  not 
very  sharply  defined  and  vary  in  size  and  shape.  The  cellular  layer 
is  always  sharply  defined  against  the  stroma,  although  there  is  no 
true  basement  membrane,  but  toward  the  outer  layer  of  the  ectoderm 
its  boundary  is  sometimes  distinct,  sometimes  lost  in  a  gradual 
transition. 

The  outer  layer  of  the  ectoderm  of  the  frondosum  is  even  more 
variable.  As  stated  by  Kastschenko,  it  is  primitively  a  dense  pro- 
toplasmic reticulum,  with  nuclei  in  a  single  layer  and  without  any 
cell  boundaries.  In  the  chorion  frondosum  at  four  months  and  after 
I  find  spots  where  this  structure  still  prevails  either  with  or  without 
an  underlying  cellular  layer ;  in  other  spots  the  layer  is  thickened  and 


324 


THE    FCETAL    APPENDAGES. 


contains  an  increased  number  of  nuclei,  which  are  sometimes  crowded 
in  a  bunch;  elsewhere  the  layer  is  thinned  out  and  has  no  nuclei; 
in  still  other  spots  the  thickening  has  gone  on  much  further,  and 
usually  but  not  always,  where  the  outer  layer  is  much  thickened  the 
cellular  layer  under  it  is  also  thickened ;  wherever  it  is  thickened, 
and  occasionally  where  it  is  thin,  the  outer  layer  of  the  ectoderm 
shows  a  marked  tendency  to  degenerate  into  canalized  fibrin,  Fig. 
189,  Fbr,  and  Fig.  186,  fb.  It  is  not  difficult  to  assure  one's-self 
that  the  fibrin  arises  by  direct  metamorphosis  of  the  ectoderm.  I 
now  think  that  its  formation  begins  in  the  outer  layer  and  thence 


THES 


FIG.  186.  — Placental  Chorion  of  an  Embryo  of  Seven  Months ;  Vertical  Section  through  the  Ecto- 
derm and  Portion  of  the  Adjacent  Stroma.  mes,  Mesodermic  stroma;  c,  cell  layer ;  fb,  fibrin 
layer;  ep,  remnant  of  epithelium,  x  445  diams. 

spreads  into  the  cellular  layer;  for,  in  fact,  when  both  layers  are 
distinguishable,  as  in  Fig.  186,  the  fibrin  layer,  fb,  is  always  external, 
and  the  external  layer  of  nucleated  protoplasm  has  either,  totally  dis- 
appeared or  is  represented  by  mere  remnants  as  in  Fig.  186,  ep.  The 
fibrin  layer  consists  of  a  hyaline  very  refringent  substance  permeated 
by  numerous  channels,  Fig.  186,  fb;  the  substance  has  a  violent 
affinity  for  carmine  and  ha3matoxylin,  and  is  always  the  most  deeply 
colored  part  of  a  stained  section ;  the  channels  tend  to  run  more  or 
less  parallel  to  the  surface  of  the  chorion,  and  are  connected  by 


THE   HUMAN    t'HORIOX.  :>•>;> 

numerous  cross-channels;  some  of  the  channels  contain  cells  or 
nuclei.  This  complex  system  of  canals  is  by  no  means  of  uniform 
appearance  in  all  parts  of  the  placenta,  both  the  spaces  and  dissepi- 
ments varying  in  size  and  shape.  The  fibrin  often  sends,  as  shown 
in  Fig.  ISO,  long  outshoots  into  the  cellular  layers  upon  which  it 
seems  to  encroach.  The  frequency  of  these  images  in  my  prepara- 
tions led  me  to  the  opinion*  that  the  fibrin  arises  from  the  cellular 
layer  only,  and  I  concluded  that  the  ectoderm  was  first  transformed 
into  the  so-called  cellular  layer,  which  was  then  transformed  into 
fibrin.  It  still  appears  to  me  that  much  of  the  degeneration  goes 
by  these  stages ;  bat,  on  the  other  hand,  it  seems  clear  that  the  de- 
generation begins,  as  above  stated,  in  the  outer  layer.  Another 
appearance  is  presented  by  the  ectoderm  where  it  is  thickened  and 
wholly  transformed  into  the  cellular  laj^er.  In  brief:  the  ectoderm 
of  the  placental  chorionic  mesoderm  undergoes  patchwise  manifold 
changes;  it  exists  in  three  chief  forms:  1,  the  nucleated  protoplasm; 
2,  the  cellular  layer;  3,  canalized  fibrin.  A  patch  of  the  ecto- 
derm may  consist  of  any  one  of  these  modifications,  of  any  two,  or  of 
all  three,  but  they  have  fixed  relative  positions,  for  when  the  nucle- 
ated protoplasm  is  present  it  always  covers  the  free  surface  of  the 
chorion ;  when  the  cellular  layer  is  present  it  always  lies  next  the 
mesoderm ;  and  when  all  three  forms  are  present  over  the  same  part, 
the  fibrin  is  always  the  middle  stratum.  In  general  terms  it  may 
be  said  that  the  amount  of  canalized  fibrin  increases  writh  the  age 
of  the  placenta,  but  it  is  very  variable  in  its  degree  of  development. 
The  peculiar  layer  into  which  the  ectoderm  is  transformed  has  long 
puzzled  anatomists.  E.  H.  Weber  recognized  the  fibrin  layer  and 
described  its  appearance  correctly ;  it  has  probably  been  often  seen, 
but  generally  regarded  as  either  pathological  or  a  blood  coagulum. 
Robin,  for  instance,  may  be  cited,  54. 1,  70-71,  as  one  who  saw  with- 
out observing  correctly  and  understandingly  the  tissue  in  question. 
An  important  gain  was  made  when  Winkler  recognized  the  modi- 
fied ectoderm  as  a  constant  layer,  and  in  1872  directed  especial 
attention  to  it  under  the  name  of  "  Schlussplatte,"  72. 1.  Kolliker 
("Entwickelungeschichte,"  2te  Aufl.  337)  added  essentially  to  our 
knowledge  of  its  structure,  but  it  is  to  Langhans  that  we  owe  the 
first  clear  light.  Meanwhile  other  writers,  following  the  lead  of 
Ercolani  and  Turner,  79.1,  551-553,  have  been  influenced  chiefly  by 
the  presence  of  the  cellular  layer,  in  the  large  size  of  the  elements 
of  which  they  found  a  resemblance  to  the  decidual  cells  which  has 
guided  them  to  the  conclusion  that  the  cellular  layer  is  derived  from 
the  wall  of  the  uterus.  This  error  has  been  definitely  corrected  by 
Kastschenko,  as  already  stated.  In  further  support  of  the  conclu- 
sion that  the  chorionic  cellular  layer  is  not  decidual,  may  be  brought 
forward  the  fact  that  there  is  a  certain  immigration  of  decidual  cells 
into  the  placenta  at  its  margin ;  but  they  remain  entirely  distinct 
from  the  cells  of  the  cellular  layer.  This  is  readily  seen  in  radial 
sections  through  the  margin  of  a  placenta  from  a  normal  afterbirth ; 
compare,  below,  the  account  of  the  ectoderm  of  the  chorion  Ia3ve. 
The  origin  of  the  canalized  fibrin  from  blood,  which  Langhans  left 
in  his  first  paper  as  an  open  possibility,  and  which  even  so  recent  a 

*  Anatom.  Anzeiger,  ii.,  23. 


320  THE   FCETAL    APPENDAGES. 

writer  as  Ruge,  86.1,  123  and  130,  has  advocated,  cannot  be  main- 
tained. Of  course  there  may  be  a  deposit  of  blood  fibrin  (coagu- 
lum),  but  it  would  be  pathological,  and,  therefore,  to  be  distinguished 
from  the  normal  fibrin  of  ectodermal  origin.  Moreover,  the  micro- 
scopic appearance  of  a  blood  clot  or  thrombus  is  so  extremely  char- 
acteristic that  one  can  readily  distinguish  it  from  the  placental 
canalized  fibrin. 

The  ectoderm  of  the  villi  of  the  placenta  differs  from  that  of  the 
chorionic  membrane  in  several  respects :  1.  The  cellular  layer  after 
the  first  month  becomes  less  and  less  conspicuous,  and  after  the  fourth 
month  is  present  only  in  a  few  isolated  patches,  known  as  the  Zell- 
knoten,  and  carefully  described  by  Langhans  and.  Kastschenko;  both 
of  these  authors  were  impressed  by  the  resemblance  of  the  cells  to 
those  of  the  decidua  serotina ;  Langhans  concludes  that  the  Zell- 
knoten  arise  from  the  serotina,  but  Kastschenko,  having  traced  their 
development  from  the  chorionic  epithelium,  denies  his  predecessor's 
conclusion,  but,  still  clinging  to  the  idea  of  a  genetic  connection  be- 
tween the  Zellkn  ot en  and  the  decidua,  reverses  the  reasoning  and 
concludes  that  the  decidual  cells  arise  in  part,  at  least,  from  the 
Knoten.  Neither  of  these  authors  have  found  the  intermediate  forms 
between  the  two  types  of  cells,  and  when  we  examine  their  descrip- 
tions critically  we  find  that  they  have  really  no  evidence  except  the 
likeness  of  the  cells  to  offer  in  favor  of  their  genetic  relationship,  and 
accordingly  Langhans  expresses  himself  with  characteristic  caution. 
To  me  the  resemblance  appears  altogether  superficial;  hence  my 
conclusion  that  the  Zellknoten  are  remnants  of  the  cellular  layer. 
2.  For  the  most  part  the  villi  remain  covered  by  the  nucleated  proto- 
plasm, which  in  many  places  is  thickened.  In  the  later  stages  these 
thickenings  are  small  and  numerous,  constituting  the  so-called 
"Proliferations-inseln;"  compare  Fig.  183.  Many  of  the  little 
thickenings  appear  in  sections  of  the  villi,  and  here  and  there  are 
converted  into  fibrin.  I  have  interpreted  them  (Wood's  "Refer- 
ence Handbook  of  the  Medical  Sciences,"  V.,  G95)  as  commencing 
buds,  and  consider  that  in  earlier  stages  they  grow  into  branches, 
but  in  later  stages  are  in  part,  at  least,  arrested  in  their  develop- 
ment. 3.  The  proliferation  islands  are  converted  into  canalized 
fibrin,  and  at  the  same  time  grow  and  fuse,  forming  larger  patches, 
particularly  on  the  larger  stems ;  in  this  manner  are  produced  the 
large  areas  and  columns  of  fibrin  found  in  the  placenta  at  four 
months  and  after ;  they  have  been  well  described  by  Langhans,  and 
form  a  striking  feature  in  sections  of  placentae.  Some  of  the  columns, 
as  stated  by  Langhans,  stretch  along  the  villi  from  the  chorionic 
membrane  to  the  surface  of  the  serotina  as  if  to  act  as  supports. 
Ercolani  appears,  if  I  understand  his  account,  to  have  seen  the  fibrin 
columns  without,  however,  ascertaining  either  their  structure  or 
their  origin.  4.  Over  the  tips  of  the  villi,  which  are  bent  consid- 
erably where  they  are  imbedded  in  the  decidua  serotina,  the  relations 
are  not  clear;  the  epithelium  is  certainly  not  present  in  its  original 
form  over  the  imbedded  ends  of  the  villi,  which  are,  however,  sur- 
rounded by  a  hyaline  tissue  of  the  character  of  the  canalized  fibrin, 
except  that  the  canals  are  often  indistinct  or  even  wanting;  the  hya- 
line tissue  forms  an  almost  continuous  coat  over  the  decidual  surface ; 


THE    HUMAN   CH ORION.  327 

in  earlier  stages  the  ectoderm  of  the  terminal  villi  is  often  consider- 
ably expanded.  The  natural  interpretation  of  these  facts  is  that  the 
ectoderm  of  the  villi  expands  over  the  decidua  serotina  and  degener- 
ates. In  this  manner  we  account  for  both  the  absence  of  any  cellu- 
lar ectoderm  over  the  ends  of  the  villi  and  the  presence  of  canalized 
fibrin  upon  the  serotina]  surface;  but  the  hypothesis  must  await 
the  final  test  by  observation. 

The  ectoderm  of  the  chorion  Ia3ve  loses  by  the  seventh  month  all 
traces  of  the  protoplasmic  layer,  and  is  without  any  canalized  fibrin, 
except  near  the  placenta ;  cf.  infra.  It  is  transformed  into  a  Zell- 
wliirlit.  In  a  section  of  the  laeve,  in  situ,  at  seven  months,  Fig. 
1 "),  the  chorionic  epithelium,  c,  rests  directly  upon  the  decidua,  which 
has  none  of  its  own.  The  ectodermal  cells  lie  two  or  three  deep ; 
they  are  described  by  Kolliker  and  Langhans,  the  former  designat- 
ing them  as  the  chorionic  epithelium,  while  the  latter  doubtfully 
traces  their  origin  to  the  uterus.  That  Kolliker  ("  Entwickelungsge- 
schichte").  2tc  AufL,  p.  322)  is  right,  I  am  confident.  It  is  easy  to 
follow  the  layer  of  cells  in  question  at  the  edge  of  the  placenta,  and 
see  that  it  is  directly  continuous  with  the  cellular  layer  of  the  fron- 
dosum,  which  it  resembles  in  character.  On  the  other  hand  the 
ectodermal  cells  of  the  laBve  are  distinct  in  character  from  the  decid- 
ual  cells  next  to  them,  Fig.  15,  having  smaller  and  more  darkly 
stained  nuclei,  and  much  more  coarsely  granular  protoplasm;  the 
ectodermal  cells  are  much  smaller  than  the  decidual.  The  ectoderm 
is  sharply  marked  off  from  the  decidua,  but  its  surface  is  often  cor- 
rugated, and  then  the  line  of  separation  between  the  tissues  is  irreg- 
ular, and  in  sections  it  may  even  appear  that  there  is  a  true  inter- 
penetration  and  mingling  of  the  decidual  and  ectodermal  cells;  but 
it  is  only  apparent,  and  the  demarcation  is  always  preserved. 

Differentiation  of  the  Mesoderm. — The  further  hi  story  of  the 
chorionic  mesoderm  is  so  fully  given  by  Langhans  in  his  invaluable 
memoir,  77.1,  and  Kastschenko,  85.1,  that  my  own  observations 
have  afforded  little  to  be  added.  In  the  earliest  stage  I  have  been 
able  to  examine,  an  ovum  of  the  third  week,  the  matrix  of  the  cho- 
rionic connective  tissue  in  a  preparation  stained  with  cochineal  or 
hiematoxylin,  and  imbedded  in  paraffin  for  cutting,  appears  hya- 
line and  glistening,  owing  to  its  refrangibility,  Fig.  187;  it  has 
lacunae  in  which  the  cells  lie ;  the  cell  bodies  are  either  shrunken  or 
colorless,  so  that  the  lacunaB,  except  for  the  staining  of  their  contained 
nuclei,  are  clear  and  light.  This  appearance  I  find  again  in  speci- 
mens a  little  older.  The  image  is  entirely  distinct  from  that  of  the 
same  layer  later,  for  then  the  cells  are  stained  darker  than  the  matrix, 
which  at  the  same  time  has  lost  its  homogeneous  character,  and  ac- 
quired a  fibrillated  look.  Very  different  from  my  own  sections  are 
several  which  I  owe  to  the  kindness  of  Professor  Langhans  of 
Bern,  and  which  that  distinguished  investigator  informs  me  are 
from  a  three  weeks'  ovum,  which  had  been  preserved  in  osmic 
acid,  Fig.  188.  In  Professor  Langhans'  preparations  the  cells  are 
all  stained  much  deeper  than  the  matrix ;  they  have  an  elongated 
form,  and  run  in  various  directions  more  or  less  parallel  to  the  epi- 
thelium, ect ;  hence  many  of  them  are  cut  transversely  or  obliquely. 
Whether  the  differences  noted  are  due  to  the  methods  of  preparation 


328  THE    FCETAL   APPENDAGES. 

must  be  decided  by  preserving  the  same  chorion  in  part  with  osmic 
acid,  in  part  with  Muller's  fiuid  or  picrosulphuric  acid,  the  latter 
being  the  reagents  I  have  used.  In  specimens  of  the  tenth  week  the 
matrix  of  the  chorionic  mesoderm  has  quite  altered  in  character, 
being  no  longer  homogeneous,  and  at  the  same  time  it  has  increased 
in  thickness.  For  the  most  part  the  matrix  stains  lightly,  and  where 
it  is  lighter  it  contains  fibrils  of  extreme  fineness  and  running  curly 
courses;  there  are  also  streaks  of  lightly  stained  matrix,  giving  the 
impression  of  fibres  resulting  from  portions  of  the  primitive  colorable 
matrix  being  left.  In  other  parts  of  the  layer  the  primitive  matrix 
is  still  present,  and  we  find  a  homogeneous  well-colored  basal  sub- 
stance, the  cell  lacunaB  of  which  appear  light  by  contrast,  as  in  Fig. 
187.  One  can  distinguish  also  the  commencement  of  the  peri  vascular 


FIG.  187.— Section  of  the  Chorionic  Membrane  of  an  Ovum  Supposed  to  Belong  to  the  Third 
Week,  ect.  Ectoderm;  rues,  mesoderm;  a,  outer,  6,  inner  layer  of  ectoderm;  stained  with  alum- 
cochineal,  x  445  diams. 

coats,  at  least  of  the  larger  vessels,  the  matrix  being  quite  dense 
around  them  and  the  cells  elongated  almost  into  fibres,  and  possessing 
a  slightly  increased  affinity  for  coloring-matters.  The  larger  blood- 
vessels and  unmetamorphosed  part  of  the  layer  occupy  a  middle  por- 
tion between  the  two  surfaces,  but  the  smaller  blood-vessels  lie  near 
the  ectoderm  (compare  Fig.  187,  v),  thus  presaging  the  formation  of 
Langhans'  vascular  layer  (Grefdssschicht) .  The  development  of  the 
mesoderm  of  the  chorion  laBve  stops  at  about  this  stage,  or  at  the 
stage  when  the  matrix  has  completely  changed  from  its  first  stage ; 
in  the  region  of  the  frondosum,  however,  development  proceeds 
much  further  by  the  production  of  fibres  throughout  the  whole  of  the 
layer;  usually,  but  not  invariably,  the  fibres  become  much  more 
numerous  near  the  ectoderm  than  in  the  inner  parts  of  the  mesoderm, 


THE    HUMAN    CHORION. 


329 


thus  differentiating  a  well-marked  sub-epithelial  fibrillar  layer,  Fig. 
180,  fib,  from  the  deeper  and  wider  stroma,  Sir.  The  fibrillar  layer 
is  that  commonly  spoken  of  as  the  connective  tissue  layer  of  the 


TT1ES 


FIG.  188. — Section  of  the  Chorionic  Membrane  of  an  Embryo  of  Three  Weeks;  stained  with  os- 
inic  ;ifi<l.  )/«•>•.  Mfscxlerm;  ect.  ectoderm;  a  outer,  6,  inner  layer  of  ectoderm.  From  a  section 
prepared  by  Prof.  Theodor  Langhans.  X  445  diams. 

chorion;  for  details  of  its  structure,  including  the  "  Gefcissschicht," 
see  Langhans  and  Kastschenko.  The  inner  layer,  Str,  is  called  the 
( inllcrlxrli  /<•!>  t  by  many  German  writers,  and  seems  to  be  what  Kolli- 
ker  ("Entwickelungsgeschichte,"  2te  Aufl.,  p.  322)  designates  as 


FIG.  189.  -Section  of  the  Amnion  and  Placental  Chorion  of  the  Fifth  Month.  En,  Amniotic  epi- 
thelium; Am,  amniqn;  Sir,  stroma;  Fib,  fibrillar  layer  ;/br.  fibrin  layer;  c.  cellular  layer;  Vi, 
yilli.  (From  a  section  cut  in  celloidin,  and  stained  with  Weigert's  haematoxylin.  The  draw- 
ing is  only  approximately  correct  as  to  details.)  x  71  diams.  f 

"  Gallertgewebe   zwischen  Chorion  und  Amnion ;"    it  usually  con- 
tains a   considerable   number   of   large   granular  wandering   cells. 


330 


THE    FCKTAL   APPENDAGES. 


Jungbluth,  69. 1,  describes  a  network  of  capillaries  which  exist  dur- 
ing the  first  half  of  pregnancy,  apparently  in  the  upper  part  of  the 
stroma — i.  e.,  next  the  amnion — but  I  fail  to  find  any.  Where  the 
amnioii  comes  into  contact  with  the  chorion  the  adjacent  parts  of  the 
two  membranes  are  more  or  less  loosened,  forming  a  network  of 
strands  by  which  the  membranes  are  united ;  most  of  the  uniting 
strands  appear  to  belong  rather  to  the  chorion  than  to  the  amnion. 
This  loose  tissue  is  perhaps  that  which  Kolliker  designates  as  a 
Gallertgewebe  distinct  from  the  chorion. 

Although  the  chorion  bounds  the  ccelom,  I  have  observed  no  mes- 
othelium  upon  its  mesodermic  surface ;  but  I  have  not  made  search 
for  it  by  any  special  methods.  In  the  rabbit,  it  will  be  remembered, 
the  mesothelium  is  very  evident  over  the  placenta,  but  the  rabbit 
differs  from  man  by  the  absence  of  union  between  the  amnion  and 
chorion.  Nor  have  I  been  able  to  find  any  basement  membrane,  prop- 
erly so-called,  under  the  chorionic  ectoderm.  As  to  the  appearance 
which  suggests  it,  I  accept  Kastschenko's  explanation,  85.1,  455. 

The  mesoderm  in  the  villi  is  differentiated  otherwise  than  that  of 
the  membrane  of  the  chorion.  In  the  youngest  stage  I  have  exam- 
ined there  is  some  of 
1.  I.  A  •  the  primitive  matrix 

,  ,  •11* 

present  m  the  villi ; 
and  I  presume  that 
earlier  the  whole 
mesoderm  has  the 
same  character.  In 
my  specimen  (three 
weeks)  the  change  is 
progressing.  I  have 
not  succeeded  in  sat- 
isfying myself  as  to 
the  process  of  change 
which  takes  place, 
but  I  think  it  prob- 
ably essentially  as 
follows :  The  cells 
gradually  develop 
large  bodies  and  ac- 
quire a  more  decid- 
ed affinity  for  color- 
ing-matters ;  meanwhile  vacuoles  appear  in  the  matrix,  presumably 
by  its  modification  into  a  new  substance ;  the  vacuoles  increase  in 
size  and  number,  transforming  the  matrix  into  a  network  and  ulti- 
mately causing  its  total  disappearance,  leaving  the  intercellular 
spaces  filled  entirely  with  the  new  substance,  which  has  come  from 
a  metamorphosis  of  the  original  matrix;  probably  this  new  sub- 
stance is  more  or  less  fluid,  since  wandering  cells  are  scattered 
freely  through  it.  Leaving  this  half -hypothetical  history,  let  us 
pass  on  to  direct  observations.  In  the  placental  villi  of  embryos  of 
four  months  and  older,  the  mesoderm  exists  in  two  principal  forms- 
adenoid  tissue  and  fibre-cell  tissue  around  the  blood-vessels.  The 
adenoid  tissue,  Fig.  190,  is  that  of  which  the  supposed  development 


FIG.  190.— Adenoid  Tissue  of  a  Villus  from  a  Placenta  of  Four 
Months.  Ill,  Wandering  cells;  v  v,  capillary  blood -vesselb :  d, 
finer  meshwork  from  near  a  capillary.  X  352  diams. 


THE    HUMAN    CTIORIOX.  331 

has  just  been  sketched;  it  may  be  considered  as  the  proper  tissue  of 
the  villus.  It  consists  of  a  network  of  protoplasmic  threads,  which 
start  from  nucleated  masses  (cells) .  There  are  many  large  meshes 
which  are  partly  occupied  by  the  coarsely  granular  wandering  cells, 
/./,  which  are  scattered  about,  and  are  usually  present  in  large  num- 
bers. About  the  capillaries  the  network  is  much  more  finely  spun. 
Kastschenko,  85.1,  454,  found  the  wandering  cells  most  abundant 
near  the  epithelium,  but  I  have  noticed  no  such  peculiarity  except 
that  they  do  not  often  enter  the  dense  peri  vascular  tissue;  and  as  the 
blood-vessels  are  centrally  situated,  the  adenoid  tissue  and  the  wan- 
dering cells  in  it  are  of  course  more  peripheral.  It  seems  to  me  that 
the  leucocytes  are  distributed  more  or  less  evenly  throughout  the 
adenoid  tissue.  I  fail  to  recognize  any  intercellular  substance.  The 
abundance  of  nuclei  deserves  special  mention.  Around  all  the  non- 
capillary  vessels  the  mesoderm  is  very  different,  for  it  exhibits  dis- 
tinct intercellular  substance,  with  a  tendency  to  fibrillar  differentia- 
tion in  quite  a  wide  zone  around  the  blood-vessels ;  in  this  zone  the 
cells  become  elongated  and  irregularly  fusiform ;  around  the  larger 
vessels  the  cells  are  grouped  in  lamina,  making  the  structure  similar 
to  that  already  described  in  the  walls  of  the  vessels  of  the  umbilical 
cord;  after  the  perivascular  coats  acquire  a  certain  thickness  the 
cells  of  the  inner  layers  are  more  elongated,  more  regularly  fusiform, 
and  more  closely  packed  than  those  of  the  outer  layer ;  the  transition 
from  the  denser  to  the  looser  tissue  is  gradual.  We  are  perhaps 
entitled  to  recognize  in  the  denser  inner  layer  the  media,  in  the  outer 
looser  layer  the  advent  it  id,  although  neither  of  the  layers  has  by 
*  any  means  the  full  histological  differentiation  characteristic  of  the 
like-named  layers  of  the  blood-vessels  of  the  adult. 

Blood- Vessels  of  the  Chorion. — As  already  stated  the  entire 
chorion  is  vascular  at  an  early  stage,  but  the  vessels  abort  very  soon 
over  the  chorion  Ia3ve,  while  over  the  frondosum  they  acquire  a  great 
development  in  connection  with  the  formation  of  the  placenta;  it 
seems  to  me  more  convenient  to  deal  with  them  in  connection  with 
that  organ,  arid  accordingly  the  reader  is  referred  to  Chapter  XVII. 

Fluid  Contents  of  the  Chorionic  Vesicle. — In  early  stages, 
as  we  have  seen,  there  is  a  large  chorionic  cavity,  which  in  later 
stages  is  obliterated  by  the  expansion  of  the  amnion.  The  space 
between  the  chorion  on  the  one  hand  and  the  amnion  and  the  yolk- 
sac  on  the  other  is  filled  with  a  fluid,  which  is  coagulated  by  the  action 
of  the  hardening  agents,  making  a  network  of  threads.  This  obser- 
vation, which  has  been  often  verified,  is  all  that  we  know  concerning 
the  nature  of  the  chorionic  fluid ;  it  is  probably  of  a  serous  character 
and  may  very  likely  be  found  to  contain  free  connective-tissue  cells 
(wandering  cells  or  leucocytes) . 

Evolution  of  the  Chorion. — There  can  be  little  doubt,  if  any, 
that  the  chorion  arose  by  the  growth  and  expansion  of  the  abdominal 
somatopleure,  in  result  of  the  increase  of  the  yolk-material  in  the 
earliest  amniota.  It  can,  therefore,  not  be  regarded  as  originally  a 
new  organ.  When  the  amniote  type  of  development  was  evolved  a 
portion  of  the  original  chorion  was  differentiated  and  separated  as 
the  amnion  from  the  primitive  membrane,  leaving  the  rest  as  the  true 
chorion  (false  amnion  or  membrana  serosa) ,  enclosing  all  the  other 


332  THE    FCETAL   APPENDAGES. 

parts  of  the  embryo  and  making  the  chorionic  vesicle.  This  vesicle, 
therefore,  results  from  the  development  of  the  amnion,  or  perhaps 
the  formation  of  the  amnion  is  a  result  of  the  development  of  the 
vesicle.  It  is  customary  to  refer  to  the  amnion  as  playing  the  lead- 
ing role,  but  of  this  there  is  no  certain  proof,  though  the  conception 
is  natural  and  plausible.  The  possession  of  a  true  chorion  is  as 
characteristic  as  the  possession  of  an  amnion  or  allantois  in  the 
higher  vertebrates,  so  that  from  a  morphological  standpoint  the  term 
Chorionida  would  be  as  appropriate  and  justifiable  as  the  terms 
Allantoidea  or  the  more  generally  used  Amniota. 

In  the  mammalia  the  chorion,  being  the  outermost  member  of  the 
ovum,  is  brought  into  immediate  contact  with  the  uterine  wall,  and 
has  consequently  undergone  many  and  complex  modifications  in  con- 
nection with  the  evolution  of  the  placenta.  But  while  the  chorion  in 
the  placental  mammals  is  the  organ  of  communication  between  the 
mother  (uterus)  and  embryo,  its  vascular  connection  with  the  latter 
is  maintained  through  the  intervention  of  the  allantois,  which  thus 
co-operates  in  an  essential  manner.in  developing  the  placenta,  though, 
strictly  speaking,  it  does  not  participate  in  forming  the  actual  pla- 
centa, meaning  by  placenta  the  organ  produced  through  the  intimate 
union  of  foetal  and  maternal  tissues.  It  is  evident  that,  as  Minot 
has  maintained,  the  placenta  is  necessarily  chorionic.  Further  re- 
marks on  this  subject  will  be  found  in  Chapter  XVII.,  "The  Pla- 


CHAPTER   XV. 
THE  AMNION  AND  PROAMNIOX. 

Definition  of  the  Amnion. — The  amnion  is  a  thin,  pellucid, 
non-vascular  membrane,  and  is  the  innermost  of  the  envelopes  enclos- 
ing the  embryo  or  foetus.  Its  origin  and  formation  have  been 
described  already,  p.  281.  Morphologically  it  is  a  part  of  the  body- 
wall  (somatopleure)  of  the  fostus,  and  therefore  consists,  as  we  have 
seen,  of  two  layers,  one  epithelial  continuous  with  the  ectoderm  (seu 
epidermis)  of  the  embryo,  the  second  of  loose  connective  tissue  con- 
tinuous with  the  somatic  mesoderm  (outer  leaf  of  mesoderm  after 
the  appearance  of  the  body-cavity) .  The  epithelial  layer  is  turned 
toward  the  embryo,  and  the  connective-tissue  layer  consequently 
lies  upon  the  outside  of  the  amnion  away  from  the  embryo,  and 
toward  the  chorion  and  the  uterine  wall. 

Growth  of  the  Amnion. — Concerning  the  growth  of  the  am- 
nion I  know  of  no  exact  measurements.  During  the  first  three  weeks 
it  stands  off  a  little  from  the  embryo,  but  during  the  fourth  week 
the  latter  grows  so  rapidly  that  it  takes  up  nearly  the  whole  of  the 
amniotic  cavity ;  during  the  second  month  the  anmion  enlarges  rap- 
idly so  as  to  leave  considerable  space  for  the  amniotic  fluid;  the 
amnion  continues,  of  course,  to  expand  during  all  the  following 
months,  but  after  the  fourth  month  it  fits  pretty  closely  around  the 
embryo,  but  is  kept  distended  b^r  the  amniotic  fluid. 

The  amnion  does  not  grow  around  the  allantois-stalk  or  umbilical 
cord  of  man  as  it  is  commonly  stated  to  do,  but,  011  the  contrary, 
springs  from  the  stalk  in  the  same  manner  as  from  the  body  of  the 
embryo,  and  is  separated  from  the  stalk  in  the  course  of  development, 
as  is  described  more  fully  below,  in  connection  with  the  histology  of 
the  allantois-stalk. 

Histology  of  the  Amnion. — For  a  certain  period  after  it  is 
first  formed  the  amnion,  in  all  embryos  I  have  been  able  to  examine, 
consists  of  two  layers  of  cells,  both  very  thin  and  with  the  nuclei 
considerable  distances  apart,  but  sometimes  in  little  groups ;  between 
the  two  layers  is  a  distinct  space.  The  ectodermal  layer  is  the  most 
regular  and  the  best  defined  as  to  its  inner  boundary.  The  meso- 
dermal  layer  is  more  or  less  irregular  and  sends  at  intervals  a  process 
across  the  space  between  the  two  layers  to  be  attached  to  the  ectoderm. 

In  a  human  amnion  of  a  normal  two  months'  embryo,  Fig.  191, 
the  mesoderm  has  become  very  much  thicker,  and  is  readily  seen  to 
be  separated  into  two  parts,  the  thin  mesothelial  layer,  math,  cover- 
ing the  surface  of  the  amnion  toward  the  chorion,  and  a  mesenchymal 
layer,  mes,  which  makes  up  the  greater  part  of  the  membrane ;  the 
mesenchyma  is  probably  derived  from  the  mesothelium  by  prolifer- 
ation and  migration ;  I  have  noticed  many  indications  of  the  process, 
but  have  never  studied  it  carefully.  The  ectoderm,  EC,  is  very 


334 


THE    FOETAL   APPENDAGES. 


much  in  the  condition  just  described  for  the  earlier  stage,  but  in 
specimens  of  three  months'  amnia  it  has  become  thicker,  and  its  cells 
are  beginning  to  change  into  the  cuboidal  form  of  later  stages. 

No  blood-vessels  or   nerves  are  known  to  exist  in  the  amnion  of 


EC 


>les 


^ 


Msth 


FIG.  191.—  Section  of  the  Amnion  Covering:  the  Placenta  of  a  Two  Months'  Embryo. 
derm  :  mes,  mesoderm  (mesenchymal)  ;  msth,  mesothelium.     x  250  drams. 


EC,  Ecto- 


the human  embryo,  although  in  sheep  embryos  in  very  early  stages 
the  vessels  have  been  noticed  by  Bonnet  to  extend  a  short  distance  into 
the  amnion  from  the  body- wall. 

Histological  Differentiation. — The  tissues  of  the  amnion  do 
not  progress  beyond  an  early  embryonic  stage ;  the  ectoderm  remain- 
ing at  the  one-layered  stage, 
the  mesoderm  preserving 
much  of  the  primitive  mat- 
rix. Emery  ("Arch.  Ital. 
Biol./'  III.,  37)  has  directed 
attention  to  the  primitive 
homogeneous  matrix  of  the 
vertebrate  mesoderm,  and  es- 
pecially to  the  separate  sub- 
epidermal  layer  of  the  em- 
bryo, which  contains  no  cells 
at  first.  In  the  human  am- 
nion there  is  a  non-cellular 
layer  under  the  epithelium, 
^m^Tuez  as  is  well  shown  in  Fig.  192, 
lip  A  and  B.  Sometimes  this 
layer  is  invaded  to  a  certain 
extent  by  connective  tissue- 
cells,  B ;  in  other  cases  the 
portion  of  the  matrix  toward 
the  chorion  acquires  a  fibril- 
lar  character,  A,  as  if  par- 
tially resorbed,  but  in  no  case  have  I  seen  the  matrix  entirely  altered 
from  its  primitive  character.  The  cells  of  the  mesoderm  lie  in  lacu- 
na ;  they  are  flattened  in  the  plane  parallel  to  the  surface,  and  hence 
in  vertical  sections,  Fig.  192,  appear  more  or  less  fusiform.  They 
present  no  special  features,  so  far  as  I  have  observed,  to  distinguish 
them  from  other  embryonic  connective-tissue  cells.  Their  bodies 
have  little  affinity  for  coloring-matters,  hence  it  is  difficult  to  follow 
the  processes  by  which  the  cells  are  united.  Their  nuclei  are  at  first 
round  or  oval.  After  the  third  month  they  often  show  a  great  vari- 
ety of  alterations  in  shape  and  size,  Figs.  193,  194;  some  of  the 
nuclei  are  then  very  large,  with  a  distinct  network,  d;  others  are 
smaller  and  differ  but  slightly  from  the  normal ;  some  are  very  irregu- 
lar, ft,  and  others  again  strangely  elongated,  a;  many  other  forms 


FIG.  192.— Two  Sections  of  the  Placental  Amnion; 
A,  from  an  embryo  of  the  eighth  month;  B,  at  term. 
ect,  Ectoderm;  mes,  mesoderm;  a,  layer  of  meso- 
dermic  cells.  X  340  diams. 


THE    AMNIOX   AND    PROAMNIOX. 


335 


beside  those  represented  in  Fig.  193,  are  to  be  found.  The  changes 
indicated  I  consider  of  a  degenerative  character,  and  in  fact  many 
of  the  nuclei  are  breaking  down,  for  one  finds  in  some  specimens  every 
stage  between  a  nucleus  and 
scattered  granules — nuclei, 
nuclei  with  indistinct  mem- 
branes, nuclei  without  mem- 
branes, masses  of  granular 
matter,  clusters  of  granules 
crowded  together,  and  final- 
ly other  clusters  more  or  less 
scattered.  This  degenera- 
tive process  may  be  com- 
pared with  that  described 
by  Phisalix  (Arch.  Zool. 
K.rpt.,  Ser.  II.,  T.  III., 
382)  as  occurring  in  the  ^^ 
blood-cells  of  the  spleen  of 
teleosts.  Compare  also  the 
chromatine  degeneration 
observed  by  Flemming  to 
occur  in  ova  of  the  verte- 
brate ovary  (His  and  Braune's  Archiv,  1885,  221-244).  In  the 
human  amnion  the  nuclear  degeneration  described  is  not  always  to 
be  recognized  so  clearl}',  although  the  nuclei  in  all  amnia  older  than 
three  months,  which  I  have  observed,  are  more  or  less  irregular  and 
distorted.  Finally  it  is  to  be  added  that  not  infrequently  the  cells 
form  a  distinct  epithelioid  layer  upon  the  surface  of  the  amnion  next 
the  chorion  as  represented  in  Fig.  192,  B,  a.  The  epithelium  of  the 
amnion  varies  in  appearance,  as  seen  in  transverse  sections.  Usu- 
ally the  cells  are  cuboidal  or 

* 


FIG.  MB.—  A  Natural  Group  of  Nuclei  from  the  Meso- 

°f  a  *****  °f  the  Fifth  month' 


... 


low  cylinders,  Fig.  192,  A,  each 
with  a  rounded  top,  in  which  is 
situated  the  more  or  less  nearly 
spherical  nucleus;  sometimes, 
however,  the  nuclei  lie  deeper 
dov?  n .  Less  frequently  the^  epi  - 
thelium  is  thin,  Fig.  192,  B, 
and  its  nuclei,  which  are  trans- 
versely elongated,  lie  further 
apart.  It  is  probable  that  those 
differences  are  not  structural, 
but  conditional  upon  the  greater 
or  less  degree  to  which  the  am- 
nion is  stretched.  I  have  ob- 
served no  constant  differences 

hpfwppn    thp    nlflppnt«l    anrl    fhp 

remaining  amnion. 
The  most  interesting  peculiarity  of  the  epithelium  is  best  seen  in 
surface  views;    namely,  the  intercellular  bridges.      They  display 
themselves  with  a  clearness  which  I  have  never  seen  in  other  epi- 
thelia;  see  Fig.  195. 


A 


FIG.  194. — Mesodermic  Nuclei  of  the  Amnion  of  an 
Embryo  of  about  Four  Months.     X  713  diams. 


336 


THE   FCETAL   APPENDAGES. 


The  nuclei,  nil,  are  relatively  large,  rounded,  with  distinct  out- 
lines ;  they  have  a  more  or  less  well  marked  intra-nuclear  network, 
with  thickened  nodes,  and  a  small  number  of  deeply  stained  granules, 
which  are  probably  chromatin.  Each  nucleus  is  surrounded  by  a 
cell-body,  pi,  and  the  adjacent  cell-bodies  are  separated  from  one 
another  by  clear  spaces.  With  high  powers,  as  represented  in  the 

figure,  one  sees  that 
these  spaces  are  sep- 
arated from  one  an- 
other by  threads  of 
material,  pr,  stretch- 
ing acroeQ  as  bridges, 
connecting  neighbor- 
ing cells.  Examined 
attentively,  the  pro- 
toplasm of  the  cells 
exhibits  a  vacuolated 
appearance.  One 


is 

thus  led  to  view  the 
epithelium  as  a 
sponge- work  of  proto- 
plasm somewhat  con- 
densed around  each 
nucleus  ;  according 
to  this  interpretation 
the  intercellular 
spaces  are  large 
meshes  of  the  sponge- 

FIG.  19.3.— Surface  View  of  the  Amniotic  Epithelium  of  an  Em-    ,    ^   i  -i    ±1 

bryo  of  144  Days;  stained  with  alum-hsematoxylin  and  eosine.     WOrK,  ana 
PI,  Protoplasm;  pr,  intercellular  processes;  nu,  nucleus,     X  1325    cellular    bridges     are 

protoplasmic.  A  re- 
cent paper  *  by  Monsieur  Manille  Ide,  which  I  owe  to  the  kindness 
of  the  author,  brings  a  series  of  interesting  observations  to  show 
that  the  intercellular  bridge  of  the  rete  Malpighi  of  the  mam- 
malian epidermis  are  not  protoplasmic,  but  processes  of  the  cell 
membranes.  This  paper  has  led  me  to  examine  my  preparations 
of  jhe  amniotic  epithelium,  but  I  have  been  unable  to  find  in  them 
any  indications  of  membranes  around  the  cells  or  reasons  for  con- 
sidering the  intercellular  bridges  as  other  than  protoplasmatic  in 
constitution.  Whether  this  result  is  due  to  the  imperfection  of  my 
preparation,  or  is  in  accordance  with  the  truth,  must  be  decided  by 
further  investigation.  Winogradow  has  called  attention,  72.1,  to 
clear  spaces  among  the  epithelial  cells ;  these  spaces  resemble  vesi- 
cles, and  in  hardened  specimens  have  granular  contents ;  they  are  a 
little  larger  than  the  neighboring  cells,  and  seem  to  have  no  nucleus. 
As  to  the  nature  of  these  spaces  I  can  express  no  definite  opinion ; 
they  are  probably  what  some  authors  have  described  as  stomata. 
The  ectodermal  cells  seem  to  partially  degenerate  during  the  latter 
half  of  pregnancy,  for  the  cell  boundaries  become  less  distinct  and 


*  Manille  Ide, 
2me  fasc. ,  1888. 


'La  Membrane  des  Cellules  du  Corps  Muqueux  de  Malpighi,  La  Cellule,"  IV., 


THE   AMXIOX   AND    PROAMXION. 

the  nuclei  become  more  and  more  difficult  to  stain,  but  the  constancy 
and  extent  of  these  changes  have  never  been  investigated. 

Meola,  84.1,  ascribes  a  much  more  complex  structure  to  the  am- 
nioii  than  his  predecessors,  in  which  he  is  followed  by  Viti,86.1. 
Both  of  these  authors  subdivide  the  mesodermic  stratum  into  three 
layers:  a  lamina  connetivale,  next  the  ectoderm,  a  sostanza  inter- 
media, and  a  niembrana  limitante.  As  to  the  histological  details, 
Viti  differs  somewhat  from  Meola,  but  agrees  with  him  in  finding  a 
histological  distinction  between  the  three  layers  enumerated'.  The 
extent  to  which  I  can  distinguish  three  layers  is  indicated  by  the 
description  of  the  mesoderm  given  above ;  I  have  been  unable  to  find 
the  marked  structural  differences  affirmed  by  Yiti.  Viti's  paper  is 
t<  >  be  commended  for  its  excellent  historical  reviews,  particularly  for 
his  summary  of  the  various  theories  as  to  the  origin  of  the  amniotic 
fluid.  Winogradow,  72. 1,  has  described  in  chloride  of  gold  prepa- 
rations a  fine  network  of  clear  spaces,  which  suggest  the  existence  of 
lymph  channels  in  the  mesodermic  layers. 

"  Union  of  the  Amnion  and  Chorion. — At  first  there  is  a  con- 
siderable distance  between  the  amnion  and  chorion,  which  condition 
is  maintained  in  man  during  the  first  two  months,  but  during  the 
third  month  the  amnion  gradually  comes  to  lie  against  the  chorion, 
and  after  that  a  loose  connection  is  established  between  the  two  mem- 
branes, their  mesodermic  layers  becoming  gently  agglutinated.  The 
connection  remains  always  very  slight,  so  that  the  amnion  can 
al  \vays  be  readily  peeled  off.  As  to  the  nature  of  the  connection 
nothing  definite  is  known;  sections  show  that  there  is  a  space  be- 
tween the  amnion  and  chorion  filled  with  a  transparent  matrix, 
which,  at  least  in  hardened  specimens,  sometimes  presents  a  some- 
what fibrillar  appearance ;  in  this  matrix  are  scattered  a  few  cells, 
but  whether  they  are  connective  tissue  (mesenchyma)  or  wandering 
cells,  and  whether  they  are  derived  from  the  amnion  or  the  chorion, 
I  do  not  know.  The  layer  in  question  is  designated  by  Kolliker 
("  Entwickelungsgeschichte,"  322)  as  Gallertgewebe^and  his  opinion, 
with  which  I  agree,  is  that  the  layer  probably  belongs  to  the  chorion. 

Amniotic  Fluid. — The  amniotic  fluid,  known  as  the  liquor 
amnii,  the  Fmclitwasser  of  German  writers,  is  a  serous  liquid, 
which  entirely  fills  the  cavity  of  the  amnion,  and  bathes  the  embryo 
on  all  sides.  We  consider  in  this  article — 1,  its  functions;  2,  its 
quantity;  3,  its  composition ;  4,  its  origin. 

FUNCTIONS. — The  amniotic  fluid  obviously  serves  for  the  mechan- 
ical protection  of  the  foetus  against  sudden  shocks,  blows,  or  press- 
ure; assists  in  the  maintenance  of  a  constant  temperature,  and 
affords  the  foetus  scope  for  its  movements  in  utero.  When  deficient 
in  quantity  it  may  no  longer  prevent  the  pressure  of  the  uterine 
walls  from  acting  on  the  child,  in  which  case  deformities  may  result. 
It  keeps  the  skin  of  the  foetus  moist  and  does  the  same  for  the  geni- 
tal passages  of  the  mother  during  delivery;  it  is,  however,  not 
essential  to  the  act  of  birth,  as  is  shown  by,  1,  the  delivery  in  some 
cases  several  hours  after  the  outflow  of  the  fluid,  and,  2,  the  delivery 
of  the  child  with  the  membranes  intact. 

The  chief  function  of  the  fluid,  however,  appears  to  be  to  serve  as 
a  water-supply  to  the  embryo.     It  is  probable  that  during  the  early 
23 


338  THE   FCETAL   APPENDAGES. 

stages  of  foetal  life,  possibly  during  the  greater  part  or  even  the 
whole  period  of  intra-uterine  existence,  the  embryo  absorbs  consid- 
erable quantities  of  fluid  directly  through  the  skin,  but  of  this 
absorption  we  have  no  direct  certain  proof.  On  the  other  hand,  the- 
swallowing  of  the  liquor  amnii  by  the  foetus  per  os  is  well  es- 
tablished, first^  by  direct  observations  of  the  bird's  ovum;  second, 
by  the  finding  in  the  mammalian  digestive  tract  of  remnants  of 
foetal  epidermis,  hairs,  and  of  the  vernix  caseosa,  which  can  have 
reached  their  site  only  by  being  swallowed  while  floating  in  the 
amniotic  fluids.  That  the  embryo  chick  swallows  the  amniotic 
liquid  was  known  to  Harvey  (1651),  and  is  said  to  have  been  ob- 
served by  Haller ;  renewal  and  extension  of  these  observations  is 
much  needed.  As  regards  the  swallowing  by  the  mammalian  foetus 
there  are  many  observations.  Needham,  Haller,  Moriggia,  and  many 
others  have  found  meconium  in  the  stomach  of  the  foetus ;  the  pres- 
ence of  epidermal  scales  in  the  foetal  digestive  tract  appears  to  be 
constant ;  the  presence  of  hairs  and  fat  (vernix  caseosa} ,  or  of  fatty 
acids  derived  from  the  fatty  vernix,  is  very  common  in  the  meco- 
nium. The  fact  that  the  foetus  does  swallow  is  established,  and 
analogy  with  the  bird  suggests  that  it  swallows  constantly  the  liquor 
amnii,  together  with  such  detritus  as  may  be  suspended  in  it.  As 
the  fluid  contains  only  one  to  two  per  cent  solids,  it  can  hardly 
serve  as  nourishment  to  the  embryo.  The  above  considerations, 
taken  collectively,  render  the  supposition  plausible  that  the  foetus 
obtains  much  of  its  water  from  the  amniotic  fluid. 

QUANTITY. — The  amount  of  amniotic  fluid  at  full  term  has  been 
estimated  by  Fehling,  79.1,  andLevison,  76.1.  The  former  burst 
the  envelope  with  the  finger  or  with  a  trocar,  collected  and  measured 
the  outflowing  liquid ;  the  after-flow  was  collected  upon  a  tared  linen 
lying  on  a  waterproof  sheet.  The  minimum  obtained  in  any  case 
was  265  c. c. ,  the  maximum  2300  c. c.  (certainly  abnormal) .  The  aver- 
age amount  at  full  term  was  680  c.c. ;  for  foetus  from  the  middle  of 
the  ninth  to  the  middle  of  the  tenth  lunar  month,  423  c.c.  Fehling 
observed  thirty-four  cases.  Levison  found  the  average  of  twenty- 
two  cases,  821  gms. ;  Gassner  the  average  of  thirty-five  cases,  1730 
gms. ;  but  as  Gassner 's  results  seem  to  deserve  less  confidence,  we 
may  safely  conclude  that  at  full  term  there  is  usually  under  one  litre 
of  amniotic  fluid,  while  it  must  be  remembered  that  the  amount  is 
extremely  variable.  Richard  Haidlen,  85.1,  gives  a  table  of  forty- 
three  observations  of  the  amount  of  the  amniotic  fluid  determined 
according  to  Fehling's  method,  vide  supra,  and  has  recorded  also  for 
each  case  the  sex,  length,  and  weight  of  the  child,  the  weight  of 
the  after-birth,  the  length  of  the  umbilical  cord,  etc.  Combining 
his  observations  with  those  of  Fehling  (thirty-four  cases) ,  he  is  still 
unable  to  detect  any  constant  relation  between  the  amount  of  the  fluid 
and  the  weight  of  the  child,  the  weight  of  the  after-birth,  or  the  length 
of  the  umbilical  cord.  Haidlen 's  method  of  tabulation,  however, 
hardly  corresponds  to  the  requirements  of  rigid  statistics,  and  it  is 
possible  that  a  reworking  of  his  figures  will  give  different  results. 
I  find  the  average  of  his  observations  to  be  714  c.c.  of  fluid;  taking 
out  two  isolated  extreme  observations,  one  of  50  c.c.,  and  one  of 
7000  c.c.  (hydramnios) ,  the  average  of  forty-one  observations  is  only 


THE    AMNION    AND    PROAMNION. 


339 


577  c.c.  Haidlen  failed  to  find  any  proportion  between  the  percent- 
age of  solids  and  the  amount  of  the  fluid. 

The  amount  during  development  gradually  increases,  but  no  exact 
proportion  exists  between  the  stage  of  development  of  the  foetus  and 
the  amount  of  fluid.  Fehling  attempted  to  show  a  relation  between 
the  length  of  the  umbilical  cord  and  the  quantity,  but  Krukenberg, 
84.1,  demonstrated  from  Fehling1  sown  figures,  79.1,  that  this  con- 
clusion was  untenable. 

Doderlein,  90.1,  has  shown  that  in  the  cow  the  quantity  of  the 
fluid  increases  during  the  early  part  of  pregnancy  and  diminishes 
during  the  latter  part ;  the  exact  figures  are  given  in  the  table  be- 
low. It  is  probably  that  a  similar  variation  occurs  in  man. 

COMPOSITION. — The  liquor  amnii  has  the  character  of  a  serous 
fluid.  Levison  found  its  specific  gravity*  to  vary  from  1.0005  to 
l.()()70,  while,  according  to  Prochownick,  77.1,  it  varies  from  1.0069 
to  1.0082.  The  latter  found  it  to  contain  between  1.07  and  1.60  per 
cent  dry  solids,  giving  0.51  to  0.88  per  cent  ash.  With  the  increase 
of  quantity  there  is  no  constant  diminution  of  the  percentage  of 
solids.  The  following  table  compiled  from  Vogtand  Scherer,  49.1, 
indicates  the  little  that  is  known  concerning  the  changes  in  compo- 
sition during  gestation : 


3  months 

4  months 

5  months 

6  months 

10  months 

Water  ....          t 

IK?  ir 

979  45 

975  84 

990  29 

991  74 

Albumen  and  Mucin  

10.77 

7.67 

6.67 

0.82 

Extracts   

7  28 

3  69 

7  24 

0.34 

0.60 

Sails       ...     ' 

9  25 

6  09 

9  25 

2  70 

7  06 

Doderlein,  90.1,  has  investigated  the amniotic  fluid  of  the  cow; 
his  work  appears  painstaking  and  reliable.  His  chief  results  are 
embodied  in  the  following  tables : 


NaCl. 

Average  per  cent 0.586 

No.  <>f  obs 10 


AMNIOTIC  FLUID. 
NaO. 


KO. 

0.060 

7 


Ca. 
0.014 
10 


Mg. 


Embryos. 

Fluid. 

Stomach. 

1. 

2.                   3.                   4-. 

5. 

6. 

7. 

8. 

No. 

".Vt.  gnus.           c.c. 

Per    cent   of 

Total  N. 

Proteid    N. 

N—  Prot.  N. 

cc.   contents. 

Wt.of  emb. 

1 

33                   110 

333 

2 

87 

160 

183 

. 

.... 

8 

276                   750 

272 

0.039 

0.011 

0.028 

100 

4 

360               1,200 

333 

... 

8 

5 

480               1,300 

270 

.... 

6 

600               2.000 

333 

20 

7 

1,380 

2,900 

210 

0.028               0.008 

6.020 

30 

8 

1,700 

3,400 

200 

....                  .... 

.... 

30 

9 

1,800 

4.320 

240 

0.029               0.009 

0.020 

100 

10 

5,123               3,200 

62 

0.043 

0.013 

0.090 

160 

11 

(i.U'.H)                1.550 

23 

0.048 

0.01'.)                 0.029 

250 

12 

8,700 

2.500 

37% 

0.048 

0.026 

0.022 

130 

13 

*.3:.<>              1.200 

14 

0.047 

0,025 

0.022 

400 

14 

11.300 

1,800 

15 

0.060 

0.040 

0.020 

10 

15 

14,900 

1,300 

8 

0.105 

0.072                 0.033 

300 

340  THE    FCETAL    APPENDAGES. 

In  the  cow  at  term  the  per  cent,  of  albuminoids  in  eight  observa- 
tions was  0.154,  0.464,  0.280,  0.440,  0.268,  0.610,  0.247,  making  an 
average  of  0.348  per  cent.  These  figures  show  that  the  fluid  can 
have  practically  no  nutritive  value. 

It  is  clear  that  there  is  a  great  diminution  in  the  amount  of  albu- 
men, especially  toward  the  last  month,  and  there  is  apparently  a  small 
diminution  in  the  percentage  of  salts.  The  salts  are  such  as  are 
usually  contained  in  serous  fluids.  In  connection  with  the  albumen 
it  may  be  remarked  that  the  fluid  contains  no  fibrin-forming  material, 
as  has  been  shown  by  Gusserow,  78. 1.  There  is  a  small  quantity  of 
urea,  but  not  more  than  is  found  in  other  serous  fluids ;  hence,  the 
presence  of  urea  is  no  argument  in  favor  of  the  view  that  the  amni- 
otic  fluid  is  an  excretion  of  the  foetal  kidney.  Early  in  gestation 
the  amount  is  small,  but  it  gradually  increases  until  the  ninth 
month,  0.030  per  cent,  and  tenth,  0.045  per  cent  (Fehling).  The 
figures  of  various  authors  differ  greatly — sometimes  no  urea  being 
found  (cf.  Preyer,  "Specielle  Physiologie  des  Embryos,"  p.  289). 
Finally  we  have  to  note  the  presence  of  lymph-corpuscles,  but 
whether  they  are  alwa}rs  present,  and,  if  so,  in  what  numbers,  is 
unknown;  in  a  few  cases  they  have  been  found  in  large  numbers. 

ORIGIN. — It  is  a  hypothesis  of  long  standing  that  the  liquor  amnii 
is  an  excretion  of  the  foetus,  and  opinion  has  inclined  to  regard  it  as 
the  product  of  the  foetal  urinary  apparatus.  There  is,  however,  no 
satisfactory  argument  of  any  kind  in  favor  of  this  view,  but,  on  the 
contrary,  there  are  many  forcible  objections  to  it,  and,,  moreover, 
there  is  strong  evidence  to  show  that  it  is  derived  from  the  mother 
by  direct  transudation.  It  is  to  be  considered,  firstly,  that  the 
liquor  has  the  composition  of  a  serous  fluid,  transuded  from  the 
blood-vessels,  and  does  not  resemble  urine;  like  other  serous  fluids 
it  contains  a  small  amount  of  urea,  but  this  is  no  indication  whatso- 
ever of  the  urinary  origin  of  the  fluid ;  secondly,  that  the  foetal  penis 
is  completely  closed  during  the  greater  part  of  embryonic  life,  because 
after  the  closure  of  the  raphe  on  the  stalk  the  glans  remains  long 
imperforate,  so  that  in  the  male,  at  least,  the  direct  discharge  of  the 
urine  into  the  amniotic  cavity  is  impossible;  unless,  therefore,  we 
are  ready  to  attribute  the  formation  of  the  fluid  to  different  sources 
in  the  two  sexes,  we  cannot  assume  the  kidney  to  be  the  source  of 
the  fluid  in  either  sex ;  thirdly,  that  the  fluid  is  not  excreted  by  the 
epidermal  glands  is  proved  by  the  very  late  development  thereof, 
and  the  early  and  abundant  formation  of  the  fluid;  fourthly,  that 
the  amniotic  fluid  appears  very  early,  being  certainly  present  in  the 
third  week,  at  which  time  the  embryo  is  entirely  without  excretory 
or  glandular  organs  of  any  kind,  and  all  its  tissues  are  still  undiffer- 
entiated;  lastly,  that  it  seems  improbable  that  the  foetus,  which 
constantly  requires  water  for  its  own  use,  should  excrete  a  large 
quantity  only  to  swallow  it  again. 

That  the  liquor  transudes  directly  from  the  uterine  wall  or  from 
the  chorion  through  the  amnion  into  the  amniotic  cavity  is  indicated, 
first ,  by  the  composition  of  the  fluid ;  second,  by  experimental  evi- 
dence that  certain  salts  cau  pass  directly  from  the  mother  into  the 
fluid  without  passing  through  the  foetus,  at  least  during  the  latter  part 
of  pregnancy.  Zuntz,  Pfliiger's  Archiv,  XVI.,  548,  was  the  first 


THE   AMNION   AND    PROAMNION.  341 

to  make  such  an  experiment ;  he  injected  an  aqueous  solution  of 
sulph-indigotate  of  sodium  into  the  jugular  vein  of  a  pregnant  rab- 
bit ;  the  liquor  amnii  showed  a  distinct  blue  color,  while  no  trace  of 
blue  was  found  in  any  part  of  the  foetus.  Wiener,  81.1,  repeated 
and  extended  this  observation,  and  G.  ferukenberg  made  similar 
experiments  with  like  results,  with  iodide  of  potassium.  R.  Haid- 
len,  85.1,  also  repeated  Krukenberg'-s  experiment  of  giving  women 
iodide  of  potassium  in  the  early  stages  of  labor,  and  also  small  re- 
peated doses  for  several  days  before  labor ;  in  each  case  he  found  the 
salt  in  the  amniotic  fluid,  and  also  in  the  first  urine  of  the  child. 
This  experiment,  therefore,  does  not  show  whether  the  diffusion  takes 
place  from  the  uterine  wall  or  the  foetus  into  the  amniotic  cavity. 

All  the  facts  taken  collectively  led  Minot  (Buck's  "Handbook,"  I., 
141)  to  the  theory  that  the  liquor  amnii  is  a  product  of  the  osmotic 
function  of  the  amnion ;  that,  during  the  earliest  period,  the  osmosis 
takes  place  from  the  fluid  in  the  space  between  the  amnion  and  cho- 
rion ;  that  during  a  certain  interval,  namely,  while  the  superficial 
capillaries  of  the  chorion  maintain  an  active  circulation  in  that 
membrane  (cf.  Chorion) ,  the  fluid  may  come  from  the  chorion,  and, 
therefore,  indirectly  from  the  foetus ;  and  finally  that  during  at  least 
the  latter  half  of  pregnancy  the  transfusion  occurs  from  the  decidua 
through  the  chorion  and  amnion  both.  That  the  amnion  itself  pro- 
duces the  liquid  it  encloses  is  highly  probable,  but  the  exact  source  of 
supply  upon  which  the  amnion  draws  is  much  more  uncertain. 

Proamnion. — This  convenient  term  was  introduced  by  Ed.  van 
Beneden  to  designate  that  part  of  the  area  embryonalis  at  the  sides 
and  in  front  of  the  head  of  the  developing  embryo,  which  remains 
without  mesoderm  for  a  considerable  period,  so  that  the  ectoderm 
and  entoderm  are  brought  in  the  region  of  the  proamnion  into  im- 
mediate contact.  As  found  in  one  stage  of  the  rabbit,  it  has  already 
been  figured  in  this  work,  p.  183,  Fig.  106.  A  later  stage  in  the  rab-' 
bit,  as  seen  in  longitudinal  section,  is  figured  by  Kolliker  in  his 
"Grundriss,"  2te  Aufl.,  107.  We  find  that  it  had  been  observed  in 
the  chick  by  Remak,  His,  68.1,  9,  Gasser,  77.3,  463,  and  Kolliker. 
Strahl  was  the  first  to  direct  special  attention  to  it.  But  the  earliest 
description  of  the  proamnion  known  to  me  is  that  of  C.  Dareste, 
66.1,  who  gives  a  very  exact  account  of  the  expansion  of  the  meso- 
derm (feuillet  vasculaire)  in  such  a  manner  as  to  leave  an  area  in 
front  of  the  head  without  mesoderm.  Dareste  is,  therefore,  to  be 
considered  the  discoverer  of  the  proamnion.  It  has  since  been  ob- 
served by  various  writers :  Van  Beneden  and  Julin  have  described 
it  in  the  rabbit,  Heape  in  the  mole,  and  recently  its  exact  history 
has  been  admirably  worked  out  in  the  chick  by  Ravn.  The  proam- 
nion, then,  has  been  observed  in  representatives  of  the  classes  Rep- 
tilia,  Aves,  and  Mammalia;  hence  we  may  conclude  that  it  is 
common  to  all  amniota.  It  will  be  remembered  that  the  mesoderm 
grows  out  in  all  directions  from  the  blastopore,  or  hinder  end  of  the 
primitive  streak.  In  a  chick  of  twenty-seven  hours  the  front  edge 
of  the  mesoderm  is  a  somewhat  irregular  transverse  line,  which 
crosses  the  germinal  area  about  at  the  front  border  of  the  head.  This 
line  is  well  shown  in  His' drawings,  loc.  cit.,  PI.  XII.,  Fig.  14.  As 
the  mesoderm  expands  it  does  not  grow  forward  in  the  median  line,  but 


342  THE    FCETAL   APPENDAGES. 

does  grow  forward  at  the  sides  of  the  area  pellucida  in  front  of  the 
head  of  the  embryo,  p.  150.  A  space  is  thus  enclosed  between 
the  mesoderm  on  each  side ;  this  space  later  becomes  the  proamnion ; 
it  contains  no  mesoderm.  Later  on  the  lateral  portions  of  the  meso- 
derm approach  the  median  line  again,  some  distance  in  front  of  the 
head,  so  that  now  the  proamniotic  area  is  completely  surrounded 
by  mesoderm,  Fig.  156,  Pr.a.  We  see,  as  the  next  phase  of  de- 
velopment, the  head  amniotic  fold  arising  in  such  a  position  that 
the  proamnion  is  embraced  between  the  arc  of  this  fold  and  the  head 
of  the  embryo ;  the  proamnion,  therefore,  constitutes  the  floor  of  the 
pit  formed  by  the  upgrowth  of  the  head  amnion.  The  appearances 
at  this  stage,  as  seen  in  longitudinal  sections,  Fig.  106,  are  extremely 
characteristic ;  the  proamnion,  Pro-am,  springs  from  the  wall  of  the 
pericardial  chamber  and  passes  round  the  head  of  the  embryo ;  the 
proamniotic  ectoderm  passes  upward  on  the  embryo,  and  its  ento- 
derm  passes  backward  under  the  heart,  as  a  thin  layer  of  cells,  En, 
which  joins  the  lining  of  the  archenteron.  In  the  chick  the  proam- 
nion never  acquires  any  considerable  development,  but  gradually 
disappears  by  encroachments  of  the  mesoderm  upon  all  sides,  as  has 
been  well  described  by  Ravn,  whose  Fig.  3,  loc.  cit.,  PL  XXI.,  will 
serve  to  give  a  clear  general  notion  of  the  relation  of  the  proamnion 
to  the  head  and  to  the  true  amnion  in  the  chick.  The  disappearance 
of  the  proamnion  in  the  chick  involves  some  curious  appearances  in 
sections  of  embryos,  which  have  not  been  understood  hitherto,  but 
which  Ravn  had  correctly  and  fully  elucidated,  so  far  as  I  can  judge ; 
see  also  the  less  thorough  observations  of  Shore  and  Pickering,  89. 1. 

In  the  rabbit,  according  to  Van  Beneden  and  Julin,  whose  obser- 
vations have  been  confirmed  to  a  certain  extent  by  Kolliker  and 
Heape,  the  role  of  the  proamnion  is  more  considerable.  The  history 
of  the  proamnion,  as  given  by  Van  Beneden,  may  be  followed  easily 
'by  the  aid  of  the  accompanying  diagrams,  Fig.  196,  copied  from 
Van  Beneden.  In  A,  the  proamnion,  pro.  A,  is  very  small,  and  the 
allantois,  Al,  is  just  growing  out.  In  B,  the  embryo,  which  for 
greater  clearness  has  been  shaded  with  stippling,  has  grown  very 
much,  and  the  anterior  half  of  its  body  is  bent  down  at  a  sharp  angle 
into  the  yolk-sac.  The  embryo,  however,  remains  separated  from 
the  cavity,  F,  of  the  yolk-sac  by  the  proamnion,  which  forms,  as  it 
were,  a  hood,  pro.  A,  over  the  anterior  extremity  of  the  embryo. 
The-amnion  proper  is  as  yet  developed  only  over  the  posterior  end  of 
the  embryo.  For  the  further  history  of  the  amnion  see  above. 
The  proamnion,  as  can  be  seen  in  C  and  D,  retains  its  importance 
as  a  foetal  covering-  for  a  considerable  period,  during  which  the  am- 
nion, am,  and  allantois,  Al,  are  rapidly  pursuing  their  development. 
After  the  stage  shown  in  Fig.  196,  D,  by  the  expansion  of  the  cavity 
marked  Coe',  the  amnion  proper,  am,  encroaches  more  and  more  upon 
the  proamnion,  pro.  A,  until  at  last  the  embryo  is  entirely  covered 
by  the  true  amnion,  and  the  proamnion  is  altogether  lost.  It  is  to  be 
noted  especially  that  the  amnion  develops  principally  over  the  poste- 
rior end  of  the  embryo,  and  grows  forward.  To  this  fact  reference 
will  be  made  again  directly. 

So  far  as  at  present  known  the  greatest  development  of  the  proam- 
nion is  in  the  opossum,  Fig.  202,  where  it  covers  ultimately  the 


THE   AMXION   AND    PROAMNION. 


343 


entire  embryo ;  at  first  there  is  a  true  amnion  over  the  caudal  half  of 
the  rump  of  the  embryo,  but  this  gradually  disappears  and  the  pro- 
amiiion  replaces  it.  As  in  the  rabbit  the  proamnion  projects  into 
the  hollow  yolk-sac,  hence  in  the  opossum  the  embryo  may  be  said 
to  lie  in  the  proamniotic  pocket,  inside  the  yolk-sac,  as  it  were.  It 
must  not  be  forgotten,  however,  that  the  cavity  of  the  proamnion  is 


FIG  196  —Diagram  of  the  Development  of  the  Foetal  Adnexa  in  the  Rabbit.  (After  Van  Bene- 
den  and  Julin  )  A  B,  C,  D,  Successive  stages;  pro.  A,  pro-amnion;  Av,  area  vasculosa;  Coe, 
coelom;  Coe',  Coe",  extra  embryonic  portion  of  thecoelom;  En,  entodermic  cavity  of  the  em- 
bryo; Ent,  extra-embryonic  entoderm;  EC,  ectoderm:  Mes,  mesoderm;  A  pi,  area  placentalis; 
Al  allantois-  T  terminal  sinus  of  the  area  vasculosa ;  Y,  yolk-sac;  Am,  amnion;  Am',  portion 
of  the  amnion  united  with  the  wall  of  the  allantois;  Ch,  chorion. 

morphologically  outside  the  yolk-sac,  as  is  clearly  shown  in  the  dia- 
grams of  Fig.  169. 

In  certain  mammalia  there  is  no  proamnion,  owing  probably  to 
modifications  in  the  early  development  of  the  mesoderm,  leading  to 


344  THE   FCETAL    APPENDAGES. 

a  precocious  invasion  of  the  proamniotic  area  by  the  middle  germ- 
layer.  This  seems  to  be  the  case  in  all  rodents  with  inverted  germ- 
layers  (guinea-pigs,  rats,  etc.,  cf.  p.  141),  also  in  the  sheep,  Bonnet, 
89.1,  19,  and  probably  other  ruminants,  and  finally  in  man.  The 
earliest  stages  of  human  development  which  we  have  yet  obtained 
show  us  the  mesoderm  completely  developed  around  the  entire  ovum 
and  separated  over  the  whole  of  the  extra-embryonic  region  into  a 
somatic  (or  chorionic)  and  splanchnic  (or  yolk-sac)  layer,  the  amnion 
fully  formed,  and  no  proamnion.  It  is  quite  possible  that  at  an  early 
stage  in  the  formation  of  the  mesoderm  there  is  an  area  correspond- 
ing to  the  proamnion,  but  it  must  be  obliterated  almost  immediately. 

Evolution  of  the  Amnion. — That  the  amnion  is  a  modification 
of  part  of  the  extra-embryonic  somatopleure  (primitive  chorion)  is 
certain  from  its  development,  but  beyond  this  nothing  definite  is 
known  as  to  its  evolution  or  phylogenetic  origin.  Nor  do  the  specu- 
lations of  Balfour,  "  Comp.  Embryol.,"  II.,  309,  nor  of  Van  Beneden. 
and  Julin,  84.1,  425,  nor  of  J.  A.  Ryder,  86.3,  seem  satisfactory. 
Balfour  says :  "  The  origin  of  the  amnion  is  more  difficult  to  explain 
than  that  of  the  allantois;  and  it  does  not  seem  possible  to  derive  it 
from  any  pre-existing  organ.  It  appears  to  me,  however,  very  prob- 
able that  it  was  evolved,  pari  passu,  with  the  allantois,  as  a  simple 
fold  of  the  somatopleure  round  the  embryo,  into  which  the  allantois 
extended  itself  as  it  increased  in  size  and  became  a  respiratory  organ. 
It  would  be  obviously  advantageous  for  such  a  fold,  having  once 
started,  to  become  larger  and  larger,  in  order  to  give  more  and 
more  room  for  the  allantois  to  spread  into. " 

Van  Beneden  and  Julin  say :  "  Dans  notre  opinion  la  cause  deter- 
minante  de  la  formation  de  1'envellope  amniotique  reside  dans  la 
descente  de  Fembryon  determinee  elle  meme  par  le  pois  du  corps. 
C'est  par  une  acceleration  du  developpement  que  la  cavite  amniotique 
en  est  venu  a  se  former  quand  1'embryon  ne  possede  encore  qu'un  pois 
insignifiant."  Essentially  the  same  view  has  been  advocated  by 
Shore  and  Pickering,  89. 1, 16.  The  chief  objection  to  this  theory  is 
that  it  really  gives  no  cause  for  the  expansion  of  the  somatopleure 
and  chorion ;  there  is  no  proof  that  a  mere  strain  of  weight  can  cause 
the  cells  of  a  membrane  to  proliferate,  and  since  such  proliferation 
is  the  immediate  cause  of  the  growth  of  the  amnion,  Van  Beneden 
and  Julin  must  assume  for  their  theory  that  the  strain  of  weight 
does  cause  proliferation ;  but  this  assumption  lacks  support.  More- 
over they  give  no  evidence  to  show  that  the  embryo,  in  utero,  is 
situated  in  the  primitive  amniota  upon  the  upper  side  of  the  ovum, 
although  it  is  probable  such  was  the  case. 

Ryder's  theory,  86.3,  of  the  origin  of  the  amnion,  like  that  of  Van 
Beneden  and  Julin,  to  which  he  does  not  refer,  is  purely  mechanical ; 
but  Ryder  seeks  the  cause  in  a  rigid  zona  radiata  forcing  the  em- 
bryo down  into  the  yolk.  See  his  summary,  loc.  cit.,  p.  184.  So  far 
as  we  know,  however,  the  embryo  of  the  Sauropsida  cannot  be  said  to 
sink  into  the  yolk,  and  so  lead  to  the  development  of  an  amnion ; 
but,  on  the  contrary,  the  amniotic  folds  rise  up  clear  above  the  yolk. 
Moreover,  the  formation  of  the  amnion  is  really  a  very  complex  pro- 
cess, part  arising  from  the  proamnion,  part  by  a  dilation  of  the  peri- 
cardial  cavity  (Parietal- Hohle) ,  and'part  as  the  extra-embryonic  tail 


THE   AMNION   AND    PROAMNION.  345 

folds.  These  facts  speak  in  my  judgment  unequivocally  against  the 
amnion  having  arisen  by  the  sinking  of  the  embryo  into  the  yolk-sac. 
Nor  is  there  any  justification,  I  think,  for  seeking  these  simple  me- 
chanical explanations,  which  are  worthy  of  Herbert  Spencer,  since 
the  formation  of  the  amnion  depends  upon  inequalities  in  the  growth 
power  of  the  germ-layers,  and  only  such  explanation  can  be  valid 
as  explains  that  inequality — which  Ryder's  hypothesis  fails  to  do, 
so  far  as  I  can  see. 


CHAPTER   XVI. 


Md 


THE  YOLK-SAC,  ALLANTOIS,  AND  UMBILICAL  CORD. 

THE  three  structures  mentioned  in  the  heading  of  this  chapter 
have  such  intimate  relations  with  one  another  in  the  human  embryo 
that  it  is  convenient  to  study  them  together;  but  it  should  not  be 
forgotten  that  morphologically  the  yolk-sac  and  allantois  are  abso- 
lutely distinct  organs,  as  their  development  in  the  embryo  clearly 
demonstrates. 

I.  THE  YOLK-SAC. 

General  Morphology. — As  the  yolk-sac  is  the  container  of  the 
nutritive  yolk,  destined  to  be  assimilated  by  the  embryo,  the  evolu- 
tion of  the  yolk-sac  must  have  depended  primarily  upon  the  accumu- 
lation of  yolk  in  the  egg-cell.  In  the  primitive  form  of  vertebrate 

development  (Petromyzon, 
Ganoids,  Amphibia) ,  we 
find  this  material  comes  to 
lie  in  the  walls  of  the  diges- 
tive tract  between  the  heart 
and  the  allantois,  and  chiefly 
on  the  ventral  side  of  the 
canal ;  the  yolk  of  the  em- 
bryo is  situated,  in  other 
words,  in  the  region  of  the 
abdominal  cavity.  When 
the  liver  appears  it  sepa- 
rates the  heart  from  the 
mass  of  yolk  in  the  ento- 

FIQ.  197.— Longitudinal  Median  Section  of  a  Petromy-  rlprrn         A  «J  Qnrvn  n«  +ho  rnc»eo 

zon  Larva.  EC,  Ectoderm;  Md,  medulla;  nch,  notochord;  "e]  m*.    ^S  ' 

o.pl,  oral  plate;  Li,  liver  pouch,  extending  into  the  mass  derm  IS  developed  Complete- 

o^eyo*  cells;  N,  btastopore.     After  C.  Kupffer.  ]y    around   th/ oyum  ^    of 

course  separates  the  yolk  and  the  ectoderm,  and  as  soon  as  the  ccelom 
is  developed  in  the  abdominal  region  there  is  a  layer  of  mesoderm 
enclosing  the  yolk ;  now  as  the  yolk  is  entodermal  it  follows  that  the 
yolk,  together  with  the  mesoderm  layer  around,  are  morphologically 
part  of  the  splanchnopleure.  This  splanchnopleuric  bag  is  the  homo- 
logue  of  the  yolk-sac.  In  the  meroblastic  anamniota*  (elasmobranchs) 
there  is  a  separation  of  the  yolk-sac  from  the  embryo,  and  it  hangs 
down  from  the  intestinal  canal  of  the  embryo  by  a  small  stalk ;  but 
it  is  covered  by  the  somatopleure  just  as  in  the  more  primitive  types, 
so  that  the  true  yolk-sac  is  inclosed  in  a  second  membrane.  The  same 
arrangement  exists  in  the  amniota ;  there  is  an  inner  or  true  yolk-sac 
formed  by  the  vitelline  entoderm  and  splanchnic  mesoderm,  and  an 

*  For  some  further  details  see  P.  Mayer,  87.1,  346. 


O.pl 


Li 


THE    YOLK-SAC.  54V 

outer  somatopleuric  sac,  homologous  with  the  external  membrane  of 
the  elasmobranch,  but  commonly  known  as  the  inanbruna  ,sr/v>,sa  in 
Sauropsida,  and  as  the  jtriinftire  cJiorinn  in  mammals.  The  term 
yolk-sac,  as  applied  to  the  elasmobranchs,  includes  both  the  inner  or 
true  yolk-sac  and  the  outer  somatopleuric  covering,  homologous  with 
the  chorion;  but  as  applied  to  amniota,  it  commonly  refers  only  to 
the  inner  sac,  to  the  exclusion  of  the  chorion. 

Yolk-Sac  of  Sauropsida. — The  manner  in  which  the  embry- 
onic archenteron  is  separated  from  the  yolk-sac  has  already  l>een 
described,  p.  '2 ~>~>,  and  we  saw  that  the  peripheral  part  of  the  area 
pellucida,  the  whole  of  the  area  opaca,  of  the  so-called  germinal  wall 
and  of  the  yolk-mass  are  included  in  the  yolk-sac;  all  the  parts  men- 
tioned constituting  collectively  the  entodermal  lining  of  the  yolk- 
The  \vhole  of  the  vitelline  entoderm  tends  to  assume  a  distinctly 
epithelial  structure ;  the  change  begins  in  the  region  of  the  embryo 
and  thence  spreads  ^radiially  in  all  directions;  in  the  region  of  the 
area  pellucida  the  vitelline  epithelium  (Dottersackeptthei)  has  thin 
wide  cells;  in  the  region  of  the  area  opaca  the  cells  are  high  cylinder 
cells,  Fig.  IDS,  c,  of  somewhat  irregular  shape,  containing  a  loose 


Fio.  198. -Wall  of  tin-  Yolk-Sac  in  the  Area  Opaca  of  a  Chick  of  the  Second  Day.    J/™.  M-s-,- 
rm;  I",  I",  blood-vessels  containing  a  few  young  blood-cells;  Ent,  entoderm;  c,  four  entoder- 
mal o-lls  showing  distinctly.     (Compare  with  Fig.  201.) 

network  of  granular  protoplasm;  the  lower  ends  of  the  cells  are 
rounded  and  projecting  and  have  a  well-marked  border  of  dense  pro- 
toplasm; the  nuclei  are  variable  in  size,  but  for  the  most  part  large, 
often  three,  or  four  times  greater  in  diameter  than  the  neighboring 
mesodermic  nuclei;  they  have  usually  one,  sometimes  two,  conspic- 
uous nucleoli ;  the  nuclei  always  lie  on  the  upper  or  basal  ends  of 
the  cells,  generally  near  one  side — a  point  best  made  out  in  surface 
views;  the  cells  further  contain  yolk  grains,  which  appear  to  be 
undergoing  resorption ;  near  the  area  pellucida  the  cells  are  smaller, 
the  network  of  protoplasm  closer,  and  the  yolk  grains  either  absent 
altogether,  or,  if  present,  small  in  size  and  few  in  number;  the  transi- 
tion to  the  thin  entoderm  of  the  area  pellucida  is  quite  abrupt,  accord- 
ing to  H.  Virchow,  75. 1,  but  I  have  found  in  some  cases  a  gradual 
change.  Toward  the  periphery  of  the  area  opaca  the  entodermal 
cells  become  larger  and  richer  in  yolk-grains  and  pass  gradually  into 
the  germinal  wall.  The  cylinder  cells  of  the  opaca  entoderm  stand 
at  various  inclinations,  so  that  they  are  cut  obliquely  for  the  most 
part ;  consequently  only  here  and  there  can  we  recognize  them  clearly, 


3-1:8  THE   FCETAL,   APPENDAGES. 

as  in  Fig.  198,  c.  The  germinal  wall  is  the  connecting  link  between 
the  epithelium  on  the  dorsal  side  of  the  cavity  of  the  yolk-sac  and 
the  yolk  forming  the  floor  on  the  ventral  side  of  the  cavity.  The 
structure  and  metamorphoses  of  the  germinal  wall  have  been  the 
subjects  of  much  discussion,  leading  to  very  little  result,  for  many 
authors  have  sought  in  the  germinal  wall  the  origin  of  mesodermal 
and  even  of  ectodermal  cells ;  that  all  such  views  are  erroneous  was 
demonstrated  by  H.  Virchow,  75.1;  it  would  have  saved  a  great 
deal  of  confusion  if  his  admirable  little  paper  had  received  the  atten- 
tion it  deserves.  H.  Virchow  has  since  confirmed  and  amplified  his 
results  in  two  valuable  memoirs,  91.1,  92.1.*  The  germinal  wall 
is  the  transition  from  the  cellular  opaca  entoderm  to  the  non-cellular 
yolk,  hence  it  consists  of  protoplasm  charged  with  yolk  grains  and 
having  numerous  nuclei,  which  toward  the  embryo  become  situated 
in  discrete  cells,  which,  as  we  pass  to  the  opaca,  gradually  take  on  a 
more  and  more  epithelial  character ;  the  non-cellular  yolk  has  nuclei 
also,  but  they  are  further  apart  than  those  of  the  germ  wall ;  these 
nuclei  are  the  so-called  parablastic  nuclei  (see  p.  352).  As  develop- 
ment proceeds  we  see  the  area  pellucida  encroach  upon  the  opaca, 
the  area  opaca  upon  the  germinal  wall,  and  the  germinal  wall  upon 
the  yolk  proper ;  the  whole  series  of  changes  may  be  described  as  a 
centrifugal  metamorphosis. 

The  mesoderm  of  the  yolk-sac  is  a  ;thin  layer  which  gradually 
spreads  over  the  yolk,  and  so  slowly  that,  according  to  M.  Duval 
("Atlas,"  Fig.  652),  it  does  not  completely  enclose  the  yolk  until 
the  seventeenth  day  in  the  chick.  The  early  appearance  of  blood- 
vessels in  it  marks  out  the  area  vasculosa,  which  is  a  part  of  the 
yolk-sac ;  the  expansion  of  the  vascular  area  has  already  been  de- 
scribed, p.  276.  A  further  peculiarity  of  the  mesoderm  is  that  it 
sends  down  partitions  into  the  mass  of  yolk,  carrying  along  the 
blood-vessels,  and  thus  increasing  the  absorbent  surface ;  the  parti- 
tions in  the  chick  begin  to  appear  during  the  sixth  day,  and  continue 
multiplying  and  growing  for  at  least  ten  days. 

As  the  yolk-sac  contains  the  nutritive  material  for  the  embryo,  it 
diminishes  in  size  as  the  latter  grows ;  the  shrinkage  causes  the  sac 
to  become,  the  sixth  o^  seventh  day  in  the  chick,  flaccid  and  some- 
what irregular  in  shape,  two  peculiarities  which  become  more  and 
more  marked  as  development  progresses.  By  the  eighteenth  day 
the  sac  is  very  much  smaller;  by  the  nineteenth  the  reduction  is 
still  more  striking  and  the  sac  begins  to  be  withdrawn  within  the  body 
of  the  chick,  and  before  hatching  the  embryo  takes  in  the  yolk-sac 
completely  through  the  umbilical  opening;  during  its  retraction  the 
sac  has  a  characteristic  hour-glass  shape,  owing  to  the  narrowness  of 
the  umbilicus. 

Concerning  the  structure  of  the  yolk-mass  during  the  resorptioii  of 
the  yolk  material  we  know  very  little,  and  of  the  physiology  of  the 
assimilation  of  the  yolk,  almost  nothing.  Von  Baer  pointed  out, 
28.3,  that  the  yolk  becomes  more  fluid  in  the  chick,  and  H.  Rathke, 
39.1,  113,  that  in  the  snake  the  separate  yolk-granules  disappear, 
and  the  yolk  becomes  a  greenish-yellow  homogeneous  translucent 

*  I  regret  that  these  memoirs  came  to  my  hands  too  late  to  enable  me  to  incorporate  Virchow's 
results  in  the  text. 


THK    YOLK-SAC. 

fluid.  H.  Strahl,  87.1,  gives  an  important  account  of  the  yolk-sac 
in  tlic  li/ard.  showing  that  the  dissepiments  of  mesoderm  are  covered 
with  lar-v  y< >lk-cells— the  whole  yolk  apparently  bec< >in in L;  cellular 
in  later  Ma-e-;  the  cavity  of  the  sac  is  very  distinct ;  the  sac  it  >clf  be- 
comes ilatteiied  ;  and  it  is  only  on  the  inferior  side  t  hat  the«li>sej»inients 
acquire  a  considerable  development,  and  on  this  lower  side  the  cellular 
structure  is  perhaps  never  fully  attained.  The  regular  form  of  the 
yolk-sac  persists  in  tin-  lizard,  but  in  the  snake,  H.  Rathke,  39.1, 
Is:;  IM.  it  !M'<  o me s  flaccid  and  irregular. 

Yolk-Sac  of  Mammals.— In  order  to  understand  clearly  the 
development  of  the  mammalian  yolk-sac,  it  is  best  to  start  with  the 
two-layered  blastodermic  vesicle,  with  a  small  embryonic  area  in 
which  there  is  mesoderm;  the  inner  layer  of  the  vesicle  is  the  homo- 
It  i^-iio  of  the  yolk-mass  of  Sauropsida,  and  is  able  to  assume  the  cellu- 
lar Mructuro  owing  to  the  loss  of  yolk,  which  is  undoubtedly  also 
the  cause  of  the  large  size  of  the  cavity  of  the  vesicle — thi>  c.ivity 
hein-,  as  we  have  seen,  the  vitel line  cavity;  the  inner  vesicle  then 
i-  the  hoinologue  of  the  entodermal  part  of  the  yolk-sac.  The  extra 
embryonic  mesoderm  and  coelom  are  extremely  variable  in  extent  in 
the  mammalian  ovum;  in  man,  as  we  have  seen,  the  mesoderm  is 
\.i\  .  i  rly  developed  completely  around  the  yolk -vesicle,  and  so  is 
the  c.elom,  so  that  in  the  earliest  accurately  known  of  human  stages 
the  yolk-sac  and  chori  on  are  completely  differentiated.  Inthesheep, 
and  probably  in  all  ruminants,  there  is  a  similar  early  separatio1 
the  yolk  sac  and  chorion.  In  the  rabbit  the  mesoderm  never  extends 
more  than  about  half  of  the  blastodermic  vesicle,  but  thorn-loin 
extends  nearly  to  the  periphery  of  the  sheet  of  mesoderm;  hence  we 
have  a  half-way  separation  of  the  yolk-sac  and  chorion.  In  the 
•  •possum  the  mesoderm  extends  about  half-way  over  the  blastodermic 
\e>icle,  but  the  ru-lom  is  developed  only  around  the  allantois,  so  that 
there  is  only  a  very  partial  separation  of  the  yolk-sac  and  the  chorion. 
lu  both  rabbit  and  opossum  the  lower  half  of  the  yolk-vesicle  is  in 
direct  contact  with  the  ectoderm,  preserving  to  this  extent  the  con- 
dition of  the  stage  of  the  two-layered  blastodermic  vesicles. 

That  the  partial  extension  of  the  mesoderm  represents  a  modified 
condition  is  evident,  since  in  all  non-mammalian  vertebrates  both 
mesoderm  and  coelom  extend  completely  around  the  yolk.  Hence, 
the  complete  separation  of  the  yolk-sac  in  man  and  the  sheep  is  nearer 
the  ancestral  t\{>e  than  the  relations  of  the  extra-embryonic  germ- 
layers  to  one  another  in  the  rabbit  and  the  opossum.  The  question 
a<  to  what  was  the  primitive  mammalian  arrangement  must  be  left 
open ;  we  cannot  say  whether  the  opossum  or  man  most  nearly  rep- 
iv-.-nts  the  ancestral  type. 

M  AN. — The  human  yolk-sac  is  an  appendage  of  the  digestive  canal 
formed  by  the  extra -embryonic  somatopleure.  At  the  beginning  of 
the  third  week  the  diameter  of  the  yolk-sac  is  about  equal  to  the 
length  of  the  embryo.  By  the  middle  of  the  third  week  the  sac  has 
become  distinctly  pear-shaped  and  is  attached  by  its  pointed  end  to 
the  intestinal  canal  of  the  embryo,  Fig.  17.  The  sac  continues 
growing  up  to  the  end  of  the  fourth  week,  after  which  it  enlarges 
very  slightly,  if  at  all;  its  diameter  is  from  7-11  mm.  It  is  then  a 
pear-shaped  vesicle  attached  by  a  long  stalk  to  the  intestine,  the 


350  THE    FCETAL   APPENDAGES. 

stalk  having  been  formed  by  the  lengthening  of  the  neck  of  the  yolk- 
sac,  Fig.  169.  Sections  show  that  the  sac  is  hollow,  with  a  lining 
of  entodermal  cells,  and  a  thicker  layer  of  mesoderm,  containing 
blood-vessels;  the  network  of  vessels  imparts  a  characteristic  appear- 
ance to  the  external  or  mesodermic 
raes  surface  of  the  yolk-sac,  compare  Fig. 

175.  The  accompanying  Fig.  HIM 
jr  represents  a  section  of  the  yolk-sac 
of  an  embryo  of  about  1  mm.,  after 
Keibel.  The  cavity  of  the  yolk-sac 
extends  at  first  through  the  stalk  to 
the  intestine,  but  it  early  becomes 
obliterated  in  the  stalk.  The  ento- 
derm  disappears  altogether  and  quite 
early  in  the  yolk- stalk ;  thus  in  an 
embryo  of  12.5  mm.,  His  ("Anat. 
menschl.  Embryonen,"  III.,  •><>) 
found  only  remnants  of  it  in  the 

FIG.  199. -Section  of  the  Yolk-Sac  of  a      S^lk"       In    t}?e  VJ6si.cle    itself   the  en~ 

Human  Embryo  (No.  ii,  p.  29D.     Ent,    toderm  consisted   in   a.  very  young 
?SsernAfte"Fr.  BEST™'    ^  ""*'     ovum  of  a  single  layer  of  cuboid.il 

cells  (Graf  Spec,  90.1,  163),  but  is 

said  to  become  fatty  and  to  change  into  a  pavement  epithelium, 
which  is  also  thrown  up  into  vascular  villi  (Kolliker).  In  regard  to 
the  further  contents  of  the  yolk-sac,  Von  Baer  states,  37. 1,  27  '2,  that 
in  young  ova  (at  six  to  seven  weeks)  the  contents  are  sometimes  as 
thick  and  yellow  as  the  yolk  of  a  bird's  egg;  in  ova  of  this  period 
the  thinner  the  contents  the  more  rounded  and  fully  distended  is  the 
yolk-sac.  A  little  later  the  contents  are  always  fluid,  but  at  the  end 
of  pregnancy,  according  to  B.  S.  Schultze,  61.1,  when  the  sac  lias 
shrunk  to  4-7  mm.  in  diameter,  it  contains  variable  quantities 
fatty  substances  and  carbonates.  It  thus  appears  that  during  the 
first  month,  at  least,  the  yolk-sac  does  contain  more  or  less  true  yolk 
— an  idea  which  is  confirmed  by  Rauber's  observations  on  the  rab- 
bit's ovum.  It  seems  indeed  probable  that  the  rudimentary  yolk-sac 
of  man  still  performs  for  a  short  period  the  function  of  a  food  reser- 
voir for  the  embryo,  amnion,  and  the  chorion.  (B.  S.  Schultze,  61.1). 
SHEEP. — The  two-layered  blastodermic  vesicle,  with  an  embryonic 
shieW,  has  an  elongated  form;  the  mesoderm  spreads  out  gradually 
between  the  ectoderm  and  entoderm  (yolk- vesicle)  starting  from  the 
shield ;  the  ccelom  is  developed  in  it  as  it  spreads,  so  that  by  the 
thirteenth  day  (R.  Bonnet,  89.1,  Taf.  VI.,  Fig.  3)  about  a  third  of 
the  ovum  is  furnished  with  mesoderm,  and  in  this  third  the  splanch- 
nopleure  of  the  yolk-sac  is  completely  separated  from  the  chorionic 
somatopleure,  while  elsewhere  the  yolk  entoderm  is  still  directly  in 
contact  with  the  ectoderm ;  this  stage,  see  Fig.  200,  is  as  far  as  de- 
velopment progresses  in  the  rabbit.  In  the  sheep  the  development  of 
the  mesoderm  and  the  coelom  proceeds,  until  about  the  seventeenth 
day  the  yolk-sac  is  completely  separated  from  the  chorion ;  the  yolk- 
stalk  remains  short,  but  the  yolk-sac  proper  becomes  drawn  out  and 
twisted,  following  in  its  growth  the  characteristic  elongation  of  the 
ruminant  chorionic  vesicle. 


THK    FOLK-SAG, 


861 


vi  10 

Yk 


RABBIT. — Tho  development  of  the 
extni-emhryonic  inesodenn  and  CO3- 
loni  is  entirely  arrested  at  alxmt  the 
Ma-v  reached  by  the  sheep  on  tli«' 
thirteenth  day,  so  that  the  yolk-sac 
and  chorion  arc  never  differentiated 
over  more  than  lialf  the  ovum,  the 
inferior  hemisphere  of  which  re- 
mains in  the  stage  of  a  two-layered 
Mastodermic  vesicle,  and  is  said  by 
Duval  to  degenerate  and  l>e  iv>«»rbed. 
The  accompanying  diagram  will 
suffice  to  render  the  disposition  clear ; 
it  will  l>e  seen  at  once  that  the  cho- 
rion.  ( 'ho,  exists  only  part- way  round 
the  ovum.  I  introduce  here  Fig. 
.'"1  of  a  section  through  the  wall  of  the  yolk-sac  of  a  rabbit  embryo 
of  thirteen  days;  the  structure  closely  resembles  that  of  the  area 


Fio.  300.— Diagram  of  the  Embryo  and 
Yolk-Sac  of  a  Rabbit,  coe,  Ccelom:  n,,>. 
cli..i-j(,n;  Yk,  yolk-sac;  m«a,  mesodn  MI  . 

•ia  terminal. 
ectoderm. 


BL—  Vertical  s.«-ti,,,,  ,,f  the  Wull  of  th«>  Yolk-Sac  of  a  Rabbit  Embryo  of  Thirteen  Days. 
W,  Blood-vessels;  &/,  blood-cells;  men,  BMOdenn. 

opaca  of  the  bird's  yolk-sac,  Fig.  198,  except  that  the  entodermal 
cylinder  epithelium  is  composed  of  much  smaller  cells  in  the  rabhit, 

owing    to    the   absence  of 
, 

Am 

OPOSSUM.  —  Our   knowl- 

edge  rests  mainly  up°n  the 


> 


Praam 


Ent 


FIG.  202. — Diagram  of  an  Opossum  Embryo  and  its 
Appendages,  st,  Sinus  terminal  is:  CTio,  chorion ;  Am, 
amnion;  EC,  ectoderm;  j>/<.s.  iiu-s.i<|»'rm;  Emb,  embryo ; 
Pro.  am,  pro-amnion;  Kiit.  «-iit<Mlcrm;  Yk,  cavity  of 
yolk-sac;  All,  allantois;  Coe,  coelom.  After  E.  Selenka. 


observations  of  Selenka, 
87.1,  whose  diagram  I 
have  copied,  Fig.  202;  the 
embryo,  Emb,  is  almost  en- 
tirely covered  by  the  pro- 
anniion,  Pro.  am,  the  am- 
nion,  Am,  being  very  much 
reduced;  the  allantois,  AH, 
projects  also  into  the  yolk- 
sac  cavity,  Yk;  owing  to 
the  development  of  the  pro- 
amnion  and  allantois  the 
cavity,  Yk,  of  the  yolk-sac 
acquires  a  ver}r  complicated 
form;  the  extra-embryonic 
coelom,  Coe,  is  hardly  more 


352  THE   FCETAL   APPENDAGES. 

than  a  space  around  the  allantois,  and  consequently  the  true  cho- 
rion  is  reduced  to  an  insignificant  area,  Cho;  the  extra-emb^- 
onic  mesoderm,  mes,  extends  over  nearly  half  the  ovum,  from  st  to 
st,  but  contains — except  around  the  allantois — no  ccelom;  in  this 
sheet  of  mesoderm  the  blood-vessels  of  the  area  vasculosa  are  devel- 
oped ;  and  as  there  is  no  ccelom  over  the  area,  the  vessels  are  almost 
as  closely  related  to  the  ectoderm,  EC,  as  to  the  entoderm,  Ent. 
Here,  then,  we  have  the  mesoderm  spreading  out  as  in  the  rabbit, 
but  the  development  of  the  ccelom  is  arrested.  Although  the  opos- 
sum stands  low  in  the  mammalian  scale,  its  foetal  membranes  show 
many  changes  from  the  sauropsidan  type  and  are  probably  modified 
in  an  aberrant  manner,  differently  from  mammals  of  other  classes. 
For  the  peculiar  relations  of  the  yolk-sac  to  the  allantois,  see  the 
description  of  the  latter  organ. 

The  So-called  Parabiastic  Nuclei  of  the  Yolk.— In  mero- 
blastic  vertebrate  ova,  after  the  embryo  is  formed,  there  appear  in 
the  yolk  near  its  surface  underneath  the  extra-embryonic  blastoderm 
peculiar  large  nuclei,  which  are  commonly  designated  as  the  para- 
blastic  nuclei.  The  following  description  applies  to  Pristiurus.* 
The  extra-embryonic  ectoderm  is  a  thin,  much-flattened  epithelium 
lying  close  to  the  yolk ;  below  the  ectoderm  is  the  superficial  layer 
of  the  yolk,  a  broad  stratum  of,  protoplasm  with  scattered  small  yolk 
granules ;  a  little  deeper  down  a  row  of  irregular  vascular  spaces, 
and  again  a  little  deeper  a  layer  of  very  big  nuclei,  each  with  a  dis- 
tinct intranuclear  network  and  several  deeply-stained  nucleoli ;  the 
nuclei  vary  in  size,  being  from  two  to  five  times  the  diameter  of  ":he 
nuclei  in  the  embryo.  The  upper  part  of  the  protoplasmatic  stratum 
contains  numerous  small  and  a  few  larger  yolk-grains,  and  contains 
near  and  under  the  embryo  small  nuclei;  the  middle  part  of  the 
stratum  contains  the  vacuoles,  the  big  nuclei,  and  but  few  yolk 
grains ;  the  deepest  part  contains  larger  granules  and  merges  grad- 
ally  into  the  yolk  proper.  (See  also  His,  82.1,  75,  and  Riickert, 
85. 1.)  Riickert  designates  these  nuclei  as  "  Merocytenkerne,"  and 
the  cells  which  they  represent  as  " Merocyten."  H.  E.  Ziegler, 
87. 1,  states  that  the  parablastic  nuclei  of  teleosts  multiply  up  to  a 
certain  stage  by  indirect  division,  but  later  they  assume  a  peculiar 
habitus  and  multiply  by  indirect  division,  and  assume  various  shapes. 
These  changes  are  perhaps  connected  with  the  death  of  the  nuclei, 
their  active  functions  being  completed.  The  special  function  of  the 
protoplasmic  layer  appears  to  be  the  assimilation  of  the  nutritive 
yolk.  Riickert  also  maintains,  but  without  proper  evidence,  it  seems 
to  me,  that  merocytes  become  cells,  some  of  which  join  the  ectoderm, 
some  the  entoderm,  and  yet  others  the  mesenchyma.  In  the  Sau- 
ropsida  we  find  similar  nuclei  and  similar  relations  of  the  nucleated 
layer,  but  in  them  the  protoplasmic  layer  becomes  the  epithelium  of 
the  yolk — see  especially  H.  Strahl,  87. 1 — and  I  consider  it  probable 
that  these  parablast  nuclei  in  all  meroblastic  ova  belong  to  the  vitel- 
line  entoderm.  J.  Riickert,  92.3,  claims  that  some  of  the  "para- 
blast  nuclei"  are  derived  from  spermatozoa,  which  enter  the  yolk 
but  do  not  unite  with  the  female  pronucleus ;  it  is  doubtful  whether 

*  From  sections  in  the  collection  of  Professor  His,  which  he  generously  permitted  me  to 
study. 


THK    ALLANTOIB.  553 

or  not  any  of  these  spermatozoa   nuclei   share  in  the  production  of 
embryonic  tissue. 

In  holohlastic  mammalian  ova  the  vitelline  entoderm  is  cellular, 
and  no  nuclei  are  known  similar  to  large  "  parablastic  "  nuclei  of 
merohlastic  ova. 

II.  THE  ALLANTOIS. 

The  origin  of  the  allantois  \ve  have  already  described,  p.  257.  It 
ari>e>  as  an  entodermal  evaginatiaa  behind  or  below  the  blastoppre 
and  anus,  and  extending  into  the  anterior  end  of  the  primitive 
streak. 

Allantois  of  Sauropsida. — The  allantois  becomes  rapidly  dis- 
tended by  the  enlargement  ot  its  entodermal  cavity,  and  hence  comes 
to  project  freely  into  the  coelom  as  a  vesicle,  attached  by  a  pedicle  to 
the  anal  end  of  the  intestinal  canal.  This  vesicle  is  of  course  formed 
by  the  splanchnopleure,  and  therefore  lined  by  entoderm,  with  an 
outer  layer  ot  im-oderm.  In  the  chick  the  vesicle  is  about  as  large 
as  the  eye  by  the  middle  of  the  fourth  day,  and  after  that  grows  very 
rapidly,  becoming  bent  so  as  to  project  on  the  right  side  of  the  em- 
bryo: by  the  end  of  the  fourth  day  it  is  already  about  as  large  as 
tin-  mid-brain  at  that  stage  (cf.  Duval,  "Atlas,"  Fig.  122).  During 
this  expansion  its  mesoblastic  walls,  which  are  at  first  very  thick, 
become  thinner,  and  at  the  same  time  the  allantoic  blood  circulation 
becomes  important  The  blood  is  supplied  directly  from  the  dorsal 
aorta,  which  terminates  in  a  fork,  of  which  each  branch  is  an  allan- 
toic artery,  and  the  blood  is  returned  by  two  allantoic  veins,  which 
run  alon--  in  the  body  walls.  By  the  third  day  in  the  chick  they 
are  found,  after  having  united  into  a  single  trunk,  to  open  into  the 
vitelline  vein,  close  behind  the  liver.  The  allantois  continues  enlarg- 
ing, and  pushes  its  way  very  rapidly  into  the  extra-embryonic  coelom, 
between  the  amnion  and  chorion  (serosa  or  false  amnion).  Curving 
up  around  the  ri-ht  side  of  the  embryo  the  vesicle  comes  to  lie  on 
the  dorsal  side,  above  the  amnion,  and  separated  from  the  shell  by 
nothing  more  than  the  thin  chorion.  In  this  position  its  rapid  growth 
continues;  it  forms  a  flattened  bag,  covering  the  right  side  of  the 
embryo,  and  rapidly  spreading  out  in  all  direct  ions.  It  is  filled  with 
fluid,  so  that  in  spite  of  its  flattened  form  its  opposite  walls  are  dis- 
tinctly separated  from  one  another.  The  expansion  steadily  contin- 
ues, so  that  by  the  ninth  day  the  allantoic  vesicle  nearly  surrounds 
the  yolk:  during  the  eleventh  day  the  outer  wall  of  the  allantois 
be-ins  to  -row,  together  with  the  chorion;  hence  in  opening  an  egg 
dui-ing  the  later  stages  of  incubation,  there  is  much  danger  of  tear- 
in--  the  allantois  when  the  shell  membrane  is  removed.  The  embryo 
may  now  be  said  to  be  surrounded  by  two  new  membranes — the  outer 
and  inner  walls  of  the  allantois.  About  the  sixteenth  day  the  allan- 
toic sac  completely  envelops  the  ovum,  and  by  the  seventeenth  its 
edges  fuse.  The  closure,  according  to  Duval,  84.2,  takes  place  in 
such  a  manner  that  there  is  formed  a  sac  of  ectoderm,  inclosing  the 
remnant  of  white  at  the  pointed  end  of  the  ovum ;  this  sac,  as  well  as 
the  yolk,  is  inclosed  by  the  allantois. 

Histology. — Concerning  the  tissues  of  the  allantois  we  possess 
23 


354  THE    FO.TAL    APPENDAGES. 

very  little  information;  the  entodermal  lining  appears,  at  least  in 
advanced  stages,  as  a  low  cuboidal  epithelium,  while  the  mesoderm 
is  thicker  and  consists  of  more  or  less  widely  separated  mesenchymal 
cells,  covered  by  a  thin  mesothelium ;  the  mesoderm  contains  blood- 
vessels ;  and  since  contractile  pulsations  have  been  observed  in  the 
allantois  of  the  chick  toward  the  close  of  incubation,  it  is  probable 
that  some  of  the  mesenchymal  cells  assume  the  form  of  smooth  mus- 
cle fibres.  Where  the  allantois  fuses  with  the  chorion  (membrana 
serosa)  the  mesothelium  of  both  layers  disappears,  and  there  is  no 
demarcation  or  difference  between  the  allantoic  and  chorionic  mesen- 
chyma— compare  Duval,  84.2,  PI.  IX.,  Fig.  8. 

Allantois  in  Mammals. — The  allantois  is  very  variously  devel- 
oped in  the  mammalia,  being  a  distinct  vesicle  in  most  forms,  but 
never  growing  around  the  embryo  and  yolk,  as  in  birds.  In  the 
opossum,  Selenka,  87.1,  141,  the  allantois  does  not  even  come  in 
contact  with  the  chorion,  but  invaginates  the  wall  of  the  yolk-sac,  as 
shown  by  the  diagram,  Fig.  202 ;  the  wall  of  the  yolk-sac  forms  a 
pocket  in  which  the  allantoic  vesicle  is  lodged ;  the  walls  of  the  two 
organs  do  not  unite ;  the  pocket  in  the  yolk-sac  has  curious  relations 
to  the  main  blood-vessels  running  from  the  embryo  to  the  area  vas- 
culosa,  for  the  two  omphalo-mesaraic  veins  run  straight  back  from 
the  embryo  along  the  edges  of  the  mouth  of  the  pocket,  while  the 
single  omphalo-mesaraic  artery  runs  in  a  great  arch  in  the  median 
plane  round  the  bottom  of  the  pocket.  These  features  are  beautifully 
illustrated  by  Selenka,  87.1,  Taf.  XXVII.,  Figs.  1-4.  The  allan- 
toic wall  is  figured  by  him,  Fig.  4,  Taf.  XXV.,  as  consisting,  in  an 
embryo  six  days  old,  of  an  inner  layer  of  entodermal  cuboidal  epi- 
thelium, a  thin  outer  layer  of  mesothelium,  and  a  thicker  layer  of 
vascular  mesenchyma.  In  the  rabbit  (and  probably  all  rodents)  the 
alla-ntois  becomes  a  moderate-sized  vesicle,  Fig.  196,  All,  which 
grows  out  until  it  reaches  the  placenta  chorion,  with  which  it  unites 
to  co-operate  in  the  development  of  the  placenta.  In  insectivora  the 
allantois  seems  to  resemble  that  of  the  rodents,  though  it  acquires 
greater  size;  exact  investigations  are  much  needed.  In  ruminants 
the  allantois  expands  very  early,  growing  out  transversely  and  con- 
tinuing to  enlarge  with  extraordinary  rapidity  until  it  takes  up  most 
of  the  chorionic  vesicle,  thus  becoming,  relatively  to  the  embryo,  of 
enormous  size.  For  further  details  see  R.  Bonnet,  89.1,  26-36,  and 
Bischoff,  54.1.  A  few  histological  facts  may  be  gleaned  from  the 
very  verbose  article  on  the  allantois  by  A.  Dastre,  76. 1,  17-44. 

MAN. — The  allantois  in  man  and  other  primates  is  essentially 
different  from  that  of  any  other  known  amniote.  It  never  becomes 
a  free  vesicle,  but  always  remains  a  narrow  tubular  diver ticulum. 
In  man  the  embryo,  when  the  amnion  is  formed,  becomes  every- 
where separated  from  the  chorion,  except  at  the  hind  end,  where  the 
accumulation  of  mesodermal  cells  into  which  the  allantoic  diverticu- 
lum  extends,  see  Figs.  170,  180,  and  222,  constitutes  a  thick  stalk. 
This  stalk  has  been  named  the  Bauchstiel  by  W.  His ;  it  may  be 
regarded  as  a  direct  prolongation  of  the  body  of  the  embryo ;  it  per- 
manently connects  the  embryo  proper  with  the  chorion.  The  amnion 
springs  from-  the  sides  of  the  Bauchstiel  in  the  same  manner  as  from 


THK     M.I    \\ToIS. 


tin-  body  <>t'  the  embryo.  In  man,  therefore,  there  is  no  free  allan- 
t«>is,  and  tin-  history  of  tin-  oru'an  may  be  said  to  be  reduced  to  that 

•in-  entodermal  allantoic  divert  iculum.  The  ontodermal  allantois 
i-  a  Miiall,  lonij,  epithelial  tube  extending,  as  we  have  8660,  to  the 
chorion.  p.  297,  Kiic.  l«n.  The  tuU-  increases  very  little  in  diam- 
eter, after  tlie  second  month:  compare  A  and  B,  Fig.  444.  It  is 
very  ])ersi>t«'Mt,  appearing  usually  even  in  the  cord  at  full  term,  at 
least  in  the  proximal  end.  according  to  Kolliker  ("  Kntwickehmgs- 
p-srh.."  -,'te  Autl.,  ]».  :>  I).  After  the  second  month  it  is  a  small  group 
i.ithelioid  cells,  with  distinct  walls,  irregularly  granular  contents, 
and  round  nuclei;  around  the  cells, 
cut.  which  mayor  may  not 
sho\\-  a  remnant  of  the/central  cavity, 
there  is  a  slight  condensation, 
of  the  connective  tissue  to  form,  a^  it 
were,  an  envelope.  This  Mructure 
has  been  regarded  by  Ahlfeld  and 
others  as  the  persistent  yolk-sac.  I 
think  t lie  correct  interpretation  was 
suggested  by  Kolliker. 

It  lias  been  supposed  by  some 
writers  that  the  human  allantois  grew 
out  a-  a  free  vesicle.  Haeckel  oven 
went  so  far  as  to  prophecy  that  when 
a  human  embryo  of  the  right  stage 
should  he  obtained,  this  condition 
would  he  found.  Shortly  after  this 
AN'.  Krause  published  a  description, 
76.1,  of  an  embryo,  which  he  said 

was  human  and  had  a  free  allantois.  Both  Haeckel  and  Krause  were 
mi-taken,  the  former  through  h a <ty  and  unfounded  speculation,  the 
latter  through  an  error  as  to  the  identity  of  his  embryo.  W.  His 
lias  shown  that  it  was  certainly  not  human,  probably  not  even  mam- 
malian, but  avian,  80.1,  \->.  Krause  still  maintained  that  it  was 
human.  The  discussion  as  to  this  specimen  was  a  long  and  animated 
one.  hut  has  now  little  interest  except  historically.  See  Krause, 
80.1,  81.1,  2,  Kolliker,  uEntwickelungsgesch.,"  1879,  306,  1013, 
Ahlfeld  (('!>/.  /'///•  r/////fV/,-.,  1880,  No.  25),  Krause,  ib id.,  81.1,  and 
Kcker  in  His''j/vA. '/.  Anat.,  1880,  405. 

Allantoic  Fluid. — The  fluid  contents  of  the  allantois  cannot  be 
well  studied  in  man,  owing  to  the  minute  size  of  the  cavity  of  the 
organ;  but  when  the  allantoi^  heroines  a  large  sac,  as  in  the  cow  and 
pig,  the  fluid  is  readily  collected.  There  are  many  observations 
recorded  concerning  the  chemical  composition  of  the  fluid,  but  the 
best  work  known  to  me  is  that  of  Doderlein,  90.1,  on  the  fluid  in 
row  embryos.  His  results  may  be  summarized  as  follows: 


Fio.  208.—  Section  of  the  Allantois  from 
tin-    Umbilical    C..r.l    «»f    an    Knihi 
Three    Mi  in;  ienn;    me«, 

in.     X  340  diatns. 


!_•>•    ].« 

N".  of  ohs 


ALLANTOIC  FLUID  (Cow). 

NaCl.  NaO. 

...-i.-.MI 


KO 

0.093 


8 


Ca. 

0.015 
10 


0.049 


356 


THE   FCETAL   APPENDAGES. 


Embryo. 

Fluid. 

1  . 

2.                     3. 

4. 

5. 

6. 

7. 

No. 

Wt.  gnus.               (-..-. 

Per    cent    of 

Total  X. 

Proteiil  N. 

N_Pn,t.  \. 

Wt,  of  emb. 

1 

33 

78 

£87 

2 

87 

90 

108 

3 

276 

200 

72 

0.135 

0.019 

1  1.  1  ISt> 

4 

360 

400 

111 

5 

480 

850 

177 

6 

600                     850 

141 

7 

1,380 

2,400 

173 

0.124 

0.092 

0.088 

8 

1,700 

1,300 

Tfl 

9 

1,800 

1,400 

77 

0.164 

0.141 

0.023 

10 

5,123 

2,000 

H 

.     0.271 

O!l84 

0.147 

11 

6,690 

2,450 

86 

0.848 

0.141 

0.107 

12 

6,700 

3,500 

» 

0.196 

0.198 

0.074 

13 

8,350 

6,50l> 

0.202 

0.141 

0.0(31 

14 

11,300 

5,000 

44 

0.329 

0.18! 

0.148 

15 

14,900' 

6,600 

44 

0.429 

0.220 

0.209 

I 

Am 


The  allantoic  fluid  differs  markedly  from  the  amniotic — compare 
the  tables  above  with  those  on  p.  339 — and  shows  in  its  compositior 
that  it  is  an  excretory  product  of  the  foetus,  coming  from  the  Wolffiar 
bodies  and  the  kidneys.  In  the  chick,  by  the  sixteenth  day,  deposits 
of  water  become  abundant  in  the  fluid  (Foster  and  Balfour's  "  Ele 
ments,"  second  edition,  280). 

Notices  of  several  of  the  earlier  investigations  on  the  allantoic 
fluid  may  be  found  in  A.  Dastre,  76.1,  45-61,  together  with  som< 
results  of  his  own. 

III.  THE  UMBILICAL  CORD. 

Bauchstiel. — As  the  Bauchstiel  is  the  anlage  of  the  human  um 
bilical  cord,  we  must  consider  its  structure  and  relations.  As  w( 

have  already  seen,  it  is  the  proloii 

fation  of  the  tail  of  the  embryo 
ig.  16G,  Al,  running  to  the  chorior 
and  containing  the  tubular  allantoic 
diverticulum,  Fig.  170,  Al ;  it  con 
sists  mainly  of  mesoderm,  and  fron 
its  side  springs  the  amnion.  Prof 
His  ("  Anat.  menschl.  Embryonen,' 
Heft  III.,  222-226)  has  made  a  spe 
cial  comparison  which  shows  tha 
the  fundamental  morphological  rela 
tions  are  the  same  in  the  humar 
Bauchstiel  as  in  the  embryo  proper 
and  that  there  are  even  traces  of  * 
rudimentary  medullary  groove.  The 
resemblance  is  illustrated  by  the  ac 
company  ing  Fig.  204.  The  am 
nion,  Am,  arches  over  the  dorsal  side,  which  is  covered  over  bj 
thickened  ectoderm,  md,  which  His  regards  as  the  rudiment  of  the 
medullary  groove;  the  archenteron  is  represented  by  the  allantoic 
diverticulum,  All,  lined  by  the  entodermal  epithelium ;  in  the  meso 


All 

FIG.  204. — Diagrammatic  Section  of  the 
Bauchstiel  of  a  Human  Embryo,  modified 
from  W.  His.  Am,  Amnion ;  md,  medul- 
lary groove ;  V,  V,  umbilical  veins ;  A,  A, 
umbilical  arteries;  All,  allantois;  coe, 
ccelom. 


THK    IMIHLK  AL    COIII). 


357 


derm  run  the-  tw«>  laru'f  allantoic  veins.  I",  1".  and  the  two  smaller 
arteries,  A*  A;  the  space  around  the  cord  is  of  course  part  of  the 
embryonic  coelom,  coe;  tin*  MI  union  represents  the  somatopleure,  the 
walls  of  the  allantois  the  closed  splanclmopleure. 

To  convert  the  "  Kauchstiel "  into  the  umbilical  cord,  the  somato- 
pleure bends  down  on  each  side,  and  finally  closing  on  the  ventral 
side  below  the  allantois.  shutting  in  a  portion  of  the  coslom,  and 
becomes  separated  t'loni  the  amnion.  The  amnioii  separates  from 
the  embryo  first,  then  from  the  embryonic  end  of  the  Bauchstiel,  and 
last  of  all  from  the  distal  end  of  the  Bauchstiel ;  hence,  when  the 
closure  of  the  somatopleure  is  completed  the  amnion  arises  no  longer 
from  the  embryo,  but  only  from  the  end  of  the  cord,  where  it  joins 
the  chorion.  The  closure  of  the  Bauchstiel  forms  a  long  tube  run- 
ning from  the  embryo  to  the  chorion;  the  cavity  of  this  tube  is  part 
of  the  coelom ;  the  whole  tube  is  known  as  the  umbilical  cord. 

When  the  coelom  of  the  cord  is  shut  off,  it  is  shut  off  in  such  a 
way  that  the  long,  narrow  stalk  of  the  yolk-sac,  or  the  so-called 
\  itelline  duct  is  included,  compare  Fig.  222,  v.s.  This  is  possible 
owing  to  the  rolling  up  of  the  embryo,  which,  as  shown  in  Figs. 
169,  112,  175,  and  222,  brings  the  Bauchstiel  into  close  proximity 
with  the  neck  of  the  yolk-sac. 

The  development  of  the  cord  shows  that  it  is  never  covered  by 
the  amnion,  which,  on  the  contrary,  is  always  separate  from  the 
cord  proper.  This  point  is  important  to  note,  because  in  most  text- 
books the  cord  is  erroneously  described  as  covered  by  the  amnion — 
compare  Minot,  98,  :>M>. 

Development  of  the  Cord.— The  origin  of  the  cord  from  the 
P.auchstiel  has  been  described  in  the  preceding  section.  The 


Ki<;.  tfi.Y   -Srrti.ms  <>f  Human  Umbilical  Cords.     A,  Embryo  of  ;!1  mm;  15.  embryo  of  sixty- 
four  an<l  sixty-nine  'lavs;  »-,  left  umbilical  vein;  Ar,  arteries;  All,  allantois;  Coe,  crelom;  F«, 

yolk-stalk  or  vitfllin.-  duct. 

structure  and  growth  of  the  cord  may  be  best  studied  in  cross-sec- 
tions, Fig.  205.  The  coelom,  Coe,  is  a  large  cavity  and  contains 
the  yolk- stalk,  Fs,  with  its  two  vessels,  and  its  entodermic  cavity 
entirely  obliterated ;  near  the  embryo  the  coelom  may  become  much 
enlarged,  and  is  often  found  during  the  second  month,  and  even  later, 


358 


THE   FOETAL   APPENDAGES. 


to  contain  a  few  coils  of  the  intestine ;  above  the  body-cavity  is  the 
duct  of  the  allantois,  All,  lined  by  entodermal  epithelium;  and  in 
this  region  are  situated  the  two  arteries  and  single  vein ;  the  section 
is  bounded  by  ectoderm.*  The  further  development  of  the  cord  de- 
pends upon  three  factors:  1,  the  growth  of  the  connective  tissue  and 
blood-vessels ;  2,  the  abortion  of  the  coelom  yolk-stalk  and  allantois 
duct,  in  the  order  named ;  3,  differentiation  of  the  connective  tissue 
and  of  the  ectoderm. 

The  growth  and  differentiation  of  the  mesoderm  proceed  rapidly, 
encroaching  upon  the  coelom,  which  is  obliterated  (early  in  the  fourth 
month).  At  first  the  connective  tissue,  Fig.  20G,  is  composed  merely 


s 


FIG.  306.— Connective  Tissue  of  the  Umbilical  Cord  of  an  Embryo  of  21  mm. 
Stained  with  alum,  cochineal,  and  eosin. 


X  540  diameters. 


of  numerous  cells  embedded  in  a  clear  substance ;  the  cells  form  a 
complex  network,  of  which  the  filaments  and  meshes  are  extremely 
variable  in  size;  the  nuclei  are  oval,  granular,  and  do  not  have 
always  accumulations  of  protoplasm  about  them,  forming  main  cell- 
bodies,  f 

I  notice  also  a  few  cells,  which  I  suppose  to  be  leucocytes,  but  see 
no  other  structures.  By  the  end  of  the  third  month  the  cells  have 
assumed  nearly  their  definite  form ;  the  protoplasm  has  increased  in 
amount  and  forms  a  large  cell-body  around  each  nucleus,  Fig.  207. 
The  network  has  become  simpler  and  coarser,  the  meshes  bigger,  and 
the  filaments  fewer  and  thicker ;  in  the  matrix  are  numerous  connec- 

*  The  ectoderm  is  often  wanting,  owing  to  its  frequent  destruction  post  mortem. 

t  It  is  possible  that  the  reticulum  here  described  as  cellular  is,  in  part  at  least,  composed  of 
remnants  of  an  early  matrix,  which  shrinks  up  and  is  replaced  by  tne  clear  matrix  here  de- 
scribed; my  observations  do  not  settle  this  question  of  the  nature  of  the  reticulum. 


THK    t  .MT.ILK  A  I.    (  ( >RD.  :>.V.I 

tive-tissue  fibrils,  not  yet  disposed  in  bundles  except  here  and  there; 
as  thev  curl  in  all  directions  many  of  them  arc  cut  transversely,  and 
therefore  appear  as  dots.  In  older  cords  there  is  an  obvious  incr> 
ill  the  number  of  fibrils,  and  they  form  wavy  bundles.  In  tin-cord  at 
term  the  matrix  contains  mucin,  and  may  be  stained  by  alum  luema- 
to.xylin  :  at  what  period  the  reaction  is  first  developed  1  have  not  ascer- 
tained. I  have  observed  nothing  to  indicate  the  presence  of  special 
lymph-channels  in  the  cord  at  any  period,  but  1  have  not  investigated 
the  point.  Tait's  lymph-channels  are  merely  the.  intercellular  spaces. 


-.1  >7.— Connective  Tissue  of  the  Umbilical  Cord  of  a  Human  Embryo  of  about  thre*-  Mouths. 
X  511  diamcton,     Stained  with  alum,  cochineal,  and  eosin. 

The  ectoderm  is  at  first  a  single  layer  of  cells,  a  condition  which 
i>  permanent  over  the  amnion;  in  an  embryo  of  three  months  I  find 
the  two-layered  stage,  Fig.  208.  The  outer  layer  is  granular,  and  in 
>ome  parts  each  cell  protrudes  like  a  dome;*  the  inner  layer  consists 
of  larger,  clearer  cells.  By  the  fifth  month  the  stratification  of  the 
epithelium  becomes  more  evident  and  cornification  begins.  The 
ectoderm  (Ec),  therefore,  develops  like  the  epidermi  proper,  although 
much  more  slowly,  so  that  it  never  gets  beyond  the  stage  which  the 
true  epidermis  reaches  by  perhaps  the  fourth  month;  on  the  other 
hand  it  differs  entirely  from  the  amniotic  epithelium. 

*  From  the  investigations  of  r>r.   J.  T.  Bowen  on  the  development  of  the  epidermis,  which 
!   •  has  1..M-H  carrying  on  in  tin-  hist,>l<><riral  laboratory  of  the  Harvard  Medical  School,  it 
to  me  probable  that  thi^  .••aernal  layer  is  homologous  with  epitrtehiom. 


aeo 


THE    FCETAL   APPENDAGES. 


The  blood-vessels  steadily  enlarge  and  acquire  thick  muscular 
walls.  In  the  cord  of  an  embryo  of  21  mm.,  Fig.  205,  the  arterial 
muscularis  is  well  marked,  the  venous  muscularis  just  beginning 
to  show.  At  sixty-three  days  I  find  the  coat  thickened  on  all  the 
vessels ;  there  is  a  gradual  passage  from  the  muscle  cells  to  the  sur- 
rounding connective  tissue,  so  that  one  wins  the  impression  that  the 


FIG.   308.— Epithelial  Covering  of  the  Umbilical  Cord  of  an   Embryo  of  three  Months,      x  545 

diameters. 

connective-tissue  cells  are  being  directly  metamorphosed  into  mus- 
cular cells.  By  the  fifth  month  the  demarcation  of  the  muscular 
coats  is  quite  sharp,  and  it  is  probable  that  the  further  growth  of  the 
layer  depends  upon  the  growth  of  the  elements  it  already  contains 
and  not  upon  the  accretion  of  new  ones;  that  the  muscle-cells  do 
actually  become  bigger  is  easily  ascertained  by  direct  observation.* 

The  obliteration  of  the  ccelom  goes  on  rapidly  during  the  second 
and  third  months,  and  by  the  beginning  of  the  fourth  is  nearly  or 
quite  completed.  The  vitelline  duct  persists  longer,  but  seems  to 
disappear  by  the  sixth  month;  for  a  time  it  is  distinguishable  as  a 
shrunken  remnant  in  the  midst  of  the  connective-tissue  cells  of  the 
cord.  The  allantoic  duct  occupies  usually  a  position  between  the  two 
arteries;  it  attains  its  maximum  diameter  about  the  fifth  week, 
when  it  is  a  small  epithelial  tube,  Fig.  203,  of  irregular  width,  as 
which  it  remains  for  some  time  without  noticeable  alteration ;  during 
the  third  month  it  loses  this  character  and  becomes  solid,  by  the 
enlargement  of  its  epithelial  cells ;  the  duct  persists  up  to  birth  in 
this  form,  though  losing,  according  to  Kolliker,  its  complete  conti- 
nuity; after  it  becomes  solid  there  is  a  slight  condensation  of  tissue 
around  it. 

The  Human  Umbilical  Cord  at  Birth.. — The  human  cord  is 
a  long  twisted  rope  of  tissue,  whitish  in  color,  and  attached  by  one 
end  to  the  navel  of  the  embryo,  by  the  other  to  the  surface  of  the 
placenta.  Its  dimensions  are  extremely  variable  at  all  periods ;  at 
birth  it  is  usually  about  fifty-five  centimetres  long  and  twelve  milli- 
metres thick ;  it  is  said  that  cords  only  fifteen  centimetres  long  as 
one  extreme,  and  over  one  hundred  and  sixty  centimetres  long  as  the 


*  This  offers  another  example  of  the  rule  that  growth  and  cell  mutiplication  may  be  distinct 
processes.     Compare  Merk's  remarks,  "Denkschr.  Wien.  Akad.,"  liii.,  pp.  34-41,  1887. 


THE   r  \ir.ii.H   \i.  CORD, 

other  extreme,  have  been  observed.  Its  surface  is  smooth  and  glis- 
tening, except  at  tlic  constricted  fo-tal  end,  where  the  epidermis 
stivtehes  about  <>ne  centimetre  on  to  the  cord.  The  placental  end 
expands  to  fuse  \\-ith  the  chorionic  membrane.  The  placental  inser- 
tion is  generally  eccentric,  that  is,  the  cord  joins  the  placenta  at  a 
point  between  the  «•» -litre  and  margin  of  the  latter  organ  :  usually  the 
eccentricity  i<  well  marked,  and  not  infrequently  N  so  great  that 
insertion  becomes  marginal;  in  still  rarer  cases  the  cord  joins  the 
chorion  outside  the  region  of  the  placenta  (in.^-rt iu  velamenfasa). 
Occasionally  the  oord  forks  before  joining  the  chorion  (inwrtin 
fureata). 

The  twisting  of  the  cord  is  always  well  marked  externally  at  the 
time  of  birth  by  the  spiral  ridges  within  each  of  which  a  largo  blood- 
vessel runs.  1  have  observed  the  number  of  spirals  to  vary  from 
three  to  thirty -two;  the  turns,  beginning  at  the  embryo,  go  usually 
from  left  to  right,  but  sometimes  from  right  to  left.  The  cause  of 
the  twisting,  which  begins  about  the  middle  of  the  second  month, 
has  been  much  and  very  unprofitably  discussed.  Of  the  many  theo- 
ries  on  the  subject  w hid i  have  been  advanced,  there  is  not  one,  so 
far  as  1  know,  having  the  slightest  claim  to  acceptance.  These 
vagaries  have  been  collated  by  Hyrtl,  70.1,  and  also  less  fully  by 
Law -on  Tait,  76. 1,  who  adds  to  them.  All  we  can  say  is  that  the 
-els  grow  faster  in  length  than  the  cord  as  a  whole,  and  therefore 
,i— ume  the  spiral  disposition;  the  cause  of  this  inequality  is  as  com- 
pletely unknown  to  us  as  the  causes  of  all  the  other  inequalities  of 
growth  which  occur  in  the  embryo.  One  point  must  be  specially 
mentioned,  namely,  that  there  is  no  reason  to  suppose  that  the  cord 
as  a  whole  actually  twists  anymore  than  the  spiral  intestine  of  a 
shark  is  formed  by  twisting;  many  writers  have  tal>el\  a»umed  the 
occurrence  of  this  twisting  motion,  and  have  dissertated  at  no  little 
length  on  the  revolutions  of  the  embry<  >  in  nfcro.  There  is  no  evidence 
that  such  revolutions  occur,  nor  have  we  any  ground  for  assuming 
that  the  twisted  appearance  of  the  cord  is  due  to  an  actual  twisting 
like  that  of  a  rope;  if  a  long  rubber  tube  forms  a  coil  within  a  short 
gla>s  cylinder,  it  does  not  indicate  that  the  cylinder  has  been 
twisted. 

The  cord  is  covered  by  a  layer  of  epithelium  which  is  continuous 
at  the  distal  end  with  the  epithelium  of  the  amnion.  Its  interior 
consists  of  a  peculiar  embryonic  connective  tissue  known  as  Whar- 
ton's  jelly,  which  is  described  below;  in  this  jelly  are  found  at  birth 
three  large  blood-vessels,  and  usually  a  few  degenerated  remnants  of 
the  epithelium  of  the  allantois.  There  are  no  capillaries  except  close 
to  the  navel,  and,  in  spite  of  the  opinion  of  some  writers,  it  appears 
sate  to  say  that  there  are  no  lymph- vessels, *  and  no  nerves  in  the 
distal  part  of  the  cord.  Schott,  36.1,  claims  to  have  followed 
branches  of  the  hepatic  plexus  along  the  vein  three  or  four  centi- 
metres into  the  cord,  and  branches  of  the  plexus  of  the  colon  and 
uterus  an  equal  distance  along  the  arteries.  Valentin  has  found 
nerves  even  further,  8-11  ctm.  from  the  navel.  As  Kolliker  remarks 


*  Wandcrin-  cells  m-mr  in  the  intercellular  spaces  of  Wharton's  j.-lly.  and  it  is  possible  that 
there  are  lymph  ./,,//-,/•/*  in  the  matrix,  though  no  vessels.  Compare  particularly  Roster's 
paper. 


THE   FtETAI,   APPENDAGES. 

in  his  larger  text-book,  79.2,  p.  347,  the  absence  of  nerves  in  the 
distal  portion  of  the  cord  and  in  the  chorion  is  of  no  little  physiologi- 
cal interest,  since  the  blood- vessels  are  so  contractile.  In  a  cross  sec- 
tion, Fig.  209,  as  usually  obtained,  the  vessels  are  found  contracted, 

the  arteries,  A  A,  with  their  cavities 
almost  obliterated.  The  vessels  have 
thick  walls  composed  of  a  muscular  coat 
and  rudimentary  intima,  but  without 
any  special  external  connective-tissue 
layer.  The  vessels  differ  from  adult 
vessels  of  similar  calibre  in  many  re- 
spects ;  there  is  no  elastic  tissue  so  far 
as  I  have  observed  in  any  part;  the 
muscle-cells  are  short,  fusiform,  loosely 
arranged,  and  run  in  various  directions ; 
next  the  intima  the  fibres  are  longitudi- 
FIG.  209.— cross-section  of  an  Umbi-  nal  in  trend ;  in  the  rest  of  the  coat  thev 

heal  Cord  at  Term,  x  about  ixJ  dianie-  j   •      i         •  i   •    i     i  j  i  " 

ters.  F,  Remnant  of  the  aiiantois;  F,  are  grouped  in  laminae,  which  have  the 
SParte?iSesaraic  vein;^^'umbili"  fibres  obliquely  in  one  direction  or  an- 
other, thus  giving  rise  to  the  appear- 
ance of  alternating  spiral  coats,  noticed  by  Lawson  Tait,  76.1 
(p.  434  and  Plate  XIII.,  Figs.  17  and  18).  The  muscular  coat 
passes  over  without  any  sharp  demarcation  into  the  surrounding 
tissue,  known  as  Wharton's  jelly,  which  consists  of  scattered  anas- 
tomosing cells,  compare  Fig.  207,  and  a  muciparous  matrix  with 
very  numerous  connective- tissue  fibres.  The  cells  and  fibres  tend 
to  arrange  themselves  in  concentric  lines  around  the  blood-vessels 
and  parallel  to  the  surface  of  the  cord,  Fig.  209,  so  that  we  may 
speak  of  four  systems;  within  each  system  the  cells  tend  to  an 
elongated  form,  but  where  the  systems  approach  one  another  the 
cells  become  more  triangular,  as  seen  in  section,  Fig.  209,  and  show 
three  or  four  main  processes.  These  triangular  cells  form,  of  course, 
long  columns  which  are  more  or  less  distinct  from  the  tissue  encom- 
passing the  vessels;  to  these  columns  the  name  of  chordce  funicuhc 
has  been  applied  by  Hyrtl ;  they  are  said  to  have  been  noticed  by 
Woortwyck  over  a  century  ago.  The  external  covering  of  the  cord 
is  a  stratified  epithelium,  of  which  the  outer  layer  is  distinctly  cor- 
neous ;  sometimes  there  are  spaces  without  cells,  which  have  been 
regarded  as  true  lymph  stomata  (Koster  and  also  Tait) ;  the  mid- 
dle layer  is  composed  of  clear  cells,  and  the  basal  layer  of  granular 
cuboidal  cells ;  in  section  the  appearances  are  closely  comparable  to 
those  of  the  embryonic  epidermis  from  parts  where  there  are  110 
hairs,  and  at  the  time  when  the  horny  layer  begins  to  appear.  As 
there  is  no  differentiated  connective-tissue  layer  beneath  the  epi- 
thelium, the  covering  of  the  cord  is  best  described  as  embrj'onic 
skin.  According  to  current  descriptions  the  cord  is  said  to  be  cov- 
ered by  the  amnion,  but  this  is  obviously  an  error,  as  shown  by  His' 
observations  upon  the  development,  and  my  own  upon  the  histology 
of  the  cord. 

There  is  usually  to  be  seen  in  sections  of  the  cord  at  term,  accord- 
ing toKolliker,  79.2,  p.  344,  especially  in  sections  from  the  proximal 
end  and  middle  region,  a  small  group  of  epithelioid  cells,  wi+h  dis- 


THK    IMI'.ILK   Al.    <  <>RD. 

tinet  walls,  irregular  granular  contents,  and  rounded  nuclei;  around 
the  cells,  Fijj.  '.'M:;.  there  is  a  slight  condensation  of  the  connective 
tissue  to  form,  as  it  were,  an  envelope.  This  structure  has  been 
ivu-arded  by  some  writers  as  the  persistent  yolk-stalk,  as.  lor  exam- 
ple, by  Ahlfeld  (J /•(•//.  fUr  GynOk.,  VIII.,  3(53).  Kolliker,  79.2, 
]>.  :>44,  considered  it  to  be  the  remnant  of  the  allantoic  cavity — a  sup- 
position which  my  own  observations  confirm. 


CHAPTER   XVII. 

THE  PLACENTA. 

FOR  convenience  the  placenta  may  be  considered  as  an  organ  by 
itself  rather  than  as  a  derivative  of  the  chorion  and  of  the  decidua, 
which  it  must  be  considered  from  a  strictly  morphological  standpoint. 
I  give  as  full  an  account  of  the  human  placenta  as  possible. 

I.  THE  HUMAN  PLACENTA. 

Placenta  at  Full  Term. — The  human  placenta  (Mutterkuchen) 
is  a  disc  of  tissue  to  which  the  umbilical  cord  of  the  child  is  attached 
by  its  distal  end.  As  a  result  of  normal  labor  the  amnion  and  cho- 
rion, by  which  the  foetus  in  utero  is  surrounded,  are  ruptured ;  the 
child  is  then  expelled,  but  by  means  of  the  long  umbilical  cord  re- 
mains attached  to  the  uterus ;  after  an  interval  the  placenta  with 
which  the  cord  retains  its  connection  is  loosened  from  the  uterine 
wall  and  expelled ,  together  with  the  foetal  envelopes  and  portions  of 
the  decidual  membrane  (uterine  mucosa)  of  the  mother;  the  parts 
thus  thrown  off  secondarily  constitute  the  so-called  afterbirth  of 
obstetricians. 

The  placenta  at  full  term,  as  thus  obtained  by  natural  expulsion,  is 
a  moist  mass,  containing  a  great  deal  of  blood,  spongy  in  texture, 
about  seven  inches  in  diameter,  but  very  variable  in  size,  being 
roughly  proportionate  to  the  bulk  of  the  child ;  usually  oval,  some- 
times round,  but  not  infrequently  irregular  in  shape.  One  surface 
is  smooth  and  covered  by  a  pellucid  membrane  (the  amnion),  and 
reddish-gray  in  color ;  to  this  surface  the  umbilical  cord  is  attached, 
and  it  shows  the  arteries  and  veins  branching  out  irregularly  from 
the  cord  over  the  surface  of  the  placenta,  Fig.  210.  The  opposite 
surface  is  rough,  lacerated,  and  is  usually  covered  irregularly  with 
more  or  less  blood,  which  is  often  dark  and  clotted.  When  the  blood 
is  removed  the  surface  is  seen  to  be  crossed  by  a  system  of  grooves 
which  divide  the  placental  tissue  into  irregular  areas,  each  perhaps 
an  inch  or  so  in  diameter;  these  areas  are  called  cotyledons.  The 
placenta  is  about  twenty-five  or  thirty  millimetres  thick,  but  thins 
out  rapidly  at  the  edges,  and  its  tissue  passes  over  without  a  break 
into  thin  foetal  membranes,  which  accordingly  spring,  as  it  were, 
from  the  margin  of  the  placenta. 

When  in  situ  the  placenta  is  fastened  to  the  walls  of  the  uterus  by 
its  rough  or  cotyledonary  surface ;  its  smooth,  amniotic  surface  faces 
the  cavity  in  which  the  foetus  lies. 

A  more  detailed  examination  of  the  gross  appearances  of  a  placenta 
discharged  at  term  leads  to  the  following  additional  observations : 
The  color  is  a  reddish  or  purplish  gray,  varying  in  tint  according  to 
the  condition  of  the  blood,  and  mottled  between  the  divaricating 


THE    HIMAN     PLACENTA.  366 

blood-vessels  by  patches  and  networks  of  pale  yellowish  or  flesh  color. 
The  light  pattern  is  produced  by  the  tissue  of  the  villi  shining  through 
the  membrane  of  the  chorion.  These  appearances  are  less  distinct 
when  the  placenta,  as  is  usually  the  case,  is  covered  by  the  thin 
aninion.  Tin-  amnion,  however,  i-  vry  easily  detached  Bfl  tar  as 
the  insertion  of  the  umbilical  cord,  but  from  the  latter  it  cannot  be 
pulled  off.  The  blood-vessels  run  out  in  all  directions  from  the  end 
of  the  cord ;  each  vessel  produces  a  ridge  upon  the  placental  surface 


FIG.  210.— Placenta  at  full  Term,  doubly  Injected  by  Dr.  H.  P.  Quincy  to  show  the  Distribution 
of  the  Vessels  upon  the  Surface. 

so  that  its  course  is  readily  followed.  The  arteries  and  veins  are 
more  easily  distinguished  after  double  injection,  as  is  shown  in  Fig. 
210. 

The  two  kinds  of  vessels  do  not  run  together ;  the  arteries  lie  nearer 
the  surface,  the  veins  deeper ;  the  arteries  fork  repeatedly,  until  they 
are  represented  only  by  small  branches  and  fine  vessels ;  some  of  the 
small  branches  disappear  quite  suddenly  by  dipping  down  into  the 
deeper-lying  tissue  in  order  to  pass  into  the  villi.  The  veins,  Fig. 
210,  are  considerably  larger  than  the  arteries ;  they  branch  in  a  sim- 


:5<i<;  THE   FCETAL   APPEND  A  (JES. 

ilar  manner,  but  some  of  the  trunks  disappear  from  the  surface  more 
abruptly  than  is  the  case  with  the  arteries.  There  is  the  greatest 
possible  variability  in  the  vessels  of  the  placenta;  I  have  never  seen 
two  placenta  with  vessels  alike.  So  far  as  I  have  observed,  the  in- 
sertion of  the  cord  is  always  obviously  eccentric;  the  degree  of 
eccentricity  is  variable  and  is  easily  seen  to  be  related  to  the  distri- 
bution of  the  vessels. 

The  insertion  of  the  cord  may  even  be  entirely  outside  the  placenta, 
which  yet,  as  B.  S.  Schultze  has  shown,  may  otherwise  be  normally 
developed.  Such  insertions  are  called  velamentous.  The  usual  type 
is  shown  in  Fig.  210.  The  arteries  come  down  together  from  the 
cord ;  they  usually,  but  not  always,  anastomose  by  a  short  transverse 
vessel,  which  lies  about  half  an  inch  above  the  surface  of  the  pla- 
centa; it  could  not  be  shown  in  Fig.  210. 

I  have  never  noticed  any  arterial  or  venous  anastomoses  on  the 
surface  of  the  placenta.  The  arteries  there  spread  out  in  a  manner 
which  may  be  described  as  roughly  symmetrical.  The  veins  par- 
tially follows  the  course  of  the  arteries.  When  the  cord  is  inserted 
near  the  margin  the  symmetry  of  the  placental  vessels  is  greater ; 
when  the  insertion  is  nearer  the  centre  the  symmetry  is  less  than  in 
Fig.  210. 

The  reverse  or  uterine  surface  of  the  placenta  is  rough  and  divided 
into  numerous  rounded  oval  or  angular  portions  termed  lobes  or  coty- 
ledons,* as  stated  above.  These  vary  from  half  an  inch  to  an  inch 
and  a  half  in  diameter.  The  whole  of  this  surface  consists  of  a  thin, 
soft,  somewhat  leathery  investment  of  decidual  membrane,  which 
dips  down  in  various  parts  to  form  the  grooves  that  separate  the 
cotyledons  from  each  other.  This  layer  is  a  portion  of  the  decidua 
serotina,  which,  as  long  as  the  parts  are  in  situ,  constitutes  the 
boundary  between  the  placenta  and  the  muscular  substance  of  the 
uterus,  but  which  at  the  time  of  labor  becomes  split  asunder,  so  that 
while  a  portion  is  carried  off  along  with  the  placenta  and  constitutes 
its  external  membrane,  the  rest  remains  attached  to  the  inner  surface 
of  the  uterus.  If  a  placenta  is  cut  through  it  is  found  to  consist  of 
a  spongy  mass  containing  a  large  quantity  of  blood  and  bounded  by 
two  membranes,  each  less  than  a  millimetre  thick ;  the  upper  one  is 
the  chorion  covered  by  the  still  thinner  amnion,  and  greatly  thick- 
ened where  the  vessels  Ho  in  it ;  the  lower  one  is  the  decidual  tissue 
together  with  the  ends  of  the  villi  imbedded  in  it  (cf.  especially  p. 
17  and  Fig.  211) ;  it  represents  only  a  portion  of  the  decidua,  the 
other  portion  has  remained  upon  the  uterine  wall.  The  spongy  mass 
is  found  upon  examination  to  consist  of  an  immense  number  of  tufts 
of  fine  rods  of  tissue,  which  are  irregularly  cylindrical  in  shape. 
Further  examination  shows  that  they  are  twigs,  Fig.  183,  with 
rounded  ends  and  springing  from  little  branchlets,  which  in  their 
turn  arise  from  branches,  and  so  on,  until  a  large  main  stem  is 
found,  which  starts  from  the  chorion.  This  branching  system  is 
richly  supplied  with  blood  from  the  foetal  vessels  on  the  surface  of 
the  placenta.  The  villi  are  interwoven  so  that  the  twigs  of  one 
branch  are  interlaced  with  those  of  another,  and  apparently  separate 

*  The  division  of  the  placenta  into  cotyledons  is  not  primary,  but,  on  the  contrary ,  is  not  de- 
veloped until  the  fourth  or  fifth  month. 


Tin:    HI-MAN    PLACENTA. 


t\vii;>  may  grow  together  and  ^iryeflaels  anastomcMKj  but  on  this 
point  I  am  unable  to  speak  positively. 

The  villons  t  \vi\ux  n.-\t  the  surface   of  the  decidna  penetrate  that 

tissue  a  slight  distal 

Tin-  intervillous  >pa<vs  are  filled,  or  nearly  so.  with  blood:  they 
forni  a  complex  system  of  ehannels.  The  intervillous  blond,  as  we 
know  from  tlie  researches  ,.f  Farre,  Turner,  and  Waldever,  is  ma- 
lenial.  Farre  says,  in  hisarticle  in  Todd's  "  (  Velnpn-dia."  V.  Suppl., 
p.  "ir,,  in  reference  to  the  placental  I'ecidua  :  "  Numerous  valve-like 
a]»ertui-es  arr  ol»sei-\ed  upon  all  parts  of  the  surface;  they  are  the 


ft 


KK;.  -11.—  Placenta  at  Full  Term.     A,  Vertical  stvtion  through  the  margin;  D,  decidua;  vf, 

lact-iita  ;  f  '!«>.  chnrion:  si.  sinus:   !•'/,  plac»*ntal  villi;  Fib,   flbrin;  B 


ah.n-t.Ml   villi  outsidi'  th.- 
portion  of  A.  nun-.-  HIM 
(.•••lls;  <l.  with  on»-  niic 


to  sh..\v  the  il.M-i.lnal  tissue  near  b;  v<  blood -v.-sx.-i ;  </,ri',  deciduai 
with  several  nuclei. 


orifices  of  the  veins,  which  have  been  torn  off  from  the  uterus.  A 
prol>e  passed  into  any  ono  of  these,  after  taking  an  oblique  direction, 
enters  at  once  into  the  placental  substance.  Small  arteries  about 
half  an  inch  in  length  are  also  everywhere  observed  imbedded  in 
this  layer.  After  making  several  sharp  spiral  turns  they  likewise 
suddenly  open  into  the  placenta;"  and  on  p.  719  he  adds:  "These 
venous  orifices  occupy  three  situations.  The  first  and  most  numer- 
ous are  scattered  over  the  inner  side  of  the  general  layer  of  decidua 
which  constitutes  the  upper  boundary  of  the  placenta;  the  second 
form  openings  upon  the  sides  of  the  deciduai  prolongations  or 


3G8 


THE   FCETAL   APPENDAGES. 


dissepiments  which  separate  the  lobes  (cotyledons)  from  each 
other;  while  the  third  lead  directly  into  the  interrupted  channel 
in  the  margin,  termed  the  circular  sinus."  The  circular  sinus  (Fig. 
211)  is  merely  a  space  at  the  edge  of  the  placenta  which  is  left  com- 
paratively free  from  the  villi.  It  is  not  a  continuous  channel,  but  is 
interrupted  here  and  there.  Susbequent  writers  have  gone  but  little 
beyond  Farre's  account,  which  has  been  entirely  overlooked  by  most 
recent  German  investigators,  who  accordingly  announce  facts  known 
to  Farre  as  new  discoveries.  Under  these  circumstances  it  seems  no 
more  than  just  to  direct  renewed  attention  to  Farre's  masterly  article. 
To  study  the  histology  of  the  placenta  sections  are  best  made  after 
imbedding  the  organ  in  celloidin.  Fig.  211  represents  parts  of>  a 
section  of  a  placenta  at  term  from  which  the  amnion  was  removed. 
Fig.  211,  A,  represents  the  placental  margin  magnified  thirteen 
diameters ;  B,  a  portion  of  the  decidua  near  b  in  A,  but  more  highly 
magnified.  The  chorion,  Cho,  and  decidua,  .D,  are  in  immediate 
contact  at  the  left  of  the  figure,  that  is,  outside  the  placenta,  though 
remnants  of  a  few  aborted  villi,  vi,  are  still  plainly  recognizable; 
but  they  are  found  only  close  about  the  placenta.  At  the  margin  of 
the  placenta  and  in  its  neighborhood  the  chorion  and  decidua  are  not 
clearly  delimited,  but,  on  the  contrary,  the  decidual  cells  find  here 
an  opportunity  to  penetrate  the  chorionic  membrane.  The  placental 
chorion  exhibits  its  characteristic  stratification  a  short  distance 
within  the  margin.  I  have  round,  however,  that  the  distinctness  of 
that  stratification  varies  considerably,  not  only  in  different  placenta, 
but  also  in  different  parts  of  the  same  placenta.  The  decidua,  D, 

outsidethe  placenta  is 

I.       «•          I-      .A       -j  very  thick,  but  at  the 

edge  it  begins  to  thin 
out,  and,  as  it  passes 
over  the  under  side 
of  the  placenta,  rap- 
idly becomes  so  much 
reduced  as  to  be  even 
less  in  thickness  than 
the  chorion,  Cho. 
The  decidua  is  char- 
acterized by  an  im- 
mense number  of 
large  decidual  cells, 
not  scattered  about 
as  in  Fig.  10,  but 
densely  packed.  Fig. 
211,  B,  the  cells  are 
irregularly  oval  in 
outline,  clear,  or 
somewhat  granular, 
and  have  usually  a  single  nucleus ;  a  few  are  larger,  more  granular 
and  multinucleate. 

At  the  edge  of  the  placenta  the  chorion  and  decidua  separate; 
where  they  first  part  there  are  very  few  villi,  Fig.  211,  Si,  but  else- 
where the  room  between  them  is  occupied  by  innumerable  branches  of 


FIG.  212.—  Mesenchymal  Tissue  of  a  Villus.  from  a  Placenta  of 
four  Months.  II,  Leucocytes;  vv,  capillary  blood-vessels;  d, 
finer  mesh -work  from  near  a  capillary. 


THE    HIM  AX    PLACENTA. 

villi,  TV,  TV,  with  narrow  spaces  between  for  the  blood;  the  sections 
of  the  villi  are  of  all  sizes  and  shapes;  they  all  contain  blood-vessels, 
but  only  the  larger  ones  can  be  distinguished  with  the  magnification 
of  Fig.  211,  A,  wheiv  they  have  been  made  as  distinct  as  possible  by 
being  drawn  black.  The  spaces  between  the  villi  have  been  left 
white,  tlu»  blood  which  partially  filled  them  not  being  represented. 

Placenta  in  Situ. — The  placenta  in  its  natural  position  in  the 
uterus  follows  the  curvature  of  the  uterine  walls,  hence  its  free  or 
amniotic  surface  is  slightly  concave,  its  decidual  surface  is  strongly 
convex ;  it  is  thickest  in  its  centre  and  thins  out  gradually  toward 
its  edge.  There  is  no  definite  boundary  between  the  portion  of  the 
decidua  serotina  which  is  to  be  torn  off  with  the  placenta,  and  the 
part  which  is  to  remain  in  the  uterus  after  delivery. 

Vertical  sections  through  the  uterus  with  the  placenta  in  place  are 
v*  TV  instructive.  Fig.  213  represents  such  a  section  through  a  pla- 
centa of  about  seven  months.  The  thin  amnion,  Am,  clothes  the 
inner  surface  of  the  chorionic  membrane,  Cho;  this  membrane  is 
separated  from  the  decidua,  Z>,  by  a  dense  forest  of  villi;  in  the 
younger  specimens  the  distance  between  the  chorion  is  considerably 
less  than  the  thickness  of  the  uterine  wall,  D,  3/c,  but  in  the  pres- 
ent specimen,  Fig.  213,  it  is  much  greater;  in  younger  stages  the 
villi  are  much  less  numerous,  and  much  smaller  than  in  the  older 
one ;  these  differences  correspond  to  the  growth  of  the  placenta  and 
to  the  changes  in  shape  of  the  chorionic  villi  already  described,  p. 
:>!!».  The  ends  of  some  of  the  villi  touch  and  are  imbedded  in  the 
decidual  tissue ;  these  imbedded  ends  are  without  covering  epithe- 
lium,  but  their  connective  tissue  is  immediately  surrounded  by  hyaline 
substance,  which  is  probably  the  degenerated  epithelium.  The  de- 
cidua is  plainly  divided  into  an  upper  compact,  and  a  lower  cavernous 
layer,  see  p.  8.  The  section  passes  through  a  wide  arterial  ves- 
sel, Ve. 

Foetal  Circulation  of  the  Placenta. — The  following  para- 
graph refers  to  the  placenta  during  the  later  months  of  pregnancy; 
it  is  copied  almost  without  change  from  my  article  on  the  placenta  in 
Buck's  "Ref.  Handb.  Med.  Sci.,"  V.,  696-697. 

To  follow  the  course  of  the  fretal  blood-vessels  within  the  placenta, 
the  best  method  is  by  corrosion  injections.  These  may  be  made 
either  with  fusible  metal,  wax,  or  celloidin.  The  first  is  specially 
suited  for  the  study  of  the  large  trunks ;  the  latter  for  that  of  the 
smaller  vessels  also.  I  have  a  very  beautiful  celloidin  injection  by 
Dr.  S.  J.  Mixter,  which,  with  others  of  wax  and  metals,  has  served 
as  the  basis  of  the  following  description :  The  veins  leave  the  surface 
somewhat  more  abruptly  than  do  the  arteries,  which  gives  off  more 
small  branches  to  the  surface  than  do  the  veins,  Fig.  210.  Both 
kinds  of  vessels  leave  the  surface  by  curving  downward  for  a  short 
distance  into  the  trunk  of  a  villus;  the  vessels  then  divide,  and 
their  branches  again  take  a  more  horizontal  course;  the  branches 
then  curl  over  downward,  and  after  a  second  short  descent  toward 
the  decidua,  again  send  out  horizontal  branches.  The  result  of  this 
arrangement  is  a  terrace-like  appearance  in  the  course  of  the  vessels ; 
they  approach  the  uterine  side  of  the  placenta  in  this  very  character- 
istic manner.  The  number  of  terraces  is  variable ;  usually  there  are 
24 


370 


THE    FCETAL    APPENDAGES. 


;  * .  -,  --.^  •    -  v^Ttr-.       •= 

^^mj^m  jo 


FIG.  213.— Section  through  a  normal  Placenta  of  about  seven  Months,  in  situ.  Am,  Amnion; 
CTio,  chorion;  Vi,  villus  trunk;  vi,  sections  of  villi  in  the  substance  of  the  placenta;  D,  decidua: 
Me,  musculans;  D',  compact  layer  of  decidua;  Ve,  uterine  blood-vessels  Cor  gland?)  opening 
into  the  placenta.  The  foetal  blood-vessels  are  drawn  black ;  the  maternal  blood  spaces  are  left 
white;  the  chorionic  tissue  is  stippled  except  the  canalized  fibrin,  which  is  shaded  by  lines;  the 
remnants  of  the  gland  cavities  in  D"  are  stippled  dark.  (Drawn  from  nature  by  J  H.  Emerton. ) 


THE    HI"  MAX    PLACEXTA. 


871 


t\v<>  or  three,  but  sometimes  there  is  only  one,  or  they  may  number 
four  or  even  five..  Arrived  at  the  end  of  its  terraces  the  main  vessel 
takes  a  more  nearly  perpendicular  course,  and  rapidly  subdivides. 
I  inn icd lately  after  entering  the  villi,  the  arteries  and  veins  give  off 
but  few  capillaries,  but  after  a  short  course  in  the  main  stalk  of  the 
villus  the  vessel*  give  rise  to  many  branchlets,  and  gradually  the 
character  of  the  circulation  changes  until  in  the  smallest  villous 
twigs  there  are  capillaries  only,  Fig.  214.  The  vascular  trunks  have 
a  marked  tendency  to  dichotomous  divi- 
sion which  is  maintained  within  the  villi 
to  a  certain  extent ;  the  arterioles  and  vein- 
let  s  iii  the  mature  placenta  go  from  their 
trunks  at  wide  angles  for  the  most  part, 
and  subdivide  in  the  same  manner,  so  that 
they  spread  out  through  the  whole  sub- 
stance of  the  placenta.  The  vessels  next 


FIG.    214.—  Portion  of   an    injected  Villus  from  FIG.  215.— Placenta  of  about  five 

a  Placenta  of  about  five   Months;    magnified  210  Months;  Portion  of  a  small  Villus  to 

show   the  Central  Vessels  and  Su- 
perficial Capillaries,     x  105  diams. 

the  decidua  take  a  more  horizontal  trend,  like  the  top  branches  of  a 
wind-swept  tree.  As  the  vessels  run  in  the  villi,  of  course  the  way 
in  which  the  latter  branch  out  determine  the  paths  of  the  former; 
hence,  by  following  the  distribution  of  the  vessels  we  inform  ourselves 
as  to  the  ramifications  of  the  villi.  Thus  the  horizontal  course  of 
the  vessels  on  the  uterine  side  of  the  placenta  corresponds  to  the  well- 
known  fact  that  the  ends  of  the  villi  attached  to  the  uterus  become 
bent  and  adhere  by  their  sides  to  the  decidual  surface. 

The  capillaries  of  the  villi  are  remarkable  for  their  large  size,  and 
on  this  account  have  been  described  as  arteries  or  veins  by  E.  H. 
Weber,  Goodsir,  and  other  writers.  Their  calibre  is  often  sufficient 
for  from  four  to  six  blood-discs  abreast.  They  are  very  variable  in 
diameter,  and  also  peculiar  in  exhibiting  sudden  constrictions  and 
dilatations,  Fig.  214.  In  the  short  bud-like  branches  there  is  often 
only  a  single  capillary  loop,  but  as  the  branch  becomes  larger  the 


372  THE    FCETAL   APPENDAGES. 

number  of  loops  increases,  and  they  form  anastomoses.  In  branches 
large  enough  to  serve  as  a  stem,  some  one  or  two  of  the  vessels  may 
be  enlarged,  as  may  be  seen  in  Fig.  214 ;  in  the  branches  large  enough 
to  admit  of  it,  there  are  two  (or  sometimes  only  one)  longitudinal 
central  vessels,  an  artery  and  vein,  and  a  superficial  network  of 
capillaries,  Fig.  215.  Goodsir  and  other  early  writers  laid  great 
stress  on  the  formation  of  the  capillary  loops,  but  this  feature  is  a 
common  one  in  the  development  of  the  foetal  vascular  system,  as  is 
also  the  width  of  the  capillaries.  In  my  opinion  these  peculiarities 
are  characteristic  rather  of  the  foetus  than  specifically  of  the  placenta. 
In  some  of  the  older  writers  (Goodsir,  Farre,  et  al.)  it  is  asserted 
that  the  true  capillary  systems  disappear  toward  the  end  of  gesta- 
tion. I  am  unable  to  confirm  this,  but  find  instead  that  in  the  slen- 
der terminal  villi  of  the  placenta  at  term  there  is  often  only  a  single, 
sometimes  long,  capillary  loop ;  the  capillary  is  very  wide,  and  its 
width  is  probably  the  reason  of  its  having  been  held  formerly  to  be 
a  vein  or  an  artery. 

Maternal  Circulation  of  the  Placenta. — The  course  of  the 
maternal  blood  in  the  placenta  has  been  the  subject  of  nearly  con- 
stant debate  for  a  century  past,  and  the  problem  has  received  its 
final  answer  only  within  the  last  few  years.  The  discovery  of 
the  facts  belongs  to  so  many  authors  that  it  seems  not  worth  while 
to  attempt  to  cite  the  authorities  for  each  detail,  accordingly  I  give 
a  summary  of  what  is  known,  and  in  an  historical  note  refer  to  the 
principal  investigations. 

The  arteries  and  veins  both  open  upon  the  surface  of  the  decidual 
serotina,  at  least  during  the  later  half  of  pregnancy ;  concerning  the 
circulation  during  the  first  half  of  pregnancy  we  possess  no  positive 
information,  although  the  fundamental  arrangements  are  presuma- 
bly the  same.  The  blood,  which  is  poured  out  from  the  arteries, 
circulates  in  the  intervillous  spaces,  which  act  as  maternal  blood 
channels. 

Both  arteries  and  veins  change  the  character  of  their  walls  as 
they  approach  the  surface  of  the  decidua ;  when  they  enter  the  decidua 
they  are  nearly  or  quite  without  muscular  walls,  and  can,  therefore, 
be  recognized  as  arteries  or  veins,  not  by  their  histological  structure, 
but  only  by  their  size  and  their  continuity  with  undoubted  arteries 
and  veins  in  the  muscularis ;  during  their  passage  through  the  de- 
cidua their  walls  gradually  become  reduced  to  the  endothelial  layer ; 
but  the  arteries  have,  what  the  veins  do  not  have,  a  thin  clear  layer 
just  outside  the  endothelium ;  this  layer  colors  readily  with  carmine, 
contains  a  few  scattered  nuclei,  and  is  probably  the  result  of  degen- 
eration; it  ceases  before  the  artery  actually  reaches  the  surface. 
The  endothelial  nuclei  of  the  veins  project  distinctly  into  the  lumen 
of  the  vessel.  Waldeyer,  90.1,  33,  summarizes  the  differences  be- 
tween the  arteries  and  veins  as  follows:  The  arteries  are  smaller; 
they  take  a  spiral  course  and  run  within  special  columns  of  fibrous 
connective  tissue ;  they  make  numerous  turns  within  the  decidua, 
and  lie  in  the  broad  ridges  of  the  membrane ;  they  usually  do  not 
branch  but  terminate  with  a  single  opening,  which  generally  lies  in 
the  upper  or  lateral  part  of  a  decidual  ridge ;  the  opening  is  narrow 
and  the  villi  do  not  project  into  it  at  all  or  but  slightly ;  the  terminal 


THE    HUMAN   PLACENTA.  B78 

piece  of  the  artery  is  round  in  cross-section;  the  art  cry  in  the  decidua 
has  a  special  layer  outside  the  endothelium,  to  within  a  short  dis- 
tance of  the  opening.  The  veins  are,  generally  speaking,  wider; 
they  have  no  special  sheaths,  and  do  not  run  in  spiral,  but  in  direct 
courses,  more  or  less  parallel  to  the  surface ;  their  openings  lie,  for 
the  most  part,  between  the  ridges  (septa)  and  never  at  the  summits 
of  the  ridges;  from  the  border  vein  (GHrensvene,  Waldeyer)  run  out 
terminal  branches  which  open  on  the  surface  and  are  usually  numer- 
ous ;  the  chorionic  villi  project  into  the  mouths  of  the  veins  and 
reach  down  even  into  the  "  Grenzvenen ;"  the  mouths  of  the  veins 
an-  irregularly  shaped,  and  the  veins  themselves  are  irregular  in 
cross-section,  never  circular.  The  position  of  the  vascular  openings 
i-  such  that  the  arterial  blood  flows  out  from  the  septa,  while  the 
venous  blood  flows  off  through  the  surface  between  the  septa;  hence, 
as  pointed  out  by  Buinin,  90.1,  each  cotyledon  represents  a  more 
or  less  distinct  circulatory  region,  the  blood  entering  at  the  sides 
and  leaving  at  the  bottom. 

Historical  A'o/f?. — The  long  prevalent  erroneous  view  that  there 
is  a  direct  communication  between  the  maternal  and  foetal  circula- 
tions originated  I  believe  with  Haller  ("Elementa  Physiologic," 
VIII.).  It  was  revived  again  by  Flourens,  36.1,  and  though  long 
since  entirely  disproved  is  still  encountered  from  time  to  time.  The 
first  important  evidence  of  the  circulation  of  the  blood  in  the  inter- 
villous  spaces  was  brought  by  F«.  H.  Weber,  whose  investigations 
were  published  in  Hildebrandt's  "  Handbuch  der  Anatomie  des  Men- 
schen,"  4te  Auflage,  IV.,  490.  Weber's  doctrine  was  adopted  by 
most  subsequent  investigators.  The  most  important  additions  to  his 
observations  were  made  by  Farre,  58.1,  and  Turner,  73.1,  76.1, 
76.3,  77. 1,  77.2,  88. 1,  until  we  come  to  the  recent  researches  of 
Lanffhans,  77.1,  82.1,  and  his  pupils,  Nitabuch,  87.1,  Rohr, 
89.1,  etc.;  of  Waldeyer,  87.1,  90. 1,  of  Bumm,  90.1,  Minot,  98, 
Bloch,  89.1,  and  others,  which  have  finally  settled  the  problem. 
That  the  intervillous  spaces  normally  contain  blood  was  seriously 
questioned  by  Braxton  Hicks,  72.1,  whose  doubts  were  again 
brought  prominently  forward  by  C.  Ruge,  86.1.  Ruge's  position 
I  was  inclined  at  first  to  adopt  (see  Minot,  Anat.  Anzeiger,  II., 
19),  but  I  have  since  become  entirely  convinced  of  the  correctness  of 
Weber's  doctrine  as  established  by  Langhans,  Waldeyer,  etc.  A 
thorough  and  very  valuable  critical  review  of  the  whole  subject  is 
given  by  Waldeyer,  90.1,  upon  whose  citations  this  note  is  based, 
but  I  have  referred  only  to  a  few  of  the  numerous  authorities  quoted 
by  AValdeyer. 

*  Nutrition  of  the  Foetus. — The  mechanism  of  the  transfer  of 
nourishment  from  the  uterus  to  the  child  is  not  well  understood.  It 
is  evident  that  the  supply  must  come  from  the  maternal  blood  and 
reach  the  foetus  through  the  veins  of  the  umbilical  cord ;  although 
the  amniotic  fluid  may  be  a  source  of  supply,  as  some  have  main- 
tained, yet  at  most  its  role  can  be  only  secondary  and  the  main 
transfer  of  material  must  take  place  through  the  placenta.  Our  pres- 
ent knowledge  of  the  structure  of  the  organ  renders  it  unnecessary 
to  discuss  the  old  theory  recently  revived  by  Currie,  of  a  direct  com- 
munication between  the  maternal  and  foetal  vessels,  for  we  know 


374  THE   FCETAL   APPENDAGES. 

positively  that  no  such  communication  exists.  This  theory  has  been 
put  forward  again  with  the  modification  that  the  vascular  walls  will 
let  small  solid  particles  through.  Thus  Koubassoff,  on  the  basis  of 
some  inconclusive  experiments,  sought  to  maintain  that  microbes, 
and  ergo  other  solid  particles,  could  pass  from  mother  to  embryo 
(see  Comptes  Rendus  Acad.  Paris,  t.  CL,  508-510).  More  care- 
ful tests  by  Marie  Miropolsky  failed  to  confirm  this  (Arch,  de 
Physiol.,  n.  etp.,  1885,  101-108).  A  second  theory,  at  pres- 
ent the  best  accredited,  is  that  of  diffusion,  which  finds  its  chief 
basis  in  the  elaborate  arrangements  found  in  all  placental  types  for 
bringing  the  foetal  and  maternal  blood  into  immediate  proximity. 
A  third  theory  is  that  Rauber,  79,  who  attributes  the  chief  role  in 
the  nutrition  of  the  embryo  to  the  immigration,  by  way  of  the  pla- 
centa, of  maternal  leucocytes.  A  fourth  theory  attributes  an  active 
part  to  the  utricular  glands,  which  are  supposed  to  pour  out  a  nu- 
trient secretion  into  the  intervillous  spaces,  where  it  is  taken  up  by 
the  chorionic  villi.  It  is  impossible  at  present  to  form  a  final  judg- 
ment upon  these  theories.  As  we  have  seen,  the  intervillous  spaces 
are  probably  maternal  blood-channels  at  all  periods,  so  that,  from  a 
very  early  stage  on,  the  conditions  for  the  transfer  of  material,  either 
by  a  migration  of  leucocytes  or  by  simple  transfusion,  are  established. 
Rauber's  leucocyte  theory  has  not  commended  itself  to  me,  and  I 
incline  to  accept  the  transfusion  theory.  That  the  uterine  milk 
exists  in  man  has  not  been  proven,  and  the  occurrence  of  such  a 
secretion  is  not  compatible  with  the  degeneration  of  the  glandular 
epithelium  observed  by  Minot,  see  p.  10. 

II.  THEORY  OF  THE  PLACENTA.* 

Attachment  of  the  Embryo. — That  the  rabbit  embryo  is  at- 
tached to  the  surface  of  the  uterus  by  a  thickened  region  (area  pla- 
centalis)  of  the  ectoderm  of  the  germinative  area  was  first  shown  by 
Van  Beneden  and  Julin,  84. 1 ;  this  discovery  has  since  been  con- 
firmed by  Minot,  98,  Masius,  89.1,  Duval,  89.1,  and  others.  That 
a  similar  method  of  attachment  exists  in  other  mammals  has  been 
shown  by  Strahl,  89. 1,  4,  90. 1 ;  in  the  dog  it  has  been  recorded  by 
G.  Heinricius,  89.1.  In  all  these  cases  the  thickened  ectoderm  is 
found  to  be  closely  adherent  to  the  uterine  surface,  upon  which  it  is 
apt  to  remain  when  the  ovum  is  forcibly  removed ;  it  fits  exactly  to 
the  surface  of  the  maternal  epithelium ;  there  is  no  visible  layer  of 
cement,  and  we  do  not  know  how  the  adherence  is  made  so  close. 

It  is  probable  that  we  have  here  the  primitive  form  of  attachment, 
and  that  therefore  the  evolution  of  the  placenta  began  with  the  dif- 
ferentiation of  the  ectoderm  of  the  area  placentalis. 

There  is  another  type  of  attachment  found  in  the  hedgehog  and 
in  rodent  ova  with  inversion  of  the  germ-layers,  characterized  by 
the  ovum  being  so  closely  invested  by  the  uterine  mucosa  that  the 
whole  surface  of  the  ovum  comes  in  contact  with  the  maternal  tis- 
ues  (see  E.  Selenka,  84.1,  and  Hubrecht's  superb  monograph  on  the 
placenta  of  the  hedgehog,  89. 1). 

*  Reference  is  made  especially  to  the  true  chorionic  placenta. 


THKnliY    OF    THE    PLA(  KM  A. 

Degeneration  of  Uterine  Tissues. — Over  the  region  of  the 
placental  attachment,  which  varies  in  different  animals  as  to  position, 
thriv  occurs  an  extensive  degeneration  of  the  tissues  of  the  uterine 
mucosa,  affecting  both  the  covering  epithelium,  the  glands,  and  the 
connective  tissue.  The  degeneration  takes  place  most  rapidly  in  the 
epithelium  and  glands, while  the  connective  tissue  undergoes  an  exten- 
sive hypertrophic  metamorphosis,  usually  in  the  form  of  the  develop- 
ment of  decidual  cells,  before  the  degenerative  change  acquires  the 
upper  hand.  The  nature  and  extent  of  the  degenerative  changes  have 
hecoiue  known  for  various  types  by  investigations  published  since 
isss.  several  of  which  appeared  during  lsv  iMinot.  89,  98,  Masius, 
89.1,  Heinricius,  89.1,  Duval,  89.1,  Hubrecht,  89.1,  Strahl, 
89.1,  4,  etc.),  and  represent  simultaneous  and  independent  re- 
searches. In  view  of  what  we  now  know  it  must  be  considered 
probable  that  in  all  placental  mammals,  or  at  least  in  the  orders  of  the 
unguiculate  series,  the  uterine  degeneration  is  an  invariable  factor  in 
the  development  of  the  placenta. 

The  form  of  degeneration  is  not  fixed,  but  varies  greatly.  This  is 
illustrated  by  tne  history  of  the  decidua  in  man  and  in  the  rabliit. 
Other  modifications  occur  in  the  dog,  the  hedgehog,  the  mole,  and 
doubtless  in  other  animals. 

The  result  of  the  degeneration  is:  first,  to  bring  the  chorionic 
ectoderm  of  the  embryo  into  direct  contact  with  the  connective  tissue 
of  the  mucosa  uteri  in  consequence  of  the  degeneration  and  resorp- 
tion  of  the  epithelium,  including  the  glands;  secon<L  to  all«»\\-  the 
maternal  vessels  by  simple  expansion  to  come  into  contact  with  the 
foetal  chorion.  In  the  rodents  the  degeneration  goes  so  far  in  the 
neighborhood  of  the  chorion  that  all  (or  nearly  all)  the  maternal  tis- 
sue disappears,  leaving  the  maternal  blood  to  bathe  the  surface  of 
the  chorion,  or,  to  speak  more  exactly,  of  the  chorionic  villi.  It  is 
probable  that  similar  changes  take  place  in  man,  but  in  the  earliest 
stages  yet  studied  they  appear  to  have  been  already  completed,  so 
that  in  the  region  of  the  villi  the  maternal  tissues  have  completely 
disappeared,  unless  the  endothelial  layer  described  by  Keibel  be  ma- 
ternal, V.S.,  P-  322.  Heinricius  has  maintained  that  in  the  dog  part 
of  the  glandular  epithelium  remains. 

Outgrowth  of  Chorionic  Villi.— These  are  restricted  at  first 
to  the  small  placental  area,  but  as  that  area  may  itself  grow  and 
take  up  more  and  more  of  the  chorion,  we  get  various  modifications 
of  the  villous  area.  The  more  primitive  types  preserve  the  discoidal 
plan,  illustrated  by  the  rabbit ;  in  other  cases  the  placental  or  villous 
area  expands  until  it  forms  a  belt  or  zone  around  the  ovum  (carniv- 
ora) ;  but  the  development  in  the  dog  shows  that  the  discoidal  form 
is  the  earlier,  and  changes  into  the  zonary ;  in  man  the  placental  area 
spreads  over  the  whole  chorion. 

The  villi  appear  to  arise  as  outgrowths  of  the  ectoderm  only ;  after 
the  outgrowths  have  attained  a  certain  size  the  mesoderm  of  the 
chorion  grows  into  them.  The  villi  grow  into  the  maternal  tissues, 
and  acquire  great  length  and  numerous  branches,  by  which  their 
form  becomes  extremely  complicated.  Their  form  is  highly  char- 
acteristic of  the  various  orders ;  it  is  known  exactly  only  in  man,  but 
is  certainly  very  different  in  various  animals. 


376  THE   FCETAL   APPENDAGES. 

The  villi  occupy  only  a  part  of  the  mucosa,  there  being  always  a 
considerable  layer  of  decidual  membrane  left  between  the  end  of  the 
villi  and  the  muscular  is. 

The  villi,  as  here  described,  consist  of  a  core  of  mesoderm  covered 
by  foetal  ectoderm,  and  are  essentially  different  from  the  ectodermal 
outgrowths  assumed  by  Duval  *  to  exist  in  the  rabbit. 

Union  of  the  Allantois  with,  the  Chorion. — We  know  two 
principal  modifications  of  the  union  of  the  allantois  with  the  cho- 
rion :  1.  The  allantois  joins  the  chorion  early,  and  serves  as  the  stalk, 
connecting  the  embryo  with  chorion ;  in  this  type  the  allantois  brings 
the  blood-vessels  to  the  chorion  and  the  vessels  then  ramify  over  the 
chorion  itself,  which  has  therefore  its  own  circulation,  though  de- 
pendent upon  the  allantois ;  this  modiiication  is  characteristic  of  the 
unguiculate  series  of  mammals.  2.  The  allantois  grows  out  into  a 
large  vesicle,  which  has  for  some  time  no  connection  with  the  chorion 
but  maintains  a  well-developed  circulation  of  its  own ;  its  expansion 
brings  it  ultimately  into  contact  with  the  chorion,  and  its  outer  or 
mesodermic  layer  grows  together  with  the  inner  or  mesodermic  layer 
of  the  chorion  (Bonnet,  89. 1)  which  thus  becomes  indirectly  vascu- 
Jarized ;  this  modification  is  characteristic  of  the  ungulate  series  of 
mammals.  How  far  other  modifications,  distinct  from  these,  may 
exist,  we  cannot  say  at  present. 

We  have  then  two  types:  1,  the  chorion  has  its  own  vessels  (un- 
guiculates) ;  2,  the  chorion  acquires  vessels  by  growing  together  with 
the  vascular  walls  of  the  allantoic  vesicle  (ungulates) . 

In  both  cases  the  chorion  is  the  part  of  the  foetus  and  the  only 
part  in  direct  contact  with  the  uterine  wall,  and  therefore  in  both 
cases  it  is  the  essential  part  of  the  foetal  placenta.  In  unguiculates 
the  chorion,  after  it  receives  its  blood-vessels,  has  its  own  blood  sup- 
ply and  circulation,  and  therefore  suffices  to  develop  the  placenta. 
In  ungulates  the  circulation  is  entirely  allantoic,  and  the  walls  of 
the  allantois  are  essential  to  maintain  the  foetal  circulation  of  the 
placenta ;  the  chorion,  therefore,  does  not  suffice  to  develop  the  f ce- 
tal  placenta.  While  we  recognize  that  the  chorion  is  always  the 
means  of  union  between  the  mother  and  the  offspring,  we  may  con- 
veniently distinguish  the  unguiculate  type  as  having  a  true  cho- 
rionic  placenta,  and  the  ungulate  type  as  having  an  allantoic  pla- 
centa. 

Evolution  of  the  Placenta. — As  regards  the  evolution  of  the 
placenta,  our  conceptions  are  still  very  obscure.  The  opinion  was 
long,  and  perhaps  still  is,  generally  prevalent  that  the  placenta  is 
primarily  an  organ  of  the  allantois.  This  notion  was  one  of  those 
theories  which  sometimes  become  current  without  ever  having  been 
supported  by  adequate  proof,  and  are  repeated  until  tradition  has 
rendered  them  venerable  and  age  gives  them  a  dignity  their  worth  does 
not  entitle  them  to.  The  principal  support  of  this  theory  was  de- 
rived from  the  fact  that  the  allantois  is  connected  with  the  placental 
circulation.  Balfour  in  1881  ("Works,"  I.,  743)  sought  to  modify 
this  view  by  attributing  importance  to  the  relations  of  the  yolk-sac, 
which  he  believed  to  be  the  means  of  maintaining  the  circulation. 

*  Erroneously,  as  I  believe. 


THEORY    OF   THE   PLACENTA. 

In  his  "Comparative  Embryology,"  II.,  240,  Balfour  reprints  most 
of  the  article  cited.  Minot,  98,  43:>,  laid  stress  upon  the  role  of  the 
chorion  and  upon  the  fact  that  the  placenta  is  necessarily  al ways  a 
product  of  the  chorion,  and  further  upon  the  fact  that  the  allantois 
in  man  is  permanently  (and  in  the  rabhit  primarily)  merely  a  stalk  of 
connection  between  the  embryo  and  the  chorion.  The  invest  i nations 
mentioned  in  this  chapter  which  have  been  recently  published  seem 
t<  >  me  to  greatly  strengthen  my  view.  It  is  by  the  chorion  that  the 
ovum  is  attached,  except  in  certain  rodents  in  which  the  development 
has  obviously  been  modified.  It  is  from  the  chorion  that  the  foatal 
villi  grow  out.  On  the  other  hand,  it  is  evident  that  the  yolk-sac  is 
pri  mi  lively  a  product  of  the  splanchnopleure  and  distinct  from  the 
-oimitopleuric  chorion;  the  failure  of  the  mesoderm  and  ccelom  to 
spread  completely  over  the  yolk  (entoderm  of  the  blastodermic  vesicle) 
in  certain  mammals  does  not  alter  the  fundamental  relations.  It  is 
true  that  in  certain  marsupials  the  chorion  is  very  imperfectly  sep- 
arated from  the  yolk-sac,  but  it  does  not  appear  that  this  represents 
an  ancestral  stage  of  the  mammalia ;  on  the  contrary,  it  is  probably 
a  purely  marsupial  modification.  I  am  therefore  unable  to  recog- 
nize any  reason  for  connecting  the  evolution  of  the  placenta  with  the 
yolk-sac  or  vitelline  circulation.  The  role  of  the  allantois  is  second- 
ary ;  it  serves  as  a  medium  of  blood  supply,  either,  as  we  have  seen, 
as  a  carrier  of  vascular  trunks  to  supply  the  circulation  of  the  cho- 
rion (unguiculates)  or  bringing  its  own  circulation  into  play  by 
growing  together  with  a  non- vascular  chorion. 

The  question  remains  whether  the  unguiculate  or  the  ungulate 
typo  of  placenta  is  to  be  regarded  as  the  more  primitive.  At  first 
thought  the  resemblance  of  the  foetal  envelopes  of  ungulates  to  those 
of  Sauropsida  leads  us  to  conclude  that  the  allantoic  placenta  must  be 
the  more  primitive ;  the  resemblance  referred  to  consists  in  the  early 
complete  separation  of  the  chorion  (serosa)  from  the  other  parts  and 
in  the  development  of  the  allantois  as  a  large  free  vesicle.  But  the 
ungulates  dre  highly  modified  mammals  riot  related  closely  to  the 
lower  placentalia,  while  the  unguiculates  do  merge  into  a  generalized 
mammalian  type.  When  we  consider  further  that  the  lower  un- 
guiculates show  the  typical  chorionic  placenta  in  its  full  perfection, 
the  conclusion  is  unavoidable  that  this  is  the  nearer  type  to  the 
ancestral.  In  fact,  the  placenta  appears  in  animals  with  the  chori- 
onic type  of  the  organ  before  the  allantois  becomes  free,  and  the 
great  size  of  the  allantoic  blood-vessels  is  connected  primitively,  not 
with  the  allantois,  but  with  the  already  important  chorionic  circula- 
tion; the  placenta  is  here  interpolated  in  the  ontogeny  before  the 
specialization  of  the  allantois,  which  functions  as  the  vascular  path- 
way between  the  chorion  and  embryo,  both  primitively  and  perma- 
nently. The  enlargement  of  the  allantois  in  ungulate  mammals  is 
a  supervening  change,  effected  perhaps  by  an  atavistic  recurrence  to 
reptilian  ontogeny. 

Ryder,  87.6,  has  advanced  the  theory  that  the  zonary  placenta  is 
older  than  the  discoidal,  but  Minot,  98,  434,  has  shown  that  this 
view  is  untenable. 

The  degenerative  changes  in  the  uterus  occur,  so  far  at  present 
known,  only  in  connection  with  the  chorionic  placenta ;  in  the  un- 


378  THE   FCETAL   APPENDAGES. 

gulates  the  uterine  mucosa  is  modified  in  structure  in  connection 
with  the  development  of  the  placenta,  but  the  modifications  are  not 
known  to  be  degenerative;  hence  in  the  allantoic  placenta  the  ma- 
ternal blood  flows  in  maternal  blood-vessels,  and  it  is  always  sepa- 
rated by  maternal  connective  tissue  and  epithelium  from  the  chorion. 

Theory  of  the  Placenta. — According  to  the  views  explained 
in  the  preceding  pages,  I  hold  the  placenta  to  be  an  organ  of  the 
chorion ;  that  primitively  the  chorion  had  its  own  circulation,  and 
formed  the  discoidal  placenta  by  developing  villi  which  grew  down 
into  the  degenerating  uterine  mucosa ;  by  the  degeneration  of  the 
maternal  tissues  the  maternal  blood  is  brought  closer  to  the  villi,  and 
the  degeneration  may  go  so  far  that  all  the  tissue  of  the  uterus  be- 
tween the  villi  disappears ;  a  layer  of  the  mucosa  is  preserved  between 
the  ends  of  the  villi  and  the  muscularis  uteri  to  form  the  so-called 
decidua ;  the  placenta  receives  its  f cetal  blood  by  the  means  of  large 
vessels  running  in  the  mesoderm  of  the  allantois.  From  this  dis- 
coidal chorionic  placenta  the  zonary  placenta  of  carnivora,  the  diffuse 
placenta  of  the  lower  primates,  and  the  metadiscoidal  placenta  of 
man  have  been  evolved. 

A  second  type  of  placenta,  perhaps  evolved  from  the  first,  is  found 
in  ungulates,  and  is  characterized  by  a  vascular  allantoic  vesicle 
uniting  with  a  non- vascular  chorion  to  form  the  fcetal  placenta,  and 
by  the  absence  of  degeneration  in  the  maternal  tissue.  This  type  is 
the  allantoic  placenta,  which  offers  many  interesting  modifications. 


PART  V. 

THE    FCETUS. 


CHAPTER   XVIII. 

GROWTH  AND  EXTERNAL  DEVELOPMENT  OF  THE  HUMAN  EMBRYO 

AND  FCETUS. 

THE  two  sections  following  on  the  growth  of  the  foetus  and  the 
weight  at  birth  are  taken  from  my  article  on  "  Growth  "  in  Buck's 
"Reference  Handb.  Med.  Sci.,"  III.,  394.  A  more  accurate  concep- 
tion of  the  growth  of  the  embryo  can,  however,  be  gathered  from  the 
figures  in  the  latter  part  of  this  chapter. 

Growth  of  the  Foetus. — The  difficulty  of  determining  the  age 
of  the  human  foetus  and  of  obtaining  specimens  certainly  fresh  and 
normal  has  prevented  our  having  any  definite  information  on  this 
subject.  Preyer  has  compiled  the  following  table  of  the  length  of 
the  human  embryo  in  centimetres : 

Lunar  Month.                           Toldt.  Hennig.  Heckcr 

(200  obs.)  (100  obs.) 

First,  1.5(1.3)  (».::> 

Second,  3.5  4.0 

Third,  7.0                          8.4  4  to    9 

Fourth,  12.0                        16.2  10  to  17 

Fifth.  20.0                        27.5  18  to  27 

Sixth,  80.0  35.25  28  to  34 

Seventh,  35.0  40.25  ::.->  to  38 

Eighth.  40.0                        44.3  39  to  41 

Ninth,  45.0  47.2  42  to  44 

Tenth,  50.0  (49.0)  45  to  47 

If  the  absolute  length  at  the  end  of  each  month  is  divided  by  the 
increase  during  that  month  we  obtain  what  Preyer  calls  the  relative 
growth.  Hennig's  figures  give  the  following  relative  growth  for 
each  month:  First,  1,000;  second,  0.812;  third,  0.523;  fourth, 
(•.111);  fifth,  0.410;  sixth,  0.219;  seventh,  0.124;  eighth,  0.093; 
ninth,  0.069;  tenth,  0.037.  All  the  above  data  are  obviously  inexact. 
Toldt's  are  evidently  cooked  up  and  not  derived  from  observation, 
nor  do  the  lengths  mean  the  same  thing,  for  of  the  early  stages  the 
head  and  trunk  only  were  measured ;  of  the  later  stages  the  head, 
trunk,  and  legs.  A  falser  and  more  misleading  device  for  studying 
growth  has  never  been  put  in  practice.  The  foetus,  too,  being 
spirally  coiled  in  early  stages  cannot  have  its  length  determined 
accurately.  Far  better  would  it  be  to  always  determine  the  weight. 
The  growth  of  the  foetus  in  weight  has  been  most  inadequately  stud- 
ied, although  the  weight  is  the  only  available  measure  of  the  growth 
of  the  foetus  as  a  whole.  Hecker's  data  are  perhaps  the  best.  The 
weights  are  in  grammes : 


382  THE   FCETUS. 

Month.  Maximum.  Minimum.  Average. 

Third,  20  5  11 

Fourth,  120  10  57 

Fifth,  500  75  284 

Sixth,  1,280  375  634 

Seventh,  2,250  780  1,218 

Eighth,  2,438  1,093  1,569 

Ninth,  2,906  1,500  1,971 

Tenth,                      1.562                         

The  range  of  the  maxima  and  the  minima  suggests  that  errors  in 
the  determination  of  the  ages  may  have  occurred — such  errors  of  a 
month  are  not  rare  with  obstetricians. 

Appended  here  are  Hecker's  data  as  to  the  weight  of  the  placenta 
in  grammes,  and  the  length  of  the  umbilical  cord  in  centimetres : 

Month.                         No.  of  obs.  Placenta.  Cord. 

Third,                              3  36  7 

Fourth,  17  80  '19 

Fifth,  24  178  31 

Sixth,  14  273  37 

Seventh,  19  374  42 

Eighth,  32  451  46 

Ninth,  45  461  47 

Tenth,  62  481  51 

2.  Weight  of  the  Neiv-Born  Child. — It  is  subject  to  very  consider- 
able variations.  For  middle  Europe  the  average  may  be  held  to  be 
about  3,340  grammes  for  boys,  3,190  for  girls,  the  latter  being  some- 
what lighter.  The  variations  are  very  great,  ranging  from  1,000  to 
5,000  grammes.  For  instance,  the  following  table  is  given  by  Pfann- 
kuch,  who  unfortunately  jumbles  the  two  sexes  together : 

Kilos.  Obs.                                    Kilos.  Obs. 

1.50  to  2. 00  23  3. 00  to  3. 25  150- 

2. 00  to  2. 25  36  3. 25  to  3. 50  115 

2. 25  to  2. 50  52  3  50  to  3.75  79 

2. 50  to  2. 75  90  3.75  to  4.00  46 

2. 75  to  3. 00  110  4. 00  to  4. 50  13 

It  will  be  noticed  that  the  maximum  number  of  cases  (150)  falls 
between  3.00  and  3.25  kilos.,  and  that  the  further  the  weight  is 
removed  on  either  side,  above  or  below  from  this  mean,  the  fewer 
are  the  cases.  The  tables  by  other  authors  show  the  same  general 
results,  with  usually  slight  differences  in  the  quantitative  values. 
For  the  most  part  these  tables  cannot  be  combined  with  one  another, 
for  they  nearly  all  fail  to  fulfil  some  obvious  requirement  of  good 
statistics;  indeed,  amateur  statistics  are  generally  provoking  to  the 
expert.  It  is,  therefore,  not  desirable  to  attempt  an  analysis  of  the 
recorded  data.  As  an  example  of  statistics  at  once  valuable  and 
grossly  defective,  the  following  table  is  given  after  Siebold.  The 
author  gives  the  weights  in  pounds,  but  has  neglected  to  say,  as  is 
necessary  in  Germany,  what  kind  of  pounds,  hence  the  metric  equiva- 
lents cannot  be  calculated.  Moreover  the  number  of  cases  weighing 
even  pounds  and  half-pounds  is  far  in  excess  of  those  weighing 
pounds  and  one-fourth  or  three-fourths,  which  shows  inaccurate 
weighing,  of  course.  To  correct  this  the  quarter-pound  groups  of 
original  table  are  condensed  into  half-pound  groups : 


GROWTH  AND  EXTERNAL  DEVELOPMENT. 


383 


H>s. 


4.H  to  4.5 
4.5  to  5.0 
5.0  to  ff.fi 

5.5  to  6.0 
6.0  to  <;.:> 
<;.:>  td  7.0 
7.0  to  7.5 


4 

19 

-14 

L71 

290 


•  iii-ls. 

10 
M 

53 
195 
MB 

:;:,:; 
240 


Weight  in  Ibs. 
7.")  tO     8.0 

8. 0  to    8. 5 

8. 5  to    9.0 

9.0  to    9.5 

'.».:>  to  10.0 

10.0  to  10.5 

Hi.:,  to  11.0 


I '....vs. 

•>sr, 

101 

79 


Girls. 

200 

44 

42 

14 
2 

1 
1 


The  extremes  recorded  in  medical  literature  are  very  far  apart, 
Mid  statements  of  excessively  large  size  are  by  no  means  ran-,  but 
can  be  received  with  incredulity  only,  as,  for  instance,  the  case  re- 
ported of  a  still-born  child  weighing  8,250  gms.  (Berlin.  Id  in. 
11'nrln'nxrlir.,  1878,  No.  14) !  Vierordt  gives  as  the  accredited 
extremes  '.  !•'  gms.  (Riter),  and  6,123  gms.  (Wright.) 

The  factors  which  determine  the  weight  at  birth  are  very  obscure. 
It  is,  of  course,  safe  to  say  vaguely  that  it  depends  on  the  nutrition 
of  the  foetus ;  it  is  probable  that  individual  differences  in  the  rate  of 
growth  exist  before  as  well  as  after  birth,  and  it  is  probable  that  the 
length  of  gestation  is  the  most  influential  single  factor,  to  judge 
from  my  own  experiments  on  the  growth  of  mammals. 

It  has  been  demonstrated  that  the  variations  in  the  weight  of  the 
child  depend  upon  various  maternal  circumstances. 

/•'/V.s7.  It  is  correlated  with  the  age  of  the  mother,  as  is  shown  in 
the  following  table,  giving  the  weight  of  the  children  in  granini* •> 
ace.  .rding  to  three  observers: 


Of  nx.th.T. 

r>  to  19  years 
•Jn  to  24 
25  to  29 
30  to  34 
:::,  to  :»» 
40  to  44 


Ingereley. 
3,241 
3,299 
3,342 

3,428 
8.8S8 


Fassbender. 
8,271 
3,240 


. 
3,367 

3, 292 


3,451 
3,485 
3,591 
4,062 

3,676 


From  such  tables  we  learn  that  very  young  mothers  have  the 
smallest  children,  and  those  of  about  thirty-five  years  the  heaviest. 
It  is  much  to  be  regretted  that  the  tables  do  not  show  the  correlation 
by  single  years  and  also  the  number  of  observations. 

Second.  The  weight  of  the  child  increases  with  the  weight  (Gass- 
ner)  and  length  (Frankenhauser)  of  the  mother.  Gassner  states 
that  the  weight  of  the  child  is  to  that  of  the  mother  as  1  to  19.13,  or 
•V.'o  per  cent  of  the  maternal  weight.  Frankenhauser  states  that 
if  the  height  of  the  mother  is  less  than  4  feet  6  inches  the  child 
weighs  6  Ib.  15  oz. ;  if  it  is  4  feet  6  inches  to  4  feet  11  inches,  the 
child  weighs  6  Ibs.  25  oz. ;  if  it  is  more  than  4  feet  11  inches,  the 
child  weighs  7  Ib.  3  oz. 

Th  ird.  The  weight  of  the  child  increases  according  to  the  number 
of  previous  pregnancies,  as  indicated  by  the  following  table : 


Number  of  pregnancies. 

One, 

Two, 

Three, 

Four, 

Five, 

Six, 


Hecker. 

(Gnns. ) 
3,201 
3,330 
3,353 
3,360 
3,412 
3,353 


Minns.  . 

3,2,54 
3,391 
3,400 
3,424 
3,500 


384  THE   FCETUS. 

Here  again  we  encounter  faulty  statistics,  for  it  is  not  shown  that 
we  have  any  other  effect  than  that  of  age,  for  the  conclusion 
claimed  cannot  be  established  until  it  is  proved  that  primipara3  have 
smaller  children  than  multipart  of  the  same  age. 

Fourth. 'Negri  has  maintained  (Annali  di  Obstetrica,  1885)  that 
the  compilation  of  three  hundred  and  thirty -three  observations  show 
that  the  children  of  women  whose  first  menstruation  is  early  are 
larger  than  the  children  of  those  whose  first  menstruation  is  late. 

Fifth  and  Sixth.  The  influence  of  race  and  climate,  which  have 
not  yet  been  subjected  to  any  proper  exact  study. 

In  conclusion  I  may  add  that  it  seems  to  me  probable  that  all  these 
influences  produce  their  effect  principally  by  prolonging  or  abbrevi- 
ating the  period  of  gestation.  In  other  words,  the  variations  in  the 
weight  of  children  at  birth  are  to  be  referred  immediately  to  two 
principal  causes:  1,  Differences  in  the  age  at  birth;  2,  individual 
differences  of  the  rate  of  growth  in  utero. 

Measuring  the  Length,  of  Embryos. — Owing  to  many 
changes  in  the  curvatures  of  the  longitudinal  axis  of  the  human  em- 
bryo it  is  impracticable  to  employ  any  one  system  of  measurement, 
to  obtain  comparable  results  for  all  ages.  On  this  account  I  have 
adopted  the  system  of  giving  in  all  cases  the  greatest  length  along 
a  straight  line,  the  embryo  being  measured  in  its  natural  attitude — 
excluding,  however,  the  limbs  from  the  measurement. 

His  adopts  for  embryos  of  four  to  ten  weeks  what  he  calls  the 
Nackenlange  ("Anat.  menschl.  Embryonen,"  Heft  II.,  5)  or  the 
distance  in  a  straight  line  from  the  neck  bend  to  the  caudal  bend, 
but  as  this  cannot  be  measured  accurately  in  later  stages  I  have 
thought  it  best  to  give  up  this  measure.  Hence  it  results  that  the 
length  of  an  embryo  as  given  by  His  is  often  different  from  that 
given  in  this  work. 

Embryos  of  Known  Ages. — As  already  pointed  out,  we  have  to 
reckon  from  the  last  day  of  the  menstrual  period  as  the  date  of 
conception,  but  this  date  is  never  quite  certain,  hence  there  is 
always  some  doubt  as  to  the  age  of  every  embryo.  We  owe  to  Pro- 
fessor His  most  of  our  information  in  regard  to  the  form  and  size  of 
the  embryo  at  successive  ages  during  the  first  two  months,  see  his 
"  Anatomie  menschl.  Embryonen,"  Heft  II.,  1882,  especially  pp.  25 
and  72,  also  Heft  L,  166,  Heft  III.,  236-254,  and  Taf.  X.,  which 
gives  figures  at  a  uniform  scale  of  twenty-five  embryos  of  the  first 
two  months. 

The  development  of  the  embryo  during  the  first  three  weeks  has 
already  been  described  and  illustrated.  Up  to  the  end  of  the  ninth 
week  the  form  and  size  of  the  embryo  undergo  a  correlated  develop- 
ment, so  that  generally  an  embryo,  at  a  given  stage  of  development 
in  /orm,  will  agree  with  its  fellows  in  size;  but  to  this  rule  there 
are  not  infrequent  exceptions,  and  sometimes  an  embryo  is  found 
much  larger  than  others  at  the  same  stage  (His,  Z.c.,  Heft  III.,  240). 
Moreover  the  variability  of  embryos  is  very  great,  for  in  specimens 
otherwise  alike  we  find  this  or  that  organ  retarded  or  advanced  in 
development,  as  compared  with  the  embryo  as  a  whole.  Neverthe- 
less it  is  possible  with  the  information  at  present  at  command  to 
determine  the  age  of  an  embryo  within  two  days  plus  or  minus  up 


(,H«»NVTH   AND    EXTERNAL    DEVELOPMENT. 


385 


to  the  end  of  the  ninth  week.  For  the  development  during  the  third 
month  we  possess  as  yet  no  satisfactory  information,  but  oinliry.  »- 
three  months  old  are  quite  frequently  obtained,  and  my  own  collec- 
tion gives  a  good  scries  of  specimens  up  to  the  end  of  the  fifth  month. 

Tir<'itti/-fltri'i>  Dni/fi. — The  first  figure  I  give  is  that  of  His'  em- 
bryo «,  Fig.  216,  described  by  him  in  his  "Anat.  menschl.  Embry- 
onen,"  Heft  I.,  100-115.  The  specimen  was  from  a  chorionic  vesicle 
measuring  2.5  by  3.0  (in. ;  the  greatest  length  of  the  embryo  was  4 
nun.,  measured  from  the  end  of  the  hind-brain,  iv,  to  the  four- 
teenth segment  of  the  rump.  It  lay  with  its  left  side  against  tho 
chorion,  with  which  it  was  connected  by  a  short  allantoic  stalk;  the 
yolk-sac  measured  •>.;  by  :;.<>mm.,  and  had  a  short  pedicle.  His 
stati-s  that  the  probable  age  of  the  specimen  was  twenty-three  da 
and  apparently  bases  the  deter- 
mination upon  comparison  with 
slightly  y<  .uni;vr  and  older  spe- 
cimens of  known  age.  The 
shape  of  the  embryo  is  very  dif- 
feivnt  fn.m  that  of  Fig.  180,  p. 
307,  owing  to  the  whole  body 
having  become  rolled  up,  so  that 
the  dorsal  outline  describes  more 
than  a  complete  circle;  the  body 
has  a  marked  spiral  twist,  the  o- 
head  being  bent  to  the  right, 
the  tail  to  the  left;  the  bending 
of  the  body  is  specially  marked 
at  the  region  of  the  mid-brain 
(head-bend)  and  at  the  posterior 
limit  of  the  hind-brain  (neck-  FIO. 
bend,  Nackettkriim  in  u  n<f) .  The 
primitive  segments  show  externally;  the  anlages  of  both  pairs  of 
limbs  have  appeared  as  outgrowths  of  the  so-called  Wolffian  ridge, 
but  the  leg  is  less  developed  than  the  arm.  In  the  region  of  the 
head  the  divisions  of  the  cerebral  vesicles  can  be  recognized.  The 
optic  vesicles  are  indicated  by  small  protuberances.  The  oval  oto- 
cyst  lies  about  at  the  level  of  the  second  gill-cleft.  The  cephalic 
tx>rder  of  the  mouth  has  become  ridge-like;  the  dorsal  end  of  the 
ridge  joins  the  dorsal  end  of  first  visceral  arch,  which  is  known 
as  the  mandibular  arch;  the  ridge. on  the  cephalic  side  is  known  as 
the  maxillary  process.  The  second,  third,  and  fourth  gill-arches 
are  distinct  and  behind  each  the  imperforate  gill-cleft  can  be  distin- 
guished, but  the  fifth  arch  is  indistinct.  The  heart  forms  a  marked 
protuberance,  the  bulbus  aortae  showing  most  on  the  left  side,  Fig. 
'I  l »;,  and  the  ventricles  on  the  right. 

An  embryo  of  C.  Rabl's,  very  similar  to  His'  «,  just  described,  is 
figured  by  6.  Hertwig,  "Entwickelungsges.,"  3te  Aufl.,  Fig.  137. 

His  points  out  that  the  embryos  numbered  by  him  XXVI. ,  (D  2), 

LVL,  (W),  and  LVIL,  (R),  are  very  near  the  one  just  described, 

though  a  little  older.     In  the  same  group  belongs  the  embryo  of 

to,  47.1,  PI.  II.,  Fig.  5,  of  which  the   age  was   determined  at 

twenty  to  twenty-five  days;  also  Thompson's  fourth  embryo,  figured 


B.S 


816.— His'  Embryo  a,  Age  probably   t\\.-ut\ 
tlnve  Days.     X  about  10  diameters. 


386 


THE   FCETUS. 


and  described  by  Kolliker,  "Entwickelungsges.,"  p.  311,  Fig.  231; 
also  that  described  by  Hensen,  77.1,  and  finally  Ecker's  specimen, 
80.1.  Of  all  these  Coste's  most  deserves  attention  on  account  of 
the  superb  manner  in  which  it  has  been  figured.  Concerning  His' 
embryo  R,  some  data  about  the  coelom  have  been  given  by  His,  8 1 . 1 , 
311. 

Twenty-five  Days. — Embryos  of  this  age  are  extremely  rare.  Fol 
has  given  a  full  but  not  wholly  satisfactory  description,  84.2, 

of  an  embryo  presumably 
of  this  age,  though  no 
data  were  obtained  in  re- 
gard to  it.  The  embryo, 
Fig.  217,  as  compared 
with  that  at  twenty- three 
daj7s,  has  grown  rapidly ; 
its  greatest  length  is 
5.6  mm.;  its  form  has 
changed  by  the  body  hav- 
ing partially  unrolled, 
but  the  head-bend  and 
neck-bend  remain  and  are 
more  prominent  than  be- 
fore, owing  to  the  embryo 
as  a  whole  being  less 
curved.  The  region  of 
the  fore-brain  is  brought 
close  to  the  heart,  the 
head  being  still  bent  to 
the  right;  the  limbs  are 
a  little  larger  and  there 
is  a  well  developed,  dis- 
tinct tail.  The  other  prin- 
cipal change  is  that  only 
three  gill  -  arches  show 
externally,  md,  1,  2,  the 
third  and  fourth  being 
already  invaginated  in 
connection  with  the  for- 
mation of  the  cervical  sinus.  It  must  be  added  that  this  embryo 
was  not  quite  normal,  as  is  shown  especially  by  the  condition  of  its 
veins.  The  representation  of  the  external  form  of  the  head  in  the 
figure  is  probably  not  entirely  correct. 

Twenty-six  Days. — Mall,  91.3,  gives  a  superb  figure  and  com- 
plete anatomical  description  of  an  embryo,  the  probable  age  of  which 
he  fixes  at  twenty-six  days. 

Twenty-seven  to  Twenty-eight  Days. — Embryos  of  this  age  are 
characterized  by  the  extreme  development  of  the  neck-bend,  Fig. 
218,  the  apex  of  which  forms,  as  it  were,  the  summit  of  the  embryo; 
the  greatest  length  from  this  apex  is  7-8  mm.  To  the  age  of  about 
twenty-eight  days  are  to  be  assigned  the  embryo  described  by  Johan- 
nes Muller,  30.2,  one  figured  by  Coste,  47.1,  PL  III.,  one  de- 
scribed by  Waldeyer,  62.1,  and  four  embryos  in  His'  collection, 


FIG.  217. — FoFs  Embryo  of  5.6  mm.,  probably  twenty- 
five  Days  old.  op,  Position  of  the  optic  vesicle;  />-.  toeea 
rhqmboidalis;  s.s,  segments;  a. I,  anterior  limb;  pi,  pos- 
terior limb;  md,  mandibular,  1,  hyoid;  2,  bronchial  arch. 


GROWTH  AND  EXTERNAL  DEVELOPMENT. 


387 


menschl.   Embryonen,"    Heft   II.,  8), 
(A),  and  XL.   (3tt.) ;  concerning  XL. 


numbered   by  him   ("  Anat. 
I.   (B),   LXI.  (Eck.  I.),  II., 
see  His,  I.e.,  pp.  '.'4.  98,     Of 
A  and  B  His  has  published 
a   detailed    anatomical    ac- 
count    ("Anat.     menschl. 
Kmbryonen,"   Heft  L,   14- 
99). 

I  choose  for  my  illustra- 
tion Fig.  218,  His'  embryo 
A,  because  it  shows  the 
neck-lx-nd  most  perfectly; 
how  entirely  the  prominent  • 
<>i'  the  neck-bend  alters  the 
shape  of  the  embryo  will 
appear  immediately  if  Fig. 
218  be  compared  with  Fig. 
175.  As  changes  since  the 
twenty -fifth  day  we  note 
especially  the  distinctness  of 
the  olfactory  pit  (Riech- 
t/rnbe)  and  of  the  still  open 
in \  agination  to  form  the 
lens  of  the  eye,  the  deepen- 
ini;-  of  the  cervical  sinus 
I  s  i  nus  prsecervicalis  of  His) , 
and  the  partial  closure  of 
the  allantois-stalk  (Bauch- 
.s//<7)  around  the  proximal 
part  of  the  now  narrow  pedicle  of  the  yolk-sac;  the  closure  of  the 
Baiichstiel  forms  the  umbilical  cord,  but  the  cord  itself-  is  very  short 
and  in  proportion  to  the  embryo  very  thick.  In  all  parts  there  has 
lieen  an  obvious  development  since  the  twenty-fifth  day,  Fig.  217,  but 
further  details  may  be  omitted.  Comparison  of  this  embryo  with 
others  of  the  same  stage  show  that  there  is  a  considerable  variation 
as  to  the  nature  and  degree  of  curvature  of  the  back,  in  consequence 
of  which  the  specimens  differ  somewhat  in  general  form,  though 
agreeing  closely  in  structure. 

Ttrt'nty-m'tie  to  Thirty  Days. — Embryos  8-10  mm.  A  number  of 
specimens,  which  probably  belong  to  the  middle  of  the  fifth  week  are 
known.  For  my  illustration  I  give  a  drawing,  Fig.  219,  of  an  em- 
bryo sent  to  me  by  Dr.  H.  J.  Garigues  of  New  York;  the  data  suffice 
only  to  determine  the  age  as  the  fifth  week ;  the  specimen  appeared 
normal  and  well-preserved,  but  upon  microtoming  it,  it  was  found 
to  be  in  poor  condition  histologically ;  it  has  interest  because  it  shows 
with  especial  clearness  the  relations  of  the  foetal  appendages.  The 
embryo  proper  has  begun  to  straighten  its  body,  and  as  the  outline 
over  the  region  of  the  medulla  oblongata,  compare  Fig.  217,  Jf^r,  has 
become  less  curved,  the  head  begin  sto  appear  to  form  a  right  angle 
with  the  body ;  the  olfactory  pit,  ol,  has  deepened ;  the  lens  of  the 
eye,  op,  is  well  marked,  as  is  also  the  lachrymal  groove  descending 
from  the  eye;  the  cervical  sinus,  c..s,  has  deepened  but  is  still  open; 


388 


THE    FCETUS. 


c.s 


the  limbs  have  lengthened  and  in  other  specimens  begin  to  show  the 
differentiation  of  the  hand  and  foot.     About  two-thirds  of  the  allan- 

tois-stalk  has  closed  to  form  the  umbili- 
cal cord,  C7m,  from  the  end  of  which  ex- 
tends the  amnion,  Am.  The  long  j^olk- 
stalk,  Vi.s,  ends  in  the  pear-shaped  yolk- 
sac,  Vi;  the  allantois-stalk  or  Bauchstiel, 
Bs,  which  runs  to  the  chorion,  Cho. 

In  this  group  belong  the 
embryo  of  Rabl  (O.  Hert- 
wig,  "Entwickelungsges.," 
3te  AufL,  Fig.  158),  the  em- 
bryo of  10  mm.  of  which 
Phisalix  gives  a  detailed 
anatomical  description, 
88. 1, — also  seven  embryos, 
enumerated  by  His,  "  Anat. 
menschl.  Embryonen,"  Heft 
II.,  8,  and  described  there 
45-7,  and  His'  embryo 
.,  /.  c.,  p.  10,  238,  Taf. 
X.,  Fig.  8,Taf.XIIL,Fig. 
47 — this  last  by  far  the  most 
perfect  drawing  of  this  stage 
which  we  possess. 

Thirty-one  to  Thirty-two 
Days.—  Embryos  of  10-12 
mm.  (see  His,  /.c.,  Heft  II., 

Cho  47-51  and,  for  a  list  of  ten 

specimens,  p.  8-9,  Taf.  X., 
Figs.  13,  14,  and  15,  Taf. 
XIII.,  Fig.  6).  The  age  of  the  embryo  at  this  stage  can  as  yet  only 
be  estimated,  as  in  no  case  have  we  data  sufficient  for  a  reliable 
determination.  For  a  typical  illustration  of  this  stage  we  may 
take  His'  Br.  1,  I.e.,  Taf.  XIII.,  Fig.  6,  which  measured  11  mm. 
The  back  has  straightened  out,  though  the  lower  end  of  the  body 
is  still  rolled  over;  the  head  has  risen  somewhat  and  enlarged 
both  absolutely  and  in  proportion  to  the  rest  of  the  body.  Be- 
tween the  end  of  the  region  of  the  hind-brain  and  the  level  of 
the  arm  the  outline  has  become  slightly  concave;  this  concavity 
His  designates  as  the  Nackengrube.  The  cervical  sinus  is  so 
deep  that  the  second,  third,  and  fourth  gill-clefts  have  disappeared 
from  the  external  surface;  the  first  gill-cleft  remains  and  can  al- 
ready be  recognized  as  the  anlage  of  the  external  auditory  meatus ; 
it  is  separated  from  the  mouth  by  the  prominent  mandibular  arch. 
On  the  cephalic  side  of  the  mouth  the  maxillary  process  has  become 
more  prominent,  but  the  two  processes  do  not  yet  meet  in  the 
median  line.  The  primitive  segments  are  still  marked  externally. 
The  limbs  show  the  tripartite  division ;  the  fore  limb  is  more  ad- 
vanced than  the  hind  limb;  the  division  of  the  digits  of  the  hand  is 
just  indicated.  The  abdomen  bulges  out  owing  to  the  growth  of  the 
liver.  There  is  a  true  tail,  which  is  now  near  its  maximum  develop- 


FIQ.  219. — Embryo   of  9.8  mm.     Minot  Collection  No. 
145.     Probable  age  thirty  days.     X  5  diams. 


<,i;«>\\  I'M    AND   EXTERNAL  DEVELOPMENT. 


389 


ment.  The  umbilical  cord  has  lengthened  and  shows  the  commenc- 
ing spiral  twisting;  UK-  amnion  springs  from  the  end  of  the  cord, 
leaving  only  a  >lmrt  stretch  of  the  allant<>is-stalk  bet \veen  the  cord 

E  roper  and  the  clmrion;  the  amnion  lies  close  to  the  embryo.     In  the 
-h  specimen  something  can  be  seen  of  the  shape  of  the  brain; 
especially  noteworthy,  among  the  points  thus  to  be  recognized,  is  the 
sharp  bend  (Bruckenkrummung)  at  the  deep-lying  anterior  end  of 
the  hind-brain  or  region  of  the  sinus  rhomboidalis. 

In  embryos  a  little  older  than  these  the  changes  in  form  above 
mentioned  have  progressed  further.  The  specimens  measure  12-13 
mm.  The  Ixxly  is  straighter;  the  head  is  larger  and  lias  risen  so  as 
to  1 .( •  at  about  right  angles  to  the  body ;  the  concavity  below  the  hind- 
brain  in  the  outline  of  the  neck  (Nacfcenkr&tnmung)  ismore  marked; 
the  liml»  are  longer,  the  fingers  more  distinctly  marked;  the  tail  is 
at  its  maximum  development  as  a  free  appendage;  where  the  man- 
dibles meet  in  the  median  line  the  separation  of  lip  and  chin  has 
he^-im  ;  the  second  gill-cleft  is  invaginated  into  the  cervical  sinus  and 
can  no  longer  be  seen  on  the  outside. 

Thii •///-/// -c  Days. — Embryos  of  14  mm.  The  correlation  of  age 
and  si/r  of  this  stage  cannot  be  regarded  as  absolute,  though  we  can 


Put.  Wi. 


Einliryo  of  about  14  mm.     Minot  Collection  No.   120.    As- 
sumed age,  thirty-five  days.    X  5  diams. 


FIG.  221.— Dorsal  View 
of  an  Embryo  of  about  14 
mm.  Minot  Collection, 
No.  120.  Assumed  age, 
thirty -five  days.  x  5 
diams.  (Compare  Fig. 
220). 


say  (His,  I.e.,  Heft  III.,  239)  that  embryos  of  this  length  are  about 
five  weeks  old.  The  body  is  now  nearly  straight;  the  limbs  project 
beyond  the  outline  of  the  body  in  profile  views ;  the  abdomen,  owing 
to  the  large  size  of  the  heart  and  liver,  bulges  far  out ;  in  side  views 
the  area  of  the  head  is  about  equal  to  that  of  the  rest  of  the  body;  the 
outline  of  the  head  shows  the  head-bend  and  neck-bend  most  clearly 
marked ;  the  neck-bend  is  characterized  by  the  prominence  at  that 


390 


THE    FCETUS. 


point ;  the  prominence  is  often  less  than  in  Fig.  220.  The  umbilical 
cord  frequently  contains  one  or  several  coils  of  the  intestine  and 
makes  one  or  two  spiral  turns.  The  stalk  of  the  yolk-sac  is  long, 
and  projects  quite  far  from  the  end  of  the  cord  between  the  amnion 
and  chorion.  In  a  dorsal  view  we  can  see  that  the  limbs  are  some- 


what flattened  and  in  a  plane  nearly  parallel  with  the  longitudinal 
axis  of  the  embryo,  but  the  planes  of  the  arms  are  inclined  so  as  to 
meet  above  the  head,  and  the  planes  of  the  legs  are  inclined  to  as  to 
meet  below  the  tail.  Owing  to  the  flattening  of  the  limbs  we  can 
already  distinguish  the  inner  or  palmar  surfaces  from  the  outer. 


GROWTH    AND    KXTKRNAL   DK\  KL<  U'.MENT. 


391 


Noteworthy  is  the  irregularly  crenulated  appearance  of  the  walls  of 
tin-  medullary  tube  or  spinal  cord. 

Kii;.  '!•>.->  is  copied  from  Coste,  and  is  valuable  on  account  of  the 
very  large  number  of  anatomical  facts  which  it  records.  Coste  gives 
no  data  but  states  that  the  specimen  was  u about  thirty-five  days  old." 

Thtrfy-rit/ltf  Dni/ft.— Kmhryo  of  15  mm.,  in  a  chorionic  vesicle  of 
45  by  40  mm.  The  age  of  this  specimen,  Fig.  223,  is  known  by 
estimate  only.  It  has  been  su- 
perbly figured  by  His  (**  Anat. 
inrnschl.  Embryonen,"  Taf. 
XIV.,  Fig.  5).  This  stage 
represents  the  transition  from 
the  embryo  to  the  foetus,  be- 
cause after  the  fortieth  day  the 
form  is  distinctly  human.  The 
head  lias  risen  considerably, 
and  the  back  has  straightened 
still  more,  the  rectangular  neck- 
Ix-nd  thus  becoming  empha- 
si/cd.  The  body  has  become 
still  more  protuberant  on  the 
ventral  side,  and  in  side  views 
the  arms  no  longer  reach  to  the 
outline  of  the  body. 

Forty  Days. —  Embryos  of 
I1.'  mm.  The  hea<J  has  risen 
far  toward  its  definite  position, 
with  the  result  of  a  very  rapid 
apparent  increase  in  the  length 
of  the  embryo.  The  change  of 
position  of  the  head  results  in 
bringing  the  mid-brain  finally 
directly  above  the  hind-brain, 
the  embryo  being  conceived  as  having  the  body  vertical.  Durink 
the  elevation  of  the  head  the  concavity  (Nackengrube)  at  the  bacg 
of  the  neck  is  gradually  obliterated.  In  both  head  and  rump  the 
external  modelling,  which  in  earlier  stages  indicated  more  or  less 
the  position  of  the  internal  organs,  has  become  blurred  and  in  the 
next  stage  is  found  to  have  nearly  or  quite  disappeared.  The  max- 
illary processes  have  met  and  united  in  the  median  line.  The  an- 
lages  of  the  eyelids  have  developed.  The  concha  of  the  ear  is  indi- 
cated. The  arm  reaches  beyond  the  heart;  the  fingers  appear  as 
separate  outgrowths. 

Fifty  Days. — Embryo  of  21  mm.  I  have  a  fair  specimen  which 
came  into  my  possession  with  no  history  whatever,  but  it  agrees  very 
closely  with  Fig.  23,  Taf.  X.,  in  His'  "Anat.  menschl.  Embryonen," 
of  His'  embryo  Ltz,  of  which  he  fixes  the  probable  age  as  just  over 
seven  weeks.  The  head  is  nearer  its  final  position  than  in  Fig.  223, 
and  relatively  larger  in  proportion  to  the  body.  In  the  eye,  cornea 
and  conjunctiva  are  clearly  separated;  the  face  has  the  foetal  form, 
the  nose,  mouth,  and  chin  being  fully  marked  off.  The  arms  are 
clearly  divided  into  upper  and  lower  segments ;  the  five  digits  are 


FIG.  223 -His*  Embryo  XXXIV.  (Dr.),  ir,  mm. 
long  from  the  Neck-bend  to  the  Coccygeal  Bend. 
Age  estimated  at  thirty-seven  to  thirty-eight 
days,  x  5  diams. 


392 


THE   FCETUS. 


well  developed ;  the  hands  rest  over  the  heart  and  nearly  touch  one 
another.  In  the  specimen  figured  the  outline  of  the  abdomen  is  ab- 
normal. The  leg  shows  the  tripartite  division ;  the  toes  are  just  be- 
ginning to  be  free,  but  the  hind  limb  is  much  less  advanced  than  the 
fore  limb.  The  tail  is  still  a  freely  projecting  appendage. 

Fifty-three  Days. — Embryo  of  22  mm.  The  specimen,  Fig.  224, 
is  probably  not  quite  normal,  but  except  for  the  extreme  and  unusual 
curvature  of  the  back  it  agrees  closely  with  His'  embryo  Zw,  which 
is  figured  by  him,  Z.c.,  Fig.  24,  Taf.  X.,  as  a  normal  embryo  of  pre- 
sumably about  seven  and  one-half 
weeks.  My  specimen  I  received  in 
1884  with  the  following  history: 
"  Menstruation  began  January  20th. 
February  and  March  slight  show 
every  few  days.  Abortion  March 
30th,"  which  is  insufficient  to  de- 


FIG  224. — Embryo  of  22  mm.  Minot  Collec- 
tion, No.  54.  Probable  age,  fifty-three  days. 
X  3  diams 


FIG.  225.  —Embryo  of  28  mm.  No.  144  of 
Minot  Collection.  Assumed  age,  sixty  days. 
X  3  diams. 


termine  the  age.  As  compared  with  the  last  stage  there  are  com- 
paratively few  changes  of  external  form ;  the  most  noteworthy  are 
perhaps  the  increased  development  of  the  legs  and  feet  and  the  com- 
mencing disappearance  of  the  free  tail.  At  this  time  the  protrusion 
of  the  coils  of  the  intestine  into  the  coelom  of  the  umbilical  cord  is 
about  at  its  maximum. 

Sixty  Days. — Embryo  of  28  mm.  The  specimen  figured  re- 
sembles closely  in  form,  though  larger  than,  His'  embryo  Wt  (Fig. 
25,  Taf.  X.,  Z.c.),  which  he  has  determined  as  a  normal  embryo  of 
about  eight  and  one-half  weeks.  My  specimen,  Fig.  225,  came  to 
me  with  no  data.  The  head  is  still  larger  in  proportion  to  the  body 
than  in  Fig.  223.  The  face  shows  the  two  lines,  which,  as  seen  in 
profile,  mark  the  two  ridges  which  run  over  the  cheek,  one  alongside 
the  nose  to  the  corner  of  the  mouth,  the  other  from  the  eye  $  these 
ridges  are  highly  characteristic  of  the  ninth  week,  and  traces  of  them 


i,K<>\YTH    AND    KXTKKNAL    DEVELOPMENT. 


393 


No   138   Minot 


not  rarely  persist  in  the  adult 
physiognomy.  The  limbs  lm\v 
grown  considerably,  the  hands 
being  lifted  toward  the  face,  at 
the  elbow  there  is  a  considerable 
bend;  the  toes  are  all  free  and 
the  soles  of  the  feet  are  turned 
toward  one  another.  The  tail 
lias  disappeared  as  a  free  appen- 
dage. The  external  ^vnitaliaare 
considerably  developed;  the  cli- 
toris-penis projects  some  dis- 
tance. 

>  lij-fnnr  Days.—  Embryo  of 
32  mm.    The  specimen,  Fig. 
came  to  me  with  the  following 
history  :  "  January  4th,  1886,  last 
flow  began;    March    13th,   1886, 
abortion  ;  "     between 
these  two  dates  are 
sixty-eight  days  ;  but 
as  the  flow  took  place 
conception  probably  occurred  af- 
ter menstruation,  therefore  if  we 
deduct  four  days,  making  the  age 
sixty-four  days,  we  shall  probably 
not  be  far  wrong.     It  will  be  no- 
ticed that  the  head  has  not  yet 


diauis 


a«mimpfl    ifa  final  nncrlo  with 
Probable  age  sixty-four  days.     x3  imai  angle  W1U1 


Fio.  226  —Embryo  of  82mm 

-  , 

body.     On   the   other  hand   the 
protuberance  of  the  abdomen  is 

much  reduced,  so  that  the  body  as  a  whole  has  begun  to  have  a  more 

slender  form  than  in  earlier  stages.  In. 
this  specimen  the 
eyelids  have  not 
even  begun  to 
meet;  in  another 
I  have  they  have 
met,  Fig.  227,  ex- 
cept just  in  the 
centre  where  is 
still  a  loophole. 
This  specimen  was 
brought  to  me 
with  the  statement 

that    it   was 

just     sixty 

days.     I  en- 

deavored, 

unsuccess- 

fully, to  get 
exact 


.-;.—  Embryo  of  34  mm.      No.  39  Minot 
Collection.    Front  View  of  Face.    xSdiams. 


Fio 

mm.  No.  97  Minot  Collection. 
Assumed  age,  seventy  -  five 
days.  Natural  size. 


894 


THE    FCETUS. 


data.  The  large  size,  43  mm.,  and  advanced  development  of  the 
embryo  led  me  to  consider  the  age  given  as  erroneous,  and  to  be- 
lieve the  true  age 
to  be  perhaps  six- 
ty-seven days. 

Seventy-  five 
Days . — Embryo 
of  55  mm.  I  fig- 
ure next,  Fig.  228, 
a  foetus  concerning 
which  I  possess  no 
data.  Comparison 
with  embryos  of 
two  and  three' 
FIG  230. -Front  view  of  months  leads  me 

the   Head  and  Face  of  the    ,  .,          ,.,,- 

Embryo,  Fig.  229.  to  place    it   a    little 

under  half-way  be- 
tween them.  The  specimen  has  essentially 
the  configuration  of  the  young  child;  but 


FIG.  229. — Embryo  of  78  mm. 
No.  74  Minot  Collection.  Age 
three  months. 


the  head  is  very  large,  and 
the  body  slender ;  the  posi- 
tion of  the  limbs  is  typi- 
cal ;  the  upper  arm  is  bent 
down,  the  forearm  extends 
toward  the  chin ;  the  knee 
is  bent  so  as  to  throw  the 
foot  toward  the  median 
line;  the  soles  of  the  feet 
are  placed  obliquely  facing 
one  another;  the  anlages 
of  the  nails  can  be  recog- 
nized on  both  the  fingers 
and  toes. 

Embryos  of  the  eleventh 
and  twelfth  weeks  are  very 
rarely  obtained.  I  have 
never  had  a  normal  one  of  this  period  with  data  to  determine  the  age. 

Three  Months. — Embryos  of  78-80  mm.     In  my  experience  there 


FIG.  231.— Embryo  of  about  120  mm.  No.  61  Minot  Col- 
lection. Assumed  age,  one  hundred  and  ten  days.  Nat- 
ural size. 


(,i;o\\TH     AM)    KXTKKNAL    I  >KV  KL<  U'.M  KM  . 


is  no  other  age  at  which  abortion  of  normal  embryos  occurs  so  fre- 
quently as  at  three  months,  and  I  possess  a  number  of  specimens  of 
this  age,  which  agree  very  closely  with  another  in  size  and  form. 
The  fcBtus  drawn  in  Fig.  22'.'  may  be  taken  to  represent  very  accu- 
rate!}' the  form  and  size  of 
the  human  embryoat  three 
months.  The  position  of 
the  limbs  is  typical  for  this 
age,  but  there  are  slight 
variations  in  that  the 
ha  in  Is,  one  or  both,  may 
project  more  or  be  higher 
or  lower:  usually  the  right 
foot  lies  in  fronted  the  left, 
but  not  always.  Fig.  \':'." 
gives  the  front  view  of 
face  of  the  same  embryo 
to  show  the  closed  eyelids, 
the  broad  triangular  nose, 
the  thick  lips  and  pointed 
chin. 

Thrt't'  <  i  tut  One -lidlf 
M  o  n  f  h  ft .  —  Embryos  of 
108-110  mm.  I  have  sev- 
eral specimens  which  rep- 
resent this  age.  I  figure 
two  of  them,  one  to  show 
the  natural  attitude,  Fig. 
231,  in  utero,  the  other  to 
show  the  natural  attitude 
assumed  by  the  embryo 
when  released  from  its 
membranes.  The  fi  r  s  t 
specimen  came  to  me  with 
no  history,  but  as  it  is  cer- 
tainly a  little  larger  than 
several  other  foetus  of 

about  one  hundred  and  six 

days  it  is  probably  a  little 

older.     The  head  is  bent  forward,  Fig.   231; 

the  back  is  curved;  the  arms  and  legs  are  both 

raised  toward  the  face ;  the  right  leg  is  nearly 

straight  so  that  the  toes  are  brought  against 

the  forehead,  while  the  left  leg  is  bent  at  the 

knee,  bringing  the  left  foot  against  the  right 

thigh.     In  this  attitude  the  embryo  fills  out  as       ^  ^..Embryo  of  m 

perfectly   as  possi  ble  an  oval  space,  and   fits     mm.    NO.  is  Mmot  coiiec- 

therefore  the  cavity  of  the  uterus.    The  second     s£nday*ei  Natura^ze.*111' 

specimen,  Fig.  232,  shows  the  attitude  assumed 

by  the  embryo  when  free,  and  proves  that  the  position  in  utero,  Fig. 

231,  is  a  constrained  one.     This  foetus  was  received  November  30th, 

1883.     The  delivery  took  place  on  the  morning  of  that  day,  and  the 


396 


THE    FCETUS. 


last  menstruation 
had  ceased  one  hun- 
dred and  six  days 
previously ;  the  re- 
markably fresh  con- 
dition of  the  foetus 
indicated  that  it  had 
been  dead  only  a  very 
short  time,  so  that 
we  cannot  be  far 
wrong  in  putting  its 
exact  age  at  one  hun- 
dred and  six  days. 

Four  Months.  — 
Embryo  155  mm. 
The  foetus,  shown  in 
Fig.  233,  came  to  me 
in  a  very  fresh  con- 
dition, January  2d, 
1887,  with  the  state- 
ment :  "  Conception 
said  to  have  taken 
place  September  1st, 
188G ;  foetus  came 
away  January  2d, 
about  noon."  The 
embryo  is  typical  in 
size  and  development 
for  four  months,  ex- 
cept that  the  crown 
is  higher  than  usual, 
and  the  antero-poste- 
rior  diameter  of  the 
head  somewhat  be- 
low the  average. 

The  natural  atti- 
tude in  utero  is  sim- 
ilar to  that  of  Fig. 
231,  the  attitude 
shown  is  that  as- 
sumed by  the  foetus, 
when  released  from 
the  membranes. 


FIG.  233.— Embryo  of  155  mm.  No.  180 
Minot  Collection ;  Age,  one  hundred  and 
twenty-three  days.  Natural  size. 


CHAPTER  XIX. 
THE  MESENCHYMAL  TISSUES. 

As  the  numerous  tissues  which  result  from  the  differentiation  of 
the  mesenchyma  enter  to  a  greater  or  less  extent  into  the  formation 
<  »t  the  organs  of  which  the  main  parts  are  derived  from  the  ectoderm, 
entoderm,  or  mesothelium,  it  is  desirable  to  begin  the  study  of  the 
organs  with  a  general  review  of  the  mesenchyma.  The  development 
of  the  skeleton  is  treated  in  the  next  chapter,  p.  422. 

Classification  of  Mesenchymal  Tissues. — The  fundamental 
and  essential  characteristic  of  the  mesenchyma  is,  that  the  cells  are 
some  distance  apart,  but  connected  together  by  their  own  protoplas- 
matic processes.  The  tissue  is  made  up  of  anastomosing  cells.  The 
spaces  left  between  the  cells  are  filled  with  intercellular  substance, 
which,  owing  to  the  size  of  the  spaces,  constitutes  a  large  part  of  the 
tissue.  In  this  respect  the  mesenchyma  offers  a  marked  contrast  to 
all  epithelia,  for  the  latter  have  the  intercellular  substance  reduced  to 
a  minimum.  The  intercellular  substance  is  an  extremely  important 
factor  in  the  differentiation  of  the  mesenchymal  tissues;  in  fact 
so  important  that  it  affords  a  better  basis  for  the  classification  of  the 
tissues  than  the  cells  themselves.  To  these  fundamental  conceptions 
I  attribute  a  great  value. 

In  the  primitive  stage  we  have  cells  with  small  protoplasmatic 
bodies,  connected  by  few  processes  and  imbedded  in  a  homogeneous 
matrix  (intercellular  substance) .  We  can  distinguish  in  subsequent 
changes  three  main  sets  of  modifications:  1,  those  which  are  spe- 
cially characterized  by  changes  in  the  basal  substance;  2,  those  char- 
acterized chiefly  by  changes  in  the  cells ;  3,  those  characterized  by 
the  special  arrangement  of  the  tissues  produced  by  the  differentiations 
of  the  mesenchyma. 

In  the  first  series  I  put  the  development  of  connective-tissue  fibrils 
and  fibres,  of  the  intercellular  network  both  elastic  and  non-elastic,  of 
niucin,  as  in  Wharton's  jelly,  of  cartilage  (chondrification),  of  bone 
(ossification),  and  also  the  disappearance  (or  liquefaction?)  of  the 
Uisal  substance,  and  finally  its  hypertrophy. 

In  the  second  series  I  put  the  development  of  the  blood-vessels,  of 
the  lymphatic  vessels,  muscle-cells,  fat-cells,  pigment-cells,  and  of 
the  marrow  of  bones. 

In  the  third  series  I  put  the  development  of  the  connective-tissue 
cavities  such  as  the  synovial,  bursal,  and  subarachnoid,  and  the 
formation  of  special  layers  such  as  the  subepithelial  basement  mem- 
branes, the  submucosa,  the  cutis,  and  so  forth.  What  little  there  is 
to  be  said  in  regard  to  the  special  layers  will  be  found  in  connection 
with  the  history  of  the  special  organs  of  which  they  form  parts. 

The  following  table  gives  the  classification  adopted.     It  must  be 


398  THE    FCETUS. 

borne  in  mind  that  the  classification  is  somewhat  arbitrary,  since  in 
all  the  tissues  modifications  occur  in  both  the  cells  and  the  inter- 
cellular substance;  moreover,  several  differentiations  may  occur 
simultaneously  or  successively  in  the  same  part;  for  instance,  the 
fibrillse  and  network  are  usually  found  together ;  cartilage  may  or 
may  not  have  fibrillse  and  elastic  tissue. 

MESENCHYMAL  TISSUES. 

First  Series.                                  Second  Series.  Third  Series. 

(Changes  in  matrix) .           (Changes  in  Cells. )  (Special  arrangements) . 

1.  Fibrils.  1.  Blood-vessels.  1.  Cavities. 

2.  Network.  2.  Lymphatics.  a.  synovial. 

a.  yellow  elastic.         3.  Muscle-cells.  b.  bursal. 

b.  white  non-elastic.  4.  Fat-cells.  c.   subarachnoid. 

3.  Mucin.  5.  Pigment-cells.  2.   Membranes. 

4.  Chondrification.  6.  Marrow.  a.  basement. 

5.  Ossification.  b.  submucous. 

6.  Disappearance.  c.  dermal. 

?  by  liquefaction.  etc. 

7.  Hypertrophy.  3.  Ligaments. 

4.  Tendons. 

Embryonic  Mesenchyma. — Concerning  the  very  early  history 
of  the  mesenchyma  we  have  little  satisfactory  knowledge  beyond  the 
fact  that  the  cells  of  the  mesoderm  are  at  first  closely  crowded  and  as 
they  move  apart  are  seen  to  remain  connected  together  by  proto- 
plasmatic processes. 

As  regards  the  shape  of  the  cells  I  distinguish  two  stages,  of  which 
the  earlier  has  not  hitherto  been  definitely  recognized.  In  the  first 
stage,  which  I  have  observed  to  occur  to  elasmobranchs,  birds,  and 
mammals,  the  protoplasm  forms  a  complex  network  in  which  the 
nuclei  are  scattered  at  irregular  intervals ;  around  the  nuclei  there 
is  often  little  or  no  condensation  of  protoplasm,  so  that  there  are, 
properly  speaking,  no  perinuclear  cell  bodies.  The  tissue  corre- 
sponds, therefore,  very  poorly  to  our  conventional  conceptions.  This 
stage  is  well  represented  by  the  mesoderm  of  the  umbilical  cord  in  a 
human  embryo  of  about  seven  weeks,  Fig.  206,  p.  358.  The  form 
of  the  cells — or,  if  the  expression  be  preferred,  of  the  nodes  of  the 
reticulum — varies  greatly,  but  in  a  definite  manner  in  the  various 
regions  of  the  embryo ;  the  variations  depend  chiefly  upon  the  den- 
sity of  the  tissue  and  its  trend;  for  instance,  in  amniote  embryos 
with  two  to  four  gill-clefts  there  is  always  a  distinct  contrast  between 
the  dermal  mesenchyma,  which  is  of  loose  texture  and  with  no  de- 
cided trend,  and  the  mesenchyma  between  the  muscle-plate  and  the 
medullary  tube,  which  is  dense  and  has  elongated  cells.  The  differ- 
ences have  never  been  comprehensively  studied,  and  we  can  only 
say  that  they  are  the  expression  of  unlike  conditions  of  origin  and 
growth  of  the  various  parts  of  the  mesenchyma.  In  the  second  stage, 
which  seems  to  be  reached  by  all  the  cells  of  the  mesenchyma  sooner 
or  later  in  all  vertebrates,  the  protoplasm  has  formed  distinct  cell- 
bodies  around  the  nuclei,  and  there  are  no  considerable  accumula- 
tions of  protoplasm  except  around  the  nuclei.  This  stage  is  illus- 
rated  by  the  human  umbilical  cord  at  three  months,  Fig.  207,  p.  359, 
and  is  still  more  typically  and  characteristically  shown  by  the  mes- 
oderm of  a  chick  of  the  third  or  fourth  day,  or  in  a  rabbit  embryo 


THE    Ml.SKNCHYMAL   TISSUES. 


899 


^\  ,:  - 


Fio.  234.— Mesenchyma  of  a  Chick  Embryo  of 
the  Third  Day  from  Close  to  the  Otocyst.  A,  a 
iiiH  1-  us  in  karyokinesis ;  the  chromatin  loops 
are  seen  in  cross-section. 


( >f  ten  or  eleven  days ;  in  the  dog-fish  this  stage  is  not  reached  until 
considerably  later  in  t lie  development  than  in  the  amniote  embryo. 
In  the  chick,  Fig.  •?•'>  I,  the  cells  have  a  large  nucleus  of  rounded  form, 
with  a  distinct  intranuclear  reti- 
culum  of  protoplasm  and  one  or 
several  granules  of  chromatin; 
the  nucleus  is  surrounded  by 
granular  protoplasm,  constitut- 
ing small  cell-body,  which  sends 
off  tapering  processes  to  unite 
with  similar  processes  of  other 
cells ;  the  processes  are  sometimes 
very  short,  but  vary  in  length 
up  to  two  or  three  times  the 
diameter  of  the  cell- bodies.  The 
length  of  the  processes  also  varies 
in  different  regions,  so  that  the 
cells  in  one  region  are  more  or 
less  crowded  than  in  others ;  the  cells  also  vary  in  shape,  being 
elongated  in  certain  districts;  these  differences  are  all  significant  as 
the  results  of  previous  development  and  as  establishing  conditions 
for  the  subsequent  development.  In  young  mammalian  embryos  the 
cell-bodies  are  less  well  marked  than  in  the  chick,  and  the  processes 
form  a  network  of  fine  threads  between  the  cells,  as  can  be  seen  in 
places  in  rabbit  embryos,  as  late  at  least  as  the  seventeenth  day. 

The  matrix  is  perfectly  clear,  homogeneous,  colorless,  and  structure- 
less ;  it  is  of  slight  consistency,  and  scarcely  stains  with  any  of  the 
most  used  histological  dyes. 

Intercellular  Differentiation. — The  means  by  which  differ- 
entiation of  the  mesenchymal  matrix  is  effected  are  little  understood. 
If  we  accept  the  view,  which  is,  however,  as  yet  by  no  means  be- 
yond doubt,  that  the  fibrils  and  network  arise  from  the  cells,  we  can 
account  for  a  part,  but  only  for  a  part,  of  the  intercellular  structures. 
If,  on  the  other  hand,  we  hold  that  all  intercellular  structures  are  of 
intercellular  origin,  then  we  can  assume  that  there  is  some  general 
principle  in  accordance  with  which  they  are  all  produced.  Even  in 
this  case  the  cells  must  have  some  influence,  since  their  presence 
and  vitality  are  essential  conditions. 

Experiments  published  by  Harting  are  suggestive  in  this  connec- 
tion. 

Connective-Tissue  Fibrils. — The  fine  fibrils  of  the  adult  con- 
nective tissue  appear  quite  early  in  the  embryo  in  the  intercellular 
substance.  There  are  two  theories  of  the  origin  of  the  fibrils:  1, 
they  arise  from  cells;  2,  they  arise  from  the  matrix.  Their  origin 
from  cells  was  the  view  of  the  founder  of  the  modern  cell  theory, 
Theodore  Schwann,  39.1,  who  thought  that  the  cells  grew  in  length 
and  split  into  bundles  of  fibrils.  Various  modifications  of  this  theory 
have  since  appeared;  thus  Obersteiner  (Sitzungsber.  Wien.  Akad., 
LVL,  251)  traces  the  fibrillae  to  outgrowth  of  spindle-shaped  mesen- 
chymal cells.  Max  Schultze  (Reichert's  Archiv,  1861,  13)  thought 
that  the  cells  fused  together  and  their  fused  parts  formed  the 
fibrillaB  as  well  as  the  intercellular  substance,  thus  tracing  the  fibrillae 


400 


THE    FCETUS. 


to  a  differentiation  of  the  peripheral  parts  of  the  cells — a  view  which, 
somewhat  modified,  has  been  revived  by  B.  Lwoff,  89.1,  who 
maintains  that  the  fibrillse  arise  from  the  surface  of  the  cells,  nearly 
the  whole  body  of  each  cell  being  converted  into  fibrilla?,  which  ex- 
tend along  whole  rows  of  cells  and  along  their  processes,  enveloping 
the  protoplasm.  The  origin  of  the  fibrils  by  deposition  in  the  matrix 
was  first  maintained  by  Henle  ("  Allgemeine  Anatomie,"  Erste 
Aufl.,  379)  and  was,  in  my  judgment,  demonstrated  by  Rollet's 
investigations,  recorded  in  Strieker's  "  Gewebelehre, "  1871,  62-07, 
upon  the  omentum,  and  by  Ranvier's  later  observations  ("Traite 
technique  d'Histologie,"  405-411).  Kolliker,  whose  judgments  upon 
histological  problems  are  rarely  mistaken,  has  accepted  in  his 
"Gewebelehre,"  Gte  Aufl.,  123,  the  intercellular  origin  of  the  fibrils. 
If  we  examine  a  tissue  in  which  the  fibrils  are  just  beginning  to 
appear,  as,  for  instance,  the  human  umbilical  cord  toward  the  end  of 
the  third  month,  Fig.  207,  p.  359,  or  the  omentum  of  a  sheep  embryo 

of  17  cm.,  we  find  the  fibrils 
running  singly  and  in  every 
direction,  both  parallel  with 
the  cells  and  their  processes 
and  at  all  angles  with  them. 
The  omentum,  as  pointed 
out  by  Rollet,  is  a  particu- 
larly favorable  object,  for 
we  are  sure  of  having  the 
entire  length  of  the  fibres. 
The  cells  of  the  omentum 
gradually  assume  (sheep 
embryos  4-7  cm.)  an  elon- 
gated spindle  form,  remain- 
ing connected  together  only 
by  very  few  processes, which 
arise  chiefly  from  the  end  of 
the  cells ;  the  nuclei  become 
oval,  and  when  stained  with 
hsematoxylin  have  a  dis- 
tinct membrane,  and  consist 
of  a  clear  outer  layer  and  a 
dark  granular  central  part. 
Between  the  cells,  and  for 
the  most  part  remote  from 
them,  appear  the  fibrils, 
which  grow  in  length  and 
number.  In  later  stages, 
Fig.  235,  the  cells  of  the 
omentum  are  more  attenu- 
ated, and  their  ends  are 
united  so  as  to  form  a  net- 
work, though  some  of  the  cells  appear  to  terminate  without  any  con- 
nection with  their  fellows ;  the  nuclei  are  more  finely  granular  and 
have  lost  the  clear  outer  zone,  characteristic  of  earlier  stages.  The 
fibrillaB  have  grown  in  length  and  increased  enormously  in  number; 


FIG.  235 —Omentum  of  a  Human  Embryo  of  five 
Months,  c  c,  Connective  cells  forming  a  network ;  leu, 
leucocyte;  fb,  fibrillae  x  363  diams. 


THE   MESENCHYMAL   TISSUES.  401 

they  form  bundles  which  take  a  wavy  course ;  these  bundles  frequently 
subdivide  and  unite,  so  that  they  form  a  network;  their  course  and 
arrangement  are  independent  of  the  trend  of  the  cells,  and  there  is 
nothing  to  suggest  any  genetic  connection  between  the  cells  and  the 
bundles  of  fibrils.  Scattered  about  there  are  also  usually  a  few 
leucocytes,  Fig.  235,  leu,  which  are  readily  distinguishable  from  the 
true  mesenchymal  cells  or  so-called  connective-tissue  corpuscles,  c  c. 
Tlie  bundles  of  fibrils  correspond  to  the  connective-tissue  "fibres"  of 
the  adult ;  each  fibre  consists  of  a  large  number  of  fibrils.  The  em- 
bryonic fibrils  differ  from  those  of  the  adult  in  staining  much  more 
readily.  The  growth  of  the  fibres  depends  upon  multiplication  of 
the  fibrils  for  Harting  ("  Recherches  micrometriques sur  le  developpe- 
inent  des  Tissus,"  etc.,  1845,  p.  53)  found  that  the  fibrils  measured 
o.nniu-o.OOU  mm.  in  the  fcetus  and  from  0.0007-0.0017  mm.  in  the 
adult ;  as,  therefore,  the  fibrils  do  not  thicken  they  must  increase  in 
number  as  the  bundles  or  fibres  enlarge. 

Ranvier,  I.e.,  finds  that  the  fibrillae  have  no  connection  with  the 
cells  in  three  tissues,  which  he  has  studied  in  regard  to  this  point, 
namely,  the  embryonic  dermis,  the  developing  tendon,  and  the  scle- 
rotic cartilage  of  rays.  E.  A.  Schafer  (Quain's  "Anatomy,"  ninth 
edition,  II.,  72)  writes  as  follows:  "The  view  which  supposes  that  a 
direct  conversion  of  the  protoplasm  of  the  connective-tissue  cells  takes 
place  into  fibres,  both  white  and  elastic,  has  of  late  years  been  widely 
adopted,  but  it  seems  to  rest  less  upon  observation  than  upon  a  desire 
to  interpret  the  facts  in  accordance  with  the  conceptions  of  Beale  and 
M.  Sch ult xe,  according  to  which  every  part  of  an  organized  body 
consists  either  of  protoplasm  (formative  matter),  or  of  material  which 
lias  been  protoplasm  (formed  material),  and  the  idea  of  deposition  or 
change  occurring  outside  the  cells  in  the  intercellular  substance  is 
excluded.  But  it  is  not  difficult  to  show  that  a  formation  of  fibres 
may  occur  in  soft  substances  in  the  animal  organism,  independently 
of  the  direct  agency  of  cells,  although  the  materials  for  such  forma- 
tion may  be  furnished  by  cells.  Thus  in  those  coelenterate  animals 
in  which  a  low  form  of  connective  tissue  first  makes  its  appearance, 
this  is  distinguished  by  a  total  absence  of  cellular  elements,  the 
ground-substance  being  first  developed  and  fibres  becoming  formed 
in  it.  Again,  the  fibres  of  the  shell-membrane  of  the  bird's  egg 
are  certainly  not  formed  by  the  direct  conversion  of  the  protoplasm 
of  the  cells  which  line  the  oviduct,  although  it  is  probably  in  matter 
secreted  by  those  cells,  and  through  their  agency,  that  the  deposit 
occurs  in  a  fibrous  form." 

Intercellular  Network  or  Elastic  Tissue. — The  intercellu- 
lar substance  of  the  adult  contains  in  most  parts  of  the  mesenchyma 
a  network  which  varies  greatly  in  appearance.  This  network  has 
hithero  been  described  usually  as  being  formed  of  elastic  fibres ;  now 
since  the  material  which  forms  the  network  does  not  always  resem- 
ble fibres,  but  often  rather  lamella?,  and  since,  as  shown  by  F.  Mall, 
88.3,  91.1,  some  parts  of  the  network  do  not  contain  elastin,  it 
seems  very  undesirable  to  continue  the  use  of  the  term  elastic  fibres, 
which  is  entirely  -misleading.  I  shall  therefore  speak  of  the  two 
forms  of  tissue  as  yellow  elastic  network  and  white  non-elastic 
network  respectively.  Mall  states  that  there  is  a  non-elastic  mate- 
26 


402  THE   FCETUS. 

rial  which  alone  forms  the  white  network,  but  which  in  the  yellow 
network  forms  a  sheath  around  the  elastic  core. 

Concerning  the  development  of  the  network  we  possess  little  accu- 
rate knowledge.  Just  as  with  regard  to  the  intercellular  fibrils,  p.  399, 
there  are  two  theories:  according  to  one,  the  network  arises  by 
metamorphosis  of  the  cells;  according  to  the  other,  by  differenti- 
ation of  the  matrix.  The  origin  from  ramifying  cells  was  the  old 
theory  and  seems  at  first  thought  plausible — see  Bonders'  remarks 
in  Zeit.  wissensch.  Zool. ,  III. ,  358 — for  if  we  assume  the  cell  processes 
to  be  converted  into  elastin  a  network  would  result.  The  attempt, 
however,  to  demonstrate  the  actual  metamorphosis  has  hitherto  been 
unsuccessful.  Kuskow,  87.1,  found  that  in  the  ligamentum  nucha3  of 
the  embryo,  after  digestion  in  cold  pepsin  solution,  the  elastic  fibres 
could  be  seen  uniting  with  the  elongated  mesenchymal  nuclei,  usually 
with  the  ends,  sometimes  with  the  sides  of  the  nuclei.  Heller,  whose 
paper  I  know  only  from  the  abstract  in  Hofmann-Schwalbe's  Jahres- 
bericht  for  1887,  126-127,  is  said  to  have  seen  the  connection  with 
nuclei  both  in  the  ligamentum  nuchse  and  in  the  very  young  arytenoid 
cartilage  of  the  embryo;  in  the  cartilage  of  the  ear,  on  the  other 
hand,  Heller  states  that  there  is  no  connection  of  the  elastic  fibres 
with  either  the  nuclei  or  the  cells.  These  observations  show  that  the 
elastic  tissue  may  enter  into  special  relation  to  the  nuclei,  but  throw 
no  light  on  the  significance  of  the  connection ;  we  do  not  yet  know 
whether  the  fibres  develop  independently  and  afterward  unite  with 
the  nuclei,  or  are  united  with  them  from  the  start.  Kuskow's  sug- 
gestion that  the  elastic  network  is  formed  by  the  nuclei  is  not  likely 
to  be  verified,  because  nuclei  never  form  outgrowths  or  unite  with 
one  another  to  make  reticula,  so  far  as  heretofore  known. 

If  the  connection  with  the  nuclei  is  secondary,  then  the  network 
may  be  intercellular  in  origin. 

Ranvier,  "Traite  technique,"  401,  411,  has  shown  that  the  elastic 
tissue  first  appears  in  the  form  of  rows  of  granules  or  minute  glob- 
ules, probably  of  elastin,  which  subsequently  fuse  together  into  a  net- 
work the  lines  of  which  are  marked  out  by  the  original  deposition  of 
the  globules.  To  form  an  elastic  membrane  the  globules,  instead  of 
being  arranged  in  lines,  are  deposited  in  small  patches,  and  by  their 
fusion  form  a  plate.  In  elastic  cartilage  the  granules  first  make  their 
appearance,  it  is  true,  in  the  immediate  neighborhood  of  the  carti- 
lage cells.  This  renders  it  not  improbable  that  the  deposition  of  the 
granules  is  influenced  by  the  cells,  but  does  not  prove  that  they  are 
formed  by  a  direct  conversion  of  the  cell-protoplasm.  Indeed  the 
subsequent  extension  of  the  fibres  into  those  parts  of  the  matrix  that 
were  previously  clear  of  them  and  in  which  no  such  direct  conversion 
of  the  protoplasm  seems  possible  is  a  strong  argument  in  favor  of  the 
deposition  hypothesis.  For  an  admirable  discussion  of  the  two  views 
see  H.  Rabl-Ruckhard,  63.1. 

As  to  the  time  when  the  elastic  fibres  appear  we  may  say  in  gen- 
eral that  it  is  quite  late.  They  appear  in  the  ligamentum  nucha3  of 
cow  embryos  of  15  cm. ;  in  the  cartilage  of  the  ear  in  embryos  of 
30  to  32  cm.,  and  human  embryos  of  five  months,  Rabl-Ruck- 
hard, 63.1,  43;  in  the  arytenoid  cartilage  in  cow  embryos  of  55 
cm. ;  in  adult  fibro-elastic  cartilage  the  elastic  network  is  still  de- 


THK     MKSKN'CHYMAL    TISSl    KS.  403 

veloping,  and  is  not  formed  at  all  in  the  sheaths  of  nerves  until  adult 
life. 

The  elastic  network  grows  by  thickening  the  fibres  and  plates, 
which  are  found  much  larger  in  diameter  in  the  adult  than  in  the 
foetus.  In  this  respect  it  forms  a  striking  contrast  with  the  inter- 
cellular fibrillse,  which  grow  princially  by  multiplication. 

Concerning  the  development  of  the  white  non-elastic  network  we 
kno\v  almost  nothing. 

Mucous  Tissue  or  Wharton's  Jelly. — In  man  this  tissue  oc- 
curs only  in  the  umbilical  cord.  It  is  characterized  by  the  develop- 
ment of  mncin  in  the  intercellular  substance.  The  tissue  has  alreadjr 
been  described,  p.  358,  and  I  have  only  to  add  that  the  mucin  is 
].n -sent  in  a  diffuse  form,  and  has,  so  far  as  yet  known,  no  special 
structural  arrangement.  Mucous  tissue  is  said  to  occur  in  various 
parts  of  the  body  in  fishes,  but  unless  it  contains  intercellular  mu- 
ciu  it  cannot  be  regarded  as  true  mucous  tissue,  in  the  sense  here 
considered. 

Cartilage. — Cartilage  begins  to  be  differentiated  earlier  than  any 
other  of  the  mesenchymal  tissues,  except  the  blood-vessels,  which  aro 
developed  much  earlier,  and  perhaps  the  smooth  muscle-cells.  It  is 
probably  older  phylogenetically  than  any  of  the  other  tissues  of  the 
-n >up  except  the  two  mentioned,  for  not  only  does  it  appear  very 
early  in  the  embryo,  but  is  found  in  invertebrates.  It  is  for  conven- 
ience only  that  I  consider  cartilage  after  the  fibrillse  and  elastic  net- 
work, for  both  of  these  intercellular  structures  appear  in  certain  forms 
of  cartilage.  In  this  section  the  history  of  cartilage  is  considered 
under  the  following  heads:  1,  condensation  of  the  mesenchymal  tis- 
sue to  form  the  anlage  of  the  cartilage ;  2,  appearance  of  the  matrix ; 
:;,  young  cartilage;  4,  growth  of  cartilage;  5,  mature  cartilage; 
ii,  tibrillar  cartilage;  7,  elastic  network  cartilage. 

1.  Condensation  of  t/ir  Tixxue. — This  takes  place  simply  by  the 
cells  becoming  very  much  more  closely  crowded  together  than  in  the 
surrounding  mesoderm;  at  first  merely  the  central  portion  of  the 
anlage  is  thus  marked  out  and  there  is  a  very  gradual  transition  to 
the  looser  mesenchyma  about ;  for  every  piece  of  cartilage  there  is 
a  separate  anlage,  which  is  distinct  from  the  start ;  one  exception  to 
this  rule  occurs  in  the  case  of  the  vertebra,  as  has  been  stated  by 
Gegenbaur,  and  has  been  shown  with  great  precision  for  birds  and 
mammals  by  A.  Froriep,  83. 1,  86. 1.  Another  exception  is  offered 
by  cartilages  of  the  limbs  of  amphibia,  which  Goette  and  H.  Strasser, 
79. 1,  have  shown  to  be  coalesced,  when  they  first  appear. 

As  development  progresses,  the  anlage  becomes  more  and  more 
sharply  defined  until  at  last  its  limit  can  be  determined  within  a  cell 
or  two.  The  cartilage  cells  are  now  so  crowded  that  the  nuclei,  as 
lias  been  observed  in  all  classes  of  vertebrates,  seem  almost  to  actu- 
ally touch  one  another — see  H.  Strasser,  79.1,  245,  and  A.  Froriep, 
86.1,  73. 

When  the  anlage  is  completed  its  peripheral  cells  become  elon- 
gated and  form  the  anlage  of  the  perichondrium ;  while  the  central 
cells,  by  taking  on  the  rounded  form,  begin  their  metamorphosis  into 
cartilage  cells;  the  perichondrium  is  a  thin  layer.  C.  Hasse,  79.1, 
2,  thinks  that  the  cells  assume  a  spindle  shape  first,  and  afterward 


404  THE   FCETUS. 

take  on  the  rounded  form,  at  least  in  elasmobranchs.  It  is  uncertain 
whether  the  two  stages  can  be  distinguished  in  the  higher  vertebrates. 
The  first  cartilaginous  anlages  appear  in  the  chick  during  the  fifth 
day,  and  in  the  rabbit,  I  think,  about  the  thirteenth  day.  The  ver- 
tebral are  probably  always  the  first  cartilages  to  be  indicated  by 
completed  anlages.  The  other  cartilages  become  recognizable  later ; 
the  exact  times  need  to  be  determined  by  closer  study  than  has  yet 
been  attempted. 

2.  Appearance  of  the  Matrix. — Prce-cartilage  (prochondrium, 
Vorknorpel). — The  intercellular  substance,  as  the  cells  begin  to  move 
apart  and  lose  their  connections  with  one  another,  gradually  assumes 
a  greater  density  and  finally  becomes  highly   retractile  and  quite 
resistant  mechanically  and  chemically,  and  at  the  same  time  acquires, 
at  least  in  many  cases,  a  great  affinity  for  carmine  and  hsematoxylin. 
Hasse,  79.2,  states  that  this  color-reaction  always  appears  in  the 
young  cartilage  of  elasmobranchs,  and  therefore  he  proposes  to  dis- 
tinguish the  stage  as  a  distinct  one,  since  the  matrix  of  the  fully 
differentiated  hyaline  cartilage  does  not  stain ;  for  the  young  car- 
tilage with  colorable  matrix  he  proposes  the  term  Vorknorpel,  which 
I  have  rendered  prce-cartilage.     Hasse  states  that  in  the  pra3-carti- 
lage  of  elasmobranchs  the  matrix  consists  of  numerous  fibrillse  held 
together  by  a  cementing  substance.     This  is  now  generally  held  to 
be  the  structure  of  the  matrix  in  adult  hyaline  cartilage — see,  for  in- 
stance, Spronck,  87.1,  and  Kolster,  87.1,  who  both  give  references 
to  the  preceding  literature.      Hasse  further  states  that  in  pra3-carti- 
lage  the  matrix  is  of  uniform  structure  throughout,  and  that  there 
are  no  capsules  around  the  cells.     The  cells  of  young  cartilage  are 
said  to  contain  glycogen;  Rouget  claims  to  have  found  it  in  the 
sheep  embryo  at  two  months.     Many  authors  have  held  that  the 
matrix  was  formed  as  a  series  of  capsules,  one  around  each  cell ;  the 
capsules  grow  and  fuse.     In  support  of  this  view  there  are  no  satis- 
factory observations  known  to  me.     As  it  is  adopted   in   Quain's 
"Anatomy"  by  Schaefer  (ninth  edition,  II.,  84),  I  presume  it  rests 
upon  some  good  authority,  which  I  have  overlooked. 

When  the  condensed  mesenchyma  is  beginning  to  change,  dark  ir- 
regular masses  appear  among  the  cells ;  these  are  the  "  prochondral 
elements"  of  H.  Strasser,  79.1.  Alice  Johnson,  83.1,  400,  states 
that  they  may  be  seen  in  the  hind  limb  of  the  chick  about  the  sixth 
day,  and  she  interprets  them  as  degenerated  cells  which  have  lost 
their  nuclei. 

3.  Young  hyaline  cartilage  differs  but  little  from  that  just  de- 
scribed, except  that  the  matrix  has  increased  and  the  cells  are  slightly 
larger.     It  is  to  be  considered  as  the  primitive  form  of  tissue,  from 
which  all  the  modifications  of  adult  cartilage  are  derived.     In  the 
thyroid  cartilage  of  a  three-months  human  embryo  I  find  the  cells 
farther  apart  and  a  little  larger  than  in  younger  stages;  the  cells  are 
still  small  and  are  here  and  there  in  groups  of  two ;  they  are  not 
round  but  more  or  less  compressed  in  shape,  and  some  of  them  appear 
to  contain  fat  granules.     In  the  same  cartilage  at  four  months  the 
general  appearance  is  the  same  as  before,  but  the  matrix  stains  un- 
evenly ;  around  the  cells  it  is  light,  but  between  the  cells  intervenes 
a  darker-colored  portion  which  forms  a   network  through  the  tissue. 


THE    MESENCHYMAL  TISSUES.  405 

In  the  neighborhood  of  the  prochondrium  the  matrix  is  altogether 
light  and  the  cells  are  in  part  larger,  rounder,  and  with  distinct  spher- 
ical nuclei.  In  the  tracheal  cartilage  of  embryos  of  about  seven 
months  the  cells  are  decidedly  larger  than  those  of  the  thyroid  just 
described;  the  rounded  nuclei  are  very  distinct ;  the  protoplasm  is 
granular  and  entirely  tills  the  cell  space  (lacuna)  of  the  matrix  ;  the 
cells  exhibit  only  a  very  slight  tendency  to  form  cell  groups  as  in 
mature  cartilage,  nor  are  there  any  signs  I  can  recognize  as  such,  of 
the  degenerative  changes  which  can  be  seen  in  the  adult. 

4.  (jroirthof  Cartilage. — The  matrix  presumably  grows  by  in- 
tu>susception,  and  not,  as  some  authors  have  maintained,  by    the 
continual  conversion  of  the  superficial  protoplasm  of  the  cells  into 
matrix.     If  such  a  conversion  took  place  we  should  expect  to  see  the 
cells  diminish  in  size,  whereas  they  increase.     The  cells  increase  in 
number  by  division,  and  by  additions  from  the  perichondrium ;  of 
the  two  factors  the  latter  is  probably  the  more  important. 

The  division  of  cartilage  cells  has  been  especially  studied  by  W. 
Schleicher,  79.1.  The  division  is  indirect.  The  nuclear  membrane 
tirst  of  all  disappears  or  is  converted  into  filaments  which  soon  be- 
come lost  among  the  other  filaments  developed  within  the  nucleus. 
The  filaments  are  at  first  short  and  irregular,  but  soon  take  on  a 
stellate  arrangement,  and  the  chromatin  becomes  grouped  into  an 
equatorial  plate,  which  divides,  one  group  of  chromatin  elements 
moving  toward  one  pole,  the  other  toward  the  opposite  pole.  The 
division  of  the  protoplasm  is  not  effected  as  usual  in  animal  cells,  but 
1  >y  means  of  a  cell-plate,  as  in  many  vegetable  tissues ;  the  cell-plate 
forms  a  partition  in  the  middle  of  the  elongated  binucleate  cell;  the 
plate  grows  and  becomes  the  matrix  between  the  two  daughter  cells. 
As  the  plate  thickens  slowly  the  cells  remain  near  together  for  some 
time,  and  one  or  both  them  may  again  divide  with  the  result  that. 
there  is  a  ^%'o'ip  of  three  or  four  cells.  This  grouping  is  highly 
characteristic  of  adult  cartilage,  but  exactly  when  it  first  appears  I 
do  not  kn«»w.  It  does  not  appear  in  embryonic  cartilage,  so  that 
we  must  assume  either  that  in  the  embryo  the  cartilage  cells  do  not 
divide,  or  else  that  they  divide  and  move  apart  very  rapidly.  In 
either  case  the  grouping  of  the  cells  remains  a  sign  of  age,  and  ought 
perhaps  to  be  regarded  as  the  expression  of  a  diminished  vitality. 

Concerning  the  exact  history  of  the  perichondrial  cells  as  they 
change  into  cartilage  cells  special  investigation  is  needed.  At  pres- 
ent we  can  say  hardly  more  than  that  the  change  takes  place. 

5.  ]\I<  if  arc  Hyaline  Cartilage. — The  hyaline  cartilage  of  the  adult 
exists  in  two  principal  modifications,  both  characterized  by  the  great 
development  of  the  matrix  and  by  having  the  cells  for  the  most  part 
in  groups  of  two,  three,  or  four,  but  distinguished  by  having  in  the 
one  case  large  cells  with  round  nuclei  and  well-developed  protoplas- 
matic bodies,  and  in  the  other  cells  which  have  shrunk  somewhat  and 
are  often  compressed,  with  nuclei  which  are  often  indistinct  and  irreg- 
ular, and  protoplasm  which  frequently  contains  fat  globules.  I  believe 
that  we  have  to  do  with  two  stages  in  the  life-history  of  cartilage, 
and  that  the  first  modification,  in  which  the  cells  are  large,  is  the 
earlier  stage,  and  represents  the  maximum  of  development,  while  the 
second,  in  which  the  cells  are  shrunk  and  fatty,  represents  a  later 


406  THE   FGETUS. 

stage,  with  more  or  less  degeneration.  Dekhuyzen,  whose  papers  I 
know  only  through  the  abstract  prepared  by  himself  for  Hofmann- 
Schwalbe,  Jahresbericht  f.  1889,  82-83,  was  the  first  to  interpret  the 
mature  cartilage  as  a  degenerating  tissue.  In  deciding  upon  the 
order  of  the  two  stages  I  have  been  guided  chiefly  by  my  observa- 
tions upon  the  growing  cartilages  of  the  lung  in  rodents,  for  in  them 
the  large,  round,  protoplasmatic  cells  lie  between  the  connective-tissue 
cells  on  the  one  hand,  and  the  fatty,  compressed  cartilage  cells  on  the 
other,  and  clearly  present  a  transitional  stage  of  the  transformation 
of  the  perichondrial  cell  into  the  old  cartilage  cell,  and  by  the  fur- 
ther observation  that  in  the  child  at  birth  the  bronchial  cartilage 
consists  entirely  of  large,  rounded  cells  with  spherical  nuclei.  The 
changes  which  are  here  noted  as  degenerative  begin  very  early; 
thus  Dekhuyzen  states  that  they  are  well  advanced  in  the  epiglottis  of 
the  dog  at  birth. 

Little  has  been  done  upon  the  development  of  the  matrix,  but 
numerous  researches  have  been  made  upon  the  structure  and  chemical 
composition  of  the  adult  matrix.  A  little  upon  the  chemical  develop- 
ment after  birth  may  be  found  in  Moner  (Schwalbe's  Jahresber. 
f.  1889,  81-82). 

6.  Fibro~cartilage  appears  first  in  the  form  of  hyaline  cartilage, 
and  the  fibrilla3,  which  appear  in  the  matrix  and  seem  to  be  homolo- 
gous with  the  ordinary  intercellular  connective-tissue  fibrillaa,  are 
developed  earlier  or  later. 

7.  Elastic  cartilage  also  appears  as  hyaline  cartilage,  in  which 
an  elastic  network  is  subsequently  developed. 

Degeneration  of  Ossifying  Cartilage. — Besides  the  changes 
of  a  degenerative  character,  above  referred  to,  the  skeletal  cartilages 
undergo  a  complete  resorption,  whenever  in  the  course  of  develop- 
ment they  are  to  be  replaced  by  bone,  except  that  in  a  few  parts  the 
cartilage  is  changed  directly  into  bone.  There  are  twofforms  of  the 
resorption  of  cartilage,  the  direct  and  the  indirect.  The  direct  re- 
sorption occurs  in  only  a  few  cases,  as  for  instance  in  the  greater 
part  of  Meckel's  cartilage,  and  is  characterized  by  the  gradual  dis- 
appearance of  the  cartilage  without  any  preceding  striking  change 
in  it.  The  indirect  resorption  occurs  whenever  the  development  of 
bone  begins  in  the  interior  of  a  cartilage,  and  is  accompanied  by  very 
remarkable  structural  alterations  in  the  cartilage.  So  far  as  I  know 
no  exact  study  of  the  direct  resorption  of  cartilage  has  yet  been 
made,  while  the  indirect  resorption  has  been  investigated  again  and 
again. 

The  indirect  resorption  begins  in  the  centre  of  the  cartilage;  the 
first  step  in  the  process  is  an  enlargement  of  the  single  cartilage  cells, 
without  much  or  any  change  in  the  amount  of  the  matrix  between 
them,  but  the  matrix  assumes  a  granular  appearance  and  acquires  a 
gritty  feel  to  the  knife  owing  to  the  formation  of  calcareous  deposits. 
Meanwhile  the  cartilage  above  and  below  the  centre  of  degeneration 
becomes  enlarged  and^  pi  led  up  in  elongated  groups  or  columns  which 
radiate  from  the  centre  for  a  certain  distance  toward  either  end.  The 
radiating  columns  of  cells  taper  toward  their  ends  away  from  the 
centre,  the  end  cells  being  smaller.  In  the  matrix  between  the  col- 
umns calcification  takes  place,  so  that  calcified  partitions  separate 


THE   MESENCHYMAL  TISSUES.  1(>T 

the  columns  from  one  another.  Turning  now  to  the  cells  w<  •  find  that 
they  are  undergoing  a  hypertrophic  degeneration,  for  their  enlarge- 
ment precedes  their  breaking  down.  There  has  been  no  sufficient 
study  of  the  changes  in  the  cells,  but  so  far  as  my  own  observations 
enable  me  to  judge  the  changes  are  probably  as  follows,  Fig.  ^:>s.  The 
cell  enlarges  and  its  protoplasm  becomes  granular;  the  enlargement 
continues  and  the  cell  appears  to  encroach  upon  the  matrix  more  and 
more  until  ultimately  adjacent  cell-cavities  coalesce;  while  this  cor- 
rosion of  the  matrix  is  progressing  the  protoplasm  of  the  cell  becomes 
vacnolated  ;  its  nucleus  becomes  irregular  and  indistinct,  and  sooner 
or  later  disintegrates;  the  cell  then  contracts  and  forms  a  flattened 
body,  which  stains  more  or  less,  but  exhibits  no  distinct  structure, 
unless  no\v  and  then  some  trace  of  the  original  nucleus;  after  the 
cells  have  shrunk  their  cavities  fuse  together,  and  sooner  or  later 
the  cells  break  down  into  mere  granular  detritus.  The  coalescence 
of  the  cell-cavities  does  not  take  place  equally  in  all  directions,  but 
principally  as  shown  in  Fig.  -.':}*,  along  radiating  lines;  hence  there 
arise  a  series  of  radiating  cavities  separated  by  partitions  formed  by 
the  calcified  matrix.  While  these  changes  are  going  on  in  the  inte- 
rior of  the  cartilage,  columns  of  the  surrounding  connective  tissue 
go  into  the  cartilage  at  various  points,  but  always  toward  the  de- 
generating tissue;  each  column  contains  blood-vessels  als<5.  As  to 
why  or  how  these  columns  penetrate  the  firm  cartilage  with  their 
own  soft  tissues,  we  know  nothing.  The  columns  reach  the  centre 
of  degeneration  just  as  the  cells  of  the  cartilage  break  down  and  the 
ingrowing  new  connective  tissue  at  once  fills  the  spaces  formed  in  the 
cartilage  and  proceeds  in  its  new  site  to  produce  bone.  The  degen- 
erative process  now  extends  toward  both  ends  of  the  cartilage  and  is 
followed  by  the  formation  of  bone.  The  whole  series  of  changes  is 
commonly  termed  the  ossification  of  cartilage,  but  this  is  incorrect, 
for  the  cartilage  is  destroyed,  not  ossified.  The  conjunction  of  the 
two  sets  of  processes,  Fig.  238,  creates  very  singular  microscopical 
pictures,  which  for  a  long  time  puzzled  investigators.  For  further 
details  see  the  following  section  on  ossification. 

Ossification. — Bone  is  a  mesenchymatous  tissue,  in  which  the 
cells  have  a  characteristic  shape  and  the  matrix  or  intercellular  sub- 
stance is  large  in  amount  and  calcified.  It  is  derived  always  by 
a  direct  metamorphosis  of  embryonic  connective  tissue  or  of  embry- 
onic cartilage,  and  of  periosteum.  The  ossification  of  cartilage 
plays  a  small  part — for  instance,  at  the  angle  of  the  jaw  it  has  been 
observed  to  occur  by  J.  Brock,  76.1,  who  found  the  cartilage  cells 
changing  into  bone  cells  there,  though  nowhere  else  in  the  mandible. 
Most  bones  are  formed  by  the  ossification  of  the  connective-tissue 
cells  or  undifferentiated  mesenchyma,  and  by  layers  of  bone  added 
by  the  ossification  of  the  periosteum.  Bony  tissue  after  it  is  once 
formed  does  not  grow  except  by  additions  to  its  surface.  In  the  sim- 
plest form  of  ossification  we  have  a  layer  or  membrane  of  connective 
tissue,  in  which  the  tissue  changes  into  bone;  this  is  called  intra- 
membranous,  direct,  or  metaplastic  ossification.  The  direct  ossi- 
fication of  cartilage  should  also  be  placed  under  this  head.  As  a 
modification  of  the  simple  ossification  we  must  regard  the  ossification 
to  replace  cartilage,  which  is  termed  the  intra-cartilaginous,  indirect, 


408 


THE   FCETUS. 


V. 

FIG.  230. —Parietal  Bone  of   a  Human   Embryo  of  fourteen 
Weeks.    After  Kulliker.     x  18  diams. 


or  neoplastic  ossification.  In  both  types  the  actual  processes  of 
ossification  are  essentially  the  same,  and  the  bone  is  completed  by  the 
co-operation  of  the  periost. 

Metaplastic  Ossification. — This  may  be  conveniently  studied  in 
the  parietal  bone  of  the  human  embryo.     About  the  end  of  the  third 

month,  in  the  parietal 
region  of  the  membran- 
ous skull  there  appear 
minute  calcified  spic- 
ules,  which  rapidly  in- 
crease in  number  and 
grow  both  in  diameter 
and  length  so  that  they 
soon  fuse  together  and 
form  an  irregular  net- 
work, Fig.  236.  The 
meshes  of  the  network 
are  filled  with  mesen- 
chymal  cells,  which  are 
continually  forming 
bone  upon  the  surface 
of  the  spicules.  Some- 
what later  the  fibrous  periosteum  appears  upon  the  surface  of  the 
young  bone,  and  adds  osseous  tissue  to  it. 

The  transformation  of  the  mesenchyma  into  bone  is  illustrated  by 
Fig.  237,  which  represents 
a  transverse  section  of  the 
foetal  mandible.  The  man- 
dible is  closely  invested  by 
the  fibrous  periosteum,  per, 
which  is  in  part  artificially 
separated  from  the  bone,  Os, 
the  irregular  bars  of  which 
have  already  acquired  con- 
siderable thickness ;  the 
spaces  in  the  interior  of  the 
bony  mandible  are  filled 
with  a  loose  mesenchyma, 
the  cells  of  which  have  large 
rounded,  finely  granular 
nuclei,  with  but  little  proto- 
plasm forming  cell-bodies; 
the  cells  are  connected  by  a 
rich  network  of  fine  granu- 
lar threads  with  one  an- 
other. Some  of  the  cells  lie 
directly  against  the  bone, 
either  just  touching  it,  or 


obi' 


FIG.  237.— Transverse  Section   of   the   Mandible  of   a 
-  Human  Embryo  of  the  tenth  Week.     Minot  Collection, 

half    Or  wholly  imbedded  in    No.  13&     Os,  Bone;  wes,  mesenchyma;  obi,  obi',  osteo- 

it;  those  which  are  in  the  blasts;  per<  perios 

bone  are  true  bone  cells,  and  are  easily  recognized  as  modified  embry- 
onic connective-tissue  cells,  which  have  gradually  accumulated  more 


THE   MESENCHYMAL   TISSUES.  409 

and  more  protoplasm  so  that,  since  the  cells  begin  to  enlarge  as  soon 
ua  they  touch  the  bone,  they  are  found  to  have  grown  considerably 
1  »y  the  time  they  are  completely  imbedded.  The  connective-tissue 
cells,  which  lie  against  the  bone,  are  known  as  osteoblasts,  a  name 
proposed  by  Gegenbaur  in  1804;  though  often  close  together  they 
always  arc  separated  by  distinct  spaces  from  one  another;  they  are 
rounded,  polyhedral,  or  triangular  in  form,  and  frequently  are  so 
crowded  <>VIT  the  surface  of  the  bone  that  they  present  a  distinctly 
epithelioid  arrangement;  the  nucleus  usually  lies  toward  one  side  or 
cm  I  of  the  cell.  The  osteoblasts  become  imbedded  in  the  bony  matrix 
and  thereby  converted  into  bone  cells,  not  by  migration,  but  by  the 
Ljr<  .\vth  of  the  calcified  matrix,  the  formation  of  which  goes  on  first 
nn  the  side  of  the  osteoblast  toward  the  bone,  and  gradually  advanc- 
ing overgrows  the  osteoblast  and  continues  beyond  it.  The  hist*  >ry 
of  the  intercellular  threads  of  protoplasm  during  the  transformation 
<>f  the  connective-tissue  cell  into  an  ostenblast,  and  then  into  a  bone- 
(vll,  lias,  so  far  as  I  am  aware,  never  been  followed  out.  It  seems  to 
me  probable  that  the  threads  are  preserved  and  lead  to  the  develop- 
ment of  the  canaliculi,  just  as  the  cell-bodies  produce  the  so-called 
lacuna1.  Whether  threads  of  protoplasm  run  through  the  canaliculi 
in  tin- mature  bone  or  not  is  still  under  dispute.  Beside  the  osteo- 
blasts in  the  interior  of  mandible  there  are  others,  Fig.  237,  obi,  which 
are  derived  from  the  cells  of  the  periost,  per,  and  although  the  peri- 
osteal  cells  are  of  a  very  different  character  from  those  of  the  mesen- 
chvma,  mes,  in  the  interior  of  the  mandible,  yet  all  the  osteoblasts 
are  alike.  E.  A.  Schafer  has  directed  attention  to  what  he  calls 
the  ostcot/fiH'fic  fibres*  Upon  close  observation  of  the  growing 
-picules  of  the  parietal  bone  the  calcified  parts  appear  granular,  and 
lion i  them  Schafer  finds  that  there  run  out  for  a  little  way  soft  and 
]  )liant  bundles  of  transparent  fibres.  They  exhibit  a  faint  fibrillation 
and  have  been  compared  to  bundles  of  white  connective-tissue  fibrils, 
with  which  in  some  situations  they  appear  to  be  continuous.  But 
although  similar  in  chemical  composition,  they  are  somewhat  differ- 
ent from  these  in  appearance,  having  a  stiffer  aspect  and  straighter 
course,  besides  being  less  distinctly  fibrillated.  The  fibres  become 
calcified  by  the  deposition  within  them  of  earthy  salts  in  the  form  of 
minute  globules,  which  produce  a  darkish  granular  opacity,  until  the 
interstices  between  the  globules  also  become  calcified,  and  the  minute 
globules,  becoming  thus  fused  together,  the  bone  again  looks 
comparatively  clear.  It  is  stated  that  the  fibrils  themselves  are  not 
calcified,  but  the  calcification  affects  the  portion  of  matrix  which 
unites  them  into  the  osteogenic  fibres,  so  that  these  may  be  described 
as  being  calcified.  The  bundles  of  osteogenic  fibres  which  prolong 
the  bony  spicules  generally  spread  out  from  the  end  of  each  spicule 
so  as  to  come  in  contact  with  those  from  adjacent  spicules.  When 
this  happens,  the  innermost  or  proximal  fibres  frequently  grow  to- 
gether, while  the  other  fibres  partially  intercross  as  they  grow  further 
into  the  membrane.  The  ossific  process  extends  into  the  osteogenic 
fibres  pari  passu  with  their  growth,  and  thus  new  bony  spicules 
become  continually  formed  by  calcification  of  the  groups  or  bundles 

*  This  account  of  the  osteogenetic  fibres  is  taken  with  some  slight  changes  from  Quain's 

"Anatomy."  ninth  edition. 


410  THE    FCETUS. 

of  osteogenic  fibres.  The  earthy  deposit  not  only  involves  the  osteo- 
genic  fibres,  but  also  the  ground-substance  of  the  tissue  in  which 
they  lie.  It  occasionally  appears  in  an  isolated  patch  here  and  there 
on  some  of  the  osteogenic  fibres  in  advance  of  the  main  area  of  ossi- 
fication. The  osteogenic  fibres  become  comparatively  indistinct  as 
they  and  the  substance  between  them  calcifies;  they  appear,  how- 
ever, to  persist  in  the  form  of  decussating  fibres,  such  as  are  seen  in 
the  adult  bone,  although  in  the  embryonic  bone  their  disposition  is 
less  lamellated,  the  bony  matter  having  a  somewhat  coarsely  reticular 
structure. 

Neoplastic  Ossification. — When  bone  replaces  degenerated  car- 
tilage, the  method  of  bone  formation  is  essentially  the  same  as  when 
of  ossification  takes  place  in  connective  tissue,  except  for  one  fea- 
ture, namely,  that  the  bone  is  first  deposited  against  the  calcified 
remnants  of  the  cartilaginous  matrix  as  soon  as  the  cartilage  cells 
have  disappeared.  A  section  through  an  ossifying  long  bone  or  ver- 
tebra, Fig.  238,  presents  a  highly  characteristic  picture,  and  if  the 
section  be  made  as  in  the  figure,  parallel  to  the  columns  of  cartilage 
cells,  all  the  phases  can  be  seen  in  a  single  successful  preparation. 
The  section  figured  was  stained  with  Beale's  carmine  and  alum 
ha3matoxylin,  by  which  method  not  only  are  the  cells  and  nuclei 
brought  out  clearly,  but  also  the  calcified  cartilage  is  made  deep  blue, 
while  the  bone  is  deep  red.  In  the  upper  part  of  the  figure,  C,  the 
cartilage  cells  are  just  forming  groups  or  columns,  which  a  little 
lower  down,  C',  are  very  distinct;  lower  down  again,  C",  the  cavi- 
ties, in  which  the  columns  of  cartilage  cells  lie,  have  fused  together 
into  large  spaces ;  in  these  spaces  the  cartilage  cells,  c,  are  scattered 
in  various  stages  of  disintegration ;  the  adjacent  spaces  are  separated 
from  one  another  by  partitions  formed  of  ossified  cartilaginous  ma- 
trix, Ma,  which  appears  a  deep  blue  in  marked  contrast  to  the  un- 
calcified  matrix  of  the  upper  part  of  the  figure,  where  the  matrix  is 
almost  uncolored.  The  remnants  of  calcified  matrix  extend  far  be- 
low the  cartilage.  At  the  level  indicated  by  the  bracket,  L,  the  new 
mesenchyma,  mes,  is  found  penetrating  the  spaces  between  the  blue 
partitions,  Ma;  the  mesenchyma  is  accompanied  by  blood-vessels, 
which  are  easily  recognized,  V,  by  their  endothelial  walls.  Some  of 
the  invading  mesenchymal  cells  lay  themselves  against  the  surfaces 
of  the  calcified  partitions,  become  osteoblasts  and  produce  bone,  which 
thickens  by  additions  to  its  surface.  Thus  the  calcified  remains  of 
the  cartilage  become  coated  with^bone,  which  in  the  preparation  de- 
scribed has  a  rich  red  stain.  As  in  the  lower  part  of  the  figure  the 
development  is  more  advanced,  we  find  there  the  layer  of  bone,  B, 
much  thicker  than  nearer  the  cartilage.  Fig.  239  is  a  very  accurate 
drawing  of  part  of  a  section  of  a  vertebra  of  a  four-months'  embryo  so 
made  that  the  columns  of  cartilage  cells  are  cut  at  right  angles ;  the 
level  of  the  section  corresponds  to  the  lower  part  of  bracket  L,  Fig. 
238.  The  cartilage  cells  have  disappeared  and  have  been  replaced 
by  the  invading  mesenchyma ;  the  network  of  partitions  formed  by 
the  remnants  of  the  calcified  matrix,  Ma,  of  the  cartilage  is  at 
once  recognized,  as  can  also  be  recognized  the  transformation  of 
the  cells  into  osteoblasts,  obi,  and  the  deposit  of  bone,  B,  upon 
the  partition;  noteworthy  are  also  the  osteoclasts,  Osc,  to  which 


THE   MESENCHYMAL   TISSUES. 


411 


fuller  reference  is  made  in  the  following  paragraph  on  the  growth  of 
bone. 

In  the  long  bones  the  periosteal  ossification  has  great  importance, 

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FIG.  238.— From  a  Section  of  an  Ossifying  Vertebra  of  a  Human  Embryo  of  four  Months. 
Minot  Collection,  No.  35.  C,  C".  C',  Cartilage;  6',  region  where  the  cells  are  beginning  to  form 
rows;  (",  region  of  the  cells  in  columns;  C",  region  where  the  cells  are  breaking  down,  and 
wlit-iv  the  c.-ll  spaivs  are  separated  by  calcified  cartilaginous  matrix.  Ma,  c,  degenerated  carti- 
lage cell;  #,  layer  of  bone;  F,  blood-vessels;  mes,  ingrowing  mesenchyma;  L,  level  of  ossifica- 
tion, x  172  diams. 

and  as  it  proceeds  very  rapidly  at  first  in  the  central  part  of  the  bone, 
most  of  the  shaft  is  formed  from  the  periost — compare  Quain's 
"Anat.,"  ninth  edition,  II.,  Fig.  109. 


412 


THE   FCETUS. 


We  have  learned  that  the  development  of  bone  may  take  place 
from  embryonic  connective  fibrillar  tissue  (periost),  or  from  cartilage, 
but  whatever  its  origin,  it  has  always  nearly,  if  not  quite,  the  same 
structure.  This  is  true  both  of  the  cells  and  the  matrix. 

Historical  Note. — I  have  purposely  abstained  from  attempting 
a  full  history  of  ossification.  For  full  and  comprehensive  accounts 
I  refer  to  Quain's  "Anatomy,"  Ranvier's  "  Traite  technique  d'Histo- 
logie,"  Kolliker's  "  Gewebelehre,"  Krause's  "  Anatomie,"  etc.  For  a 
good  review  of  the  literature  up  to  1858,  seeH.  Miiller,  58.2,  and  for 


^M^Jimm 


.Si: -obi 


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FIG.  239.— Sectton  of  a  Vertebra  of  the  same  Embryo  at  right  Angles  to  the  Plane  of  Fig. 
238,  and  corresponding  in  level  to  the  lower  part  of  the  bracket  i,  Fig.  238.  Osc,  Osc',  Osc", 
osteoclasts;  I?,  bone;  obi,  osteoblasts:  I/a,  calcified  matrix  of  cartilage.  Stained  with  haeina- 
toxylin  and  cosine.  X  356  diams. 

references  to  the  more  important  later  authorities  see  Ranvier's 
"Traite,"  Rollett's  chapter  in  Strieker's  "Gewebelehre,"  and  Mas - 
quelin's  "Memoir." 

Growth  of  Bone. — It  is  a  well-known  fact  that  the  bones  do  not 
grow  in  the  ordinary  sense ;  the  bone  cells  cannot  multiply ;  the  ap- 
parent growth  of  bone  is  accomplished  by  the  destruction  of  the  bone 
already  formed  and  the  production  of  new  bone.  The  destruction  of 
the  bone  is  effected  by  means  of  large  multinucleate  cells,  Fig.  239, 
Osc,  which  are  derived  from  the  mesenchymal  cells,  but  just  how  is 
not  clear.  The  cells  in  question  have  been  named  myeloplaxes  (or 
myeloplacques),  by  Robin  and  French  histologists,  and  osteoclasts 


THE   MESENCHYMAL  TISSUES.  413 

(bone-destroyers)  by  Kolliker.  They  are  frequently  found  against 
the  surface  of  the  bone,  on  cartilage,  Fig.  239,  Osc',  and  in  that 
case  lie  in  a  little  concavity  formed  by  the  eating  away  of  the  bone. 
As  the  development  of  these  cells  is  not  known  and  as  their  functions 
have  been  but  little  studied  in  the  embryo,  the  detailed  examination 
of  their  structure  and  history  may  be  omitted  here.  Full  accounts 
of  the  growth  of  bone  may  be  found  in  all  the  standard  histologies. 

Disappearance  of  Intercellular  Substance. — In  the  adult 
there  are  various  spaces  in  the  mesenchymal  tissues,  which  are  in 
the  natural  condition  filled  with  fluid,  such  as  the  so-called  lymph 
spaces  and  lymph  channels ;  these  spaces  have  no  cellular  walls.  In 
the  lymph  glands  also  there  is  much  fluid  between  the  cells  and  retic- 
ulum  of  the  gland.  We  must,  therefore,  assume  that  the  intercel- 
lular substance  has  in  some  way  been  replaced,  but  whether  it  has 
been  liquefied,  or  resorbed  and  fluid  supplied  in  its  stead,  or  simply 
cavities  developed  in  it,  we  do  not  know.  "We  can,  therefore,  do 
nothing  more  than  note  the  gap  in  our  knowledge. 

Hypertrophy  of  Intercellular  Substance. — By  this  I  do  not 
mean  the  increase  which  occurs  in  connection  with  the  development 
of  fibrillse,  elastic  network,  or  cartilage,  but  the  hypertrophy  of  the 
clear  homogeneous  matrix  of  the  young  mesenchyma  or  embryonic 
connective  tissue.  Such  an  hypertrophy  occurs  in  the  amnion,  in 
the  young  cutis,  and  elsewhere,  and  it  is  probably  the  most  impor- 
tant factor  in  the  histogenesis  of  the  vitreous  and  aqueous  humors ; 
as  to  how  this  hypertrophy  is  effected  nothing  is  known.  For  the 
history  of  the  vitreous  humor  see  Chapter  XXVIII. 

Blood-Vessels  are  the  earliest  of  the  mesenchymal  tissues  to  be 
differentiated.  Their  history  has  already  been  given  in  full.  See 
Chapter  X. 

Lymphatic  System  consists  of  lymph  spaces,  lymphatic  ves- 
sels, and  lymph  glands.  The  lymph  spaces  are  merely  channels  in 
the  intercellular  substance,  concerning  the  development  of  which 
nothing  has  been  ascertained,  and  not  much  is  known  concerning  the 
development  of  the  vessels  or  glands. 

/,//////>//'///>  Vessels. — Kolliker  ("Gewebelehre,"  5te  Aufl.,  599- 
600)  states  that  in  tails  of  tadpoles  the  lymph  vessels  can  be  seen  de- 
veloping, in  similar  manner  to  the  blood-vessels,  by  the  hollowing 
out  of  mesenchymal  cells.  Klein  has  come  to  the  same  conclusion 
from  the  study  of  the  development  of  lymphatics  in  serous  mem- 
branes. According  to  Klein  a  vacuole  is  formed  within  one  of  the 
cells  of  the  connective  tissue,  and  becomes  gradually  larger,  so  as 
ultimately  to  produce  a  cavity  filled  with  fluid,  while  the  protoplasm 
of  the  cell  thins  out  to  form  the  wall  around  the  cavity.  He  also 
adds  that  from  this  wall  portions  bud  inward  into  the  cavity,  and 
detaching  themselves  become  lymph  corpuscles — but  this  history 
cannot  be  accepted  without  better  foundation.  To  form  vessels  the 
vesicular  cells  become  connected  together.  The  protoplasmatic  walls 
become  multinucleate  and  are  differentiated  into  the  lining  endothe- 
linm.  A.  Budge's  incompleted  investigation  of  the  development  of 
the  lymphatics  in  the  chick,  87.1,  was  published  posthumously  by 
Professor  W.  His,  and  is  an  admirable  piece  of  thorough  work.  The 
main  part  of  the  published  memoir  is  devoted  to  the  history  of  the 


414 


THE   FCETUS. 


formation  of  the  coelom  by  the  fusion  of  a  network  of  channels  in  the 
mesoderm,  see  p.  151.  Budge  states  that  after  the  coelom  is  devel- 
oped some  of  the  channels  are  still  found  in  the  somatopleure,  and 
represent  the  primitive  lymphatics ;  the  somatopleure  at  this  stage  has 
no  blood-vessels  and  the  splanchnopleure  no  lymph-vessels.  The 
primitive  lymph-vessels  communicate  directly  with  the  ccelom. 
Later  on  the  ductus  thoracicus  appears  and  establishes  the  commu- 
nication between  the  lymphatics  and  the  blood-vessels.  Unfortunately 
the  published  paper  contains  no  details  about 
the  development  of  the  ductus.  In  a  short 
note  (Centralbl.  Med.  Wiss.,  1881,  No.  34) 
Budge  has  reported  that  in  the  allantois  of  a 
chick  of  eighteen  to  twenty  days  there  are 
abundant  lymphatics  which  can  be  injected 
with  a  subcutaneous  syringe.  The  vessels 
accompanying  the  arteries,  forming  networks 
around  them,  Fig.  240,  extend  along  the  arte- 
rise  umbilicales  to  enter  the  body  and  run  along 
the  aorta  (see  Budge,  87.1,  60,  and  Taf.  VI., 
Fig.  2)  as  paired  ducti,  which  are  connected 
with  one  another  by  smaller  cross  stems,  and 
unite  in  the  upper  part  of  the  thorax  into  a 
single  duct,  which,  however,  again  forks  and 
has  a  double  opening  into  the  veins.  The 
right  umbilical  lymph  stem  appears  to  atrophy 
later.  In  connection  with  the  allantoic 
l3*mphatics  Budge  has  found  (see  His  and 
After"  Aibrecht  Braune's  Arch.  f.  Anat.,  1882,  350)  in  chick 
embryos  of  ten  to  twenty  days,  lymph  hearts, 
which  lie  in  the  angle  between  the  pelvis  and  coccyx. 

Lymph- Glands. — Concerning  the  development  of  the  glands  I 
know  of  three  papers,  Sertoli,  66.1,  Chievitz,  81.1,  and  a  disserta- 
tion by  Orth  (Bonn,  1870) ,  which  last  I  have  not  seen.  Kolliker  quotes 
also  Breschet  ("  Le  Systeme  lymphatique, "  Paris,  1836, 185)  and  Engel 
(Prag.  Viertelj.,  II.,  Ill,  1850)  as  maintaining  that  the  glands  arise 
each  as  a  plexus  of  lymph  vessels — a  view  which  the  observations  of 
Sertoli  have  set  aside.  To  study  the  early  stages,  glands  must  be 
chosen,  the  exact  position  of  which  in  relation  to  other  parts  can  be 
determined,  in  order  that  the  condition  of  the  tissue  before  the  differ- 
entiation of  the  gland  can  be  ascertained.  With  this  in  view  Sertoli 
selected  the  mesenterial  glands  in  cow  embryos  and  Chievitz  the 
same  in  the  pig  and  the  inguinal  gland  in  man.  Sertoli  found  in 
four-inch  embryos  fissures  in  the  connective  tissue  of  the  mesentery 
where  the  glands  were  to  appear ;  in  four-inch  embryos  these  spots 
were  further  marked  out  by  the  crowding  of  nuclei  around  them. 
In  six-and-a-half-inch  embryos  the  anlages  were  pear-shaped,  the 
pointed  end  being  toward  the  radix  mesenterii ;  the  pointed  end  alone 
contains  lymph  spaces,  while  the  blunt  end  in  which  the  nuclei  are 
crowded  is  the  anlage  of  the  future  cortex  of  the  gland.  Somewhat 
later  the  fibrous  envelopes  of  the  glands  are  differentiated,  and  as 
soon  as  their  formation  begins,  the  growth  of  the  glands  by  accession 
from  the  surrounding  mesenchyma  ceases.  Chievitz  studied  the 


FIG.  240.— Artery  from  the 
Allantois  of  a  Chick,  sur- 
rounded  by  a  Network  of 
Lymphatics. 
Budge. 


THE    MESENCHYMAL   TISSUES.  415 

human  inguinal  gland;  its  anlage  is  clearly  recognizable  at  about 
three  months,  and  at  three  and  one-half  months  the  cortical  portion 
with  crowded  nuclei  can  be  distinguished  from  the  medullary,  in 
which  there  are  spaces;  the  gland  is  separated  from  the  surrounding 
tissue  by  a  fissure  which  is  crossed  by  a  few  threads;  the  fissure  does 
not  extend  across  the  part  of  the  gland  corresponding  to  the  future 
hilus;  the  cells  of  the  glands  have  large  granular  nuclei,  and  are 
easily  distinguished  from  the  lymphoid  cells,  which  are  much 
smaller  with  spherical  refringent  nuclei;  at  first  there  are  very  leu 
lymphoid  cells,  but  they  increase  in  number.  Concerning  the  devel- 
opment of  thereticulum,  which  Ranvier  ("  Traite  technique, "  678)  has 
shown  to  be  distinct  in  the  mature  glands  from  the  branching  cells, 
\ve  have  no  information. 

Spleen. — Although  the  development  of  the  spleen  must  offer 
many  points  of  great  interest,  it  has  received  very  little  attention.  In 
O.  Hertwig's  text-book  no  mention  of  the  spleen  is  made;  Kolliker, 
in  both  his  text-books,  dismisses  the  organ  with  a  single  brief  para- 
graph. A  little  fuller  is  the  notice  by  W.  Muller,  in  Strieker's 
"  Handbuch  der  Gewebelehre,"  260.  Of  special  investigations  there 
are  three  short  ones,  Peremeschko,  67.1,  2,  and  F.  Maurer,  90.1, 
ami  the  longer  article  on  the  spleen  in  fishes  by  E.  Laguesse,  90. 1. 

The  spleen  is  developed  out  of  a  mesenchymal  anlage,  which  be- 
comes recognizable  in  the  human  embryo  toward  the  end  of  the  sec- 
ond month.  In  all  amniota  it  is  situated  in  the  mesogastrium  near 
the  pancreas,  and  close  to  large  arterial  vessels.  Its  first  differen- 
tiation appears  to  be  due  to  an  accumulation  of  rather  large  lymph- 
oid cells  with  large  granular  nuclei,  and  to  the  moving  apart  of 
the  mesenchymal  cells,  which  are  much  smaller  than  the  lymphoid. 
Concerning  the  origin  of  the  lymphoid  cells  we  have  only  the  obser- 
vations of  F.  Maurer,  90,  who  found  in  tadpoles,  measuring  from 
6-8  mm.  from  mouth  to  anus,  of  the  frog,  Rauu  1<'mjHtr<iri«,  that  the 
entoderm  gives  off  cells  which  pass  into  the  mesenchyma  and  give 
rise  to  the  first  lymphoid  cells.  Maurer  also  obtained  evidence  that 
the  same  process  occurs  in  the  tailed  amphibians.  During  the  third 
month,  in  man  (Kolliker),  the  blood-vessels  penetrate  the  organ, 
which  soon  becomes  rich  in  bloods  W.  Muller  states  that  the  fur- 
ther development  proceeds  rapidly,  so  that  in  the  human  foetus  of 
eight  centimetres  in  length  the  various  constituents  are  already  differ- 
entiated. The  cells  lying  beneath  the  peritoneal  epithelium  'become 
elongated,  and  form  fusiform  nucleated  bodies,  and  similar  ones  at 
an  early  period  invest  the  larger  vessels.  From  both  small  processes 
are  given  off  which  grow  toward  one  another  and  represent  the  com- 
mencement of  the  trabecular  system.  Along  the  arterial  branches 
denser  accumulations  of  small  nucleated  cells  may  already  be  dis- 
cerned, which  are  conspicuous  in  tinted  preparations  by  their  deep 
color,  and  these  form  by  far  the  chief  constituent  of  the  pulp.  This 
consists  of  cells  with  from  one  to  three  nuclei  and  a  delicate  inter- 
cellular substance,  forming  plexuses,  the  interstices  of  which  are 
constantly  filled  with  blood-corpuscles.  According  to  Peremeschko, 
there  are  now  developed  larger  protoplasmic  corpuscles  in  the  tissue 
of  the  pulp  containing  from  two  to  six  nuclei,  that  are  capable  of 
performing  amoeboid  movements,  and  which,  toward  the  end  of 


416  THE   FOETUS. 

embryonic  life,  atrophy.  In  the  further  course  of  development  the 
several  constituents  increase  in  volume,  and  a  part  of  the  fusiform 
cells  of  the  capsule  and  the  vascular  sheaths  develop  into  smooth 
muscular  tissue.  The  arterial  sheaths,  containing  numerous  cells, 
are  clearly  distinguishable  from  the  pulp,  and  from  the  middle  of 
embryonic  life  the  Malpighian  corpuscles  are  recognizable.  Concern- 
ing the  size  of  the  foetal  spleen  I  know  only  of  the  statement  by 
Kolliker,"  Grundriss,"  380,  that  in  man  by  the  eighth  week  the  anlage 
measures  0.62x  0.31  mm.,  and  in  the  third  month  1.7  x  1.13  mm. 

In  the  embryo  at  six  months  the  spleen  already  has  its  triangular 
form  in  outline;  the  fibrous  sheath  or  capsule,  C,  is  differentiated; 


FIG.  241.— Section  of  the  Spleen  of  a  Human  Embryo  of  six  Months.     C,  Capsule;  Hi,  hilus; 
V  V)  blood-vessels.     (The  Embryo  is  Minot  Collection,  No.  8.) 

the  hilus,  Hi,  is  wide ;  the  main  blood-vessels  are  remarkable  for 
their  size,  and  are  encased  in  the  sheaths  of  muscle  fibres  as  in  the 
adult ;  the  differentiation  of  the  Malpighian  corpuscles  is  indicated 
by  the  scattered  areas,  in  which  the  cells  are  more  crowded,  which 
therefore  appear  darker  in  the  stained  specimen.  In  a  thin  section 
(0.01  mm.)  of  a  some  what  younger  spleen  the  reticulum  of  the  spleen, 
the  abundant  blood  capillaries,  and  the  immense  number  of  pulp- 
cells  I  find  all  well  shown ;  the  pulp  cells  have  round,  finely  granular 
nuclei  with  a  very  small  amount  of  protoplasm ;  I  see  also  a  much  less 
number  of  smaller  oval  nuclei,  which  seem  to  belong  to  the  reticulum. 
Laguesse's  monograph,  90. 1,  on  the  spleen  of  fishes  is  a  conscien- 
tious and  valuable  work.  The  spleen  appears  late,  some  time  after 
the  pancreas,  in  the  mesenchymal  wall  of  the  duodenum  close  to 
and  on  the  left  side  of  the  insertion  of  the  mesentery,  and  in  close 
relation  with  the  subintestinal  vein.  The  anlage  is  first  recognizable 
by  the  condensation  of  the  tissue  and  the  accumulation  of  free  cells 
in  its  meshes.  The  developing  spleen  gradually  comes  into  closer 
relations  with  the  stomach  and  separated  from  the  duodenum,  and 


THK    MKSKNCHVMAL    TISSUES.  41*' 

is  ultimately  situated  in  the  mesogastrium.  The  origin  of  tin*  free 
cells  was  in  >t  ascertained,  but  the  author  is  inclined  to  trace  them 
to  the  mesenchyma  rather  than  to  accept  F.  Maurer's  view.  They 
are  small,  have  rounded  granular  nuclei  (Laguesse's  noyau  </'or/f//'//r  i 
and  very  little  protoplasm ;  according  to  Laguesse  they  give  rise 
some  to  leucocytes,  others  to  red  cells;  but  in  regard  to  this  1  think 
there  is  need  of  further  evidence,  for  in  other  cases  we  know  that 
leucocytes  and  n-d  blood-cells  (heniaties)  have  different  origins.  The 
network  is  produced  in  *////  by  the  mesenchymal  cells,  the  proce- 
of  which  gradually  l>ecome  more  resistant,  refringent,  and  homoge- 
neous, while  the  nuclei  gradually  disappear  more  or  less  completely. 
This  confirms  the  view  so  long  defended  by  Kolliker,  as  to  the  nature 
<  it  t  he  reticulum  of  the  spleen.  The  cavities  of  the  spleen  form  a 
rich  network,  which  very  soon  enters  into  direct  communication 
with  branches  whi<-h  develop  from  the  subintestinal  (portal)  vein, 
but  the  similar  connection  with  the  arteries  is  not  established  until 
later ;  after  the  arteries  have  penetrated  there  is  a  circulation  through 
the  spleen  and  many  of  its  free  cells  are  carried  off,  but  in  places 
aside  from  the  currents  there  remain  accumulations  of  multiplying 
free  cells ;  such  accumulations  are  found  especially  around  the  large 
arteries.  The  veins  in  the  spleen  consist  only  of  an  endothelium, 
but  in  the  adult  are  in  part  encased  in  a  sort  of  basement  membrane 
formed  by  condensation  of  the  spleen  reticulum  around  the  larger 
vessels. 

Smooth-Muscle  Fibres. — That  these  are  simply  modified  mes- 
enchymal cells  seem  to  me  no  longer  open  to  doubt,  as  explained  in 
Chapter  VI.  on  the  mesoderm.  This  implies  that  the  hypothesis  so 
1<  >ng  upheld  by  His,  that  the  muscles  are  genetically  distinct  from  the 
c(  miiective-tissue  elements,  must  be  definitely  laid  aside.  His  classed 
the  muscles  as  archiblastic  elements  His'  pupil,  Erik  Miiller,  has 
sought  in  a  special  article,  88.1,  to  justify  His'  view,  but  the  his- 
tory he  gives  is,  that  the  inner  mesothelium  of  the  primitive  segment 
breaks  up  into  mesenchyma,  and  that  some  of  these  mesenchymal  cells 
form  the  peri-endothelial  walls  of  the  aorta — a  fact  I  can  verify  from 
my  o wn  observations  on  birds  and  mammals — but  others  of  the  cells, 
coming  from  the  inner  wall  of  the  segment,  form  connective  tissue, 
so  that  in  this  instance  we  have  a  proof  of  the  identical  mesenchymal 
origin  of  the  two  tissues.  So  also  in  the  umbilical  cord,  it  can  be 
s< 'on  alter  the  third  month  that  the  vessels  are  surrounded  by  smooth 
muscle  cells,  which  gradually  pass  into  mesenchymal  cells  proper;  as 
the  muscular  walls  thicken  with  age  it  seems  evident  that  the  transi- 
tion represents  an  actual  transformation  of  the  connective- tissue  cells 
into  muscle  cells,  but  the  details  of  the  process  have  still  to  be  worked 
out.  The  earliest  definite  proof,  known  to  me,  that  no  line  can  be 
drawn  between  smooth  muscle  and  connective  tissue  is  that  afforded 
by  Flemming's  observations,  78.2,  on  the  bladder  of  salamanders,  in 
which  both  tissues  with  all  intermediate  forms  occur. 

Concerning  the  histogenetic  transformation  of  mesenchyma  into 
smooth  muscle  we  possess  no  detailed  or  accurate  information. 

Fat-cells  first  appear  in  the  human  embryo,  it  is  said,  about  the 
fourteenth  week,  and  after  their  first  appearance  gradually  increase 
in  size  and  number  up  to  the  time  of  birth,  when,  however,  the  fat 
27 


418 


THE   FCETUS. 


cells  are  still  much  smaller  than  in  the  adult.  The  fat  cells  are 
derived  from  the  embryonic  connective-tissue  cells  or  mesenchyma,  as 
has  been  demonstrated  by  Flemming,  71.1,  71.2,  whose  view  was 
questioned  by  L.  Ranvier  ("  Traite"),  and  the  Hoggans,  79. 1.  Ran- 
vier's  observations  were  incomplete,  in  that  he  did  not  ascertain  the 
origin  of  the  cell  which  forms  the  fat-cells,  as  Flemming  has  pointed 
out  in  his  reply,  79. 1,  to  the  criticisms  upon  his  work.  The  inves- 
tigations of  the  Hoggans  appear  to  me  untrustworthy. 

The  fat  cells  are  always  developed  in  groups  or  clusters,  and  each 
cluster  is  supplied  with  an  abundant  network  of  blood  capillaries. 


FIG.  242.  —Fat  Island  from  the  Skin  of  a  Human  Embryo  of  five  Months.  Minot  Collection, 
No.  95.  Ve._  Blood-vessel;  Mes,  mesenchyma  or  embryonic  connective  tissue;  F,  fat  cells.  X  310 
diams. 

The  fat  cells  always  occur  in  the  neighborhood  of  blood-vessels,  so 
that  one  is  almost  compelled  to  conclude  that  superabundant  food  sup- 
ply is  an  essential  condition  of  their  development.  Some  interesting 
studies  on  the  circulation  in  fat  tissue  have  been  published  by  J. 
Schobl,  85.1.  The  clusters  of  fat  cells  may  be  called  fat  islands,  a 
term  less  likely  to  mislead  students  than  that  of  fat  globule,  which 
has  been  used.  Fig.  242  represents  a  section  of  a  fat  island  in  the 
embryonic  cutis,  drawn  very  exactly  from  the  preparation,  which 
had  been  stained  with  alum  cochineal  and  eosine ;  the  mesenchymal 
cells,  Mes,  are  scattered  around  and  completely  isolate  the  fat  islands 


THE    MESENCHYMAL   TISSUES.  1  1  '.» 

from  one  another;  the  fat  evils,  F,  form  a  group  by  themselves;  each 
cell  has  a  large  globule  of  fat  surrounded  by  a  thin  layer  of  proto- 
plasm, which  is  thickened  on  one  side,  where  the  nucleus  is  situated; 
the  smaller  the  cell  the  more  distinctly  does  the  layer  of  protoplasm 
stand  out;  the  nuclei  are  compressed,  smaller  than  those  of  the  sur- 
rounding me&enchyma,  and  more  darkly  stained  ;  the  difference  be- 
t  \veen  the  staining  of  the  fat-cell  and  the  other  nuclei  is  exaggerate  >d 
in  the  drawing.  By  their  subsequent  growth  and  expansion  the  fat 
islands  may  fuse  together,  thus  forming  a  more  or  less  continuous 
fatty  layer. 

A-  regards  th-k  history  of  the  single  cells  our  knowledge  restfl 
chiefly  on  the  admirable  re3earches  of  Flemming,  I.e.  The  cells  lose 
their  connections  with  one  another  and  assume  a  somewhat  rounded 
form,  and  the  amount  of  protoplasm  increases;  the  nucleus  comes  to 
lie  on  one  side  of  the  cell  either  before  the  fat  granules  are  developed 
or  just  as  they  are  beginning  to  appear  ;  according  as  the  nucleus 
is  peripheral  or  central  the  fat  is  at  first  on  one  side  or  around  the 
Periphery  of  the  cell.  In  either  case  the  fat  soon  collects  in  one  main 
globule,  \\ithotlier  small  ones  about  it  in  the  protoplasm,  and  thus 
the  condition  of  the  young  fat  cells,  as  in  Fig.  242,  is  attained.  Soon 
alter  the  nucleus  has  been  forced  to  one  side  by  the  fat  the  membrane 
of  the  cell  appears.  It  is  probable  that  the  fat  is  accumulated  within 
the  oils  before  it  becomes  microscopically  visible  as  granules,  for 
Stolnikow  (Ardi.  Anat.  u.  PhysioL,  SuppL,  1887,  p.  1),  has 

h 


that  the  fat  in  the  liver  cells  of  frogs  after  phosphorus 
poisoning  may  be  present  in  considerable  quantities  without  appear- 
ing in  granules.  Upon  this  stress  has  been  laid  by  Gaule,  90.1,  as 
indicating  that  the  fat  is  bound  to  some  other  compound,  perhaps  leci- 
thin. This  lends  support  to  the  suggestion  of  Poljakoff,  88.1,  that 
the  dull  (;*  imittcn")  granules,  which  appear  in  the  protoplasm  before 
or  along  with  the  first  minute  fat  granules  and  disappear  as  the 
fat  increases,  are  made  up  of  fat  combined  with  some  albuminoid. 

The  degeneration  or  regression  of  fat  cells  has  been  studied  by 
Fl  <  -n  i  in.  i  ng  and  Poljakoff,  but  as  the  change  does  not  occur  before  birth 
it  does  not  fall  within  our  scope,  beyond  noticing  the  suggestion  that 
Ehrliclf  s  -Mast/ellen  (plasma  cells)  are  regressive  stages  of  fat  cells. 

Pigment  Cells.  —  Concerning  their  development  in  the  embryo  I 
know  of  no  exact  investigation.  What  Goette  gives,  75.  1,  521-522, 
is  largely  speculative.  Flemming,  90.1,  has  shown  that  the  pig- 
ment cells  multiply  by  indirect  division  in  salamander  Iarva3,  and 
that  the  scission  of  the  protoplasm  may  be  delayed.  K.  W.  Zim- 
mennann,  90.1,  has  given  some  further  details.  The  divisional 
process  offers  several  interesting  features. 

The  pigment  granules  which  give  color  to  the  epidermis  are  not 
of  epidermal  origin,  but  arise  in  mesenchymal  cells,  which  wander 
in  from  the  underlying  cutis.  The  source  of  the  pigment  was  dis- 
covered by  Aeby,  85.  1,  whose  observations  have  been  extended  and 
confirmed  by  Kolliker,  87.2,  3,  u  Ge  webelehre,  "  Gte  Aufl.,  202,  List, 
89.1,  and  Piersol,  90.2.  Kodis,  89.  1,  on  the  other  hand,  has 
maintained  that  the  pigment  cells  are  formed  in  the  epidermis  and 
wander  thence  into  the  cutis,  but  Kodis  fails,  I  think,  to  prove  his 
point.  In  amniota  the  first  pigment  appears  in  small  granular  cells 


420  THE    FCETUS. 

in  the  basal  layer  of  the  epidermis  (lizard,  40  mm. ;  chick  of  ten 
days;  cat, 47  mm.).  These  cells  resemble  leucocytes  so  much  that 
Kodis  has  designated  them  as  "  leucocytoide  Zellen;"  they  lie  be- 
tween the  true  epidermal  cells;  the  protoplasm  is  small  in  amount 
when  the  pigment  begins  to  appear,  but  as  the  pigment  increases 
the  cell  enlarges  and  passes  from  an  apparently  round  to  a  distinctly 
stellate  form.  In  mammals  the  bodies  of  the  cells  are  composed  at 
first  of  clear,  homogeneous,  faintly  granular  protoplasm,  in  the  midst 
of  which  sharply  defined  oval  nuclei  are  seen ;  in  short,  they  resem- 
ble the  cells  of  the  underlying  cutis  and  are  probably  immigrant 
mesenchymal  cells.  The  earliest  pigment  particles  are  sparingly  and 
irregularly  distributed,  but  soon  evince  a  tendency  to  aggregate 
about  the  nucleus,  around  which  a  brown  wreath  is  soon  formed. 
Subsequently  pigment  cells  appear  also  in  the  cutis  and  exhibit  a 
strong  tendency  to  collect  beneath  the  epidermis  and  to  form  there 
rich  networks.  These  cells  send  processes  into  the  epithelium,  to 
be  followed  often  by  the  greater  part  of  the  cell ;  it  is  thus  that  the 
pictures  of  immigrating  pigment  cells  arise. 

As  to  the  source  of  the  pigment  granules,  they  seem  to  be  formed 
within  the  pigment  cells  and  not  to  be  taken  up,  as  some  writers 
have  suggested,  as  preformed  particles  from  outside.  It  is  possible 
that  the  pigment  is  connected  genetically  with  the  haemoglobin,  but 
of  this  there  is  no  definite  proof.  For  a  discussion  of  the  source  of 
pigment  granules  see  Maass  (Arch.  f.  mikrosk.  Anat.,  XXXIV., 
452)  and  Piersol,  I.e. 

Marrow. — The  marrow  of  bone  is  derived  from  the  mesenchyma, 
which,  as  above  described,  p.  410,  enters  the  space  left  by  the  degen- 
erating cartilage;  some  of  these  mesenchymal  cells  become  osteo- 
blasts,  while  the  remainder  produce  the  marrow  of  the  future  bone. 
The  marrow  has  a  very  complex  structure  in  the  adult,  and  numerous 
investigations  upon  its  adult  structure  have  been  published.  In  these 
publications  are  scattered  a  good  many  observations  on  the  foetal 
marrow,  but  as  they  have  never  been  properly  collated,  and  as  there 
is  no  comprehensive  research  upon  the  development  of  the  foetal 
marrow,  I  reluctantly  forego  the  attempt  to  describe  the  histogenesis 
of  the  tissue — a  subject  which  would  certainly  well  repay  competent 
thorough  study.  I  will  only  add  that  the  suggestion  made  by  Raii- 
vier  ("  Traite  technique,"  439),  that  the  cells  of  the  degenerating  carti- 
lage produce  marrow  cells,  cannot  in  my  opinion  be  upheld,  for  it 
appears  to  me  unquestionable  that  the  cells  of  the  cartilage  are  dis- 
integrated. 

Mesenchymal  Cavities. — Under  this  head  I  do  not  include  the 
blood-vessels,  nor  lymph-vessels,  nor  the  lymph  channels  of  the  inter- 
cellular substance  and  lymph  spaces  of  the  lymphatic  glands,  but 
only  those  spaces  which  have,  so  to  speak,  passive  functions,  are  filled 
with  serous  fluid,  and  are  entirely  bounded  by  mesenchyma.  For 
example :  the  channels  around  the  membranous  labyrinth  of  the  ear 
(compare  the  second  division  on  the  ear  in  Chapter  XXVII.),  the 
subarachnoid  space,  the  synovial  and  bursal  cavities.  These  are 
probably  all  formed  by  the  cells  breaking  apart,  and  are  further 
characterized  by  the  tendency  of  the  layer  of  mesenchymal  cells  im- 
mediately round  the  cavity  to  become  crowded  until  they  form  a  dis- 


THK  .MKM-:N<  u  V.MAI.  TISSIKS. 

tinrt  lining  endothelinm.  The  degree  to  which  this  tendency  is 
evinced  varies  extremely,  and  we  may  have  the  cells  either  simply 
somewhat  crowded,  or  converted  into  an  endothelium  in  patches,  or 
wliolly  eiidotheliuin.  The  transition  from  one  form  of  tissue  to  the 
other  can  be  seen  in  the  adult  synovial  cavities,  and  is  important  as 
additional  evidence  of  the  slight  real  difference  between mesendiy ma 
and  epithelium. 

1  kno\\  no  observations  on  the  development  of  the  arachnoid  spaces. 

Xijnorial  and  JJurtHtl  ( 'tin'fit'K. — The  development  of  the  synovial 
cavities  has  been  studied  by  Hagen-Torn,  82. 1.  Between  the  car- 
tilages of  the  limbs  there  is  left  un differentiated  mesenchyma.  which 
very  early  acquires  blood-vessels  and  shows  later  an  increased  vascu- 
larity.  The  formation  of  the  cavity  begins  in  the  centre  between 
the  cartilages,  and  is  first  indicated  by  the  tissue  becoming  less  dense 
there  (rabbit  embryos  1(.  »-•>(.)  mm.) ;  some  of  the  central  cells  undergo 
a  mucoid  degeneration  and  disappear,  others  become  spindle-shaped 
and  change  into  cartilage  cells,  with  the  result  that  the  ends  of  the 
skeletal  ca  tillages  are  now  separated  from  one  another  only  by  a 
slight  space.  At  the  sides  of  the  cavity  the  mesenchyma  forms  the 
•vial  membrane,  which  is  merely  very  vascular,  fibrillar  con- 
nective tissue;  upon  the  synovial  surface  patches  of  endothelium 
are  developed.  Villi,  if  formed  at  all,  appear  in  later  stages  and  al- 
ways at  the  sides  of  the  cavity  by  the  synovial  membrane  proper. 

Membranes. — The  development  of  the  various  membranes  and 
special  mesenchymal  layers,  such  as  the  submucosa,  dermis,  etc..  is 
considered  in  connection  with  the  various  organs,  to  which  they  he- 
long.  Tl ie re  is  one  general  feature  which  may  be  mentioned  here, 
namely,  the  so-called  basement  membranes.  By  this  term  is  now 
generally  understood  the  layers  of  endothelioid  cells  found  imme- 
diately underneath  various  epithelia;  for  instance,  under  the  ento- 
derm  (epithelium)  of  the  intestine,  around  the.  Graafian  follicles  of 
the  ovary,  around  the  seminiferous  tubules,  and  the  urinary  tubi^o. 
These  membranes,  often  designated  as  tunica  propriae,  are  undoubt- 
edly the  product  of  the  mesenchyma,  though  nothing  is  known  of 
their  development.  They  have  the  general  morphological  interest  of 
demonstrating  the  tendency  of  the  mesenchyma  to  revert  to  the  epi- 
thelioid  type. 

Ligaments  and  Tendons. — Both  structures  are  modifications 
of  fibrilke  and  elastic  connective  tissue.  The  tendons  consist  almost 
wholly  of  fibrillae  running  all  in  the  same  direction.  The  ligaments 
vary  more,  and  may  consist  either  of  fibrillar  or  elastic  tissue  or 
both.  The  development  of  the  ligaments  has  scarcely  been  studied; 
that  of  tendons  has  been  investigated  by  L.  Ranvier,  74.1,  also  his 
"  Traite  technique,"  407;  the  regeneration  and  growth  of  the  tendon 
tissue  in  the  adult  has  been  studied  by  several  authorities — see  A. 
Beltzow,  83.1.  We  learn,  however,  little  beyond  the  fact  that 
where  tendon  is  to  be  formed  the  cells  arrange  themselves  in  rows, 
parallel  with  the  length  of  the  future  tendon ;  the  fibrilla3  are  devel- 
oped between  the  rows  and  parallel  to  them,  and  gradually  increase 
until  they  occupy  the  entire  space  between  the  cells.  By  what  stages 
the  cells  pass  from  the  condition  of  simple  mesenchyma  to  the  sin- 
gular shapes  of  the  adult  tendon  cells  is  unknown. 


CHAPTER   XX. 
THE  SKELETON. 

THE  literature  of  the  skeleton  is  very  extensive  as  regards  both  its 
development  and  comparative  anatomy.  The  ease  with  which  skele- 
tons can  be  prepared  and  the  importance  of  the  hard  parts  to  the 
palaeontologist  has  long  given  the  skeleton  a  prominence  in  morpho- 
logical research  far  in  excess  of  its  importance  as  compared  with  the 
other  systems.  Athough  the  skeleton  is  in  the  mechanical  sense 
the  framework  of  'the  body,  it  is  not  so  in  the  morphological  sense, 
because  so  far  is  it  from  being  the  framework  upon  which  the  body 
is  built  up,  that  its  development  is  entirely  subsidiary  to  the  devel- 
opment of  other  systems,  and  is  dominated  by  the  arrangement  of 
other  tissues  and  organs,  which  have  been  formed  and  arranged  be- 
fore the  skeleton  even  begins  to  appear. 

In  this  chapter  there  is  no  attempt  to  give  an  exhaustive  treatise 
upon  the  development,  but  by  following  the  summaries  given  by 
Kolliker  ("  Entwickelungsgeschichte, "  2te  Aufl.,  401-502),  Hertwig 
("Lehrbuch,"  3te  Aufl.,  492-543),  and  W.  K.  Parker  ("Morphology 
of  the  Skull"),  and  consultation  of  the  more  important  original  au- 
thorities, I  have  endeavored  to  write  a  comprehensive  account  of  the 
subject. 

Stages  of  the  Skeleton. — We  must  distinguish  between  the 
stages  of  the  skeleton  as  a  whole,  and  the  stages  in  the  histogenesis 
of  the  bones.  It  must  also  be  constantly  borne  in  mind  that  the  verte- 
brates have  two  morphologically  distinct  skeletons,  the  primary 
cartilaginous  skeleton,  which  in  the  higher  forms  becomes  partly 
ossified,  and  the  secondary  skeleton  composed  of  dermal  bones. 

1.  Not ochordal  Stage. — Permanent  in  amphioxus.     In  this  stage 
the  only  skeleton  is  the  axial  rod  of  the  notochord,  and  it  is  found  to 
be  the  first  stage  in  all  vertebrate  embryos. 

2.  The  Membranous  Stage. — The  second  stage  in  all  true  verte- 
brates and  the  permanent  one  in  marsipobranchs.     The  mesenchyma 
is  condensed  around  the  notochord  and  strengthens  thus  the  axis. 

3.  The  Primary  Cartilaginous  Stage. — The  principal  parts  of 
the  primary  skeleton  are  represented  by  separate  cartilages. 

4.  The  Completed  Cartilaginous  Stage. — In  which  all  the  parts 
of  the  primary  skeleton  are  present  in  the  form  of  cartilages.     No 
definite  line  can  be  drawn  between  this  stage  and  the  preceding,  nor 
between  it  and  the  following. 

5.  Stage  of  the  dermal  skeleton,  characterized  by  the  develop- 
ment of  sundry  bones  in  the  dermis.     Dermal  bones  begin  to  develop 
before  the  cartilages  ossify,  and  are  present  in  cartilaginous  fishes, 
hence  they  must  be  considered  as  older,  and  therefore  belonging  to 
an  earlier  stage,  than  the  bones  replacing  cartilages. 


THE   SKELETON. 

•  ;.  N/m//'  n-ith  osseous  t>riintiri/  .s-AWr/o//,  characterized  by  the 
primary  cartilages  being  replaced  by  bone.  The  replacement  is  very 
gradual  ami  never  becomes  complete;  it  begins  in  some  of  the 
cartilages  before  others  are  developed ;  it  is,  accordingly,  impossible 
to  establish  any  definite  limit  in  time  for  this  stage. 

The  most  logical  treatment  would  be  to  deal  with  these  six  stages 
in  their  natural  sequence,  but  it  has  appeared  to  me  more  convenient 
to  give  the  complete  history  of  the  notochord  by  itself  (see  p.  181), 
to  add  a  section  upon  the  membranous  stage,  and  then  to  present  the 
entire  history  of  the  primary  skeleton  under  two  main  heads,  the 
axial  skeleton,  p.  424,  and  the  appendicular,  p.  448;  leaving  the 
dermal  skeleton  till  the  last,  p.  4(51,  although  it  is  ontogenetically 
and  i (hylogenetically  older  than  the  osseous  primary  skeleton.  The 
chapter  closes,  p.  405,  with  some  general  remarks  on  the  morphology 
of  the  skull. 

Membranous  Stage. — As  we  have  already  seen,  the  mesothe- 
lium  of  the  inner  side  of  the  primitive  segments  produces  the  mes- 
enchymal cells,  which  invest  the  notochord  and  medullary  canal. 
Kecent  writers  have  tended  to  regard  this  periaxial  mesenchyma  as 
Mien  ted,  and  Van  Wijhe  even  proposes  to  bestow  the  special 
name  of  sklerotome  upon  each  of  the  mesenchymal  segments.  It  is 
true  that  owing  to  its  segmented  origin  the  tissue  does  show  for  a 
time  traces  of  metameric  division,  but  the  division  becomes  unrecog- 
nisable long  before  there  is  any  mesenchymal  skeleton  indicated. 
The  primary  segmentation  plays  no  immediate  part  in  the  develop- 
ment of  the  separate  vertebrae.  These  considerations  render  it  un- 
justifiable to  regard  the  periaxial  mesenchyma  as  segmented.  We 
ought  not  to  speak  of  sklerotomes  unless  we  are  prepared  to  speak  of 
dermotomes,  because  the  anlage  of  the  dermal  mesenchyma  is  as  much 
segmented  as  the  anlage  of  the  periaxial  mesenchyma.  The  ques- 
tion under  consideration  arose  from  a  mistake  of  the  older  embry- 
ologists,  who  believed  that  the  primitive  segments  were  the  direct 
anlages  of  the  vertebra?,  and  accordingly  named  them  protovertebrae 
( ( 'nrirlwl) ;  unfortunately  this  misleading  term  is  still  in  use.  Then 
came  the  discovery  that  the  true  vertebrae  are  developed  apparently 
IK -tween  the  primitive  segments  or  in  alternation  with  them.  Re- 
mak  formulated  the  hypothesis  of  resegmentation  of  the  skele- 
ton (  Neugliederung  des  Axenskelct:*),  which  is  wrong  in  assuming 
that  the  segmentation  of  the  skeleton  is  not  parallel  with  the  primary 
it  is  right  in  assuming  that  there  is  a  primary  segmen- 
tation of  the  skeleton,  corresponding  to  the  original  mesothelial 
segments.  Remak's  conception  has  perpetuated  itself  to  this  day, 
and  is  carefully  repeated  in  current  text-books ;  were  it  correct  in 
its  entirety  then  the  mcnihranous  stage  we  are  now  considering 
would  not  occur. 

The  first  step  toward  the  development  of  the  perichordal  skeleton 
is  the  fusion  of  the  loose  mesenchyma,  derived  from  the  segmented 
mesothelium,  into  a  continuous  mass  of  cells,  which  grow  around  the 
notochord  arid  separate  it  first  from  the  entoderm  and  later  from  the 
medullary  canal,  and  grow  around  the  medullary  canal  and  close 
over  it  slowly,  and  also  grow  around  the  primitive  aortae,  see  Figs. 
161  and  103.  This  mesenchyma  is  of  a  loose  but  not  <iuite  uniform 


424  THE    FCETUS. 

character,  and  the  cells  early  begin  to  condense  in  the  immediate 
neighhorhod  of  the  notochord  and  nervous  system.  Around  the 
notoohord  the  cells  gradually  become  very  closely  crowded  and  form 
what  is  known  in  the  lower  vertebrates  as  the  chorda  sheath,  in  the 
amniote  embryo  as  the  investing  mass,  but  in  the  amniota  the  uni- 
form continuous  sheath  exists  only  around  the  anterior  end  of  the 
notochord  where  the  investing  mass  participates  in  the  formation 
of  the  cranium,  while  throughout  the  remainder  of  the  embryo,  as 
has  been  shown  by  A.  Froriep,  the  condensed  mesenchymal  an- 
lage  is  divided  from  the  start  more  or  less  distinctly  into  separate 
vertebral  masses,  which  in  stained  sections  stand  out  conspicuously. 
Froriep  has  studied  the  development  of  the  vertebrae  in  the  chick, 
83. 1,  and  mammals  (cow  embryos),  86. 1. 

I.  AXIAL  SKELETON. 

Vertebral  Column. — As  to  how  far  forward  the  vertebral  col- 
umn extends  in  the  head  we  have  no  means  of  deciding  positively, 
but  as  the  occipital  region  of  the  skull  is  developed  by  the  fusion  of 
vertebra,  and  as  these  vertebrae  appear  less  and  less  distinctly  as 
we  pass  forward  from  the  neck,  and  as  the  number  of  occipital  ver- 
tebras is  greater  in  birds  than  in  mammals,  we  cannot  avoid  the 
supposition  that  the  number  of  vertebras  fused  in  the  .head  was  once 
greater  than  now  appears  in  the  mammalian  embryo.  There  is 
accordingly  much  uncertainty  as  to  the  number  of  cephalic  vertebrae. 
But  though  the  number  of  vertebrae  is  not  exactly  known,  we  can 
fix  the  position  of  the  cephalic  end  of  the  vertebral  column,  as  coin- 
cident with  the  cephalic  end  of  the  notochord,  which  is  close  to  the 
hypophysis  or  pituitary  body.  The  notochord  becomes  invested 
almost  up  to  its  cephalic  extremity  by  the  condensed  mesenchymal 
sheath,  which  is  found  in  the  occipital  region,  as  in  the  body,  to  be 
the  blastema  out  of  which  are  differentiated  the  anlages  of  the  verte- 
brae; it  appears,  therefore,  no  mere  imagination  to  regard  this  as 
homologous  with  the  vertebral  column  throughout,  but  with  the 
development  of  the  vertebrae  inhibited  entirely  in  the  anterior,  par- 
tially in  the  posterior  occipital  region.  In  front  of  the  pituitary  body 
the  notochord  and  consequently  the  investing  mass  do  not  extend. 
We  must  in  fact  divide  the  head  into  a  prae-pituitary  unvertebrated 
and  a  post-pituitary  vertebrated  region.  The  remaining  vertebrae 
to  the  end  of  the  tail  develop  all  much  alike.  They  assume,  how- 
ever, modified  forms  in  the  various  regions,  but  the  origin  in  the 
embryo  of  the  differences  between  the  cervical,  dorsal,  and  lumbar 
vertebrae  has  never  been  worked  out.  Special  modifications  of  the 
first  and  second  cervical  vertebrae  take  place  in  mammals  to  form 
the  atlas  and  epistropheus  or  axis,  in  the  five  sacral  vertebrae  to  form 
the  sacrum,  and  in  the  caudal  vertebrae  to  form  the  coccyx. 

Typical  Development  of  a  Vertebra.— Our  exact  knowl- 
edge rests  mainly  upon  the  investigations  of  August  Froriep,  83. 1, 
86. 1,  on  chick  and  cow  embryos.  The  investing  mass  or  condensed 
perichordal  mesenchyma  forms  a  continuous  sheath  around  the  noto- 
chord. At  a  point  corresponding  to  the  centre  of  each  mesodermic 
segment,  or  a  little  on  the  cephalic  side  of  each  segment,  the  investing 


AXIAL    sKKLKTnN. 


1  •.':, 


mass  l>ecoin*^  thicker  in  diameter  and  its  tissue  moiv  c<>nden>ed ;  the 
condensation  is  V«TV  nc-ticeahle  in  stained  section>  and  is  tin-  first 
siii'M  <>f  the  vertehral  formation;  the  condensation  sj>ivad>  rapidly, 
extending  BldeWS]  9,  upward,  and  backward  with  tin-  result  of  forming 
a  h«.w  of  dense  mesenchyma,  the  primitive  vertehral  ho\v  (  Wirbel- 
IHHJCH)  of  Kr<>riep.  These  bows  are  distinct  from  the  bodies  of  the 
\t -rt ehi-M',  which  arise  later  from  separate  anlages.  The  bows  pass 
on  the  ventral  side  of  the  notochord,  and  thence  arch  on  each  side, 
Ki-.  .' !  :.  tailward  and  dorsalward,  so  as  to  end  at  the  caudal  edge 
of  the  muscle  plate  of  the  segment  to  which  they  belong,  and  ending, 
therefore,  just  in  front  of  the  intersegmental  artery,  t%  and  of  the 
spinal  nerve,  N,  from  the  sensory  ganglion  of  the  next  following  seg- 
ment. We  see  here  that  the  vertebra  are  strictly  segmental  struc- 
t  u  res  and  not  intergegmental  as  has  been  commonly  assumed  since 


FIG. 243.—  Reconstriirtioii  of  the  Last  Occip- 
ital, JIM-!  I  '  .-i-viral  V-Ttebrae  of  a 
<'<>\v  KmKryo  »f  !->  mm.,  tin-  BOtOChord  an«l 
axi-<  being  a— it:n«-<l  to  be  straight.  i,,-h. 
Notochor.l :  x.  sin-jit  h  of  notochord;  ft.  bow  of 
nri-ijiital  vi-rti-ln  -  n-ntal  artery:  N. 
nerve;  if y,  myotome.  After  A.  Froriep.  x 
88dia 


FIG.  **4. -Cross-Section  of  the  Anlage  of 
Second  C'en'ical  Vertebra  of  a  Cow  Embryo  of 
8.8  mm.  Md,  Medullary  canal :  Gl,  Kanj?lion  of 
the  second  cervical  nerv,  ;  M«.  mus.-le  plate 
of  the  s«-f»inl  n-rvicnl  wj-rmt-iit  ( I- rurii-p's  first 
•  •••rvii-iil  mus«-lf  jilat'- > ;  AV/< .  notochonl :  I-/' 
anla^e  of  the  vertebra;  Ao,  aorta.  At't.-r  l-'r-- 
riep. 


Remak.  The  course  of  the  bow,  as  compared  with  the  transverse 
plane  of  the  body  of  the  embryo,  is  oblique,  so  that  while  the  centre  of 
the  bow  next  the  notochord  is  near  the  centre  of  the  segment,  the 
tips  of  the  bow  lie  at  the  caudal  limit  of  the  segment  and  ultimately 
separate  the  muscle  plate  of  its  own  segment  from  that  of  the  next 
following.  The  obliquity  of  the  bow  appears  to  me  to  be  deter- 
mined primarily  by  the  arrangement  of  the  spinal  ganglia,  the  dor- 
sal ends  of  which  fill  out  the  width  of  the  segment,  while  the  lower 
pointed  end  is  carried  forward  to  the  anterior  border  of  the  segment ; 
this  disposition  leaves  the  caudal  side  of  the  segment  free  for  the  mes- 
enchyma  and  the  differentiation  of  the  vertebral  bow ;  the  obliquity 
is  further  assisted  by  the  form  of  the  muscle  plate,  as  can  be  seen 
in  Fig.  243.  The  portion  of  the  bow  underneath  the  chorda  in  the 
median  line  is  termed  the  hypochordal  brace  (Spange)  and  in  its 
ultimate  development  differs  considerably  from  the  rest  of  the  bow. 
The  investing  mass  around  the  notochord  on  the  caudal  side  of  the 


426  THE   FCETUS. 

bow  and  above  it  becomes  later  the  anlage  of  the  body  of  the  verte- 
bra. The  vertebral  bow  may  be  regarded  as  the  primitive  stage; 
it  is  found  in  the  chick  from  the  middle  of  the  fourth  to  the  middle 
of  the  fifth  day;  in  cow  embryos  of  7-11  mm. 

The  vertebral  bow  is  destined  to  form  the  processes  of  the  verte- 
bra, and  the  manner  in  which  its  ends  spread  out  against  the  muscle 
plate  can  be  well  seen  in  a  cross-section,  Fig.  244.  At  the  time  the 
bow  is  differentiated  the  muscle  plate  has  become  protuberant 
toward  the  notochord,  and  when  the  dense  mesenchyma  forming  the 
bow  spreads  out  it  is  forced  by  the  muscle  plate  to  grow  dorsal  ward, 
and  ventral  ward,  and  thereby  to  become,  as  it  were,  branched ;  the 
dorsal  branch  is  the  anlage  of  the  neural  arch;  the  ventral  branch 
the  anlage  of  the  transverse  or  costal  process,  because  it  grows  out 
still  farther  to  form  the  anlage  of  the  rib. 

There  follows  a  transitional  state  which  is  characterized  by  the 
gradual  development  of  the  cartilaginous  vertebra.  This  stage  ex- 
tends in  the  chick  from  the  middle  of  the  fifth  to  the  middle  of  the 
sixth  day,  and  is  found  in  cow  embryos  of  12-17  mm.  The  noto- 
chord exhibits  signs  of  retrogressive  change,  and  is  contracted  at 
the  level  of  the  vertebral  bow.  The  part  of  the  investing  mass  (peri- 
chordal  mesenchyma)  immediately  over  the  centre  of  the  bow  or 
hypochordal  brace  becomes  the  anlage  of  the  intervertebral  liga- 
ment, its  cells  becoming  first  less  crowded  and  then  acquiring  an 
elongated  form;  out  of  this  anlage  the  adult  ligament  is  slowly 
differentiated,  chiefly  by  the  development  of  connective-tissue  fibril- 
Ise.  The  investing  mass  behind  the  hypochordal  brace  develops  into 
the  cartilaginous  body  of  the  vertebra,  in  the  mammal  before,  in  the 
bird  after,  cartilage  begins  to  appear  in  the  vertebral  bow.  In  the 
mammals  there  are  two  centres  of  chondrification,  which  may  be 
recognized  in  the  bird  also,  although  they  are  in  the  latter  connected 
with  one  another  under  the  chorda.  The  process  of  chondrification 
continues  until  out  of  the  investing  mass  tha  separate  vertebral  body 
is  differentiated.  Meanwhile  the  chondrification  goes  on  in  the  ver- 
tebral bow,  and  in  birds  the  whole  bow  is  converted  into  cartilage 
and  unites  with  the  body  to  form  the  completed  vertebra.  In  mam- 
mals except  in  the  occipital  and  anterior  cervical  vertebrae  the  cen- 
tral part  does  not  form  cartilage  but  remains  as  a  dense  mesenchymal 
band,  which  can  be  recognized  as  a  more  or  less  distinct  structure 
for  some  time,  but  is  ultimately  lost  in  the  substance  of  the  inter- 
vertebral  ligament.  A  median  longitudinal  section  of  a  cow  embryo 
a  little  more  advanced,  Fig.  245,  shows  the  persistence  of  the  hypo- 
chordal brace. 

The  permanent  stage  is  reached  by  the  fusion  of  the  cartilage  of 
the  bow  with  that  of  the  body,  which  may  be  said  to  be  completed 
in  the  chick  by  the  middle  of  the  seventh  day,  and  in  cow  embryos 
of  22  mm.  In  the  chick  the  whole  bow  is  differentiated  into  carti- 
lage, and  its  central  part  fuses  with  the  vertebral  body.  In 
mammals  this  fusion  does  not  take  place  except  in  the  occiput,  but 
the  two  ends  of  each  bow  become  cartilaginous  and  fuse  with  the  cor- 
responding vertebral  body,  except  in  the  case  of  the  first  cervical 
vertebra,  see  p.  430.  The  central  portion  of  the  bow  in  all  vertebrae 
below  the  first  cervical  disappears  and  is  lost  in  the  intervertebral 


AXIAL    sKKI.KTnN. 


Eps" 


li^aim-nt.  In  a  longitudinal  section,  Fit;-.  '.M.'),  it  can  be  seen  tliat 
the  first  how  is  a  well -developed  cartilaginous  piece,  .sv-,  while  the 
M'cond,  is  only  partially  chondrified,  while  the  third  and  fourth  an- 
almost  loM  in  the  intervertebral  ligament.  The  first  bow,  as  just 
Mated,  forms  the  alia-.  1  Miring  the  development  of  the  cartilage  the 
vertebra  continues  urowini;-  and  the  arches  extend  farther  from  the 
body  ;  the 
neural  arches 
i;1  r  a  d  nail  \ 
cloM-  over  the 
nied  u  1 1  a  ry 
canal,  the 
do>ure  tak- 
ing ] »  i  a  <  •  e 
much  earlier 
in  tlie  chick 

than  in  the  mammal.  In  the 
human  emhryo  the  neural 
arches  extend  at  eight  weeks 
only  ;t  short  distance  up  the 
side  of  the  spinal  cord ;  at  three  months 
they  have  come  in  contact  on  the  dor- 
sal side  in  the  dorsal  region,  but  are 
still  quite  far  apart  in  the  lumbar  and 
sacra]  regions  (  Kolliker,  "Grundriss," 
I'.il )  and  by  the  fourth  month  all  the 
arches  have  <•!«  >sed.  The  development 
of  the  spinous  process  needs  to  be  fur- 
ther investigated.  The  ventral  pro- 
cesses, Fig.  244,  spread  downward  and 
are  brought,  owing  to  the  primitive 
inclination  of  the  vertebral  IM>W,  to  the 
caudal  boundary  of  the  segment  to 
which  they  belong,  and  as  they  lie  at 
the  caudal  edge  of  the  muscle  plate  of 
their  respective  segments,  they  con- 
tribute to  separate  that  plate  from  the 
next  follow  inn1.  These  processes  lose 
their  continuity  with  the  vertebra  proper,  but  remain  connected  with 
it  by  ligaments;  they  thus  become  the  independent  anlages  of  the 
i  ibs,  where  true  ribs  are  developed. 

Another  point  deserving  attention  is  the  relation  of  the  vertebrae  to 
the  vertebral  artery  which  arises,  as  described  in  Chapter  XXIV.,  as 
a  series  of  longitudinal  anastomoses  between  the  intersegmental  ar- 
teries; the  vertebral  artery  begins  to  appear  in  cow  embryos  of  12 
mm.,  and  is  a  continuous  stem  in  those  of  15  mm.  The  vessel  form- 
ing the  anastomosis  grows  through  the  mass  of  the  vertebral  bow 
during  the  transitional  stage,  while  the  mesenchyma  is  not  very  dense 
at  the  point  penetrated  by  the  artery.  This  discovery,  which  we 
owe  to  Froriep,  sets  aside  the  statement,  which  has  become  traditional, 
that  the  developing  vertebra  grows  around  the  artery,  and  shows 
instead  that  the  artery  grows  through  the  developing  anlage  of  the 


c« 


Fio.  245.— Longitudinal  Median  Sec- 
tion of  the  Upj>er  Portion  of  the  V 
hral  (-..liium  «-f  a  Cow  Embryo  of  2J.5 
nun.     ych,  notochord ;  Oe,  occipital  car- 
ti I, ; i -.'.-:    .\rt.b,    arteria    hasilaris;    Eps, 
f'3,  c«,  bodies  of  the  first  and  sec- 
>  i-vical   vertebrae;    C\  C'4,  bodies 
<>t  third  and  fourth  vertebrae;  «c,  anlage 
of  Atlas.     After  Froriep. 


428  THE    FCETUS. 

vertebra.  The  artery,  by  its  position,  may  be  said  to  mark  approx- 
imately the  boundary  between  the  neural  and  costal  processes  of  the 
vertebra. 

The  ossification  of  the  vertebrce  does  not  alter  the  morphology 
of  the  cartilaginous  stage,  and  it  is  doubtful  whether  it  is  accom- 
panied by  any  noteworthy  change  in  the  form  of  the  single  skeletal 
pieces.  The  ossification  begins  with  two  centres,  one  in  each  neural 
arch,  and  is  continued  by  a  third  centre  in  the  body  of  the  vertebra. 
The  centres  in  the  neural  arches  lie  near  the  body  proper ;  that  of 
the  body  appears  in  man  about  the  seventh  week.  The  centres  of 
ossification  of  the  body  become  recognizable  first  in  the  dorsal  region, 
and  from  there  their  differentiation  progresses  successively  from 
vertebra  to  vertebra,  both  headward  and  tailward.  The  centre  is 
situated  at  first  on  the  dorsal  side  of  the  chorda  (Robin),  but 
as  the  centre  extends  it  incloses  the  notochord,  which  is  gradually 
obliterated  so  that  it  can  no  longer  be  distinguished  after  the  actual 
formation  of  bone  has  commenced.  The  progress  of  ossification  is 
very  slow;  thus  the  preliminary  degeneration  covers  the  period,  in 
cow  embryos,  in  which  their  length  increases  from  2.2  to  6.0  cm.,  and 
it  is  not  until  the  latter  length  has  been  attained  that  the  actual 
deposit  of  bone  begins  (Froriep,  86. 1,  130).  In  man  the  centres  do 
not  attain  the  surface  of  the  cartilage  until  the  fourth  or  fifth  month. 
Ultimately  *  the  three  deposits  of  bone  fuse  into  a  single  osseous 
vertebra,  but  for  a  long  period  before  this  cartilage  remains  between 
the  bony  arches  and  the  bony  body,  and  on  the  dorsal  side  between 
the  arches;  these  cartilaginous  areas  act  as  growing  zones.  The 
epiphyses  are  separate  centres  of  ossification,  which  appear  one  on 
the  cranial  side,  one  on  the  caudal  side  of  the  body  of  each  vertebra, 
but  not  until  after  birth.  The  development  of  the  epiphyses  and 
their  fusion  with  the  main  body  have  been  investigated  by  Schwegel, 
58.1.  To  complete  the  adult  bony  vertebra  there  are  five  centres  of 
ossification  requisite. 

Summary. — Every  vertebra  is  developed  within  the  limits  of  a 
single  segment,  that  is,  out  of  the  mesenchyma  produced  from  the 
inner  wall  of  a  single  segment.  This  point  is  especially  important 
because  it  is  commonly  stated  that  each  vertebra  is  derived  from  ad- 
jacent parts  of  two  segments.  Each  vertebra  has  two  distinct  parts, 
the  vertebral  bow  ( Wirbelbogen)  and  the  vertebral  body  ( Wirbel- 
korper) ;  both  parts  in  their  first  stage  consist  of  condensed  mesen- 
chymal  tissue.  The  bow  appears  first  and  is  an  arched  band  of  tissue 
passing  under  the  notochord,  thence  running  obliquely  backward 
and  terminating  on  the  caudal-  side  of  the  muscle  plate  of  the  seg- 
ment. The  body  appears  later  in  each  segment  just  behind  the  me- 
dian part  of  the  bow.  The  bow  and  the  body  both  chondrify  and  fuse 
with  one  another,  except  in  the  first  cervical  segment;  in  birds  the 
whole  bow  becomes  cartilaginous,  but  in  mammals  the  middle  part  of 
the  bow  atrophies,  except  in  the  first  cervical  segment.  The  lateral 
portions  of  the  bow  form  both  the  neural  and  costal  arches ;  the  distal 
parts  of  the  latter  separate  from  the  vertebra  proper  to  form  the  an- 
lages  of  the  ribs.  The  morphology  of  the  vertebral  column  is  com- 

*  During  the  first  year  after  birth  the  arches  unite  dorsally,  between  the  third  and  eighth 
year  the  arches  unite  with  the  body. 


AXIAL    SKKI.KTON.  I'.".) 

pletely  determined  while  it  is  in  tin*  cartila^inou-  stage;  «•— itiratioii 
is  merely  a  supplementary  process  and  produces  no  important  change 
in  the  form  or  anatomical  relations  of  the  vertebra-. 

Froriep's  discovery  that  the  vertehral  l»o\v  and  Ixxly  are  distinct 
pieces  must  h  >  considered  very  important,  and  at  once  suggests  com- 
parison with  those  paheozoic  reptiles  in  which  centra  and  iutercentra 
have  heen  dist i ii^uishei  1  in  the  vertebral  columns,  but  this  compari- 
son has  yet  to  be  worked  out.  For  a  general  paper  on  the  intercen- 
trum  see  Cope,  86.4,  also  G.  Baur,  86.1,  for  a  discussion  of  the 
niorplmgeny  of  vertebra?  from  the  palaeontological  point  of  view. 

Evolution  of  Vertebrae. — We  have  no  positive  knowledge,  nor 
even  valuable  theori  the  causes  which  first  led  to  the  evolution 

of  vertebra?,  though  unscientific  hypotheses  have  been  abundant. 
There  is  one  important  consideration  which  has  been  rather  neglected, 
though  almost  self-evident,  namely,  that  vertebrae  have  arisen  within 
the  vertebrate  series,  the  perichordal  mesenchyma  in  the  lowest 
vertebrates  not  being  divided  into  vertebra?,  there  being,  in  short, 
so-called  vertebrates  without  vertebra?.  As  the  higher  fishes  have 
vertebrae,  it  is  evident  that  the  vertebral  column  was  evolved  within 
the  class  of  fishes. 

Theembr\oloe,ical  development  of  the  vertebra?  indicates  that  they 
are  compound  bodies,  as  above  shown  We  are  thus  led  to  distin- 
guish four  stages  in  the  differentiation  of  the  axial  skeleton : 

1.  Notochordal  stage. 

2.  Periehordal  stage. 

3.  Froriep's  stage  (vertebral  bow  and  centre  not  united). 

4.  Vertebral  stage  (vertebral  bow  and  centre  united). 

The  first  stage  is  permanent  in  Amphioxus;  the  second  is  perma- 
nent in  Petromyzon ;  the  third  will  perhaps  be  found  permanent  in 
Chimsera;  the  fourth  is  permanent  in  Amphibia  and  Amniota.  The 
skull  may  be  looked  upon  as  in  part  a  modification  of  the  second  stage 
in  the  head  region. 

Occipital  Vertebrae. — The  occipital  bone  of  the  adult  is  the 
final  outcome  of  the  fusion  and  ossification  of  an  uncertain  number 
of  vertebra?.  The  investing  mass  of  the  cephalic  portion  of  the 
notochord  forms  the  anlage  of  the  occipital  skeleton.  This  anlage 
terminates  a  short  distance  behind  the  hypophysis.  In  birds  and 
mammalia  it  may  be  divided  into  two  regions,  comprising  each 
about  half  the  length  of  the  anlage;  the  anterior  or  pituitary  half 
does  not  offer,  even  in  the  earliest  embryonic  stages,  so  far  as  known, 
any  trace  of  division  into  separate  vertebral  masses;  the  posterior  or 
cervical  half  does  show  clear  division  at  an  early  stage  into  four 
vertebra?  (in  the  chick  into  five  vertebrae) ,  but  of  these  only  the  last 
appears  as  a  perfectly  distinct,  well-differentiated  vertebra?,  but  even 
this  vertebra,  when  its  chondrification  begins,  merges  into  the  gen- 
eral occipital  mass  (A.  Froriep,  83.1,  86.1).  The  vertebra?  of  the 
mammalian  occiput  correspond  to  four  segments,  of  which  the 
hypn^lossus  represents  the  nerves.  Fig.  xM'i  is  a  frontal  projection 
of  the  cephalic  end  of  the  mesenchymal  vertebral  column  of  an  em- 
bryo, 15.5  mm.  long,  from  a  cow.  The  nerves,  N,  mark  the  divis- 
ions between  the  vertebra?,  as  do  also  the  intersegmental  arteries, 
v;  the  anterior  vertebra?  are  already  fused,  Oc,  but  the  fourth  is 


430 


THE    FCETIS. 


ica 


FIG.  246.  — Frontal  Projection  of  the 
Cephalic  Part  of  a  Vertebral  Column 
of  a  Cow  Embryo,  15. 5  mm.  long. 


perfectly  differentiated  and  closely  similar  to  the  succeeding  ver- 
tebrae. Three  hypoglossal  nerves  traverse  the  occipital  anlage.  In 
embryos  of  18.5  mm.  the  occipital  vertebra  is  found  to  have  fused 

with  the  occipital  mass,  though  the  ends 
of  its  vertebral  bow  project  enough  to 
still  indicate  the  original  division  of 
which  all  trace  is  lost  in  slightly  older 
embryos. 

In  the  occipital  mass  chondrification 
begins  on  each  side  of  the  notochord,  rtc/i, 
just  as  it  does  in  the  bodies  of  the  indi- 
vidual vertebra3,  and  it  begins  before  the 
fourth  vertebra  (Froriep's  occipital  ver- 
tebra) unites  with  those  in  front.  The 
result  of  the  chondrification  is  to  produce 
two  bars  of  cartilage  which  extend  along- 
side the  occipital  notochord,  but  of  couise, 
as  the  histogenetic  change  spreads,  the 
cartilage  unites  and  finally  extends 
through  the  entire  anlage.  The  bars  of 
cartilage  are  known  as  the  par achor dais,  and  are  commonly,  but 
erroneously,  described  as  the  primitive  anlage  of  the  occipital  cra- 
nium, whereas  in  reality  they  indicate  only  the  growth  of  the  cen- 
tres of  chondrification  in  the  anlage.  I  can  j-ecognize  no  grounds  at 
present  for  assigning  any  special  morphological  meaning  to  the 
parachordals. 

Atlas  and  Epistropheus. — The  first  and  second  cervical  verte- 
brae undergo  remarkable  modifications,  which  are  established  during 
the  transitional  stage  of  the  vertebrae — in  other  words,  while  the  ver- 
tebral anlages  are  chondrifying.  In  mammals  the  first  cervical 
vertebra  develops  two  cartilages,  one  of  which  is  formed  out  of  the 
whole  vertebral  bow  and  gives  rise  to  the  atlas,  and  the  other  is 
formed  out  of  the  vertebral  body.  The  later  cartilage  fuses  with 
the  second  vertebra  and  with  it  forms  the  epistropheus  or  axis.  Our 
precise  knowledge  of  the  development  of  these  two  vertebrae  rests 
principally  upon  the  admirable  researches  of  A.  Froriep,  83.1, 
86.1,  though  previous  investigators  had  established  that  the  first 
vertebra  forms  the  so-called  odontoid  process  of  the  epistropheus,  see 
Ch.  Robin,  64.1,  and  C.  Hasse,  73.1.  In  birds,  but  not  in  mam- 
mals, the  central  portion  of  the  vertebral  bow  of  the  second  cervical 
segment  also  contributes  to  the  formation  of  the  epistropheus;  in 
mammals  it  disappears  or  is  merged  in  the  intervertebral  ligament. 
Owing  to  this  difference  the  atlanto-epistrophic  articulation  is  not 
strictly  homologous  in  the  two  classes,  being  formed  in  birds  by  the 
vertebral  bow  of  the  second  segment ;  in  mammals  by  the  expanded 
caudal  part  of  the  vertebral  body  of  the  first  segment  of  the  neck. 
The  specialization  of  the  two  vertebrae  begins  when  their  chondrifi- 
cation is  well  advanced  (cow  embryos,  17-18  mm.),  for  we  see  then 
that  the  whole  of  the  first  vertebral  bow  is  changing  into  cartilage 
to  form  the  atlas,  and  that  it  does  not  grow  together  with  the  body. 
Meanwhile  in  mammals  the  body  of  the  first  vertebra  is  changing 
form,  its  cephalic  end  becoming  conical  to  make  the  anlage  of  the 


\\I.\L  SKKLF.TOV  431 

odontoid  process,  and  the  caudal  part  broadening  out,  and  making 
a  shoulder  laterally  and  ventrally  anmnd  the  base  of  the  odontoid 
process;  this  shoulder  forms  the  articulation  with  the  atlas.  The 
expansion  of  the  fir>t  vrrtehral  l>ody  forces  the  vertebral  artery  and 
the  second  cervical  nerve  out  laterally;  the  bend  of  the  artery  thu> 
produced  is  permanent ;  the  expansion  also  brings  the  first  body  into 
contact  with  the  bases  of  the  transverse  processes  of  the  second 
vertebra;  the  inter  vertebral  tissue  (ligament)  between  them  dis- 
appearing; the  two  vertebrae  unite  by  their  two  points  of  contact. 
and  thereafter  their  fusion  progresses  toward  the  median  line,  until 
all  the  tissue  of  the  intervertehral  ligament  is  obliterated  and  the 
two  cartilages  have  fused  into  one,  the  epistrophens. 

The  atlas  ossifies  from  three  centres,  two  of  which  correspond  to 
and  appear  about  the  same  time  as  those  of  the  neighboring  vertebral 
l>ows  (neural  arches),  while  the  third  does  not  appear  until  after 
birth,  and  is  situated  in  the  middle  of  the  ventral  arch  of  the  atlas 
(corresponding  to  the  primitive  hypochordal  brace,  Froriep's 
HI >«  IK ic)  Often  there  is  also  a  separate  centre  for  the  spinous 
process.  The  two  primitive  centres  unite  on  the  dorsal  side  during 
the  third  year,  and  with  the  ventral  centre  in  the  fifth  to  sixth  year. 

The  epistropheus,  in  accordance  with  its  development,  has  four 
centres,  one  for  the  body  of  its  first  vertebra  or  the  odontoid  pro- 

9,  one  for  its  own  bod}',  and  two  for  its  neural  arches.     The  t\\ 
first -named  centres   appear  during  the  fourth  or  fifth  month.     The 
fusion  of  the  centres  may  not  be  completed  until  the  sixth  or  seventh 
year,  and   up  to  that  age  the  tip  of  the  odontoid  process  remain > 

miossitied. 

Sacral  Vertebrae.— In  man  there  are  five  vertebra?,  characterized 
l>y  their  peculiar  form  and  by  their  articulation  with  the  pelvis,  and 
which  begin  at  eighteen  years  to  slowly  unite  into  a  single  bone 
known  in  anatomy  as  the  os  sacrum.  In  other  animals,  however, 
the  sacrum  is  not  formed  out  of  the  same  vertebra?,  if  we  count  from 
the  last  cervical  vertebra,  nor  out  of  the  same  number  of  vertebra?. 
Various  attempts  have  been  made  to  explain  these  divergences — see 
especially  Rosenberg,  76.1 — but  no  certain  result  has  yet  been 
reached.  Of  the  history  of  these  vertebra?  we  have  no  such  exact 
knowledge  as  Froriep's  researches  have  given  us  concerning  the 
cervical  vertebra?. 

The ' processes  form  neural  arches  and  lateral  processes  (Scil<i,- 
fnrtxiitze)  which  were  commonly  homologized  with  the  costal  pro- 
ce^ses  of  other  vertebra?,  principally  upon  comparative-anatomical 
grounds.  The  chief  embryological  evidence  in  favor  of  this  homology 
was  the  fact  that  the  lateral  processes  have  a  separate  centre  of  ossi- 
fication, making,  together  with  the  three  usual  centres,  five  primary 
centres  for  each  sacral  vertebra.  In  1875  Rosenberg,  75.1,  showed 
that  the  anlages  of  the  sacral  ribs  can  be  seen  in  human  embryos,  and 
that  the  proximal  ends  of  these  change  into  cartilage  and  fuse  with 
the  true  transverse  processes  of  the  vertebra? — very  much  as  happens 
with  the  thirteenth  rib  in  man. 

Coccygeal  and  Caudal  Vertebrae. — Behind  the  sacrum  there 
are  nine  segments  to  be  found  in  the  human  embryo  of  8-0  mm.,  a- 
discovered  by  H.  Fol,  85.1.  From  the  sacrum  tailward  they  are 


432  THE    FCETUS. 

found  progressively  more  and  more  rudimentary,  and  only  from 
three  to  five  of  the  segments  immediately  following  the  sacrum  devel- 
oped ossified  vertebrae.  These  are  the  so-called  coccygeal  vertebrae, 
concerning  the  embryology  of  which  we  know  nothing.  It  is  prob- 
able that  some  of  the  segments  behind  the  coccyx  form  at  least 
mesenchymal,  if  not  cartilaginous,  vertebrae,  and  Fol's  observations 
suggest  that  the  last  coccygeal  vertebra  is  really  the  product  of  the 
fusion  of  several  caudal  vertebrae. 

Only  the  first  coccygeal  vertebra  begins  to  ossify  before  birth. 
This,  the  thirtieth  vertebra,  has  been  shown  by  E.  Rosenberg,  76. 1, 
to  be  in  the  embryo  really  a  sacral  vertebra,  but  it  separates  in  the 
course  of  development  from  the  sacrum,  and  becomes  the  first  of  the 
coccygeal  series. 

Ribs  and  Sternum. — The  ribs  and  sternum  are  vertebral  struc- 
tures, and  therefore  strictly  segmental.  This  statement  seems  to  me 
an  unavoidable  deduction  from  Froriep's  observations  on  the  devel- 
opment of  the  costal  processes  of  the  vertebrae,  but  it  is  directly 
opposed  to  the  conception  current  among  morphologists,  according 
to  which  the  ribs  are  inter  segmental.  That  the  sternum  is  a  mor- 
phological product  of  the  ribs  is,  I  believe,  the  accepted  opinion  of 
both  comparative  anatomists  and  embryologists.  That  it  is  so  in 
man  has  been  put  beyond  doubt  by  G.  Ruge's  investigations,  80. 1 , 
see  also  C.  K.  Hofmann,  80. 1. 

1.  RIBS. — Comparative  anatomy  renders  it  probable  that  every 
vertebra  had  ribs  primitively,  and  most  of  them  have  still  in  the 
human  embryo  the  anlages  of  ribs.  In  man  there  are  only  twelve 
vertebrae  (eighth  to  nineteenth)  of  which  the  costal  anlages  are  repre- 
sented in  the  adult  by  true  ribs;  traces  of  a  thirteenth  pair  of  ribs 
belonging  to  the  twentieth  vertebra  appear  in  the  human  embryo, 
and  as  a  rare  anomaly  the  thirteenth  pair  occurs  in  the  adult.  In 
the  cervical  region  there  are  found  costal  processes  of  the  vertebrae, 
also  in  the  lumbar  and  sacral  region ;  in  the  last-named  region  the 
processes  acquire  a  certain  independence,  but  soon  lose  it  and  fuse 
with  the  vertebrae  proper.  These  variations  should  be  borne  in  mind 
while  reading  the  following  paragraph,  which  attempts  to  give  the 
general  history  of  a  typical  rib. 

The  ends  of  the  vertebral  bows  grow  out  as  shown  by  Froriep,  86.1, 
until  they  come  in  contact  with  the  muscle  plates  of  their  own  seg- 
ments. By  the  bulging  of  the  plate  the  end  of  the  bow  is  forced  to 
expand  dorso-ventrally,  and  there  is  thus  given  the  primary  divi- 
sion into  dorsal  or  neural,  and  ventral  or  costal  process.  The  spinal 
ganglion  forces  the  end  of  the  bow,  compare  Fig.  243,  p.  425,  to  grow 
toward  the  posterior  limit  of  the  segment,  and  this  permits  the  costal 
process  to  grow  out  past  the  caudal  edge  of  the  muscle  plate  and  to 
there  become  the  anlage  of  the  rib,  which  is  not  therefore  an  inter  - 
segmental  structure,  as  current  tradition  has  it,  but  truly  segmental ; 
the  rib  and  the  myotome  headward  of  it  belong  to  the  same  som- 
ite, and  the  rib  owes  its  apparently  intersegmental  position  to  its 
situation  at  the  caudal  limit  of  the  segment,  behind  the  muscular  an- 
lage. Whether  the  costal  anlage  is  produced  as  an  actual  outgrowth 
of  the  condensed  mesenchyma  of  the  vertebral  blastema  or  by  differ- 
entiation of  the  mesenchyma  in  loco,  we  do  not  know ;  nor  do  we 


AXIAL   SKELETON.  433 

know  what  limits  the  rib  in  tin*  transverse  plane  so  that  it  is  merely 
a  rod  and  not  a  wide  and  high  partition  wall.  In  this  sta^v  tin-  rib 
i>  directly  continuous  with  the  vertebra,  but  when  by  changing 
into  cartilage  it  passes  into  the  next  stage,  it  separates  from  the  ver- 
tebra by  the  development  of  a  fibrous  ligament,  forming  the  primary 
articulation  between  the  rib  and  spinal  column.  The  division  takes 
place  obliquely,  thus  allowing  the  head  of  the  rib  to  come  in  contact 
with  the  body  of  the  vertebra,  and  to  articulate,  by  its  dorsal  sur- 
face, with  the  ventral  surface  of  the  future  transverse  process.  In 
the  course  of  its  further  development  the  single  primitive  articulation 
becomes  divided  and  the  secondary,  or  adult  condition,  is  established 
with  one  articulation  with  the  transverse  process,  and  a  second  with 
the  body  of  the  vertebra.  In  the  case  of  the  ribs,  which  become 
rudimentary,  the  development  ceases  at  this  stage,  and  only  the 
proximal  end  of  the  rib  chondrifies ;  the  small  remnant  of  cartilage 
unites  with  the  transverse  process  of  the  vertebra,  re-establishing  by 
a  secondary  union  the  primary  connection. 

The  true  ribs,  as  those  belonging  to  the  dorsal  vertebrae  of  mam- 
mals are  called,  extend  a  considerable  distance  through  the  somato- 
pleure  toward  the  median  ventral  line,  but,  as  discovered  by  H. 
Rathke,  38.2,  365,  before  they  reach  the  middle  ventral  line  the  ribs 
produce  the  anlages  of  the  sternum,  and  of  the  intercostal  ligament,  at 
first  as  condensed  mesenchyma,  which  afterward  becomes  histolog- 
ically  differentiated — see  the  next  section  on  the  sternum.  The  ribs 
extend  to  unequal  distances,  the  first  coming  nearest  the  ventral 
line,  the  last  terminating  farthest  from  it.  In  the  human  embryo 
of  from  2  to  3  cm.  there  is  present  a  thirteenth  true  rib  (Rosen- 
berg, 75.1,  89-91);  the  proximal  end  chondrifies  and  fuses  with  the 
vertebra.  This  valuable  observation  shows  that  the  so-called  first 
iumbar  vertebra  of  man  is  really  the  last  dorsal  vertebra,  and  in  its 
embryonic  stage  is  strictly  comparable  with  the  thirteenth  dorsal 
vertebra  of  Troglodytes.  As  in  Hylobates  the  twenty-first  vertebra 
sometimes  has  ribs,  the  evidence  within  the  primates  suffices  to 
prove  that  the  lumbar  region  was  evolved  at  the  expense  of  the 
dorsal. 

The  ribs  are  only  partly  ossified,  hence  the  osseous  rib  of  the  adult 
represents  only  a  portion  of  the  whole  primitive  rib,  the  most  distal 
part  of  which  has  been  reserved  to  contribute  to  the  sternum 
(or  intercostal  ligament),  and  another  part  of  which  remains  in  the 
cartilaginous  stage  to  unite  the  costal  t>one  with  the  sternum 
or  intercostal  ligament.  Each  primitive  rib  is  therefore  divided  into 
three  parts:  1,  the  proximal  bony  division,  the  rib  of  human  anat- 
omists; 2,  the  middle  cartilaginous  division,  the  costal  cartilage; 
3,  the  distal  sternal  or  ligamentous  division.  By  the  differentiation 
of  fibrillar  tissue  out  of  the  original  costal  anlage  articulations  are 
developed  for  the  costal  cartilages  at  their  proximal  ends  with  the 
bony  ribs,  and  at  their  distal  ends  with  the  sternum.  The  exact 
history  of  these  differentiations  has  still  to  be  worked  out. 

The  ossification  of  the  ribs  begins  during  the  second  month,  ac- 
cording to  Kolliker,  and  there  is  but  a  single  centre.  Schwegel, 
58.1,  states  that  epiphyseal  centres  appear  eight  to  fourteen  years 
after  birth  in  the  head  and  tubercle,  that  is,  for  both  vertebral  artic- 
28 


434  THE   FCETUS. 

ulations ;  the  epiphyses  do  not  unite  with  the  main  bone  until  later ; 
often  not  until  the  twenty-fifth  year. 

2.  STERNUM. — The  breast  bone  is  developed  from  the  ends  of  the 
ribs,  but  the  early  stages  have  still  to  be  ascertained  by  following 
out  the  relations  while  the  anlages  are  in  the  mesenchymal  stages. 
Hitherto  investigations  have  begun  only  with  the  cartilaginous 
stage.  It  seems  probable  that  the  costal  anlages  grow  beyond  the 
ventral  limits  of  the  muscle  plates  and  then  bend  headward,  and  by 
uniting,  form  a  longitudinal  sternal  anlage  on  each  side  at  some  lit- 
tle distance  from  the  median  line.  The  cartilaginous  half-sternum 
apears  in  rabbits  the  seventeenth  day ;  they  are  still  separate  in  chicks 
of  the  eighth  day,  in  pig  embryos  of  about  27  mm.  in  human  em- 
bryos of  24  mm.  In  the  chick  the  halves  are  uniting  during  the 
seventh  day,  and  in  pig  embryos  of  about  50  mm.,  the  halves  are 
fully  united.  The  sternal  anlages  (Ruge's  Sternalleisten)  arise 
from  the  ends  of  the  first  to  seventh  ribs,  and  accordingly  are 
nearest  together  toward  the  head  and  diverge  tailward.  My  own 
observations  lead  me  to  think  it  probable  that  the  connection  really 
extends  to  all  the  ribs,  but  between  the  seventh  and  twelfth  ribs  it 
becomes  fibrillar,  and  gives  rise  to  the  intercostal  ligament,  which, 
therefore,  is  morphologically  the  prolongation  of  the  sternum.  The 
sternal  halves  gradually  coalesce,  beginning  at  their  upper  ends.  In 
many  mammals  the  sternum  shows  plainly  its  metameric  origin  and 
consists  of  separate  pieces  metamerically  arranged,  and  there  is  a 
•separate  centre  of  ossification  for  each  piece.  In  man,  on  the  con- 
trary, the  originally  continuous  cartilage  forms  three  pieces,  the 
uppermost  of  which  belongs  only  to  the  first  sternal  segment  or  first 
pair  of  ribs  according  to  G.  Ruge,  80.1,  but,  according  to  W.  K. 
Parker,  also  is  formed  partly  at  the  expense  of  the  aborted  last  cer- 
vical rib ;  the  middle  piece  corresponding  to  the  second  to  seventh 
segment;  and  the  third  piece,  which  remains  chiefly  or  wholly  car- 
tilaginous. The  first  piece  is  the  manubrium,  the  second  piece  is 
the  body  of  the  sternum,  and  the  third  piece  is  the  ensiform  or 
xiphoid  cartilage.  G.  Ruge,  80.1,  found  in  human  embryos  two 
small  suprasternal  cartilages  which  fuse  with  one  another  and  then 
with  the  manubrium ;  the  significance  of  these  cartilages  is  uncertain. 

The  sternum  ossifies  with  one  centre  in  the  manubrium,  and  in 
man  with  an  irregular  number  of  centres  in  the  body.  Its  ossifica- 
tion does  not  begin  until  the  sixth  month. 

The  double  origin  of  the  sternum  and  its  dependence  upon  the  ribs 
was  discovered  by  H.  Rathke,  38.2,  363.  This  discovery  was  con- 
firmed and  extended  twenty  years  later  by  W.  K.  Parker,  58.2,  and 
more  recently  by  A.  Goette,  Hofmann,  80.1,  and  G.  Ruge,  80.1; 
the  last  is  an  admirable  investigation  of  the  development  of  the 
sternum  in  man. 

Trabeculae  Cranii. — H.  Rathke  discovered  that  at  the  same  time 
that  the  cartilaginous  tissue  develops  in  the  occipital  skeleton  there 
appear  two  curved  bars  of  cartilage  in  front  of  the  notochord.  These 
cartilages  by  their  fusion  and  expansion  form  the  whole  of  the  pra3- 
chordal  chondrocranium,  and  were  named  by  Rathke  the  trabecula3 
cranii.  All  subsequent  writers  have  made  Rathke 's  discovery  the 
starting-point  of  their  accounts  of  the  development  of  the  anterior 


AXIAL   SKELETON. 


435 


part  of  the  skull.  But  the  morphological  differentiation  of  the  skel- 
eton, as  we  have  already  seen  in  the  case  of  the  vertebrae,  etc.,  is  given 
by  condensed  mesenchyma,  and  the  cartilage,  when  it  first  appears 
and  tor  a  considerable  period  afterward,  does  not  by  any  means  cor- 
respond to  the  real  shape  of  the  skeletal  piece.  Now  nearly  all  the 
information  \\-e  possess  as  to  the  early  stages  of  the  skull  is  concern- 
ing the  progress  of  the  so-called  chondrocranium,  and  since  this  is 
really  for  a  considerable  period  merely  the  history  of  the  progress  of 
choiidrification  in  the  already  formed  mesenchymal  skeleton  of  the 
cranium,  it  results  that  concerning  the  early  stages  of  the  skull  we 
have  almost  no  available  information,  nor  can  we  hope  to  understand 
the  morphology  of  the  skull  until  its  developmental  history  through 
the  mesenchymal  stages  shall  have  been  followed,  as  has  been  that  of 
the  cervical  vertebrae  by  Froriep.  Concerning  the  history  of  the  car- 
t  ilage  of  the  skull  we  possess  an  immense  fund  of  information,  owing 
chiefly  to  the  long  series  of  splendid  monographs  by  W.  Kitchen  Par- 
ker (18GG-188G),  the  chief  results  of  which  have  been  summed  up  by 
himself  and  Mr.  Bettany  in  a  single  comprehensive  volume,  77. 1. 

From  what  has  been  said  it  is  clear  that  the  shape  of  the  prse- 
chordal  cartilaginous  skull  has  very  little  morphological  significance 
until  the  mesenchymal  skull  is  completely  chondrified;  until  then  the 
growth  of  the  cartilage  represents  merely  the  advance  of  a  histological 
modification  within  the  skeletal  piece. 
rnt'<  irtmmtely  it  is  impossible  at  present 
to  say  when  the  cartilage  does  begin  to 
represent  the  shape  of  the  cranium. 
As  the  history  of  the  early  stages  of  the 
prae-chordal  cartilage  has  very  little 
morphological  value  it  may  be  very 
briefly  given.  ; 

The  trabeculae  cranii  of  the  pig*  may      / 
be  taken  as  typical   representatives  of     /< 
the  mammalian   trabeculae,    and  show    1  . 
essentially  the  same  arrangement  as  are  /     Q 
found  in  all  other  vertebrates,  although  I 
the  form   and  proportions  vary  from  \ 
class  to  class.     In  pig  embryos  of  about    \ 
1G  mm.,  the  trabeculae  cranii  appear  as 
two  curving  rods  of  cartilage,  united  in 
front,  but  separated  behind ;  in  general 
shape  they  resemble  calipers;  they  lie 
anteriorly  between    the  olfactory  pits 
and  the  brain,  and  form  from  the  start 
a  skeletal  partition  between  these  two 

Structures.       As  shown  in  Fig.   247,   TV,    hyoid    cartilage f~»er,~  periotic    cap- 

the  trabecul*  separate  posteriorly  some  SS&oS':  TjSdH  iSSSTfe«Sl 
distance  in  front  of  the  hypophysis,  H,  ftgjf8**'  10'  vagus-  After  *•  K 
then  curving  toward  the  median  line, 

taper  and  end  in  points  immediately  behind  the  hypophysis.  The 
anterior  end  of  each  trabecula  is  bent  over  outward  and  down- 
ward, forming  the  cornu  trabeculce,  and  causing  a  projection  on 

*  I  follow  the  account  given  in  Parker  and  Bettany 's  "Morphology  of  the  Skull,"  chapter  viii. 


per 


TlGll 


FIG.    247.— Embryo  Pig  of  about  16 
mm.     Cranial  elements  seen  from  be- 


436 


THE   FCETUS. 


either  side  of  the  palate  in  the  mouth  cavity  behind  the  olfactory 
pits.  These  pits  are  situated  entirely  in  front  of  the  trabeculaB  at 
this  stage,  but  between  them  there  is  an  internasal  septum  of  mes- 
enchyma,  and  into  this  septum  there  already  extend  two  cartilagi- 
nous lamina  which  are  the  prolongations  of  the  trabecula?.  In  the 
course  of  their  further  development  the  trabeculse  fuse  throughout 

their  entire  extent.  In  pigs 
one  inch  long  the  internasal 
cartilages  have  nearly  or 
quite  fused  into  a  single 
median  piece,  and  the  trabe- 
culse  proper  are  united  also 
except  around  the  hypophy- 
sis, which  they  closely  em- 
brace. At  this  stage  we  see 
further  that  the  trabecular 
cartilage  is  extending  side- 
ways, outward  and  upward 


.per 


J>.  sp       chy 


b.o<j 


Fio.  348. — Embryo  Pig,  One  and  one-third  Inch  long; 
Median  Longitudinal  Section  of  the  Head;  the  nasal 
septum  and  brain  have  been  removed.  After  W.  K. 
Parker.  (For  explanation  of  lettering  see  text. ) 


around  the  brain,  outward 
and  downward  around  the 
olfactory  pits.  In  embryos 
an  inch  and  a  third  long 
the  posterior  ends  of  the 
trabeculaB  have  united  with  the  anterior  end  of  the  occipital  car- 
tilage, thus  forming  a  continuous  floor  of  cartilage,  which  under- 
lies the  brain,  and  in  front  overlies  the  olfactory  pits,  and  has  also 
odevelped  under  the  hypophysis,  which  thus  becomes  definitely  sepa- 
rated from  the  mouth  cavity  and  inclosed  within  the  brain  case. 
We  find  at  this  stage  also  that  the  cartilaginous  periotic  capsules 
have  begun  to  fuse  with  the  lateral  portions  of  the  occipital  cartilage, 
thus  making  one  continuous  skeletal  piece,  which  is  known  as  the 
primitive  chondrocranium,  but  it  does  not  correspond  to  the  real  cra- 
nium at  this  stage,  for  beyond  the  limits  of  the  cartilage  the  skeleton 
around  the  brain  and  olfactory  pits  is  already  formed  as  condensed 
mesenchyma.  The  general  arrangement  and  the  outgrowths  from 
the  trabecular  mass  are  shown  in  Fig.  248.  The  hypophysis,  Hy, 
lies  in  a  deep  fossa,  which  remains  in  the  adult  and  is  known  as  the 
sella  turcica;  on  the  caudal  side  of  the  hypophysis  the  fused  ends 
of  the  trabecula3  have  risen  as  a  transverse  plate,  the  posterior 
clinoid  ridge,  p.cl,  and  in  front  of  the  hypophysis  is  the  much 
smaller  anterior  clinoid  ridge;  p.sp  indicates  the  region  of  the  future 
pra3-sphenoid  bone,  the  cartilage  of  which  is  continued  directly  for- 
ward in  the  nasal  septum  as  the  ethmoidal  plate;  from  the  sides  of 
cartilage  there  spring  two  lateral  plates,  which  curve  upward  and 
outward  around  the  brain ;  the  anterior  and  larger  of  these  plates  is 
the  orbi to-sphenoid,  O.sp,  which  spreads  out  between  the  brain  and 
the  eyeball,  and  extends  far  back  toward  the  periotic  capsule,  per  ; 
during  its  development  the  orbito-sphenoid  cartilage  grows  around  the 
optic  nerve,  thus  forming  the  optic  foramen,  Op,  which  is  near  the 
base  of  the  plate ;  the  smaller  of  the  plates  is  the  ali-sphenoid,  and 
springs  from  the  region  of  the  two  clinoid  ridges ;  it  is  snort  and 
thick  and  has  a  downward  process,  which  exterids  to  the  palato- 


AXIAL   SKELETON.  437 

pterygoid  bar  and  represents  the  external  pterygoid  cartilage ;  this 
process  being  external  does  not  show  in  the  figure.  Between  the 
ali -sphenoid  and  the  periotic  capsule  is  a  shallow  fossa  for  the 
Gasserian  ganglion,  and  from  the  ganglion  the  main  stem  of  the 
fifth  or  trigeminal  nerve  passes  out  through  a  foramen.  The  sup- 
erior maxillary  division  of  the  trigeminal  passes  out  between  the 
orbito-  and  ali-sphenoids.  The  nasal  cavities  are  large  and  com- 
plex ;  tlu-y  already  occupy  more  than  half  the  length  of  the  head,  and 
in  part  underlie  the  brain;  the  partition  which  separates  the  nasal 
ravin-  from  the  overlying  olfactory  lobes  is  composed  of  undifferen- 
tiated  mesenehyma,  which  is  traversed  by  the  olfactory  nerve  fibres, 
but  at  the  present  stage,  or  a  little  later,  the  partition  chondrifies  by 
an  extension  of  the  cartilage  of  the  ethmoidal  plate,  with  the  result 
<  >t  producing  the  cribriform  plate,  cr.  p.  The  shape  of  the  nasal 
ehanihrrs  is  rendered  complex  by  the  turbinal  prominences  on  the 
lateral  wall  of  each  chamber  as  described  in  Chapter  XXVIII. 
A 1  ready  in  the  previous  stage  the  median  ethmoidal  plate  had  sent 
nutumuini;-  laminae  of  cartilage  one  on  each  side  over  the  top  and 
<l<>\vn  on  the  outside  of  each  nasal  cavity,  and  from  the  lateral  car- 
tilage there  appear  ingrowths  into  each  turbinal  prominence.  The 
relations  of  the  cartilage  to  the  nasal  chambers  can  be  more  readily 


•  "-"  :'•• .  "  •": 


Oh* 


Fro.  249.  —Section  of  the  Anterior  Portion  of  the  Snout  of  an  Embryo  Pig.  Ethm,  Median 
Hlmioidal  plate;  lot,  lateral  nasal  cartilage;  t. tb.  inferior  turbinal  prominence  into  which  the 
cartilage  has  begun  to  penetrate;  N,  nasal  cavity. 

understood  in  a  cross-section,  Fig  249,  which  calls  for  no  further  de- 
scription than  is  afforded  above  and  in  the  explanation  of  the  figure. 
As  partly  indicated  by  Fig.  248,  there  are  five  turbinal  promi- 
nences? the  ali-nasal,  the  inferior,  it,  the  middle,  nt,  and  the  upper 
//.  tb — the  last  two  mentioned  being,  however,  hardly  distinct  from 
one  another  at  this  stage.  It  now  remains  only  to  add  that  at  the 


438  THE    FCETUS. 

ventral  side  of  the  anterior  edge  of  the  ethmoidal  plate  the  cornua 
trabecularum  are  still  present ;  the  cornua  are  the  anlages  of  the 
ali-nasal  cartilages. 

In  man  the  history  of  the  chondrocranium  is  very  similar  to  that 
just  given  for  the  pig,  as  we  know  through  the  investigations  of 
Spondli,  46.1,  Vrolik,  73.1,  Virchow,  57.1,  and  Van  Noorden, 
87.1,  and  others. 

The  general  significance  of  the  chondrocranium  is  discussed  in  the 
section  on  the  morphology  of  the  skull,  p.  465. 

Periotic  Capsules. — This  name  has  been  employed  by  Huxley, 
and  may  be  conveniently  retained,  to  designate  the  independent  car- 
tilages, which  appear  very  early  around  the  otocysts,  and  later  be- 
come integral  parts  of  the  primitive  chondrocranium  by  coalescing 
with  the  occipital  cartilage.  In  pig  embryos  of  about  16  mm.  they 
appear  as  two  rounded  masses,  Fig.  247,  per,  close  alongside  the 
anterior  half  of  the  occipital  cartilage,  against  which  they  lie  with 
a  nearly  straight  margin,  while  the  rest  of  their  outline  is  rounded. 
The  aqueductus  vestibuli  is  left  as  an  opening  in  the  cartilage  on  the 
upper  and  inner  edge ;  the  facial  nerve,  7,  e.nters  the  capsule  a  little 
behind  this,  its  passage  being  the  aqueductus  Fallopii.  As  regards 
the  inclosed  otocyst  we  find  that  the  semicircular  canals  and  cochlea 
are  just  budding  forth.  At  this  stage  there  is  sort  of  plug  of  non- 
cartilaginous  mesenchyma  still  left  in  the  external  wall  of  the  cap- 
sule. The  neighboring  cranial  nerves  show  a  characteristic  relation 
to  the  capsules.  The  trigeminus  passes  out  between  the  capsule  and 
caudal  extremities  of  the  trabeculaB.  In  the  angle  between  the  cap- 
sule and  the  occipital  cartilage  there  pass  out  three  nerves,  the  glosso- 
pharyngeal,  0,  the  vagus,  10,  and  the  hypoglossus,  11.  In  embryo 
pigs  of  one  inch  the  capsules  have  begun  to  coalesce  posteriorly  with 
the  occipital  cartilage,  and  in  those  an  inch  and  a  third  long  they 
are  found  coalesced  along  nearly  the  whole  line  of  con  tact  bet  ween  the 
capsules  and  the  basilar  plate. 

Concerning  the  origin  of  the  periotic  capsules  we  possess  no  accurate 
knowledge,  and  cannot  even  say  whether  they  represent  primarily 
distinct  skeletal  pieces  or  merely  separate  centres  of  chondrification 
in  a  larger  mesenchymal  skeletal  piece.  The  latter  appears  to  me 
the  more  probable  alternative,  and  it  may  be  further  suggested  that 
the  capsules  are  differentiations  of  the  lateral  outgrowths  of  the  in- 
vesting mass  of  the  cephalic  notochord.  The  questions  raised  can  be 
answered  only  by  a  careful  investigation  of  the  mesenchymal 
cranium. 

Ultimate  History  of  the  Chondrocranium. — The  primitive 
cartilaginous  skull  is  formed  by  the  fusion  and  expansion  of  the  oc- 
cipital cartilage,  the  trabecula3  cranii,  and  the  periotic  capsules.  It 
occupies  the  floor  of  the  cranial  cavity  and  the  roof  of  the  olfactory 
cavities,  and  has  certain  lateral  expansions.  The  arrangement  of 
these  can  be  understood  from  the  accompanying  Fig.  250,  although 
the  figure  represents  a  stage  in  which  ossification  has  begun.  Be- 
tween the  nasal  cavities  lies  the  mesethmoid  septum  from  the  dorsal 
side  of  which  spring  the  ali-nasals,  oln,  covering  the  dorsal  and 
lateral  parts  of  the  nasal  cavities ;  from  the  mesethmoid  extend  also 
the  plates  forming  the  ali-ethmoids  and  middle  turbinal,  mfb ; 


AXIAL    SKKLKToX. 


139 


ol.n 


mtb 


obs 


also  tho  cribriform  plate,   rr,   through  which  the    olfactory  nerve 

passes.     The  orbito-sphenoidal  wings,  obs,  are  large  and  rise  from 

the  prse-sphenoid ;  the  ali-sphenoidal  wings  are  smaller,  al ;  between 

the    t\vo    sphenoid    wings    is    the    foramen   lacerum;    the  periotic 

capsules  are  large  and   fill  out  nearly  the 

whole  space  between  the  ali-sphenoids  and 

the  wings  of  the  occipital.     The  occipital  has 

expanded    completely    around    the    foramen 

magnum,  /.///,  through  which  the  spinal  cord 

enters  tho  brain-case,  so  as  to  form  on  the 

dorsal  side  the  supra-occipital,  s.oc. 

In  the  fishes  the  chondrocranium  passes 
through  a  stage  corresponding  closely  to  that 
just  described,  except  that  in  them  there  is 
no  bone  formed ;  but  whereas  in  the  mammal 
the  chondrocranium  does  not  pass  beyond 
this  stage,  in  the  fishes  it  continues  growing 
until  the  brain  is  completely  inclosed  and 
there  is  a  perfect  cartilaginous  skull,  at  least 
in  the  lower  forms,  marsipobranchs,  ganoids, 
and  selachians.  We  must,  then,  distinguish 
two  types  of  chondrocranium,  according  as 
it  does  or  does  not  completely  encase  the 
brain.  The  latter  is  the  type  exclusively 
found  in  mammalia. 

The  mammalian  chondrocranium  is  repre- 
sented in  the  adult  by  a  number  of  distinct 
bones,  which  represent  also  a  still  larger 
number  of  bones  of  lower  types.  As  to  how 
the  originally  continuous  cartilage  becomes 
divided  into  separate  bones,  our  notions  are 
somewhat  vague.  In  the  division  the  centres  of  ossification  play  a 
leading  role,  of  course,  but  not  in  the  sense  that  every  centre  invariably 
results  in  the  formation  of  a  separate  bone.  The  second  important 
factor  is  the  development  of  the  sutures,  which  form  the  boundaries 
of  the  bones.  The  sutures  are  of  two  kinds,  those  marked  out  by 
the  edges  of  the  chondrocranium  itself,  and  those  produced  in  the 
cartilage.  Although  a  knowledge  of  the  history  of  the  sutures  must 
be  considered  of  the  utmost  importance  for  the  elucidation  of  the 
morph-  "v  of  the  skull,  such  knowledge  appears  never  to  have  been 
sought.  Besides  those  parts  of  the  cartilaginous  skull  which  make 
bones  there  are  certain  others,  few  in  number  and  small  in  size,  which 
atrophy.  We  have  then  to  present  the  history  of  the  ossification  and 
partial  atrophy  of  the  chondrocranium. 

OSSIFICATION. — The  occipital  region  begins  to  ossify  during  the 
early  part  of  the  third  month  in  human  embryos;  comparative  anat- 
omy teaches  that  the  occipital  bone  of  man  is  homologous  with  five 
bones — the  median  ventral  basi-occipital  bordering  the  front  or  ven- 
tral side  of  the  foramen  magnum,  the  paired  lateral  ex-occipitals  bor- 
dering the  sides  of  the  foramen  and  including  the  condyles  by  which 
the  occiput  articulates  with  the  axis,  and  the  paired  supra-occipitals, 
which,  however,  are  often  united  into  a  dorsal  median  bone;  in 


f.m 

Fio.  250.  —  Embryo   Pi  . 
Iiicli.-s   Long.     Partly   <  • 
Chondrocranium      seen     from 
;il...v,-.    ,,lu.  Ali  -nasal  :  etli.  t-tti- 
moid;    n,.t/>.  middle   turbirml; 
cr.cribriform  plate;  obs,  orbito- 
sphenoid  ;  <//.  ali-sphenoid  ;  per, 
6.oc,basf-oc- 


c'ipital  ;  /m,  foramen  inagimm. 
«.  or,  supra-occipital.  Natural 
size.  After  W.  K.  Parker. 


440  THE    FCETUS. 

agreement  with  this  homology  there  are  five  centres  in  the  occipital 
cranium,  namely,  the  basi-occipital,  the  two  ex-occipital  or  condylar, 
and  two  supra-occipital,  which,  however,  very  soon  unite;  according 
to  Kolliker  there  is  also  later  a  small  deposit  of  dermal  bone  added  to 
the  supra-occipital.  The  ex-occipitals  do  not  unite  with  the  supra- 
occipitals  until  one  or  two  years  after  birth,  nor  with  the  basi-occip- 
itals  until  the  fifth  or  sixth  year.  In  the  sphenoid  region  ossifica- 
tion begins  during  the  second  half  of  the  third  month  in  the  human 
embryo,  and  takes  place  from  six  principal  centres  corresponding  to 
the  six  bones  with  which  the  human  sphenoid  bone  is  homologized 
by  comparative  anatomists.  The  six  centres  are:  1,  the  basi-sphe- 
noid  in  the  neighborhood  of  the  hypophysis,  and  said  by  Kolliker  to 
be  due  to  the  fusion  of  two  minor  centres ;  2,  the  pre-sphenoid,  which 
appears  in  the  median  line  near  the  optic  foramina,  and  is  likewise 
said  to  consist  of  two  minor  fused  centres;  the  pre-sphenoid,  at  least 
in  the  pig,  is  the  last  of  the  six  centres  to  appear ;  3,  4,  the  ali-sphe- 
noid  centres,  one  in  each  wing,  Fig.  250,  al ;  they  appear  a  little 
later  than  the  basi-sphenoid  centre ;  5,  6,  the  orbi to-sphenoid  centres, 
which  unite  with  the  pra3-sphenoid  after  the  fifth  month ;  the  pra3- 
sphenoid  and  basi-sphenoid  do  not  unite  until  several  years  after  birth, 
and  even  at  thirteen  years  Virchow  has  found  remnants  of  cartilage 
between  the  two  bones.  In  the  periotic  region  there  are  three  main 
centres,  which  are  taken  to  represent  as  many  distinct  bones,  al- 
though they  unite  in  mammals  into  a  single  bone,  the  05  petrosum; 
in  man  the  petrous  bone  is  found  to  have  fused  with  the  dermal  bone, 
known  as  the  squamosum,  and  also  with  the  ring  of  bone  formed 
around  the  tympanum  of  the  ear,  and  known  as  the  annulus  tym- 
panicus ;  from  the  union  of  these  five  bones  arises  the  temporal  bone 
of  human  anatomy.  The  three  centres  which  appear  in  the  periotic 
capsules  are  termed  the  pro-otic,  opisthotic,  and  epiotic,  and  are  con- 
sidered to  represent  the  separate  bones  bearing  the  same  names  in 
lower  vertebral  js;  the  pro-otic  centre  is  by  its  position  in  close  rela- 
tion with  the  anterior  vertical  semicircular  canal,  between  which 
and  the  exit  of  the  third  division  of  the  fifth  nerve  it  lies ;  in  pig  em- 
bryos of  six  inches  it  forms  a  patch  of  bone  lying  under  the  fore  part 
of  the  cochlea  above  and  in  front  of  the  fenestra  ovalis,  and  extend- 
ing to  the  junction  of  the  anterior  and  posterior  semicircular  canals; 
the  opisthotic  centre  is  on  the  lower  and  posterior  surface  of  the 
capsule,  placed  so  that  most  of  the  bulbous  portion  of  the  cochlea  lies 
dorsal  to  it ;  one  of  its  processes  lies  between  the  fenestra  ovalis  and 
the  fenestra  rotunda,  close  in  front  of  the  head  of  the  stylo-hyal  car- 
tilage ;  the  epiotic  centre  develops  somewhat  more  tardily ;  it  is  in 
especial  relation  with  the  posterior  vertical  semicircular  canal,  and 
when  it  first  appears  (pig  embryos  of  six  inches)  is  a  small  piece  just 
above  the  stylo-hyal  process  and  foramen  rotundum,  and  behind 
both  the  foramen  ovale  and  the  above-mentioned  opisthotic  process. 
According  to  A.  J.  Vrolik,  73.1,  the  ossification  of  the  periotic  cap- 
sules proceeds  somewhat  differently  in  man,  there  being  four  centres 
which  coalesce  by  the  sixth  month  of  foetal  life.  In  the  ethmoidal 
region,  including  the  cribriform  plate,  the  lateral  nasal  and  turbinal 
cartilages,  ossification  takes  place  very  late,  and  the  morphological 
significance  or  homologies  of  the  various  centres  is  little  understood. 


AXIAL   SKELETON.  441 

In  the  pig  at  birth  the  median  cartilage  is  unossified,  the  cribriform 
jilate  is  about  to  begin  ossification,  being  invaded  by  vascular  nu's«-n- 
chyina,  the  upper  and  middle  turbinals  are  partially  ossified,  the  in- 
ferior turhiimls  almost  completely  ossified.  In  man  a  similar  con- 
dition is  reached  about  the  seventh  month  of  foetal  life.  The  human 
ethmoid  proper  does  not  ossify  until  the  first  year  after  birth. 

ATROPHY. — There  are  certain  parts  of  the  chondrocrauium  which 
do  not  ossify,  but  are  lost  in  the  adult.  The  exact  process  by  which 
they  are  resorbed  is  not  known.  The  following  parts  are  said  to 
disappear:  the  cornua  trabeculae;  2,  the  cartilage  under  the  nasals; 
:;,  Spnndirs  so-called  frontal  plate,  or  that  portion  of  the  orbito-sphe- 
noid  outside  of  which  the  frontal  bone  is  developed;  4,  the  parietal 
]  >late  or  a  small  portion  of  the  ex-occipital  outside  of  which  the  parietal 
b<  me  is  developed ;  5,  a  small  portion  of  the  ali-sphenoid  (ala  magna) 
outside  of  which  the  parietal  bone  is  developed;  6,  the  cartilaginous 
capsules  of  the  sphenoidal,  maxillary,  and  frontal  sinuses ;  7,  parts 
of  the  turbinal  cartilages. 

Dursy,  69. 1,  203,  has  maintained  that  some  of  these  cartilages  do 
n«  >t  really  disappear  by  atrophy,  but  by  becoming  ossified  and  united 
with  the  dermal  bones  overlying  them.  Kolliker  (u  Entwickelungs- 
• -Indite,"  456),  without  absolutely  denying  the  correctness  of 
1  >ursy's  view,  states  that  he  has  been  unable  to  confirm  it  by  his  own 
observations. 

The  following  description  of  the  primordial  skull  of  Tatusia  (one  of 
the  Insectivora)  in  VV.  K.  Parker's  own  words,  86. 1,  7-10,brings  out 
many  points  of  morphological  importance:*  "So  great  is  the  uni- 
formity of  the  early  chondrocranium  in  the  eutheria  or  placental 
mammals,  that  the  drawing,  Fig.  251,  made  from  the  skull  of  an 
outlying  and  low  type,  might  serve  as  a  diagram  wherewith  to 
illustrate  the  skull  at  this  stage  of  the  types  of  this  order,  and  of  all 
the  orders  above  it.  The  figure  of  a  chondrocranium  like  this,  but 
a  little  less  advanced,  before  the  osseous  centres  have  commenced  in 
it — that  of  the  mole — will  be  given  in  my  next  paper ;  and  such  a 
skull  is  very  near  to  that  of  a  shark,  or,  still  better,  of  a  skate.  The 
parts,  or  rather,  regions,  of  which  it  is  composed,  correspond  very 
exactly  with  what  is  seen  in  those  generalized,  but  not  low,  fishes; 
and  in  this  specimen  with  long  centres  appearing,  the  level  is  ob- 
tained which  is  permanent  in  the  skull  of  the  dipnoi,  and  of  such 
a  low  ganoid  as  the  paddle-fish  (Polyodon).f  As  in  cartilaginous 
fishes  and  amphibians,  the  chondrocranium  may  be  compared  to  a 
basin  or  a  boat,  the  upper  part  being  unfinished,  leaving  a  mem- 
branous fontanelle  of  greater  or  less  extent;  this  is  only  partially 
filled  in,  at  present,  by  the  investing  bones,  the  frontals  and  parie- 
tals  (f.,  p.).  The  outline  of  this  sectional  view  is  very  elegant,  and 
quite  similar  to  that  of  a  vertical  section  of  a  bird's  skull  at  a  like 
stage,  except  that  the  nasal  roof-cartilages  run  on  along  the  whole 
extent  of  the  median  keeled  bar — the  intertrabecula ;  in  the  bird  they 
stop  short,  leaving  a  free  cartilaginous  rostrum,  like  that  of  a  shark 
or  skate,  which,  however,  only  lasts  until  it  has  served  as  a  model 

*  Compare  also  Parker,  86.2,  "On  the  Skull  of  Insectivora." 

t  See  Bridge,    "On  the  Skull  of  the  Pulyodon  Felium,"  Phil.  Trans.,  1872.     Plates  55-57,  pp. 


442  THE   FOETUS. 

on  which  the  huge  premaxillaries  of  the  bird  are  formed.  In  the 
sides  of  this  hollow  cartilaginous  structure  near  the  hind  part  the  large 
oval  auditory  capsules  (a.sc,  chl)  are  seen  to  have  great  distinct- 
ness ;  they  are,  however,  confluent  with  the  chondrocranium  proper 
at  various  points — above,  behind,  and  below,  as  the  section  will 
show.  These  are  the  only  sense  capsules  displayed  in  a  preparation 
of  this  kind,  for  the  eyeballs  are  quite  free  from  the  solid  cranial 
structure  (and  are,  indeed,  outside  in  such  a  view  as  this)  and  the 
left  nasal  labyrinth  has  been  removed.  Before  describing  this  figure 
in  detail  there  is  one  remark  to  be  made,  namely,  that  here  we  have 
clearly  shown  the  true  diagnostic  mark  of  a  mammalian  skull. 
This  mark  is  the  rupture  of  the  side  walls,  due  to  the  pressure  of 
the  large  lateral  masses  of  the  cerebrum.  In  front  of  the  auditory 
capsules  there  is  a  large,  elegantly  semicircular  opening,  the  crown 


SLS.C 


so 


FIG.   251.—  Chondrocraniuin    of  an  Insectivorous   Mammal   (Tatusia).     After  W.  K.    Parker. 

Explanation  in  text. 

of  the  arch  looking  upward  and  forward.  Only  the  lower  half  of 
the  wall  has  thus  broken  outward;  this  ' fault'  forms  the  ali- 
sphenoid,  while  the  orbito-sphenoid  (o.s),  the  so-called  'lesser 
wing, '  is  many  times  its  size  and  is  continuous,  over  the  archways, 
with  the  cartilage  that  runs  on  backward,  into  the  supra-occipital 
region  (so).  There  is  nothing  similar  to  this  in  that  sauropsidan 
skull  which  comes  nearest  to  that  of  the  mammal,  the  skull  of 
the  crocodile  (see  Trans.  Zool.  Soc.,  Vol.  XI.,  Plate  65),  while  in 
birds  the  orbito-sphenoids  are  very  small,  even  when  they  are  most 
developed,  as  in  Struthio  (see  Phil.  Trans.,  1866,  Plate  7),  and  in 
that  class  the  ali-sphenoids  almost  finish  the  cranial  cavity,  being 
turned  inward  toward  each  other,  on  each  side  of  the  back  part  of 
the  orbital  septum.  I  lay  special  stress  upon  this  rupture  outward  of 
the  ali-sphenoid,  and  on  the  fact  that  the  nasal  roofs  utilize  the 
whole  of  the  huge  high-crested  intertrabecula,  because  these  are  the 
most  distinctive  marks  of  the  mammalian  skull,  and  they  arise  out 
of  two  things  in  which  the  mammal  shows  its  great  superiority  to 
even  the  highest  Sauropsida,  namely,  the  huge  volume  of  the  cere- 
brum, and  the  tenfold  complexity  of  the  nasal  labyrinth.  A  third 
clear  diagnostic  is  seen  in  this  very  figure ;  this  is  the  peculiar  de- 
velopment of  the  antero-inferior  part  of  the  oblique  auditory  capsule, 


AXIAL   SKELETON  .\  \:} 

due  to  tin-  development  of  the  coils  of  the  cochlea.  So  that,  at  once 
correlated  with  the  sudden  expansion,  so  to  speak,  of  the  cerebrum, 
we  have  these  ncir  and  most  important  improvements  in  tin-  organs 
of  smell  and  of  hearing.  At  first  sight,  seeing  how  large  the  median 
bar  (intertrabectila)  is,  with  its  internasal  crest  (perpendicular  eth- 
moid and  septum  nasi — pe,  s.tt),  it  might  be  supposed  that  the 
mammalian  skull  was  of  the  lu'f/h  kind,  like  that  seen  in  many 
tc -It MI stcan  iishrs,  in  lizards,  and  in  birds.  It  is  not  so,  however,  but 
belongs  to  the  /o/rkind,  seen  in  selachians  and  amphibians;  and,  like 
theirs,  is  hinged  on  the  spine  by  a  pair  of  occipital  condyles.  Hence 
the  eyeballs  are  kept  far  apart,  instead  of  coming  very  near  each 
other  as  in  most  birds,  where  often  nothing  but  a  membranous 
ieiiestra  is  found  between  the  right  and  left  capsules  and  their  spe- 
cial miiH-uljir  apparatus.  But  the  face  as  well  as  the  skull  of  the 
mammal  shows  marks  of  excellence,  such  as  are  not  seen  in  the 
Sauropsida,  even  in  the  higher  kinds  as  crocodiles  and  birds.  The 
uieat  development  of  the  nasal  organs  is  correlated  with  a  most 
remarkable  growth  of  the  bones  of  the  upper  jaw  and  the  palate  to 
form  the  'hard  palat  /  This  is  found  in  rudiment  even  in  the 
chelonia  and  in  birds;  but  especially  in  the  crocodilia,  where,  how- 
ever, its  excessive  development — as  in  certain  Edentata,  e.  (/.  Myr- 
nuTuitliaqa — is  not  dependent  upon  or  correlated  with  any  great 
improvement  in  the  organs  of  smell,  but  has  to  do  with  the  peculiar 
manner  in  which  these  monsters  take  their  prey." 

Branchial  Skeleton. — Every  branchial  arch  contains  a  skeletal 
element,  which  in  its  primitive  form  in  all  vertebrate  embryos*  is  a 
bar  or  rod  of  condensed  mesenchyma,  which  very  early  changes  into 
cartilage.  The  number  of  these  bars  of  course  depends  upon  the 
number  of  gill-arches,  compare  p.  263,  and  hence  in  the  mammalia 
there  are  five  branchial  cartilages  on  each  side,  which  begin  dorsally 
near  the  cranium,  and  curving  around  the  sides  of  the  pharynx  end 
near  the  median  ventral  line,  Fig.  177.  The  position  of  the  carti- 
lage can  be  seen  in  a  section  of  a  branchial  arch,  Fig.  152,  to  be 
alongside  of  the  artery  or  aortic  arch,  and  on  the  pharyngeal  side  of 
the  coelom  of  the  branchial  arch.  The  constant  recurrence  of  the 
simple  stage  just  described  in  all  vertebrates  (except,  perhaps,  in 
marsipobranchs),  renders  it  highly  probable  that  forms  existed  at  one 
time  with  such  a  branchial  skeleton;  but  no  such  forms  are  known 
to  exist  at  the  present  day. 

It  will  be  convenient  to  state  the  divisions  which  comparative 
anatomy  teaches  us  may  be  considered  typical  for  each  branchial 
cartilage.  The  divisions  are  usually  given  as  four:  1,  pharyngo- 
branchial,  or  dorsal  segments,  which  has  usually  a  horizontal  course; 
\\  the  epi-branchial,  and,  3,  cerato- branchial,  both  at  the  sides  of  the 
pharynx ;  4,  the  hypo-branchial  or  ventral  segment,  which  typically 
articulates  with  a  median  unpaired  cartilage  known  as  the  basi- 
branchial,  or  copula.  In  the  aquatic  vertebrates  the  bars  usually  send 
out  supporting  cartilages  into  the  branchial  lamellae,  but  in  mammals 
there  is  no  trace  of  any  similar  outgrowths  even  during  embryonic 
periods. 

*  E\-cfpt.  perhaps,  in  the  marsipobranchs.  the  branchial  skeleton  of  which  is  possibly  not 
homologous  with  that  of  the  higher  vertebrates.  See,  however,  Anton  Dohrn,  84.  1. 


444  THE    FCETUS. 

In  mammals  the  earliest  stage  of  the  branchial  skeleton  has  never 
been  accurately  described ;  this  is  because  investigators  have  hitherto 
been  content  to  begin  with  the  cartilaginous  stage,  instead  of  the 
mesenchymal  stage,  and,  consequently  we  are  left  with  no  definite 
information  as  to  the  bars  of  the  fourth  and  fifth  arches,  and  with 
insufficient  information  as  to  the  origin  of  the  bars  of  the  first  to 
third  arches.  In  selachians,  according  to  Anton  Dohrn,  84. 1,  110- 
111,  the  differentiation  of  the  cartilage  of  the  branchial  arches  begins 
shortly  after  the  branchial  filaments  have  appeared  as  a  condensa- 
tion of  the  mesenchyma,  Fig.  152,  O,  situated  on  the  pharyngeal 
side  of  the  arch  and  tailward  of  the  mesothelial  anlage,  lu.m,  of  the 
inner  muscles.  For  the  further  history  see  Dohrn,  /.  c.,  114.  In 
regard  to  the  history  of  the  branchial  skeleton  from  the  cartilaginous 
stage  on,  we  have  very  full  information,  chiefly  owing  to  the  exten- 
sive investigations  of  W.  K.  Parker,  also  in  part  through  Kolliker, 
Dollo,  Salensky,  80. 1,  Fraser,  82. 1,  and  others.  Each  pair  of  bars 
passes  through  a  distinct  series  of  modifications,  therefore  it  will  be 
convenient  to  present  the  history  of  each  pair  separately.  We  shall 
call  the  skeleton  of  the  first  arch  the  mandibular  bars,  that  of  the  sec- 
ond the  hyoid  bars,  of  the  third  the  thyro-hyal  bars. 

MANDIBULAR  BARS. — The  adaptations  of  both  the  mandibular  and 
hyoid  bars  to  functions  entirely  different  from  those  which  they 
primitively  served,  are  most  remarkable.  In  mammals  the  mandib- 
ular bar  becomes  primarily  divided  into  two  parts,  a  dorsal  piece 
corresponding  to  the  palatoquadrate.of  comparative  anatomy,  and  a 
ventral  piece  known  as  Meckel's  cartilage.  The  commencement  of 
the  corresponding  division  of  the  mandibular  bar  may  be  seen  in  a 
dog-fish  embryo  of  about  23  mm.,  the  upper  end  of  the  bar  being 
enlarged  and  sending  out  a  process  which  runs  forward  on  the  cranial 
side  of  the  mouth  and  later  joins  the  trabecula ;  this  process  is  the 
palato-pterygoid ;  another  process,  the  meta-pterygoid,  runs  upward ; 
the  wider  part  uniting  the  two  processes  is  homologous  with  the 
quadrate;  in  elasmobranchs  the  meta-pterygoid  process  becomes 
ligamentous.  In  mammals  the  early  stages  have  not  been  worked 
out.  Parker  states  that  in  embryo  pigs  of  about  16  mm.  the  cartila- 
ginous palato-pterygoid  bars,  Fig.  247,  are  less  definitely  developed 
than  the  other  skeletal  elements  present  at  this  stage,  but  are  more 
or  less  distinct  from  the  rest  of  the  mandibular  bar ;  the  palato-ptery- 
goids  are  situated  in  the  maxillary  process,  so  that,  starting  from  the 
dorsal  end  of  the  mandibular  arches,  they  run  obliquely  downward 
and  forward  toward  the  anterior  end  of  the  trabecula3 ;  anteriorly, 
they  converge  toward  the  median  line,  but  do  not  meet.  In  the 
mandibular  arch  itself  is  the  rod-like  Meckel's  cartilage,  Fig.  247, 
Md.  Between  the  pterygoid  plate  and  the  cartilage  of  Meckel  is  a 
space  in  which  Parker  figures  no  skeletal  element,  but  which  is  oc- 
cupied by  the  quadrate  element,  which  in  mammals  is  the  anlage  of 
incus.  At  the  same  stage  (embryo  pig,  16  mm.)  the  lower  divisions 
of  mandibular  bar  or  the  Meckel's  cartilages  are  much  stouter  and 
are  better  differentiated  from  the  mesenchyma  than  the  palato-ptery- 
goids;  they  are  situated  in  the  mandibular  processes,  and  do  not 
meet  in  the  median  line.  Each  Meckel's  cartilage  is  a  rounded  rod, 
but  its  dorsal  extremity  forms  a  hook,  is  somewhat  enlarged,  and  is 


AXIAL   SKELETON.  445 

situated  close  to  the  upper  border  of  the  first  branchial  cleft.  In 
pigs  a  little  older  (•>:*  nun.)  the  hook  is  longer  and  the  end  of  the 
cartilage  is  thicker,  making  it  easy  to  recognize  in  it  the  anlage  of 
the  malleus,  the  hook  being  the  future  mannbrium  or  handle  of  the 
malleus.  In  pigs  t\v<>  and  one-half  inches  long  the  malleus  is  sepa- 
rately  ossified,  but  is  not  separated  from  the  cartilage  of  the  jaw. 
When  the  final  separation  takes  place  I  do  not  know. 

M*  i -leers  cartilage  proper  may  be  defined  as  the  ventral  segment 
of  the  first  branchial  bar.  In  mammals  the  two  cartilages  a hvays 
unite  in  the  median  line,  although  in  man  the  actual  union  is  said 
not  to  have  been  observed.  The  lower  portions  of  the  cartilage  ossify 
metaplastically  but  not  to  the  median  line,  and  this  ossification  be- 
gins in  man  during  the  third  month.  The  bony  part  is  incorporated 
in  the  permanent  mandible,  but  the  rest  of  the  cartilage  atrophies 
and  entirely  disappears  except  a  small  portion  of  the  end  next  the 
malleus,  which  becomes  changed  into  fibrillar  ti>sue  and  remains, 
according  to  Kolliker,  "Grundriss,"  320,  as  the  ligamentum  laterale 
in ternum  maxillae  inferioris.  Meckel's  cartilage  is  the  homologue 
of  the  cartilaginous  mandible  of  the  lower  fishes,  but  is  not  homolo- 
gous with  the  bony  mandible  of  the  amniota,  which  is  developed 
later  and  belongs  to  the  class  of  the  dermal  bon<  •-. 

Summitry. — The  primitive  cartilaginous  rod  of  the  first  branchial 
arch  gives  rise  first  to  a  palato-quadrate  dorsal  segment  and  a  ven- 
t  r;  1 1  or  Meckelian  segment.  The  palato-quadrate  segment  subdivides 
into  the  palato-pterygoid  plate  and  the  quadrate  or  incus.  In  the 
earliest  accurately  known  mammalian  stage  the  palato-pterygoid 
and  incus  are  already  separate,  but  it  may  be  safely  assumed  that  in 
a  still  earlier  stage  they  constitute  one  piece.  The  Meckelian  seg- 
ment subdivides  into  the  malleus  and  the  Meckelian  cartilage  proper ; 
the  latter  unites  in  the  median  ventral  line  with  its  fellow.  One 
inevitably  inclines  to  homologize  the  parts  with  a  typical  branchial 
arch  as  follows :  The  palato-pterygoid  is  the  pharyngo-branchial ;  the 
incus  is  the  epi-branchial;  the  malleus  is  the  cerato-branchial ;  the 
Meeker s  cartilage  is  the  hypo- branchial;  the  united  ends  of  the 
cartilages  are  the  copula.  These  homologies  are,  however,  some- 
what hypothetical,  principally  because  the  homologies  of  the  malleus 
are  not  clearly  ascertained,  and  we  cannot  say  what  element  of  the 
lower  vertebrates  it  represents. 

The  course  of  the  palato-pterygoid  at  such  a  marked  angle  to  the 
Meckel's  cartilage  is  probably  due  to  the  head-bend.  Very  likely 
the  head-bend  is  causally  connected  also  with  the  peculiar  forms 
assumed  by  the  incus  and  malleus. 

HYOID  BARS,  or  ReicherVs  cartilages,  as  they  have  been  named 
1  >y  Kolliker,  are  the  skeletal  elements  of  the  second  or  hyoid  branch- 
ial arch,  and  they  are  typically  divided,  like  the  other  bars  in  the 
lower  vertebrates,  into  four  parts,  the  dorsal  one  of  which  (pharyngo- 
branchial)  fuses  quite  early  with  the  cartilaginous  periotic  capsules, 
and  becoming  ossified  appears  in  the  human  adult  as  the  styloid 
process;  the  second  part  (epi-branchial)  becomes  partly  ligamentous 
in  all  placental  mammals,  and  perhaps  wholly  ligamentous  in  man ; 
the  third  part  (cerato-branchial)  and  fourth  part  (hypo- branchial) 
both  become  cartilaginous  and  ossify  early,  so  as  to  form  a  single 


446  THE    FCETUS. 

piece  of  bone,  which  perhaps  includes  also  some  bone  derived  from 
the  second  part  also.  This  single  piece  of  bone  is  known  in  the 
adult  as  the  lesser  horn  of  the  hyoid.  The  adult  hyoid  bar  then 
comprises  the  styloid  process,  the  stylo-hyal  ligament,  and  the  lesser 
hyoid  cornua.  The  main  body  of  the  hyoid  probably  belongs  to  the 
next  branchial  arch,  but  the  hyoid  bars  unite  with  it  very  early. 

It  was  long  maintained  by  Huxley,  69.1,  and  W.  K.  Parker 
(Parker  and  Bettany,  "  Skull")  that  the  incus  was  derived  from  the 
hyoid  bar,  but  since  Salensky,  80. 1,  showed  that  the  incus  is  devel- 
oped from  the  mandibular  bar,  Parker,  86.1,  10,  has  retracted  his 
former  opinion.  Reickert,  37.1,  thought  that  the  stapes  was  de- 
rived from  the  hyoid  bar,  but  recent  investigations  show  that  this  is 
not  the  case,  although  Rabl,  87.1,  has  shown  that  the  stapes  is  de- 
veloped within  the  territory  of  the  second  branchial  arch.  O.  Hert- 
wig  ("Lehrbuch,"  3te  Aim.,  509)  suggested  that  the  stapes  was  a 
double  bone,  one  part  of  which  is  derived  from  the  branchial  skele- 
ton, but  Staderini,  91.1,  has  proved  that  this  suggestion  cannot  be 
adopted— see  Chapter  XXVIII. 

The  following  quotation  from  W.  K.  Parker,  86.1,  10, 11,  gives 
some  insight  into  the  discussion  about  the  incus,  which  may  be  said 
to  have  ended  with  the  admissions  made  in  the  course  of  the  quoted 
sentences.  "  But  that  great  improvement  just  spoken  of  as  appear- 
ing in  the  organ  of  hearing  in  the  mammal  has  wrought  a  change  in 
the  hinder  face  that  has  two  most  important  bearings.  From  the 
first  promise  of  an  ear-drum  in  the  tailed  Amphibia,  to  its  highest 
fulfilment  in  the  noblest  of  the  oviparous  tribes — the  birds  that  nes- 
tle on  high  {'aves  altrices') — the  only  element  from  the  visceral 
arches  that  is  used  for  carrying  the  vibrations  of  the  air  inward  to 
the  organ  of  hearing  is  the  uppermost  part  of  the  hyoid  arch — the 
'  pharyngo-branchial '  element  of  the  second  postoral  arch,  to  speak 
morphologically.  From  the  salamandroids  to  the  singing  birds,  all 
through  the  Amphibia  and  Sauropsida,  the  first  postoral  arch  which 
forms  both  the  upper  and  lower  jaw  is  only  segmented  once,  that 
is,  into  an  epi-branchial  and  a  cerato-branchial  element  or  joint. 
The  upper  piece  is  specially  termed  the  '  quadrate  '  and  the  lower  the 
'articulo  Meckelian;'  the  one  forms  the  swinging  piece,  hinge,  or 
pier,  to  the  '  compound  lower  jaw, '  and  the  other  its  axis  or  pith,  the 
part  which  becomes  covered  with  more  or  fewer  'investing  bones.' 
In  these  low  '  Eutheria  '  and  also  in  both  the  '  Metatheria  '  and  the 
'  Prototheria '  (Marsupials  and  Monotremes) ,  the  modified  visceral 
rod  that  runs  through  the  drum  cavity  has  two  new  elements  added 
to  the  one  (single  or  variously  segmented)  element  derived  from 
the  hyoid  arch.  This  is  an  apparently  sudden  change,  for  we  have 
it  in  the  lowest  or  teatless  mammals ;  their  ancestry  that  should  show 
us  the  earlier  steps  of  the  change  are  unfortunately  all  extinct.  In 
this  dilemma  not  only  zoology,  but  paleontology  also,  fails  us  utterly, 
but  embryology  comes  in  with  every  stage  and  every  link.  I  have 
worked  out  the  early  conditions  of  these  parts  in  several  kinds  of 
Marsupials,  and  in  the  young  of  Ornithorhynchus ;  but  even  in  the 
lower  Euthreia,  the  Edentata,  now  to  be  described,  and  in  the  large 
and  varied  group  of  the  Insectivora,  I  have  been  able  to  trace  every 
step  in  the  transformation  of  these  parts.  I  am  now  satisfied  that 


AXIAL   SKELETON.  447 

the  inrns  is  the  upper  element  of  the  first  or  mandibular  arch;  both 
Professor  Salensky's  and  Professor  Eraser's  researches  put  this,  I 
think,  l>eyond  doubt;  and  my  own  attempts  for  a  long  time  to  make 
the  hyoid  theory  of  this  part  agree  with  facts,  only  kept  the  subject 
in  hopeless  confusion.  The  new  elements  of  the  ear-chain  art-  thru 
the  arrested  quadrate  or  incus,  and  the  arrested  and  um/tntuft^l 
articular  region  of  the  articulo-Meckelian  rod  or  primary  lower  jaw. 
The  bony  part  of  the  'ramus'  is  the  well-known  deiitary  with  the 
con  moid  and  splenial  bones  in  a  sub-distinct  state;  the  cartilage  for 
the  in-ir  (trltrnliitf<>H,of  the  lower  jaw  is  derived  from  a  large  super- 
ficial slab — a  '  lower  labial ' — the  like  of  which  is  not  found  again 
until  we  get  as  low  down  as  the  ChimaBroids.  From  this  is  derived 
the  hinder  half  of  the  ramus  by  transformation  of  its  substance  into 
bone ;  and  from  this  we  get  the  cartilage,  both  of  thecondyle  and  the 
glenoid  cavity,  and  also  of  the  intervening  'meniscus.'  Of  course 
the  drum  cavity  is  the  'first  cleft,'  and  the  concha  auris  with  its 
segmented  meatus-tube — the  tympanic  bone,  the  tympanic  bulla,  and 
the  cartilaginous  lining  of  the  Eustachian  tube — all  these  are  parts 
of  a  curiously  specialized  opercular  growth  belonging  to  the  hinder 

<  «l-e  of  the  first  visceral  fold  and  arch.     This  last  assertion  has  not 
been  made  as  a  stride  across  the  types  from  the  mammal  to  the 
elasmobranch,  but  is  the  result  of  a  very  slow  step-by-step  process, 
nmdo  during   many  years  'along  all  the  lines'  of  vertebrate  mor- 
phology." 

THYRO-HYOID  BARS. — Whether  these  bars  extend  in  the  mesen- 
rhymnl  stage  through  the  entire  length  of  the  third  branchial  arches 
or  not  is  not  known,  but  their  lower  ends  are  chondrified  and  later 

<  >ssitied  to  form  the  principal  part  of  the  hyoid  bone.     There  appears 
very  early  a  median  azygous  element  or  copula,  which  in  pigs  of  16 
nun.  is  already  cartilaginous  and  united  with  not  only  the  thyro- 
hyoid  bars  but  also  with  the  recurved  ends  of  the  hyoid  bars.     This 
copula  is  called  the  basi-hyal,  and  is  the  anlage  of  the  main  body  of 
the  hyoid  bone;  it  is  said  to  belong  to  the  third  branchial  arch,  al- 
though the  hyoid  bars  unite  with  it.     It  is  at  first  small  in  size,  but 
as  development  progresses  it  enlarges  considerably,  while  the  ventral 
ends  of  the  hyoid  bars  grow  but  little;  it  results  that  the  relative 
size  of  the  parts  is  changed,  and  the  rudiments  of  the  hyoid  bars, 
which  start  nearly  equal  in  diameter  to  the  basi-hyal,  appear  in  the 
adult  as  the  lesser  horns.     The  thyro-hyoid  cartilages,  on  the  other 
hand,  grow  at  about  the  same  rate  as  the  basi-hyal  and  become  the 
greater  horns  of  the  adult  hyoid  bone. 

The  hyoid  bone  of  mammals  is  formed  by  the  ventral  portions  of 
the  hyoid  bars  (lesser  cornua) ,  the  ventral  portions  of  the  thyro-hyoid 
bars  and  the  copula  of  the  third  pair  of  branchial  arches.  In  ac- 
cordance with  its  development  the  hyoid  bone  has  five  centres  of 
ossification,  one  for  the  body  and  one  for  each  of  its  four  horns. 
Ossification  begins  in  man  in  the  great  cornua  and  body  during  the 
last  month  of  foetal  life,  and  in  the  small  cornua  during  the  first 
year  after  birth.  The  great  cornua  and  body  do  not  unite  until  mid- 
dle life,  and  the  lesser  cornua  usually  remain  distinct,  though  some- 
times found  united  with  the  body  at  advanced  ages. 


448  THE   FCETUS: 

II.  THE  LIMBS  AND  APPENDICULAR  SKELETON. 

Origin  of  Vertebrate  Limbs. — The  morphological  value  of 
the  limbs  of  vertebrates  has  long  been  the  subject  of  discussion  and 
speculation,  and  at  the  present  time  the  solution  of  the  problem  is 
theoretical  rather  than  positive.  It  is  unnecessary  to  give  a  resume 
of  the  older  hypotheses  as  to  the  archtype  of  the  limbs,  though  I 
may  refer  those  interested  to  Owen's  article  "  On  the  Nature  of 
Limbs,"  and  Goodsir's  essay  "  On  the  Morphological  Constitution  of 
Limbs,"  Edinburgh,  Neiv  Philos.  Journ.,  1857.  Gegenbaur  has  ad- 
vanced an  hypothesis  of  the  origin  of  limbs  in  support  of  which  his 
memoir,  76.1,  brought  very  scanty  evidence.  According  to  this 
hypothesis  the  limbs  are  modified  branchial  skeletons,  the  shoulder 
and  pelvic  girdles  representing  the  branchial  bar,  and  the  skeletal 
pieces  of  the  limbs  proper  representing  branchial  rays ;  the  central 
ray  formed  the  axis  of  the  limb,  and  the  remaining  rays  gradually 
became  articulated  with  the  axial  ray,  and  thus  produced  the  type  of 
limb  found  in  Ceratodus,  and  which  Gegenbaur  regards  as  the  primi- 
tive type  from  which  all  vertebrate  limbs  are  derived.  This 
theory,  which  was  adopted  by  Huxley  (on  Ceratodus,  Proc.  Zool.  Soc., 
London,  187G),  has  attracted  great  attention,  although  it  has  been 
definitely  set  aside  by  the  observations  of  Balf our,  81.1,  on  the  de- 
velopment of  the  limbs  of  Scyllium,  which  demonstrated  that  the 
limbs  arise  as  parts  of  a  longitudinal  fold,  which  runs  along  the 
side  of  body,  both  fore  and  hind  limb  being  part  of  the  same  fold. 
Were  Gegenbaur 's  hypothesis  correct,  the  limbs  should  arise  as 
transverse  or  vertical  folds.  Under  these  circumstances  it  seems  to 
me  that  Gegenbaur 's  theory  has  merely  historical  interest. 

The  only  theory  having  any  standing  at  present  is  the  one  adopted 
by  Balfour  ("  Comp.  Embryology,"  II.)  according  to  which  the  limbs 
are  specialized  portions  of  a  lateral  fin-fold,  similar  to  the  dorsal  and 
ventral  median  fin-folds  of  fishes.  The  resemblance  of  the  lateral 
fins  or  true  limbs  to  the  median  fins  in  general  structure  is  obvious 
in  many  fishes,  and  especially  in  teleosts,  and  renders  direct  compari- 
son very  natural.  Such  comparison  is  suggested  by  several  writers, 
but  was  first  definitely  worked  out  by  J.  K.  Thacker,  77.1,  and  at 
about  the  same  time  advocated  by  St.  George  Mivart,  79.1,  both 
these  authors  basing  their  conclusions  upon  comparative  anatomical 
studies.  Their  general  result  was  that  the  structure  of  limbs  could 
be  explained  by  assuming  that  they  are  specialized  portions  of  lateral 
fin-folds,  having  a  structure  similar  to  that  of  the  median  fin-folds. 
At  about  the  same  time  appeared  the  chapter  of  Balf  our 's  mono- 
graph on  the  development  of  elasmobranch  fishes,  in  which  he  ad- 
vocated a  similar  theory  upon  embryological  grounds,  and  by  his 
observations  put  the  theory  upon  a  firm  basis.  It  is  a  remarkable 
coincidence  that  the  same  hypothesis  was  formulated  independently 
and  published  at  about  the  same  time  by  three  investigators.  These 
views  were  attacked  by  Yon  Davidoff,  79. 1,  then  a  pupil  of  Gegen- 
baur's,  and  to  Davidoff's  paper  Gegenbaur  added  a  note  upholding 
his  theory ;  these  criticisms  were  adequately  answered  by  Balfour, 
81.1  ("Reprinted  Works,"  L,  714). 

From  the  manner  of  their  development  it  is  obvious  that  the  limbs 


TIM-:     1.1MMS    ANh     A.PPENDICULAR    SKKLKTON. 


149 


have  a  ilattened  1'nrm  and  a  dorsal  (or  extensor)  surface,  and  a  ven- 
tral (or  flexor)  surface,  and  as  soon  as  they  project  from  the  body, 
as  they  do  at  right  angles,  there  is  an  anterior  or  cranial  border  and 
a  posterior  or  caudal  border.  The  development  of  the  limbs  in  Scyl- 
lium.  as  described  by  Balimir,  throws  important  light  on  the  primitive 
position  of  tin-so  borders.  Balfour  (k*Comp.  Embryol.,"  II.,  ur.') 
says:  "  The  direction  of  the  original  ridge  which  connects  the  two 
lins  of  each  side  is  nearly,  though  not  quite,  longitudinal,  sloping 
Knnewhat  obliquely  downward.  It  thus  comes  about  that  the  attach- 
ment of  each  pair  of  limbs  is  somewhat  on  a  slant,  and  that  the  pel- 
vic pair  nearly  meet  each  other  in  the  median  ventral  line  a  little 
way  hehind  the  anus.  The  elongated  ridge,  forming  the  rudiment 
of  each  tin,  gradually  projects  more  and  more,  and  so  becomes 
broader  in  proportion  to  its  length,  but  at  the  same  time  its  actual 
attachment  to  the  side  of  the  body  becomes  shortened  from  behind 
forward,  so  that  what  was  originally  the  attached  harder  becomes 
in  part  converted  into  the  posterior  border.  This  process  is  much 
more  completely  carried  out  in  the  case  of  the  pectoral  fins  than  in 
that  of  the  pelvic,  and  the  changes  of  form  undergone  by  the 
pectoral  fin  in  its  development  may  be  gathered  from  my  figure- 
In  Scy Ilium  the  development  of  both  the  pectoral  and  pelvic  tins  is 

similar.  In  both  fins 
the  skeleton  in  its  earliest 
st  age  consists  of  a  bar  spring- 
ing from  the  posterior  side  of 
the  pectoral  or  pelvic  girder, 
and  running  backward  paral- 
lel to  the  long  axis  of  the 
1  >« « ly .  The  outer  side  of  this 
bar  is  continued  into  a  plate 
which  extends  into  the  fin, 
and  which  becomes  very 
early  segmented  into  a  series 
of  parallel  rays  at  right  an- 
gles to  the  longitudinal  bar. 
In  other  words,  the  primitive 
skeleton  of  both  the  fins  con- 
si  sts  of  a  longitudinal  bar 
running  along  the  base  of  the  fin  and  giving  off  at  right  angles  a 
series  of  rays  which  pass  into  the  fin.  The  longitudinal  bar,  which 
may  be  called  the  basi-pterygium,  is,  moreover,  continuous  in  front 
with  the  pectoral  or  pelvic  girdle  as  the  case  may  be.  My  obser- 
vations show  that  the  embryonic  skeleton  of  the  paired  fin  con- 
sists of  a  series  of  parallel  rays  similar  to  those  of  the  unpaired  fins. 
These  rays  support  the  soft  part  of  the  fin,  which  has  the  form  of  a 
longitudinal  ridge,  and  are  continuous  at  their  base  with  a  longitudi- 
nal bar.  which  may  very  probably  be  due  to  secondary  development. 
As  pointed  out  by  Mivart,  a  longitudinal  bar  is  also  occasionally 
formed  to  support  the  cartilaginous  rays  of  unpaired  fins. " 

Balfour's  observations  show  that  there  was  a  primitive  longitudi- 
nal skeletal  piece  at  the  base  of  the  limb- fold,  and  that  from  this 
rays  are  developed  which  run  out  into  the  fold ;  Mivart  assumed 
29 


-~p.fi 


Fio.  258.— Pectoral  Fin  of  a  Yotmj?  Embryo  of  Sycl- 
liiun  in  Longitudinal  and  Horizontal  S,-<:ti..n  The 
•>ls.-l.-t.,n  (.f  tii,-  tin  was  still  in  the  condition  of  em- 
hr\,mir  cartilage:  b.p.^  basi-ptery^iuin  (••v.-ntuul 
iiu-ta -pt»>ryKuim) :  /V,  fln  rays:  p.y.  MOtonU  irirdle  in 
rse section;  /,  foramen  in  pectoral  girdle-  p  c. 
wall  of  peritoneal  cavity. 


450  THE    FCETUS. 

that  the  rays  were  primitive  and  the  longitudinal  piece  the  product 
of  the  fusion  of  the  bases  of  the  rays.  As  the  limb  grows  out  its 
base  becomes  free  and  constitutes  the  posterior  border,  and  the  basal 
skeletal  piece  appears  as  the  axis  of  the  limb,  while  the  fin-rays  run 
off  from  one  side  toward  the  primitive  outer  or  ultimate  cephalic 
border  of  the  fin ;  on  the  caudal  side  of  the  axis  there  are  necessa- 
rilly  no  fin-rays.  If  we  assume,  as  we  must,  that  Scyllium  illustrates 
the  general  type  of  fin  development,  then  a  condition  in  which,  as 
in  the  fins  of  the  adult  Ceratodus,  there  are  rays  on  both  sides  of  the 
axis  must  be  considered  a  secondary  condition.  The  Ceratodus 
type  is  known  as  the  archipterygium,  and,  as  already  stated,  has  been 
held  by  Gegenbaur  to  be  the  ancestral  form  of  vertebrate  limbs. 
But  our  knowledge  of  the  development  and  morphology  of  fins  ren- 
ders it  impossible  to  accept  this  view,  at  least  at  present. 

The  archipterygium  may  be  defined  as  a  skeletal  limb  axis  with 
rays  coming  off  on  both  sides;  no  such  fins  are  known  among  the 
lower  fishes,  but  only  among  the  higher  (Dipnoi) ;  this  fact  offers 
another  serious  obstacle  to  regarding  the  archipterygium  as  the  prim- 
itive ancestral  form,  but  suggests  that  it  may  represent  the  ancestral 
form  of  the  pentadactyle  limbs  of  amphibia  and  mammals.  I  think 
much  may  be  said  in  favor  of  this  suggestion,  and  indeed  it  is  largely 
on  account  of  the  possibility  of  deducing  the  pentadactyle  limbs 
from  it  that  the  archipterygium  has  been  regarded  as  an  archtype 
by  Gegenbaur  and  his  followers. 

The  cheiropterygium  is  the  archtype  or  ancestral  form  of  the 
pentadactyle  limb.  Its  essential  characteristic  is  the  division  into 
four  segments : 

Upper  arm.  2   j  Forearm.  o    <  Carpus.  4    \  Hand. 

Upper  leg.  ''  /  Lower  leg.  j  Tarsus.  \  Foot. 

The  upper  segment  contains  one  long  bone  (humerus  or  femur) ;  the 
second  segment  contains  two  long  bones  (radius  or  tibia,  and  ulna  or 
fibula) ;  the  third  segment  contains  nine  small  bones  (carpals  or  tar- 
sals)  ;  the  fourth  segment  consists  of  separate  digits,  five  in  number, 
hence  the  term  pentadactyle  applied  to  this  type  of  limb ;  each  digit 
has  a  proximal  or  basal  bone  (metacarpal  or  metatarsal)  upon  which 
follows  a  linear  series  of  phalanges,  separate  bones  variable  in  num- 
ber. It  is  convenient  always  to  count  the  digits  in  the  same  way, 
commencing  from  the  radial  or  tibial  side ;  thus  the  thumb  is  the 
first  digit  of  the  hand,  the  great  toe  the  first  digit  of  the  foot. 

The  arrangement  of  the  carpal  and  tarsal  bones  is  greatly  modified 
not  only  in  the  amniota  but  also  in  many  of  the  amphibia,  both  by 
the  suppression  of  some  of  the  nine  bones  and  by  fusions  among 
them.  The  nine  bones  are  the  intermedium  between  the  distal 
ends  of  the  radius  and  ulna,  the  radiale  and  ulnare  at  the  distal 
ends  of  the  radius  and  ulna  respectively ;  the  two  centralia,  on  the 
distal  side  of  the  ntermedium ;  between  these  four  and  the  meta- 
carpals  (or  metatarsals)  follow  the  five  carpalia  or  tarsalia, .  In 
most  pentadactyle  limbs  the  two  centralia  are  fused  into  one  bone, 
the  centrale.  In  many  cases  some  of  the  bones  are  suppressed.  The 
following  table  shows  the  homologies  in  man : 


TMK    LIMBS    AND    A  I'I'KN  I  >I(  T  LA  r,    SK  K  1  ,KT<  >N.  451 

itiKtilai  ( 'uin-it'orm*'.  (  alt-am-uni. 

Inl.-riiM-.liiuii.  Liuiaiv.  1  A«*tr-i"-ilus 

Ka.lial.'  (til.iale).  Srapl.oi.l.  \A 
Ontralia.  ui'-ulare. 

1.  1.    Carjiali-.  1.   Tars:il««. 

•J. 

a.    (Tarealia  3.  :;. 

>  Unciforme.  >  Cuboides. 

The  pisiforme  i-  moid  bone  developed  in  the  tendon  of  the 

r    carpi    ulnaris,   and    has   nothing   to   do  with   the   primitive 
carpus. 

It  is  generally  taught  that  there  is  one  series  of  bones  which 
represent-  tin-  true  axis  »>f  the  limb,  and  that  the  other  bones  repre- 
sent a  series  of  rays  coming  off  from  it.  This  supposed  axis  begins 
with  the  humenis  (fcinnri.  is  continued  through  the  ulna  (fibula), 
and  terminates  with  one  of  the  dibits,  but  which  digit  authorities 

!i«»t  agr 1;  t hus  Gegenbanr  carries  the  axis  through  the  ulnare 

fifth  metacarpal  and  fifth  digit,  which  makes  the  first  ray  pass  off 
from  the  humerus  and  include  the  radius,  radiale,  first  carpal,  and 
fir>t  digit:  the  >.-c,,nd  ray  arises  from  the  ulna  and  includes  the 
intermedium,  one  centrale,  and  the  second  digit ;  the  third  ray  springs 
from  the  ulnare  ;md  includes  one  centrale  and  the  third  digit :  the 
fourth  ray  springs  from  the  fifth  carpale  and  includes  the  fourth  car- 
pale  and  the  fourth  digit:  similarly,  changing  the  names,  in  the 
hind  limb,  see  Gegenbaur,  "Grundriss  d.  vergl.  Anatomic,"  1878, 
.")!•.',  Fig.  \'M.  Wiedersheim,  on  the  contrary,  carries  the  axis  (see 
his  -Mii-midri^  d-T  v.-rgl.  Anatomie,"  2te  Aufl.,  Fig.  110)  through 
the  ulna  (fibula),  intermedium,  both  centralia,  second  carpale  (tar- 
.  and  second  digit.  Such  divergences  of  opinion  raise  doubts  as 
to  the  existence  of  any  true  axis  at  all. 

ill  discussion  of  the  morphology  of  the  limbs  does  not  fall 
within  the  scope  of  this  work,  because  our  conceptions  are  not  based 
upon  embryological  observations.  1  shall,  therefore,  merely  refer  to 
the  recent  papers  of  G.  B.  Howes,  87.1,  J.  A.  Ryder,  87.1. 
D'Arcy  Thompson,  86.1,  llatschek,  89.1, and  E.  E.  Prince,  90.1. 

/»V/////o//  /"  ///f  .s<>///  //ex.  Kadi  limb  arises  along  the  territory  of 
several  somites,  and  receives  outgrowths  from  the  muscle  plat* 
several  successive  segments,  and  with  these  outgrowths,  which  pro- 
duce the  muscles  of  the  limbs,  come  the  nerves  of  several  segments, 
so  that  the  fact  that  the  limb  arises  along  a  considerable  length  of 
the  body  explains  several  important  features  in  the  development  of 
limbs— features  which  remain  inexplicable  if  we  accept  Gegenbaur 's 
theory  of  the  evolution  of  the  limb  from  a  branchial  arch,  because 
this  t  henry  confines  the  primitive  limb  to  a  single  segment,  whereas 
at  its  very  earliest  stage  it  is  already  related  to  several  segments. 
As  to  the  exact  number  of  limb  somites  we  are  in  doubt.  Balfour's 
observations  indicate  that  each  limb  was  originally  attached  along 
a  considerable  number  of  segments,  but  that  on  the  caudal  side  the 
attachment  becomes  shortened.  As  it  is  not  until  this  restriction  of 
the  base  has  taken  place,  that  the  muscle  plates  penetrate  the  limb, 
it  follows  that  the  muscles  of  the  limb  are  derived  from  a  less  num- 
ber of  segments  than  corresponded  to  the  primitive  attachment. 


452  THE    FCETUS. 

This  reduced  number  is  probably  five  in  the  amniota,  but  certainty 
011  this  point  is  yet  to  be  reached. 

Concerning  the  position  of  the  limbs,  as  regards  their  distance 
from  the  head  and  the  segments  to  which  they  belong,  we  have  little 
exact  knowledge.  A.  M.  Paterson,  91.2,  holds  that  the  position  in 
this  sense  is  not  uniform  among  the  mammalia ;  he  bases  this  opin- 
ion upon  the  innervation  which  is  variable.  The  variation  is  much 
less  for  the  fore  than  the  hind  limb;  the  former  is,  as  a  rule,  inner- 
vated from  the  lower  cervical  and  upper  thoracic  segments;  the 
twenty-fifth  spinal  nerve  is  the  only  one  invariably  present  in  the 
hind  limb  of  mammals,  while  the  nerve  plexus  may  begin,  according 
to  the  species,  with  any  of  the  nerves  from  the  twenty-first  to  the 
twenty-fifth,  and,  as  it  usually  comprises  five  or  six  spinal  nerves, 
it  ends  with  the  twenty-fifth  to  twenty-ninth  nerve.  It  is  thus  prob- 
able that  the  hind  limb  readily  shifts  its  position.  As  the  sacrum 
is  always  developed  in  connection  with  the  limb  it  follows  that  the 
number  of  praa-sacral  vertebrae  must  vary,  although  there  is  no  in- 
tercalation or  obliteration  of  vertebras. 

Position  of  the  Limbs. — The  primitive  position  of  the  limbs  is 
at  right  angles  to  the  body  in  a  plane  nearly  parallel  with  the  longi- 
tudinal body-axis.  The  first  change  is  the  appearance  of  two  bends 
which  give  the  limb  the  position  which  is  permanent  in  amphibia ; 
the  bends  are  similar  in  the  fore  and  hind  limbs.  The  first  bend 
(elbow  or  knee)  is  at  the  end  of  the  upper  limb  (humerus  or  femur) , 
and  is  such  that  the  lower  limb  is  flexed  downward  (ventralward) 
and  toward  the  median  line ;  the  second  bend  is  at  the  carpus  (tar- 
sus) and  is  in  the  opposite  direction  or  outward.  Thus  the  ventral 
aspects  of  the  forearms  and  lower  legs  come  to  look  inwardly  and 
their  dorsal  aspects  outwardly;  while  the  ventral  aspects  of  the 
hands  and  feet  look  downward  and  their  dorsal  aspects  upward. 
This  change  is  obviously  correlated  with  the  change  from  aquatic  to- 
terrestrial  life  and  the  consequent  substitution  of  legs  for  fins.  When 
the  position  of  the  limbs  has  been  no  further  altered  than  this,  the 
radius  and  tibia  are  found  on  the  cranial  side,  the  ulna  and  fibula 
on  the  caudal  side  of  their  respective  limbs.  The  second  step  is  the 
torsion  of  the  limbs,  which  is  similar  in  both  pairs  and  occurs  in  all 
mammalia,  the  result  of  which  is  that  the  digits  point  headward,  the 
first  digit  being  in  both  hind  and  fore  limbs  toward  the  median  line. 
This  is  the  arrangement  which  is  permanent  in  the  reptilia  and  in 
the  lower  mammalia.  The  torsion,  by  which  the  change  is  effected, 
does  not  take  place  in  the  arm  or  leg  itself,  but  at  the  shoulder  or  hip. 
The  third  change  is  the  torsion  of  the  upper  arm  (not  known  to  occur 
in  the  leg)  by  which  the  distal  end  of  the  humerus  is  twisted  over; 
through  an  angle  in  man  of  nearly  or  quite  one  hundred  and  eighty 
degrees ;  by  this  torsion  the  head  of  the  radius,  which  before  the  change 
was  on  the  inner  side  of  the  arm,  is  brought  across  in  front  of  the  ulna 
to  the  outer  side,  with  the  result  that  if  the  hand  is  kept  in  its  primi- 
tive position,  palm  down,  the  forearm  is  twisted  in  the  reverse  direc- 
tion to  the  upper  arm ;  this  third  change  is  accompanied  by  accessory 
modifications  in  the  joints  and  muscles  by  which  the  radius  becomes 
so  movable  that  it  can  be  employed  to  turn  the  hand  with  the  palm 
either  up  (supination)  or  down  (pronation) . 


THK    I.IMI'.S    AM:    APl'KNDlCl  l.Ai:    SKELETON. 


Anatomists  are  not    entirely   agreed   as  to   the  alterations   in   the 
positions  of  the  limbs.      The  ahove  fonnularization  of  the  ehang' 
based   partly  upon  that   given   by  Huxley  in  his  "  Anatomy  of    \Yr- 
tehrated  Animals,"  32-33,  and   on    Hatschek,  89. 1,  partly'  on  a 
observations  I  have  made  on  skeletons. 

It  is  to  be  expected  that  the  limbs  of  the  higher  mammalia  pass 
through  the  three  stages  of  limb  position  which  may  be  conveniently 
designated  as  amphibian,  reptilian,  and  mammalian.  Unfortu- 
nately there  are  no  observations  as  yet  to  show  whether  this  is  the 
or  not.  This  gap  in  our  knowledge  offers  a  favorable  opportu- 
nity for  a  research. 

Shoulder  Girdle. — The  anlage  of  the  shoulder  girdle  is  probably 
continuous  in  all  vertebrates,  as  it  has  been  shown  to  be  in  the  fishes, 
with  the  anlage  of  the  base  of  the  limb,  but  in  the  amniota  it  early 
hecomes  a  separate  cartilage,  lying  in  one  plane  and  extending dorso- 
ventrally.  In  mammals  there  is  a  large  dorsal  segment  of  this  car- 
tilage ahove  the  articulation  with  the  humerus  (glenoid  fo^a)  and  a 
much  smaller  segment  below  the  articulation.  The  dorsal  segment 
develops  into  the  large  shoulder  blade,  while  the  ventral  segment 
forms  merely  the  small  coracoid  process,  although  it  is  the  homo- 
logue  of  the  Lugo  and  independent  coracoid  bone  of  sauropsida  and 
amphibia.  It  is  to  be  noted  that  Sabatier,  80.1,  has  homologized 
the  "coracoid"  process  with  the  pra?-coracoid,  and  holds  that  the 
upper  third  of  the  mammalian  glenoid  fossa,  which  ossifies  from  a 
separate  centre,  represents  the  true  coracoid,  but  this  view  has  not 
been  accepted. 

Little  is  known  concerning  the  development  of  the    scapula    in 
mammalia  beyond   what  is  given  in  W.  K.  Parker's    monograph, 
68. 1,  a  work  which  has  by  no 
means  received  the  attention  it  -  c' 

•ves.  Owing  to  the  reduc- 
tion of  the  coracoid  in  mam- 
malia the  history  of  the  scapula 
is  practically  that  of  the  entire 
•boulder  girdle.  Parker,  /.  <•., 
p.  '!~y.\  -\|-M,  records  some  obser- 
vations on  the  scapula  of  hu- 
man embryos.  In  an  embryo 
o.l  in  dies  long,  the  scapula, 
Fig.  •>:>:;.  already  has  much  of 
its  persistent  form  and  is  ossi- 
fied through  about  half  its 
extent,  b;  the  small  size  of 
the  pra3-scapular  region,  p.sc, 
and  the  great  size  of  the  acro- 
mion, Acr,  are  features  in 
which  the  embryonic  shoulder 
blade  differs  strikingly  from 

that  of  the  adult;  in  an  earlier  stage,  embryo  of  2£  inches  the 
scapula  is  proportionately  still  smaller,  and  the  acromion  thicker  and 
more  curved  toward  the  clavicle.  The  meso-scapular  ridge,  msc, 
is  the  thick  prolongation  of  the  acromion.  The  coracoid  process, 


S.Sc 


Fir..  "Jr.!.  Sr.ipulu  of  a  Human  Kml>ry<>  of  five 
and  one-half  Inclu's.  I >< •!•>;»!  Vic\v.  Natural  ^i/>-. 
itte'a  t-pisl.-nial  fl.-m«Mit  (Tarkrr's  oin.. 
stt-nuiiii);  r,  cartilage  at  end  of  clavicle;  C7, 
termed  by  Parker  prae-coracoid  :  <• '.  cartilage  at 
scapular  end  of  clavicle,  tcnneil  by  Parker  meso- 
scapular  segment:  Acr,  acromion;  CV,  coracoid 
process;  gl.f,  glenoid  fossa :  l>.  bony  i».rti(»n: 
?HSC,  meso-scapula:  x.  x<-.  supra-scapula ;  p.ac. 
prae-scapula.  After  W.  K.  Parker. 


454  THE    FCETUS. 

Or,  is  small  and  slightly  curved ;  it  is  connected  by  a  fibrous  band 
with  the  end  of  the  clavicle,  but  the  cartilaginous  end  of  the  clavicle 
(Parker's  so-called  meso- scapular  segment)  is  articulated  by  a  synovia! 
joint  at  this  stage  with  the  end  of  the  acromion.  The  coraeoid  has 
its  own  centre  of  ossification,  to  which  are  added  at  the  time  of 
puberty  two  epiphysal  centres  (Rambaud  and  Renault) — its  ossifica- 
tion thus  indicating  its  morphological  individuality.  The  acromion 
has  two,  sometimes  three,  centres,  which  appear  between  the  four- 
teenth and  sixteenth  years  and  soon  coalesce,  but  the  ossified  acro- 
mion does  not  unite  with  the  scapula  until  eight  to  ten  years  later. 
There  is  a  separate  centre  for  the  inferior  angle  (supra-scapular)  and 
for  the  upper  part  of  the  glenoid  cavity. 

Clavicle. — Opinions  differ  as  to  whether  the  clavicle  is  a  dermal 
bone  or  an  integral  portion  of  the  scapular  arch.  It  is,  as  discovered 
by  C.  Bruch,  53.1,  371-372,  the  first  bone  formed  in  the  human 
embryo  its  ossification  going  on  during  the  seventh  week.  Geg- 
enbaur,  has  shown  that  the  bone  commences  by  ossification  of 
mesenchyma ;  then  cartilaginous  masses  appear  at  each  end,  which 
are,  however,  softer  and  have  less  basal  substance  than  most  em- 
bryonic cartilage ;  these  cartilages  serve  to  maintain  the  growth  in 
length  of  the  clavicle.  Kolliker  states  ("  Entwickehmgsgeschichte," 
1879,  p.  495)  that  he  has  verified  on  rabbit  embryos  Gegenbaur's 
observations,  though  he  regards  the  tissue  of  the  anlage  as  inter- 
mediate between  mesenchyma  and  true  cartilage.  Kolliker  adds 
that  there  is  a  separate  centre  of  ossification,  which  may  be  com- 
pared to  an  epiphysis  at  the  sternal  end.  This  epiphysal  piece  was 
first  described  by  W.  K.  Parker,  68.1,  223-224,  and  was  shown  by 
him  to  become  distinct  while  still  cartilage ;  Parker  terms  it  the  pra3- 
coracoid,  although  this  name  is  properly  applied  to  an  entirely  differ- 
ent bone.  These  peculiarities  in  the  development  of  the  clavicle, 
together  with  Rathke's  statement  that  the  clavicular  anlage  is  at 
first  continuous  with  that  of  the  coraco-scapular  arch,  and  certain 
observations  of  his  own,  have  led  Alex.  Goette,  77.1,  to  maintain 
that  the  clavicle  is  an  element  of  the  shoulder.  Goette 's  observa- 
tions have  been  in  part  confirmed  by  C.  K.  Hofmann,  79. 1.  Gegen- 
baur  regards  the  mammalian  clavicle  as  a  compound  bone  homolo- 
gous with  both  the  true  dermal  clavicle  (Decknochen  des  Procora- 
coids)  and  the  cartilaginous  procoracoid  of  fishes,  the  two  originally 
separate  skeletal  elements  having  united  with  one  another ;  by  this 
double  homology  Gegenbaur  explains  the  peculiar  development  of 
the  bone;  compare  his  "Grundriss  d.  vergl.  Anatomie,"  2te  Aufl., 
501.  It  is  possible,  however,  that  we  attribute  too  great  morphological 
meaning  to  the  appearance  of  cartilage,  and  that  partial  chondrifi- 
cation  of  the  clavicular  anlage  does  not  mean,  as  Gegenbaur  thinks, 
a  separate  element  of  the  skeleton,  or,  as  Goette  thinks,  connection 
with  the  shoulder  girdle,  but  is  merely  a  modification  of  the  histo- 
genetic  development — compare  the  paragraph  on  the  mandible,  p. 
444.  We  cannot  hope  to  understand  the  homologies  of  the  clavicle 
until  its  development  shall  have  been  completely  traced,  beginning 
with  the  earliest  mesenchymal  stage. 

Episternum. — Whether  there  is  any  episternum  in  the  human 
embryo  is  uncertain.  Perhaps  the  suprasternal  cartilages  just  men- 


T1IK    I.IMI'.N    AND    AI'I'KXDK   fl.Al;    s  K  K  |.I-;T<  >X.  455 

tioned  as  having  been  (It-scribed  by  G.  Rug1*1,  80.1,  arc  its  repre- 
sentative-. K.  r>ardeleben,  79.1,  has  sought  to  honiologi/e  the 
deep  portion  ».f  tin-  interclaviciilar  li ganu -n t  as  the  r udi n icii t  < >f  the 
human  episternum.  A.  Goette,  win >  has  worked  out,  77.1,  the 
development  of  the  parts  more  fully  than  any  other  anatomist, 
finds  that  "paired  interelavicular  elements  grow  out  backward 
from  the  ventral  ends  of  the  clavicles,  and  uniting  together  form 
'inewhat  T-shaped  interclavicle  overlying  the  front  end  of  the 
sternum.  This  condition  is  permanent  in  the  Ornithodelphia  ex- 
cept that  the  anterior  part  of  the  sternum  undergoes  atrophy.  But 
in  the  higher  forms  the  interclavicle  becomes  almost  at  once  divided 
into  three  parts,  of  which  the  two  lateral  remain  distinct,  while  the 
median  element  fuses  with  the  subjacent  part  of  the  sternum  and 
cMii-iit i He-  with  it  the  presternum  (mamitn'imn  .s7r/-///).  If  Goette Js 
facts  am  to  be  trusted,  and  they  have  been  toa  large  extent  confirmed 
by  Hofmann,  his  homologies  appear  to  be  satisfactorily  estab- 
lished." (Halfour.) 

Pelvic  Girdle. — The  pelvic  girdle  resembles  the  pectoral;  it 
consists  of  a  bar  of  cartilage  which  articulates  with  the  femur;  the 
articular  cavity  is  known  as  the  acetabulum  and  divides  the  girdle 
into  a  dorsal  and  ventral  segment,  as  the  glenoid  fossa  divides  the 
scapular  arch.  The  dorsal  pelvic  division  is  called  the  iliac-  section, 
the  ventral  division  the  pubic  section.  The  iliac  section  has  no 
connection  with  the  vertebral  column  in  fishes,  but  is  articulated  with 
the  sacral  vertebra  in  amphibia  and  amniota.  The  pubic  section 
meets  its  fellow  in  the  median  ventral  line;  in  amphibians  it  be- 
comes  more  expanded  and  plate-like,  and  there  appears  an  inter- 
ruption of  the  cartilage  by  which  the  obturator  foramen  is  formed; 
this  foramen  divides  the  pubic  section  into  a  cephalic  portion  or 
pubis,  and  a  caudal  portion  or  ischium.  In  mammals  the  foramen 
is  enlarged  so  that  ischium  and  pubis  are  more  distinct  than  in  am- 
phibia. 

Balfour  ("Comp.  Embryology,"  II.,  606)  found  that  the  mode  of 
development  of  the  pelvic  girdle  in  Scyllium  is  very  similar  to  that 
of  the  pectoral  girdle.  There  is  a  bar  on  each  side  continuous  on  its 
posterior  border  with  the  basal  element  of  the  fin  (Figs.  345  and  :>47). 
This  bar  meets  and  unites  with  its  fellow. 

Concerning  the  early  development  of  the  girdle  in  amniota  I  know 
of  only  the  observations  of  A.  Bunge,  whose  dissertation  I  have 
not  seen,  and  those  of  Alice  Johnson,  83.1,  on  the  chick.  The  lat- 
ter shows  that  the  girdle  is  continuous  with  the  femur  at  first ;  the 
ischium  and  pubis  grow  out  separately  from  the  acetabular  region, 
both  growing  ventral  ward,  the  former  on  the  caudal,  the  latter  on 
the  cephalic,  side  of  the  crural  nerve;  if  the  ischium  and  pubis  were 
to  unite  distally,  wrhich,  however,  they  do  not  do  in  the  chick,  they 
would  inclose  a  space  homologous  with  the  obturator  foramen.  This 
observation  renders  it  improbable  that  the  ischium  and  pubis  are 
together  homologous  with  the  pubic  section  of  the  girdle  in  fishes, 
and  indicates  that  one  of  them  is  a  new  element — added  in  the 
amphibia  perhaps.  The  pubis  sends  out  a  process  headward  from 
just  below  the  acetabulum;  this  process  is  the  pre-pubis;  it  is  well 

*"Entwick.  Beckengttrtels. "    Inaug.  Diss.,  Dorpat,  1880. 


4f)li  THE   FCETUS. 

developed  in  the  Ornithorhynchus,  but  is  rudimentary  in  the  higher* 
mammalia. 

Skeleton  of  the  Arm. — Our  knowledge  of  the  development  of 
the  skeleton  of  the  fore  limb  in  mammalia  is  very  imperfect.  It 
rests  chiefly  on  the  data  furnished  by  Henke  and  Reyher,  74.1, 
supplemented  by  E.  Rosenberg's  valuable  investigations  of  the  cen- 
trale  carpi  in  man,  75. 1,  and  a  few  observations  recorded  by  Kolli- 
ker  in  his  "Entwickelungsgeschichte,"  2te  AufL,  and  by  C.  Emery, 
90. 1.  To  these  references  ought  to  be  added  one  to  the  paper  on  the 
development  of  ungulate  limbs  by  Alexander  Rosenberg,  73.1, 
which,  however,  has  less  direct  interest  for  us. 

The  skeleton  of  the  arm  in  mammals  (as  in  amphibia  also,  H. 
Strasser,  79. 1)  in  its  earliest  mesenchymal  stage  forms  an  uninter- 
rupted anlage  (Kolliker,  /.c.,  401),  with  no  indication  of  its  future 
subdivision,  and  is,  moreover,  probably  continuous  with  the  anlage 
of  the  pectoral  girdle.  As  soon  as  chondrifications  begin  the  indi- 
vidual skeletal  pieces  are  indicated  by  corresponding  separate  centres 
of  chondrification,  which  begin  near  the  centre  of  each  piece  and 
spread  toward  its  periphery.  The  separation  of  each  digital  series 
is  given  in  the  primitive  mesenchymal  anlage,  which  also  shows, 
according  to  C.  Emery,  90. 1,  296,  traces  of  a  sixth  digit  (pra3-pollux) 
in  front  of  the  thumb ;  the  sixth  digit  persists  as  a  rudiment  and  only 
fora  short  time.  The  condensed  mesenchyma  between  two  adjacent 
cartilages  becomes  fibrillar  and  produces  the  articulations.  On  the 
development  of  the  joints,  see  p.  460.  When,  however,  two  car- 
tilages fuse  into  one,  as  occurs  in  man  with  several  of  the  carpals, 
the  fusion  takes  place  very  early  and  no  articulation  is  formed.  It 
may  be  noted  here  that  the  joints  are  not  differentiated  until  six  or 
eight  weeks  after  chondrification  begins. 

In  the  human  embryo  at  six  weeks  nearly  all  the  skeletal  pieces 
are  present ;  the  ends  of  the  humerus  are  somewhat  enlarged ;  the 
ulna  has  a  processus  ancona9us  already ;  the  radius  shows  both  head 
and  neck;  the  metacarpals  are  beginning  to  chondrify.  By  the 
eighth  week  the  phalanges  are  cartilaginous,  having  begun  to  chon- 
drify (Kolliker,  "Entwickelungsgeschichte,"  2te  Aufl.,  491)  when 
the  five  digits  became  distinctly  indicated  by  marginal  notches  in 
the  hand,  and  in  the  humerus  the  calcification  of  the  cartilage,  pre- 
liminary to  its  degeneration  and  replacement  by  bone,  has  begun ; 
the  articular  surfaces  of  the  cartilages  are  becoming  more  sharply 
defined  (Henke  u.  Reyher,  74.1,  224-230).  These  authors  discov- 
ered, 1.  c.,  p.  268,  that  the  centrale  exists  as  a  separate  structure  in 
embryos  of  the  second  month.  E.  Rosenberg,  75.1,  172-191,  has 
traced  out  the  history  of  the  centrale  very  carefully ;  it  is  character- 
ized by  having  less  intercellular  substance  than  the  other  carpal  car- 
tilages, and  by  never  changing  into  bone,  except  as  a  rare  anomal}^; 
normally  it  is  gradually  absorbed  in  older  embryos  and  disappears, 
the  space  it  occupied  being  taken  up  by  the  enlargement  of  the 
radiale  (scaphoid) .  Henke  and  Reyher  have  observed  a  tenth  carpal 
also  which  was  perhaps  merely  a  transitory  (Gegenbaur's  "  radial 
sesamoid")  bone — at  least  this  suggestion  of  E.  Rosenberg's  is  a 
plausible  explanation. 

OSSIFICATION. — "In  the  humerus  a  nucleus  appears  near  the  mid- 


THE    UMI'.s    AM>    AI'I'KNMt   ILAK    SKKLKTnN.  4.")? 

die  of  the  shaft  in  the  eighth  week.  It  gradually  extends,  until  at 
birtli  only  the  cuds  of  the  bone  are  cartilaginous.  In  the  first  year 
the  nucleus  of  the  head  appears,  and  during  the  third  year  that  for 
the  great  tuhernsity.  The  lesser  tuliernsity  is  either  ossified  from 
adistinct  nucleus,  which  appears  in  the  fifth  year,  or  by  extension  of 
ossification  from  the  great  ttiU-msity.  These  nuclei  join  together 
about  the  sixth  year  to  form  an  epiphysis  which  is  not  united  to  the 
shaft  till  the  twentieth  year.  In  the  cartilage  of  the  lower  end  of 
the  bone  four  separate  nuclei  are  seen,  the  first  appearing  in  the 
capitellnm  in  the  third  year.  The  nucleus  of  the  internal  condyle 
appears  in  the  fifth  year,  thatof  thetrochlear  in  the  eleventh  or  twelfth 
year,  and  that  of  the  external  condyle  in  the  thirteenth  or  fourteenth 
year.  The  nucleus  of  the  internal  condyle  forms  a  distinct  epiphl 
which  unites  with  the  shaft  in  the  eighteenth  year :  the  other  three 
nuclei  coalesce  to  form  an  epiphysis.  which  i-  united  to  the  shaft  in 
the  sixteenth  or  seventeenth  year. 

"  The  radius  is  developed  from  a  nucleus  which  appears  in  the  mid- 
dle of  the  shaft  ;n  the  eighth  week,  and  from  an  epiphysal  nucleus 
in  each  extremity  which  only  appears  some  time  after  birth.  The 
nucleus  in  the  carpal  extremity  appears  at  the  end  of  the  second  year, 
while  that  of  the  head  is  not  seen  till  the  fifth  or  sixth  year.  The 
superior  epiphysis  and  shaft  unite  about  the  seventeenth  or  eigh- 
teen th  year;  the  inferior  epiphysis  and  shaft  unite  about  the  twen- 
tieth year. 

"  The  iihm  is  ossified  similarly  to  the  radius  but  beginsalittle  later. 
The  nucleus  of  the  shaft  appears  about  the  eighth  week,  that  of  the 
carpal  extremity  in  the  fourth  or  fifth  year.  The  upper  extremity 
grows  mainly  from  the  shaft,  but  at  the  end  of  theolecranon  a  small 
epiphysis  is  formed  from  a  nucleus  which  appears  in  the  tenth  year. 
This  epiphysis  is  united  to  the  shaft  about  the  seventeenth  year;  the 
inferior  epiphysis  about  the  twentieth  year. 

*'  From  what  is  stated  above  it  appears  that  in  the  bones  of  the  arm 
and  forearm  the  epiphyses  which  meet  at  the  elbow-joint  begin  to 
fv  later,  and  unite  with  their  shafts  earlier,  than  those  at  the 
opposite  ends  of  the  bones;  whereas  in  the  bones  of  the  thigh  and  leg 
the  epiphyses  at  the  knee-joint  are  the  soonest  to  ossify  (except  in  the 
tibula)  and  the  latest  to  unite  with  their  shafts.  In  the  bones  of  the 
arm  and  forearm  the  arterial  foramina  are  directed  toward  the  el- 
bow; in  those  of  the  thigh  and  leg  they  are  directed  away  from  the 
knee.  Thus,  in  each  bone  the  epiphysis  of  the  extremity  toward 
which  the  canal  of  the  medullary  artery  is  directed  is  the  first  to  be 
united  to  the  shaft.  It  is  found  also  that  while  the  elongation  of 
the  long  bones  is  chiefly  the  result  of  addition  to  the  shaft  at  the 
epiphysial  synchondroses,  the  growth  takes  place  more  rapidly,  and 
is  continued  longer,  at  the  end  where  the  epiphysis  is  last  united; 
and  the  oblique  direction  of  the  vascular  canals  is  due  to  this  in- 
equality of  growth,  which  causes  a  shifting  of  the  investing  perios- 
teum, and  so  draws  the  proximal  portion  of  the  medullary  artery  to- 
ward the  more  rapidly  growing  end. 

"  The  carpus  is  entirely  cartilaginous  at  birth.  Each  carpal  bone  is 
<>— ified  from  a  single  nucleus.  The  nucleus  of  the  os  magnum  ap- 
pears in  the  first  year;  that  of  the  unciform  in  the  first  or  second 


458  THE   FCETUS. 

year;  that  of  the  pyramidal  in  the  third  year;  those  of  the  trapezium 
and  the  lunar  bone  in  the  fifth  year ;  that  of  the  scaphoi  in  the  sixth 
or  seventh  year ;  that  of  the  trapezoid  in  the  seventh  or  eighth  year ; 
and  that  of  the  pisiform  in  the  twelfth  year. 

"  The  metacarpal  bones  and  phalanges  are  usually  formed  each 
from  a  principal  centre  for  the  shaft  and  one  epiphysis.  The  ossi- 
fication of  the  shaft  begins  about  the  eighth  or  ninth  week.  In  the 
inner  four  metacarpal  bones  the  epiphysis  is  at  the  distal  extremity, 
while  in  the  metacarpal  bone  of  the  thumb  and  in  the  phalanges  it  is 
placed  at  the  proximal  extremity.  In  many  instances,  however, 
there  is  also  a  distal  epiphysis  visible  in  the  first  metacarpal  bone  at 
the  age  of  seven  or  eight  years,  and  there  are  even  traces  of  a  prox- 
imal epiphysis  in  the  second  metacarpal.  In  the  seal  and  some  other 
animals  there  are  always  tvvoepiphyses  in  these  bones.  The  epiphy- 
ses  begin  to  be  ossified  from  the  third  to  the  fifth  year,  and  are 
united  to  their  respective  shafts  about  the  twentieth  year.  The 
terminal  phalanges  of  the  digits  present  the  remarkable  peculiarity 
that  the  ossification  of  their  shafts  commences  at  the  distal  extremity, 
instead  of  in  the  middle  of  their  length,  as  is  the  case  with  the  other 
phalanges  and  with  the  long  bones  generally  (F.  A.  Dixey)."  (G. 
D.  Thane  in  Quain's  "  Anat.,"  tenth  edition.) 

Skeleton  of  the  Leg. — The  primitive  mesenchymal  anlages  of 
the  skeleton  of  the  leg,  like  that  of  the  arm,  is  continuous  throughout 
in  amphibia,  H.  Strasser,  79.1,  and  birds,  Alice  Johnson,  85.1, 
and  therefore  probably  in  mammals  also,  and  in  birds  it  is  continu- 
ous also  with  the  pelvic  girdle,  which  appears  as  an  outgrowth  of 
the  skeletal  anlage  of  the  limb  proper.  As  in  the  arm  chondrifica- 
tion  blocks  out  the  separate  skeletal  pieces.  The  formation  of  car- 
tilage begins  in  the  chick  the  sixth  day  and  becomes  well  marked  by 
the  seventh  day,  when  Strasser's  "prochondral  elements,"  p.  404, 
have  already  disappeared  (Johnson,  I.  c.). 

In  the  human  embryo  at  six  weeks  all  the  skeletal  parts  are 
mapped  out  in  cartilage,  except  the  terminal  phalanges,  which  are 
still  entirely  mesenchymal.  The  plan  of  structure  is  essentially  the 
same  as  in  the  arm  at  the  same  age,  but  the  differentiation  is  less 
advanced;  the  femur  has  already  neck  and  trochanter,  is  slightly 
curved,  and  its  lower  end  is  enlarged,  with  two  condyles  and  the 
incisura  intercondyloidea  recognizable ;  the  tibia  has  broad  condyle& 
at  its  upper  end  and  is  suddenly  restricted  immediately  below,  and 
slowly  increases  in  diameter  toward  the  tarsus,  to  end  with  a  sur- 
face so  oblique  as  to  be  nearly  parallel  with  the  length  of  the  limb ; 
the  astragalus  (talus)  consists  of  a  lower  main  portion,  the  homo- 
logue  of  the  tibials,  and  an  upper  process  lying  between  the  tibia 
and  fibula,  and  homologous  with  the  intermedium;  the  fibulare 
(calcaneum)  is  not  so  long  as  the  astragalus,  and  is  separated  by 
articular  mesenchyma  from  both  the  fibula  and  astragalus,  alongside 
of  which  last  it  is  situated,  but  this  situation  is  found  to  alter  grad- 
ually, beginning  to  alter  in  embryos  but  little  over  six  weeks.  In 
the  digital  rays  the  metatarsi  and  first  phalanges  only  are  differen- 
tiated (Henke  and  Key  her,  74.1,230-234). 

In  an  embryo  of  nearly  six  months  the  ankle  has,  I  have  found, 
essentially  the  adult  form.  As  shown  in  a  vertical  section,  Fig.  254,, 


THK    LIMMS    AM)    AI'I'KNDK   T  LA  K    SKKI.KION. 


\-  Fb 


the  lower  ends  of  tin-  til.ia,  77>,  and  fibula,  P6,  are  still  cartilaginous; 

the  astragalus.  .!*//•.    and    calcaiieum   or  os   calcis,  Cut.  are   wholly 

cartila.^inous.    although  penetrated   l>y    vessels  preparatory   to    their 

lat.-r  ossification.      Tlie  astraga- 

lus.   J.s-//\    is    in   quite  different 

relations  from  those  found  at  six 

weeks;   it  underlie-  the  tibia,  and 

sho\\>  rlearly  the   Bubdn 

it>     tibial    articulation     into    tin- 

joint    with    the   in-iin   shaft,  Tb, 

and  \vitli  the  internal  malleolns, 

///  .   by  its  external  surface  it  ar- 

ticulates   with    the   fibula,     l''ih: 

by  its  l..\ver  -ui-face  with  the  os 

calcis.  C«l.     A  11  of  these  articu- 

lations  are    \\-eil     differentiated. 

ita  lower  internal  angle  the 

•'laire  .  ,f  th"  aM  raisins  is  in- 
terrupted to  allow  the  irruption 
of  the  vascular  mesenchyma. 

(  >SSIFI<  ATIMX.  —  "The  femur 
is  developed  from  one  principal 

Ifl  centre  for  the  shaft  which 
appears  in  the  seventh  week,  and 


Col 


\ 


four  epiphyses,  the  centres      FIG.  254.-v.-ni.-ai  S^-HOM  <>(  n,,-  Ankle  of  a 

f.  ir  \vliieh  -i  nin-  -ir  in  tin*  f<»11i»\vin<r    Human    Kmhryo  of   nearly   six  Month*.     Miu«.t 

lOWing     (,lll(M.ti()n.  No;  m,.      -/•/,.  Tihiu.     Ph.  filmla:  ,„. 
Order:    A    Sinfifle    nUCleUS    for   the     iiit«-niHlinall«'olusof  tlu-tibia  :A*t>\  astragalus; 

lower  srtmmty  appears  shortly   ' 

before  birth,  one  for  the  head  appears  in  the  first  year,  one  for  the  great 
trochanter  in  the  fourth  year,  and  one  for  the  small  trochanter  in 
the  thirteenth  or  fourteenth  year.  These  epiphyses  become  united  to 
the  shaft  in  an  order  the  reverse  of  that  of  their  appearance.  The 
small  trochanter  is  united  about  the  seventeenth  year,  the  great  tro- 
chanter about  the  eighteenth  year,  the  head  from  the  eighteenth  to 
the  nineteenth  year,  and  the  lower  extremity  soon  after  the  twen- 
tieth year.  The  neck  of  the  femur  is  formed  by  extension  of  ossifi- 
cation from  the  shaft. 

"  The  //7m/  and  Jilmlfi  each  present,  besides  the  principal  centre  for 
the  shaft,  a  superior  and  an  inferior  epiphysis.  In  the  tibia  the  cen- 
tre for  the  shaft  appears  in  the  seventh  week;  that  for  the  upper  ex- 
tremity including  both  tuberosities  and  the  tubercle,  appears  most 
frequently  before,  but  sometimes  after  birth  ;  and  that  for  the  infe- 
rior extremity  and  internal  malleous  appears  in  the  second  year. 
The  tubercle  is  occasionally  formed  from  a  separate  centre.  The 
lower  epiphysis  and  shaft  unite  in  the  eighteenth  or  nineteenth  year, 
the  upper  epiphysis  and  shaft  in  the  twenty-first  or  twenty-second 
year.  In  the  fibula  the  centre  for  the  shaft  appears  rather  later  than 
in  the  tibia;  that  for  the  lower  extremity  appears  in  the  second 
year,  and  that  for  the  upper,  unlike  that  of  the  tibia,  not  till  the 
third  or  fourth  year.  The  lower  epiphysis  and  shaft  unite  about  the 
twenty-  tir^t  year,  the  upper  epiphysis  and  shaft  about  the  twenty- 
fourth  year. 


460  THE    FCETUS. 

u  The  tar  sal  bones  are  ossified  in  cartilage,  each  from  a  single  nu- 
cleus with  the  exception  of  the  os  calcis,  which  in  addition  to  its 
proper  osseous  centre  has  an  epiphysis  upon  its  posterior  extremity. 
The  principal  nucleus  of  the  os  calcis  appears  in  the  sixth  month  of 
foetal  life ;  its  epiphysis  begins  to  be  ossified  in  the  tenth  year,  and 
is  united  to  the  tuberosity  in  the  fifteenth  or  sixteenth  year.  The 
nucleus  of  the  astragalus  appears  in  the  seventh  month ;  that  of  the 
cuboid  about  the  time  of  birth;  that  of  the  external  cuneiform  in 
the  first  year ;  that  of  the  internal  cuneiform  in  the  third  year ;  that 
of  the  middle  cuneiform  in  the  fourth  year,  and  that  of  the  navicular 
in  the  fourth  or  fifth  year. 

"  The  metatarsal  bones  and  phalanges  agree  respectively  with  the 
corresponding  bones  of  the  hand,  in  the  mode  of  their  ossification. 
Each  bone  is  formed  from  a  principal  piece  and  one  epiphysis ;  and 
while  in  the  four  outer  metatarsal  bones  the  epiphysis  is  at  the  dis- 
tal extremity,  in  the  metatarsal  bone  of  the  great  toe  and  in  the 
phalanges  it  is  placed  at  the  proximal  extremity.  In  the  first  meta- 
tarsal bone  there  is  also  to  be  observed,  as  in  the  first  metacarpal,  a 
tendency  to  the  formation  of  a  second  or  distal  epiphysis  (A.  Thom- 
son) .  In  the  metatarsal  bones  the  nuclei  of  the  shafts  appear  in  the 
eighth  or  ninth  week.  The  epiphyses  appear  from  the  third  to  the 
eighth  year,  and  unite  with  the  shafts  from  the  eighteenth  to  the 
twentieth  year.  The  nuclei  of  the  shafts  of  the  phalanges  appear 
in  the  ninth  or  tenth  week.  The  epiphyses  appear  from  the  fourth 
to  the  eighth  year,  and  unite  with  the  shafts  from  the  nineteenth  to 
the  twenty-first  year."  (G.  D.  Thane  in  Quain's  "Anatomy," 
ninth  edition.) 

Joints  of  the  Limbs. — Our  knowledge  of  the  development  of 
the  joints  is  based  chiefly  upon  the  researches  of  Henke  and  Reyher, 
74.1,  Bernays,  78.1,  and  Hepburn,  89.1;  Hagen-Torn's  article, 
82.1,  is  chiefly  on  the  histogenesis  of  the  synovial  membrane,  see 
p.  421.  Where  a  joint  is  to  be  formed  the  cells  become  elongated  at 
right  angles  to  the  axis  of  the  anlage  (synarthrodial  stage),  the 
tissue  becomes  fibrillar  and  in  its  midst  the  cavity  appears  (diarthro- 
dial  stage) ;  chondrification  soon  extends  to  the  cavity,  the  articu- 
lating surfaces  thus  becoming  cartilaginous.  The  development  of 
the  joints  is  very  gradual,  but  by  the  end  of  the  third  month  there 
are  true  articulating  surfaces,  which  gradually  become  better  devel- 
oped; the  development  of  the  joints  progresses  distally,  thus  the 
elbow-joint  is  developed  much  earlier  than  the  finger-joints;  the 
articulations  of  the  arm  appear  sooner  than  the  corresponding  ones 
in  the  leg,  thus  the  knee-joint  appears  later  than  the  elbow-joint. 
Bernays,  78. 1,  states  that  the  synarthrodial  stage  of  the  knee  begins 
in  a  human  embryo  of  2  cm.,  and  still  persists  in  one  of  3  cm.; 
in  the  latter,  although  there  is  still  no  articular  cavity,  yet  the  artic- 
ular ends  of  the  femur  and  tibia  are  shaped  nearly  as  at  birth — an 
important  observation  because  it  shows  that  the  articulating  surfaces 
are  shaped  before  any  free  motions  can  begin.  In  the  three-centi- 
metre embryo  the  growth  of  the  lateral  tibial  condyle  has  already 
forced  the  fibula  out  of  its  intimate  connection  with  the  femur,  which 
is  characteristic  both  for  the  earlier  stage  in  man  and  for  ancestral 
types.  By  comparative  anatomy  Bernays  has  sought  to  prove  that 


hKKMAL   BONES.  -Ml 

Bjrnarthrodial  joints  are  characteristio  ol  the  fishes,  imperfect  diar- 
tnrodial  joints  of  the  amphibia,  perfect  ours  of  tin*  amniota.  H«-j.- 
l)iirn,  89.1,  adds  but  little  to  <»ur  knowledge,  hut  his  paper  is 
valuable  for  an  admirable  synopsis  of  the  stages  of  joint  different  i- 
at  ion  and  of  the  classification  of  joints  from  the  emln-yolngiral  Mand- 
point.  Hepburn's  classification  i-  essentially  as  follows:  S\  mle>- 
inoHs,  synchondrosis,  primitive  articular  cavity,  amphiartnn 
diarthrosis  (simple,  doiil.le  with  meniscus);  the  diarthroses  show 
the  following  stages :  i.  surf aoes  become  cartilaginous;  ".'.  capsular 
ligament  formed ;  '•*>,  other  ligaments  formed;  4,  synovial  memorane 
developed. 

III.  DERMAL  BONES. 

It  has  been  long  known  that  not  all  the  bones  are  prseformed  in 
< -artilage,  and  that  some  of  them,  especially  of  the  head,  are  devel- 
oped from  soft  tissue.  The  latter  were  known  to  the  older  anato- 
miVts  as  nn'inln'(im'  IHHICN.  In  the  years  1845-50  the  origin  of  the 
membrane  bones  was  actively  debated,  and  at  that  time  the  term 
',/dnri/  IHHK-S  was  substituted  for  the  earlier  designation,  and  the 
tenna  Belegknochen  and  l)wl:kmH-ln'n  were  introduced  by  Kolliker, 
whose  investigations  played  the  principal  part  in  demonstrating  that 
the  membrane  hones  are  developed  by  the  direct  ossification  of  young 
c<  nmective  ti»i  ie.  <  .r — as  we  should  now  say — of  mesenchyma.  Those 
who  wish  to  follow  this  discussion  are  referred  to  Kxilliker,  50.2, 
where  references  are  L?iven  to  various  authorities  of  the  time,  and 
also  to  Kolliker's  "  Bericht  dor  Zootom.  Anstalt  in  Wvirzburg,"  and 
his  u Entwickelungsgeschichto,"  2te  Aufi.,  4i;:>.  The  dermal  bones 
of  the  head  may  lie  close  against  the  cartilage  (or  bone)  of  the  pri- 
mordial skull,  and  in  that  case  are  often  called  splint  bones  or 
splenial  I ><> n  ex. 

In  the  lower  vertebrates  the  membrane  bones  acquire  a  greater 
development  than  in  higher  form>,  and  in  certain  ganoids  and  tele- 
-  are  developed  over  nearly  the  entire  body,  whereas  in  the  amni- 
ota  they  are  confined  to  the  head. 

O.  Hertwig's  brilliant  researches,  74.1,2,  76.1,  79.1,  have 
demonstrated  that  the  dermal  bones  are  homologous  with  the  pi 
formed  by  the  fusion  of  epidermal  teeth  or  so-called  placoid  scales. 
The  placoid  scales  are  true  teeth  developed  in  the  skin  and  supported 
by  a  base  of  bone ;  by  the  fusion  of  adjacent  bony  bases  we  may 
have  an  osseous  plate  developed  in  the  cutis.  In  tailed  amphibia 
several  of  the  membrane  bones  arise  as  dentiferoufl  plates,  but  later 
in  the  development  the  teeth  are  resorbed  leaving  merely  the  bony 
plate,  but  in  anoura  the  homologous  bones  are  developed  without 
teeth  being  formed  at  all.  The  inevitable  conclusion  from  these  facts 
is  that  the  dermal  skeleton  has  been  evolved  through  three  principal 
stages:  1,  scattered  independent  dermal  teeth  (placoid  scales);  2, 
teeth -bearing  plates  formed  by  the  fusion  of  the  expanded  bases  of 
adjacent  teeth  (exo-skeleton) ;  3,  membrane  bones  developing  without 
teeth  appearing  (dermal  bones  of  tailless  amphibia  and  amniota). 

The   plates  or  bones  of  the  dermal   skeleton  are  not  the   same 
throughout  the  vertebrate  series;  among  the  fishes  there  are  numer- 


4<;-2  THE   FCE'lTS. 

ous  modifications,  the  homologies  of  which  have  not  yet  been  thor- 
oughly elucidated;  in  the  amphibia  we  encounter  all  the  elements  of 
the  dermal  skeleton  of  the  amniote  head,  and  comparative  anatomists 
have  succeeded  in  homologizing  some  of  these  elements  with  plates 
in  fishes,  but  as  much  remains  to  be  done,  and  as  the  conclusions 
have  not  hitherto  been  based  upon  much  embryological  evidence,  I 
shall  not  attempt  to  enter  into  these  difficult  discussions. 

Typical  Dermal  Bones  of  Amniota. — In  amniota  the  dermal 
bones  are  confined  to  the  skull  and  face.  There  are,  1,  four  pairs  of 
bones  on  the  dorsal  side,  namely,  the  nasals  overlying  the  olfactory 
chambers;  the  f rentals  overlying  the  anterior  part  of  the  brain  cav- 
ity; the  par  totals  overlying  the  middle  part  of  the  brain  cavity,  and 
the  interparietals  overlying  the  anterior  part  of  the  occipital  region ; 
the  frontals,  parietuls,  and  interparietals,  together  with  the  supra- 
occipital,  constitute  the  roof  of  the  skull;  when  the  cartilaginous 
skull  spreads  upward  it  goes  under  the  territory  of  the  frontals, 
parietals,  and  interparietals,  and  when  it  ossifies  it  may  contribute 
to  a  greater  or  less  extent  to  the  bones  in  question,  so  that  they  are 
not  exclusively  membranous  in  origin  (Dursy).  Between  the  parie- 
tals and  supra-occipital  is  the  interparietal;  2.  The  small  lachrymals 
situated  between  the  nasals,  frontals,  and  the  eye  on  each  side  (in 
certain  reptiles  there  are  additional  periorbital  bones),  and  the 
squamosal,  occupying  the  space  between  the  parietals,  ali-sphenoids, 
and  occipitals,  and  overlying  that  portion  of  the  mandibular  bar 
which  forms  the  quadrate  of  reptilia  (incus  of  mammalia) ;  the  squa- 
mosal is  perhaps  the  homologue  of  the  prse-opercular  of  fishes,  as 
maintained  by  Huxley,  or  perhaps  of  the  ganoid  supratemporal  as 
suggested  by  Balfour,  "Comp.  Embryol.,"  II.,  593.  3.  The  bones 
associated  with  the  mandibular  branchial  bar ;  these  are,  first ,  those 
associated  with  the  palato-quadrate  bars  and  appearing  in  the  roof 
of  the  mouth,  the  vonier,  palatines,  and  pterygoids;  second,  a  series 
associated  with  Meckel's  cartilage,  and  consisting  primarily,  accord- 
ing to  comparative  anatomists,  of  three  dermal  bones,  the  distal  den- 
tale,  the  smaller  articulare,  and  in  the  angle  between  these  two  the 
small  angulare:  but  in  mammals  there  is  only  a  single  bone  devel- 
oped from  the  mesenchyma  around  Meckel's  cartilage,  which  evi- 
dently represents  the  dentale,  but  whether  or  not  it  also  represents 
the  articulare  and  angulare  has  not  been  definitely  settled.  4.  The 
series  associated  with  the  maxillary  processes,  four  on  each  side  form- 
ing a  row ;  beginning  at  the  ventral  end  of  the  process  these  four  bones 
are  the  prce-maxilla,  maxilla,  jugal,  and  quadrato-jugal.  5.  The 
median  para-sphenoid,  which  is  developed  in  the  roof  of  the  mouth 
in  many  fishes  (but  not  in  elasmobranchs  or  marsipobranchs),  in 
amphibia,  and  in  sauropsida,  in  which  last  it  is  less  important  and 
becomes  indistinguishably  fused  with  the  sphenoid  in  the  adult;  in. 
mammalia  it  has  not  been  found,  though  probably  morphologically 
present  in  the  sphenoid — a  probability  which  it  would  be  worth  test- 
ing by  a  special  investigation.  6.  The  tympanal  bone  formed 
around  the  drum  of  the  ear. 

The  Dermal  Bones  in  Man. — The  numerous  dermal  bones, 
mentioned  as  characteristic  for  the  amniota  at  large,  have  all  been 
identified  in  the  adult  human  skull,  except  the  articulare,  angulare, 


I>KI;.\IAL   BONKS.  4i'»:'» 

quadrato-jogolar,    mid    para-sphenoid.     The  four  bones  mentioned 

arc,  h«>\vever,  all  probably  represented  by  definite  parts  as  follows : 
the  interparietal  by  the  upper  median  portion  of  the  supra-occipital; 
th'1  articulare  and  angnlare  by  parts  of  the  adult  mandible;  the  quad- 
rato-jugular  by  one  of  the  oesificatory  centres  of  the  jugal,  and  the 
para-sphenoid  by  part  of  the  sphenoid.  The  nasals,  parietals,  lach- 
rymals, vomer,  and  jupil  remain  independent  bones,  while  the 
fmntals  an<l  palatines  are  also  independent  except  that  each  pair 
forms  but  a  single  bone.  On  the  other  hand  the  squamosals,  ptery- 
gnids,  dentals,  are  united  with  certain  parts  of  the  primordial  skull. 
Finally  the  pra'-maxillaries  and  maxillaries  fuse  into  a  single  bone. 
of  \vhich  the  part  bearing  the  four  upper  incisors  corresponds  to  the 
pr;e  maxillaries. 

A  tabular  view  of  the  Imniologi.-s  of  the  human  skull  is  given  on 
p.  ' 

The  following  data  afford  additional  information  concerning  the 
development  of  the  single  dermal  bones. 

1.  AV/W.S-  are  eaeh  ossified  from  a  single  centre  which  appears 
ab. -ut  the  eighth  week. 

•.'.  Fruitful  is  ossified  from  two  centres,  one  for  each  frontal  ap- 
pearing about  the  seventh  week.  At  birth  the  frontals  are  still  en- 
tirely distinct,  but  they  become  united  during  the  first  year  after 
birth  by  the  median  "  frontal"  suture,  which  usually  becomes  obliter- 
ated by  osseous  union  taking  place  from  below  upward  during  the 
second  year,  but  not  infrequently  the  suture  persists  throughout  life. 

:;.  l\in'fffi/fi  are  each  ossified  from  a  single  centre  which  appears 
in  the  site  of  the  parietal  eminence  about  the  seventh  week.  The 
eminence  is  very  conspicuous  in  tbe  young  bone  and  gives  a  marked 
character  to  the  form  of  the  skull  for  a  number  of  years  in  early  life. 

t.  littcriHirit'tnlx  arc  represented  by  the  upper  pair  of  centres  of 
the  supra-occipital  region;  these  centres  appear  during  the  seventh 
w.  ek  in  the,  mesenchyma  overlying  the  supra-occipital  cartilage. 
The  interparietals  usually  unite  with  the  true  occipitals,  but  oc- 
casionally they  remain  distinct  and  are  then  separated  from  the 
supra-occipital  by  a  suture  running  transversely  from  one  lateral 
angle  of  the  occipital  bone  to  the  other. 

Lucltnjinals  are  each  ossified  from  a  single  centre,  which  ap- 
pears about  the  eighth  week. 

»;.  ,sv///f////o.x7//s  are  each  ossified  from  a  single  centre,  which  ap- 
pears in  its  lower  part  about  the  seventh  or  eighth  week;  ossification 
spreads  upward  in  the  squamosum  proper,  and  outward  into  the  zygo- 
matic  process.  At  birth  the  squamosal  is  still  separated  from  the 
periotic  capsules,  but  during  the  first  year  bony  union  is  effected  and 
the  squamosal  becomes  a  part  of  the  temporal  bone  of  the  adult. 

T.  I  'outer  is  ossified  from  a  single  nucleus  appearing  at  the  hinder 
part  about  the  eighth  week.  From  this  nucleus  two  laminae  are  de- 
veloped, which  pass  up  on  either  side  of  the  median  line  and  embrace 
the  lower  part  or  the  cartilaginous  internasal  septum.  These  laminae 
gradually  coalesce  from  behind  forward  till  the  age  of  puberty,  thus 
forming  a  mesial  plate,  with  only  a  groove  remaining  on  its  superior 
and  anterior  margins. 

8.  Palatine  is  ossified  from  a  single  centre  which  appears  in  the 


464  THE    FCETUS. 

seventh  or  eighth  week  at  the  angle  between    its   horizontal   and 
ascending  parts. 

9.  Pterygoids  are  each  ossified  from  a  single  centre  which  appears 
during  the  fourth  month ;  during  the  fifth  or  sixth  month  the  ptery- 
goids  unite  with  the  ossified  pterygoid  processes    (future  external 
pterygoid  plates)   of  the  ali-sphenoids  and  thus  become  the  internal 
pterygoid  plates  of  the  adult  basi-sphenoid. 

10.  Prce-maxiUaries  have  been  studied  by  Th.  Kolliker,  82.1; 
they  ossify  later  than  the  maxillaries  and  appear  just  before  the  pal- 
ate fissure  closes,  and  after  the  fissure  has  closed  they  are  found  united 
with  the  maxillaries  so  that  the  period  of  their  independent  existence 
is  very  short;  but  in  the  ninth  week  traces  of  the  primitive  division 
are  still  present,  and  even  these  traces  disappear  by  end  of  the  tenth 
week.     The  pra3-maxillaries  carry  the  four  upper  incisors.     A  spe- 
cial interest  attaches  to  these  bones  because  their  homologies  in  man 
were  ascertained  by  Goette. 

11.  Maxillaries  begin  to   ossify  toward  the  end  of  the  second 
month  and  offer  the  peculiarity  of  starting  from  several  spots,  which, 
however,  speedily  fuse  and  cannot  be  regarded  as  separate  centres. 
This  peculiarity  was  first  recorded  by  Beclard,  20.1,  and  his  obser- 
vation has   been  confirmed  by  Rambaud  et  Renault,    and  more  re- 
cently by  Callender,  70.1,   103.     As   stated  above,  the  maxillaries 
and  praB-maxillaries  are  united  before  the  tenth  week. 

12.  Jugals,  or  malars,  begin   to   ossify   about  the  eighth  week. 
According  to  Rambaud  et  Renault,   ossification  begins  from   three 
points,  which  are  found  united  by  the  fourth  month. 

13.  Mandible.     The  mandible  of  the  adult  is  a  compound  bone, 
for  it  includes  both  the  dermal  bone  and  the  ossified  lower  ends  of 
Meckel's  cartilage,  most  of  which,  however,  is  resorbed,  and   it   is 
further  peculiar  in  having  cartilage  developed  at  the  ends  of  both 
the  coronoid  and  condylar  processes.     The  two  mandibles  are  distinct 
at  birth,  but  during  the  first  year  their  lower  or  ventral  ends  unite, 
but  in  a  pig  embryo  of  two  and  a  half  inches  Parker  ("  Morpholog}r 
of  the   Skull,"  290)  describes   the   ends  of   Meckel's  cartilages    as 
united,  and  it  is  probable  that  the  cartilaginous  jaws  of  the  human 
embryo  are  similarly  united.     The  development  of  the  human  man- 
dible has  been  studied  by  Masquelin,  78.1;  in  an  embryo  of  5  cm. 
the  cartilage  of  Meckel  is  entirely  surrounded  by  mesenchymal  bone, 
and  in  embryos  of  17  cm.  there  are  only  slight  calcified  remains  of 
the  cartilage,  except  in  the  lower  ends  near  the  syrnphysis,  where, 
as  shown  by  Kolliker,  the  cartilage  participates  in  the  ossification  of 
the  mandible;  the  cartilage  of  the  coronoid  process  was  found  in 
embryos  of  7.5  and  9.5  cm.,  and  in  the  later  cartilage   along  the 
alveolar  border ;  the  cartilage  of  the  condyle  is  developed  still  earlier ; 
the  three  cartilages  upon  each  mandible  undergo  direct  ossification. 
Strelzoff,  73.1,  was  led  by  the  observation    of    these    cartilages   to 
maintain  that  the  entire  jaw  is'preformed  in  cartilage,  but  that  this 
view  is  erroneous  was  demonstrated  in  an  admirable  paper  by  J. 
Brock,  76. 1.     It  is  evident  that  the  accessory  cartilage  of  the  man- 
dible is  morphologically  distinct  from  that  of  the  primordial  skeleton. 

14.  Tympanals  develop  during  the  third  month  each  from  a  cen- 
tre which  appears  in  the  lower  part  of  the  external  membranous 


MORPHOLOGY   OF   THE   SKULL.  4'..~> 

wall  of  the  tympanum  and  extends  upward  until  a  nearly  complete 
bnny  ring  is  tnnncd,  inclosing  the  tympanic  nicmlu-anc;  before  birth 
the  ends  of  the  open  ring  become  united  with  the  squamosal,  and 
thus  incorporated  in  the  great  temporal  bone  of  the  adult. 

The  Fontanelles. — These  are  membranous  intervals  between 
the  incomplete  angles  of  the  parietal  and  neighboring  bones,  in  some 
of  which  movements  of  the  soft  wall  of  the  cranium  may  be  observed 
in  connection  with  variations  in  the  state  of  the  circulation  and 
respiration.  They  are  at  the  time  of  birth  six  in  number,  two  me- 
dian, anterior  and  posterior,  and  four  lateral.  The  antrrinr  fon- 
tanelle,  situated  between  the  antero-superior  angles  of  the  parietal 
bones  and  the  superior  angles  of  the  ununited  halves  of  the  frontal 
bone,  is  quadrangular  in  form  and  remains  open  for  some  time  after 
birth.  The  posterior  fontanelle,  situated  between  the  postero- 
superior  angles  of  the  parietal  bones  and  the  superior  angle  of  the 
occipital  bone,  is  triangular  in  shape.  It  is  filled  up  before  birth, 
but  the  edges  of  the  bones  being  united  by  membrane  only  are  still 
freely  movable  upon  each  other.  The  lateral  fontanelles,  small  and 
of  irregular  form,  are  situated  at  the  inferior  angles  of  the  parietal 
bones.  The  fontanelles  are  gradually  filled  up  by  the  extension  of 
i(  ation  into  the  membrane  which  occupies  them,  thus  complet- 
ing the  angles  of  the  bones  and  forming  the  sutures.  The  closure, 
especially  of  the  posterior  and  lateral,  is  often  assisted  by  the  devel- 
opment of  Wormian  bones  in  these  situations.  All  traces  of  these 
unossified  spaces  disappear  before  the  age  of  four  years. 

IV.  MORPHOLOGY  OF  THE  SKULL. 

We  are  no'w  in  a  position  to  consider  several  questions  concerning 
the  skull  as  a  whole.  What  is  presented  on  these  questions  I  have 
divided  under  the  following  headings  into  sections :  1 .  Homologies 
of  the  bones  of  the  human  skull.  2.  Relations  of  the  primary  and 
secondary  skull.  3.  Position  of  the  facial  apparatus.  4.  Signifi- 
cance of  the  trabecula}  cranii.  5.  Theories  of  the  skull.  The  de- 
tailed history  of  each  element  of  the  skull  is  given,  as  fully  as 
practicable,  in  the  preceding  pages. 

Homologies  of  the  Bones  of  the  Human  Skull. — These 
have  been  discussed  in  the  preceding  pages  of  this  chapter,  but  it 
will  be  convenient  to  present  the  conclusions  arrived  at  in  a  tabular 
form : 

HUMAN  TYPICAL  AMNIOTE. 

A.  CRANIAL.  — Ethmoid  and  turbinals.    Ethmoid  and  turbinals. 

Prae-sphenoid.  Prse- sphenoid,    orbito-sphenoids     (alae) 

minores),  and  pterygoids. 

Basi -sphenoid.  Basi -sphenoid,  ali -sphenoids    (alae  ma- 

jores)  ( ?  and  para-spheaoid) . 

Occipital.  Basi -occipital,    ex-occipitals,     supraoc- 

cipitals,  and  interparietal. 

Temporal.  Periotic    capsule    (pro-otic,  opisthotic, 

epiotic)  squamosal,    annulus    tym- 
panicus,  and  styloid  process  (upper 
end  of  hyoid  bar) . 
30 


460  THE    FOETUS. 

HUMAN.  TYPICAL  AMNIOTE. 

B.  FACIAL.— Nasals.  Nasals. 

Lachrymals.  Lachrymals. 

Jugal.  Jugal. 

Superior  maxillary.  Prsemaxillse  and  maxillae. 

Vomer.  Vomer. 

Palatine.  Palatines. 

Mandible.  Dentale  (?  Articulare  and  angulare)  and 

lower  part  of  Meckel's  cartilage. 

Relations  of  the  Primary  and  Secondary  Skull. — Com- 
parative anatomy  and  embryology  alike  teach  us  that  we  must 
attribute  to  the  skull  a  double  origin,  or  rather  that  there  are  two 
skulls,  one  outside  the  other,  which  were  primitively  distinct  from 
one  another,  but  in  the  progress  of  evolution  from  the  earliest  fish 
type  to  the  higher  mammalia  the  union  between  the  two  skulls  be- 
comes more  and  more  intimate.  The  inner  skull  is  what  is  known 
as  the  primordial  skull,  with  which  I  include  the  branchial  skeleton ; 
the  outer  skull  comprises  the  series  of  dermal  bones  of  the  cranial 
and  facial  regions. 

The  primary  skull  appears  first  as  the  continuation  into  the  region 
of  the  head  of  the  axial  mesenchymal  skeleton,  which  in  the  neck 
and  rump  is  the  anlage  of  the  vertebra?.  That  the  mesenchymal 
skull  represents  in  part,  at  least,  a  series  of  vertebraB  is  certain,  and 
we  find  it  sending  dorsal  outgrowths  to  inclose  the  brain  just  as  the 
true  vertebra  cover  in  the  spinal  cord.  The  mesenchymal  skull  also 
extends  in  front  of  the  hypophysis,  where  it  produces  the  trabeculse 
cranii.  What  little  can  be  surmised  concerning  the  original  homolo- 
gies  of  this  part  of  the  skull  is  given  in  the  section  on  the  trabeculse, 
p.  434.  The  mesenchymal  skull  grows  so  as  to  completely  incase 
the  brain  and  partially  incase  the  olfactory  chambers.  While  it  is 
growing  six  centres  of  chondrification  appear  in  it :  namely,  two  tra- 
becular,  two  parachordal,  and  two  periotic;  each  centre  forms  a 
cartilage,  which  is  extraordinarily  uniform  in  shape  and  relations 
throughout  the  entire  vertebrate  series;  the  six  cartilages  remain 
distinct  for  a  very  short  time  only;  the  two  trabeculae  unite  first,  the 
two  parachordals  next,  third  the  united  parachordals  (or  occipital) 
coalesce  with  the  periotic  capsules  and  later  with  the  caudal  ends  of 
the  trabecula?,  thus  forming  a  large  floor  of  cartilage  under  the 
brain.  In  the  lower  forms  chondrification  spreads  until  the  entire 
primary  skull  becomes  cartilaginous,  and  it  is  in  this  condition  we 
find  the  skull  in  many  of  the  fishes. 

In  the  amphibia  and  amniota  there  is  a  progressive  reduction  of 
the  cartilaginous  skull  by  which  its  development  as  a  roof  over  the 
brain  is  more  and  more  diminished.  This  reduction  leaves  an  open- 
ing as  it  were  on  the  dorsal  side,  and  at  once  increases  the  impor- 
tance of  the  covering  dermal  bones — f  rentals,  parietals,  and  inter- 
parietals.  In  Sauropsida  the  opening  is  larger  than  in  amphibia, 
and  in  the  mammalia  there  is  further  progressive  increase  in  size,  as 
shown  by  Parker's  observations,  the  opening  being  larger  in  pigs 
than  in  insectivora  and  edentates.  In  mammals  there  is  a  further 
loss,  which  is  not  found  in  other  classes,  namely,  an  absence  of 
chondrification  in  the  region  between  the  ali-sphenoids  and  periotic 
capsules,  by  which  the  importance  of  the  squamosal — the  dermal 


MORPHOLOGY  OF  THE  SKULL.  467 

hone  of  the  region — is  increased;  see  W.  K.  Parker,  86.1,  S,  who 
-peaks  of  the  disappearance  of  the  cartilage  under  the  squamosal  as 
"tlu1  true  diagnostic  mark"  of  the  mammalian  chondrocranium. 
Reduction  of  the  cartilages  of  the  branchial  skeleton  also  progresses 
from  the  lower  to  the  higher  vertebrates.  This  shows  itself  in 
mammals  not  only  in  the  total  disappearance  of  the  cartilages  of  the 
fourth  and  fifth  arches,  but  also  in  the  partial  disappearance  of  the 
thyro-hyoid  bars  and  the  imperfect  development  of  the  hyoid  bars. 
It  -ho\vs  itself  further  in  the  reduction  of  the  mandibulars,  for  not 
only  is  tlu»  greater  part  of  Meckel's  cartilage  resorbed  as  in  all  am- 
n iot a,  hut  also  the  palato-quadrate  is  very  much  reduced.  As  the 
pa lato-quadrate  is  an  important  part  of  the  skull  in  the  amphibia, 
the  palatines  and  pterygoids  appear  as  true  splint  bones,  whereas  in 
mammalia  they  have  greater  independence.  It  is  clear  from  the 
above  that  the  evolution  of  the  mammalian  skull  has  depended  to  a 
large  extent  upon  the  reduction  or  partial  degeneration  of  the  inner 
skull,  or  primordial  chondrocranium. 

The  secondary  or  outer  skull  is  not  so  old  as  the  inner  skull,  and 
<>i -i-inated  in  the  higher  fishes  as  a  series  of  dermal  bony  plates, 
which  overlaid  the  primary  skull,  and  probably  formed  a  nearly 
complete  case  for  the  head,  including  the  face.  The  definite  arrange- 
ment of  the  plates,  as  perpetuated  and  modified  in  mammalia,  appears 
in  the  amphibia,  and  was  perhaps  evolved  during  the  transition  from 
the  fish  to  the  amphibian  type.  The  dermal  plates  (membrane  bones) 
may  either  remain  as  splint-bones,  as  for  instance  is  the  case  with 
tin*  vomer,  or  they  may  coalesce  with  the  underlying  portions  of  the 
chondrocranium,  as  for  instance  occurs  with  the  interparietals  in 
primates,  or  they  may  remain  where  the  cartilage  disappears  beneath 
thorn,  as  for  instance  the  frontals.  Already  in  the  amphibians  the 
co-ordination  and  fusion  of  the  inner  and  outer  skulls  into  one  com- 
plex skull  is  established,  and  in  the  amniota  the  welding  together  is 
carried  still  further,  and  the  elements  of  the  outer  skull,  i.e.  the 
dermal  bones,  acquire  increased  importance  as  the  inner  skull,  i.e. 
chondrocranium,  is  reduced.  In  brief,  the  evolution  of  the  mammal- 
ian skull  has  depended  largely  upon  increased  morphological  promi- 
nence of  the  dermal  bones. 

If  \ve  designate  the  formation  of  the  chondrocranium  as  ihe  first 
stage,  and  the  formation  of  the  dermal  bones  as  the  second  stage  in 
the  evolution  of  the  skull,  we  may  designate  the  ossification  of  the 
primordial  chondrocranium  as  the  fhird  stage.  As  to  what  caused 
that  ossification,  we  have  not  even  an  hypothesis,  and  we  are  equally 
in  the  dark  as  to  how  the  number  of  separate  bones,  or  centres  of 
ossification,  was  determined.  It  is  noteworthy  that  the  number  of 
the  primordial  bones  is  extraordinarily  constant. 

Finally,  let  me  emphasize  the  fact  that,  given  the  full  number  of 
bones,  there  is  a  sustained  tendency  to  reduce  them  by  fusion.  The 
number  of  skull  bones  is  less  in  the  amphibia  than  in  the  teleosts,  in 
edentates  than  in  amphibians,  in  man  than  in  edentates.  A  thorough 
comparative  study  of  the  number  of  the  skull  bones  is  much  to  be 
desired. 

Position  of  the  Facial  Apparatus. — Owing  to  the  head-bend 
of  the  embryo,  the  oral  invagination,  or  mouth  cavity,  is  brought 


468  THE   FCETUS. 

between  the  fore-brain  and  the  heart,  and  upon  the  ventral  surface, 
and  this  is  the  permanent  position  in  the  sharks.  If  we  follow 
through  the  vertebrate  series,  or  the  development  of  an  amniote,  we 
find  in  either  case  a  steady  increase  in  the  region  of  the  olfactory 
and  oral  invaginations,  in  consequence  of  which  it  projects  more 
and  more,  and  further  by  a  throwing  of  the  whole  head  upward  the 
face  is  brought  forward  and  projects  in  front  of  the  brain.  In  man 
this  condition  is  again  modified :  first,  because  the  upright  position 
renders  it  unnecessary  to  bend  the  head  as  in  quadrupeds,  and,  there- 
fore the  head  is  left  facing  ventralward;  second,  because  the 
enormous  development  of  cerebral  hemispheres  has  rendered  an 
enlargement  of  the  brain  cavity  necessary,  and  this  enlargement  has 
taken  place  by  extending  the  cavity  over  the  olfactory  regions  as 
well  as  by  enlarging  the  whole  cranium ;  third,  because  the  develop- 
ment of  the  facial  apparatus  is  arrested  at  an  embryonic  stage,  the 
production  of  a  long  snout  being  really  an  advance  of  development 
(Minot,  35),  which  does  not  take  place  in  man. 

Significance  of  the  Trabeculae  Cranii. — Concerning  the 
nature  of  the  trabeculae  we  have  no  satisfactory  conceptions.  They 
are  a  temporary  stage  of  the  chondrification  of  the  mesenchymal 
skeleton  in  front  of  the  notochord,  and  it  seems  to  me  as  improbable 
that  they  have  any  important  morphological  significance,  as  it  is 
improbable  that  the  rounded  form  of  the  bony  centre  of  a  half-ossi- 
fied vertebra  has  any  important  meaning.  It  cannot  be  too  strongly 
insisted  upon  that  the  morphological  condition  is  determined  by  the 
shape  of  the  mesenchymal  anlage,  of  which  the  trabeculaB  are  merely 
a  part.  So  far  as  I  am  aware,  not  a  single  investigator  has  described 
this  anlage  accurately  and  fully. 

I  consider  it  not  improbable  that  the  axial  perichordal  mesen- 
chymal skeleton  sends  an  outgrowth  past  the  hypophysis  to  inclose 
the  fore-brain,  and  that,  assuming  that  the  infundibulum  marks  the 
true  anterior  limit  of  the  medullary  canal,  the  trabecular  anlage  is 
not  a  prolongation  of  the  floor  of  the  cranium,  but  an  upgrowth,  which 
owing  to  the  head-bend  has  come  to  lie  in  the  line  of  the  cranial  axis. 

Theories  of  the  Skull. — It  was  noticed  a  long  time  ago  that 
the  skull  has  resemblance  to  vertebra;  the  skull  has  the  greatest 
thickness  on  the  ventral  side  of  the  brain  and  arches  over  the  central 
nervous  system,  and  thus  possesses  two  of  the  chief  characteristics 
of  the  vertebrae.  It  was,  therefore,  natural  to  seek  to  compare  the 
skull  homologically  with  vertebrae.  It  is  said  that  during  the 
eighteenth  century  this  comparison  acquired  greater  prominence  and 
was  definitely  formulated  by  Vicq  d'Azyr.*  These  comparisons  of 
Vicq  d'Azyr  and  others  proceeded  upon  a  false  basis,  and  it  was 
not  until  1872,  when  Gegenbaur,  72.1,  opened  an  entirely  new 
method  of  solving  the  morphology  of  the  head,  that  correct  views 
began  to  be  formed.  Another  great  stride  was  made  by  Froriep's 
observations  on  the  development  of  the  occiput,  p.  429.  I  have  placed 
what  I  have  to  say  under  the  three  headings  of  Vicq  d'Azyr's  the- 
ory, Gegenbaur's  theory,  and  Froriep's  law. 

1.  VICQ  D'AZYR'S  THEORY. — According  to  this  theory  the  skull 
consists  of  several  vertebrae.  Whether  d'Azyr  really  originated  it, 

*I  have  not  succeeded  in  finding  anything  in  Vicq  d'Azyr's  "CEuvres"  to  justify  this  state- 
ment. 


MORPHOLOGY  OF  THE  SKULL.  469 

I  cannot  say.  It  was  taken  up  by  Oken,  who  is  often  quoted  as  the 
founder  of  it,  and  later  also  by  Goetjfe,  who  by  some  authors  has 
been  cited  as  the  l'ath«T  of  the  theory.  The  history  of  the  theory 
and  of  the  modification  it  underwent  is  given  by  R.  Virchow  ("  Goette 
als  Natorforaoher"). 

<  >m»  of  the  earliest  suggestions  of  the  vertebral  theory  is  that  of 
Mi  ml  in,  independently  made  about  the  same  time  by  Heilmeyer. 
These  authors  compared  the  skull  to  a  single  complex  vertebra. 
( )krn  miHvived  that  there  were  four  cranial  vertebra?,  and  this  was 
the  notion  most  in  favor  until  1858.  Goetjbe  counted  six  vertebra?,  of 
which  three  belonged  to  the  facial  apparatus.  As  to  the  number  of 
tin -so  supposed  vertebra?  there  is  a  very  extensive  literature,  which 
possesses  an  interest  purely  historical.  Let  it  suffice,  therefore,  to 
state  aphoristically  that  three  vertebra?  were  advocated  by  Spix, 
Mrckel,  Burdach  and  Cams;  four  by  Oken,  Bojanus,  and  Owen ;  six 
I . \  M  <  •( '  1  i  x  • ;  seven  by  Geoffrey. 

The  death-blow  to  this  long-lived  error  was  dealt  by  Huxley  in  his 
Croonian  lecture  delivered  in  1858,  68. 1 — a  great  achievement,  for  it 
at  once  terminated  the  history  of  the  old  vertebral  theory  of  the 
skull,  and  paved  the  way  for  Gegenbaur. 

GEGENBAUR'S  THEORY. — In  1872  Gegenbaur  published  his  great 
work,  72.1,  on  the  cephalic  skeleton  of  Selachians,  in  which  he 
took  the  ground  that  the  skull  does  not  represent  a  series  of  vertebrae, 
but  that  it  arose  out  of  the  axial  or  perichordal  skeleton  before  dis- 
tiuct  vertebra?  were  formed  in  the  axial  region;  he  further  main- 
tained that  the  head  includes  a  number  of  segments,  which  he 
sought  to  ascertain  by  determining  the  segmental  arrangement  of 
the  cranial  nerves.  This  was  a  great  step  and  in  the  rigtit  direction. 
F.  M.  Balfour,  78.3,  was,  I  believe,  the  first  to  endeavor  to  trace 
out  the  actual  number  of  segments  (mesoblastic  somites)  in  the  head 
of  embryos.  A  vast  amount  of  labor  has  been  expended  by  subse- 
quent writers  in  investigating  the  development  of  the  cephalic  myo- 
tomes  and  cranial  nerves,  but  much  remains  to  be  done  before  the 
morphological  constitution  of  the  head  shall  be  understood,  but  we 
are  already  in  a  position  to  say  that  Gegenbaur's  thesis — that  the 
primary  or  inner  skull  is  developed  from  the  axial  skeleton  but  not 
from  vertebrae — is  correct  except  as  regards  the  hypoglossal  region. 
For  further  observations  on  the  segmentation  of  the  head  see  Chap- 
t.  r  XXVI. 

FRORIEP'S  LAW. — Froriep's  investigations,  p.  429,  have  demon- 
strated that  the  skull  has  extended  itself,  in  the  amniota  at  least,  by 
the  annexation  of  true  vertebra?,  corresponding  to  segments  of  which 
the  hypoglossus  represents  the  nerve.  The  head  has  grown  at  the 
expense  of  the  neck. 

PRESENT  THEORY  OF  THE  SKULL. — The  primary  skull  was  de- 
veloped out  of  the  axial  (perichordal)  skeleton,  in  the  region  of  the 
brain,  where  the  dorsal  and  ventral  nerve  roots  are  not  united  into  a 
single  nerve  for  each  segment ;  the  primary  skull  has  grown  at  least 
in  the  amniota  by  the  annexation  of  several  cervical  vertebrae;  a 
secondary  skull  was  developed  outside  the  primary  cartilaginous 
skull  by  the  formation  of  dermal  bones.  In  the  higher  forms  the 
primary  skull  partly  disappears ;  what  remains,  together  with  the 
secondary  or  dermal  skull,  constitutes  the  actual  skull  of  the  adult. 


CHAPTER  XXI. 
THE  MESOTHELIAL  MUSCLES. 

THE  muscle  fibres  fall  into  two  main  classes,  the  smooth  or  mesen- 
chymal  fibres,  which  have  been  already  considered,  p.  417,  and  stri- 
ated or  mesothelial  muscles.  The  latter  fall  into  three  groups;  1, 
the  skeletal  muscles ;  2,  the  branchial  muscles ;  3,  the  cardiac  mus- 
cles. The  first  are  developed  from  the  epithelial  muscle  plates,  the 
origin  of  which  from  the  mesothelial  primitive  segments  has  been 
already  described,  p.  205 ;  the  second  are  developed  from  the  meso- 
thelium  of  the  branchial  ccelomatic  cavities  (head  cavities  of  Balfour) 
see  p.  478 ;  the  latter  are  developed  from  the  mesothelial  wall  of  the 
heart  of  the  embryo  and  constitute  the  so-called  "  muscular  heart " 
(Muskelherz),  see  p.  227. 

The  Segmental  or  Skeletal  Muscle  Fibre. — Remak,  50.1, 
was  the  first,  if  I  am  not  mistaken,  to  show  that  the  primitive  seg- 
ment, or  as  he  termed  it  the  "proto  vertebra,"  forms  both  the  anlage 
of  the  axial  mesenchyma  and  of  the  muscles ;  he  also  thought  that 
the  "  proto  vertebra"  formed  the  spinal  ganglion,  an  error  which  was 
corrected  by  His,  68. 1.  To  the  myotome,  or  the  two  layers  of  the 
mesothelium  remaining  after  the  differentiation  of  the  periaxial 
mesenchyma  (Van  Wijhe's  sclerotome),  Remak  applied  the  term 
Riickenplatte.  After  Remak  (1850)  followed  a  series  of  investiga- 
tions and  discussions  as  to  the  histogenesis  of  the  striated  muscle 
fibre.  The  chief  differences  of  opinion  were  as  to  whether,  as  origin- 
ally maintained  by  Remak,  each  fibre  is  developed  from  a  single  cell, 
or,  as  suggested  by  Theodore  Schwann,  out  of  the  fusion  of  several 
cells.  The  latter  view  was  advocated  by  Margo,  59.1,  in  1859; 
Margo  studied  the  muscle  corpuscles,  terming  them  sarcoplasts,  and 
regarding  them  as  so  many  separate  cells  which  had  united  to  form 
the  muscle  fibre.  That  Remak  was  right  was  maintained  by  Kolli- 
ker  in  1857,  57. 1,  on  the  basis  of  his  own  observations,  and  also  by 
Max  Schultze  in  a  masterly  essay,  61.1,  which  at  the  same  time 
laid  the  foundation  of  the  modern  doctrine  of  cells,  and  anticipated 
Heitzmann's  observations  on  the  union  of  the  cells  by  over  twenty 
years.  In  the  same  year,  1861,  appeared  Deiters'  paper,  61.1,  and 
the  year  after,  F.  E.  Schulze's,  6 S.I,  who  together  with  Max 
Schultze  conclusively  established  Remak's  opinion  as  correct.  Never- 
theless we  find  the  Schwann-Margo  hypothesis  reappearing  from 
time  to  time,  although  it  has  never  had  any  sound  observational  basis 
to  rest  upon.  A  synopsis  of  various  papers  upon  the  development 
of  striated  muscle  fibres  is  given  by  Calberla,  76.1,  and  more 
fully  by  G.  Born  in  his  dissertation,  73. 1.  That  the  striated  mus- 
cles have  an  epithelial  origin  was  first  emphasized  by  the  two  Hert- 
wigs,  81.1,  61-66,  who  demonstrated  at  the  same  time  that  only  the 


THE    MESOTHELIAT,   MUSCLES. 


171 


inner  layer  of  the  myotome  forms  muscle,  not  both  plates  as  had 
been  wrongly  stated  by  Balfour,  **  Comp.  Embryology,"  II.,  to  be  the 
case  in  elasmobranchs.  Since  then  it  has  been  ascertained  beyond 
question,  that  the  outer  layer  gives  rise  to  the  dermis  (compare  p. 
),  Kaestner's  contrary  conclusions,  90.1,  being  attributable,  in 
my  judgment,  to  his  imperfect  observation. 

The  single  muscle  fibre  arises  from  a  single  epithelial  (mesotlu-lial) 
cell  of  the  muscle  plate  or  inner  wall  of  the  myotome.  In  the  am- 
phibia  each  cell  elongates  in  a  direction  parallel 
\vitli  the  axis  of  the  body  until,  as  shown  by  F. 
V..  Schultze,  62.1,  it  stretches  the  entire  length  of 
the  segment ;  it  seems  to  me  that  each  cell  extends 
the  entire  length  (cephalo-caudal)  of  the  segment 
in  sharks  and  chick  embryos  also,  but  I  have  not 
studied  the  point  sufficiently.  Paterson,  87.1, 
asserts  that  in  chicks  the  cells  lengthen  but  remain 
shorter  than  the  segment.  In  amphibia  the  cells 
are  crowded  with  yolk  granules,  which,  however, 
are  gradually  resorbed ;  thus  in  the  frog  they  at 
first  hide  the  nuclei,  but  by  the  fourth  day  are 
sufficiently  reduced  to  allow  the  nuclei  to  be  seen 
easily  in  the  fresh  unstained  specimen  (Calberla, 
76.1);  in  amniota,  on  the  other  hand,  there  are 
exceedingly  few  yolk  grains  left  in  the  muscle 
plate.  The  first  evidence  of  striation  appears  in 
the  frog  toward  the  end  of  the  fifth  day,  on  one 
side  of  the  cell,  Fig.  225,  as  first  recorded  by  F.  E. 
Schultze,  62.1,  and  since  frequently  confirmed 
i'  .f/.,  by  Calberla  and  Ranvier,  "Traite  technique 
d'Histologie,"  51G).  The  side  upon  wl}ich  the 
striation  first  appears  has  been  observed  in  elas- 
mobranchs by  C.  Rabl,  89.2,  239,  to  be  the  side 
toward  the  notochord,  or  farthest  from  the  cavity 
of  the  myotome.  In  Petromyzon,  A.  Goette, 
90.1,  50,  the  fibrillse  are  found  to  form  a  peri- 
pheral layer  so  very  early  that  it  is  doubtful 
whether  they  first  appear  on  one  side  of  the  cell 
only  or  not.  The  striation  continues  to  develop 
until  it  passes  completely  around  the  cell,  forming 
a  peripheral  layer  (Deiters,  61.1,  Kolliker,  "Ge- 
webelehre,"  6te  Aufl.,  p.  401)  as  illustrated  in  Fig. 
25G.  At  about  this  time,  perhaps  sooner  in  some 
forms  and  later  in  others,  the  nucleus  divides,  and 
by  repetitions  of  the  divisional  process  the  cell 
soon  becomes  multinucleate.  C.  Rabl,  89.2,  242, 
directs  attention  to  the  fact  that  the  nuclei  of  the  muscle-plate  in 
sharks  stain  more  lightly  than  the  mesenchymal  nuclei  and  contain 
an  elongated  chromatme  granule ;  in  the  chick  I  have  observed  the 
same  nuclear  peculiarity.  Later  the  nuclei  lose  this  main  granule 
and  have  instead  a  number  of  smaller  ones,  Fig.  256,  mn.  The 
muscle  fibre  acquires  its  membranous  sheath,  sarcolemma,  some 
time  later.  As  to  the  exact  time  I  have  found  no  positive  data; 


F I  o  .  255.  —  Isolated 
Muscle  Fibres  of  a  Frog 
Embryo.  A,  Showing 
yolk  grains  (partly  re- 
sorbed)  and  nucleus;  B, 
with  the  muscular  stria- 
tion just  appearing  on 
one  side. 


472 


THE   FCETUS. 


but  authorities  are  agreed  that  the  fibre  remains  naked  for  a  consid- 
erable period.  During  the  early  stages  of  their  differentiation,  the 
muscle  fibres  retain  the  epithelial  arrangement,  that  is,  remain 
closely  packed ;  not  long  after  the  appearance  of  the  fibrilte  and  stri- 
ation,  the  fibres  begin  to  separate  and  connective  tissue  grows  in 
between  them.  During  their  separation  the  fibres  become  massed 
in  bundles  instead  of  in  epithelial  layers.  The  central  portion  of  the 
young  muscle  fibre  is"  granular,  and  contains  not  only  the  nuclei  and 
the  remains  of  the  yolk  material, 
but  also  a  considerable  quantity  of 
glycogen  (Ranvier,  "Traite  tech- 
nique d'Histologie,"  515) .  As  this 
substance  is  very  readily  dissolved 
out,  it  is  probable  that  the  clear 
empty  appearance  of  the  central 


tnn 


FIG.  256.— A,  Transverse;  B,  Longitudinal  Section  of  Muscle  Fibres  in  the  Neck  of  a 
Human  Embryo  of  sixty-three  to  sixty-eight  Days.  Minot  Collection,  No.  138.  m.m.m,  Muscle 
fibres ;  mn,  muscle  fibres  showing  the  central  nuclei ;  mes,  mesenchymal  nuclei.  A  X  about  750 
diams. 

portion  of  the  fibres,  which  is  so  striking  in  sections  of  hardened  em- 
bryos, see  Fig.  256,  A,  is  due  to  the  loss  of  the  glycogen.  The  mantle 
of  striated  muscle-substance  gradually  increases  until  the  whole  fibre 
is  fibrillated  and  the  muscle  no  longer  appears  hollow.  The  time  at 
which  the  muscle  fibres  become  "  solid  "  varies  from  embryo  to  em- 
bryo and  from  muscle  to  muscle.  In  the  human  embryo  at  the  end 
of  the  fifth  or  beginning  of  the  sixth  month  most  of  the  fibres  of  the 
upper  extremity  are  solid,  but  it  is  not  until  the  seventh  month  that 
those  of  the  lower  extremity  become  solid,  W.  Felix,  89.1,  232. 
The  nuclei  have  at  first  an  axial  position,  but  toward  the  end  of  the 
third  month  some  of  them  lie  in  the  mantle,  compare  also  p.  474. 

The  size  of  the  fibres  is  smaller  in  the  embryo  than  in  the  adult, 
but  Felix,  89.1,  233,  points  out  that  up  to  latter  part  of  the  third 


THE   MESOTHELIAL   MUSCLES.  473 

month  in  man  the  fibres  increase  in  size,  many  having  at  two  and 
one-half  months  a  diameter  of  from  13-19;*,  but  later  they  are  smaller 
owing  probably  to  the  division  of  the  fibres,  and  it  is  not  until  birth 
that  tin-  same  diameter  is  again  reached  by  the  single  fibres. 

Fihrilln*. — Before  discussing  the  origin  of  the  fibrillse  it  is  neces- 
sary to  remove  an  unfortunate  confusion  which  has  prevailed  in  the 
use  of  the  term.  By  fibrillsB  is  sometimes  meant  the  longitudinal 
threads  of  protoplasm,  but  more  often  is  meant  the  material  between 
adjacent  longitudinal  threads.  Corresponding  to  the  two  usages  of 
tin-  \\-ord  "fibrilla,"  there  are  two  essentially  different  conceptions 
( -t  the  structure  of  the  adult  muscle  fibre.  According  to  the  older 
vi«-\\ ,  which  is  currently  repeated  in  text-books  of  anatomy  and  his- 
tology, the  "  primitive  fibrillge "  into  which  a  muscle  fibre  may  be 
mechanically  divided  under  certain  conditions  are  the  contractile 
portions  of  the  fibre;  the  ends  of  the  "primitive  fibrillse"  are  the  so- 
called  Cohnheim's  areas,  and  the  "  sarcous  elements"  are  the  divisions 
of  the  "  primitive  fibrilla?. "  Henle  ("Allgemeine  Anatomie,"  1841, 
]>.  ~>80)  recognized  that  there  was  substance  left  between  the  fibrillse; 
later  Lcydi^  (Muller's  Arch.,  1856,  156)  and  Kolliker  (Zeit.  Wiss. 
Z< ><>!..  VIII.,  316)  pointed  out  its  general  occurrence.  Max  Schultze, 
61.1,  :5,  shows  that  this  material  was  the  derivative  of  the  proto- 
plasm of  the  muscle  cells.  L.  Gerlach  *  was  the  first,  so  far  as  I 
know,  to  demonstrate  that  this  interfibrillar  material  formed  a  re- 
ticulum,  but  he  regarded  it  as  the  prolongation  of  the  nerve.  The 
reticulate  structure  appears  to  have  been  recognized  also  by  G.  Thin 
(Quart.  Journ.  Microsc.  Sci.,  1876,  XVI.  251).  No  special  signifi- 
cance seems  to  have  been  attributed  to  all  these  observations  until 
lvsl,  when  Retzius,  81.2,  proposed  the  new  conception  of  the 
structure  of  muscle  fibres,  according  to  which  the  material  between 
the  so-called  "  primitive  fibrillsB  "  is  the  essential  part  of  the  fibre ; 
this  material  is  part  of  the  protoplasmatic  network  of  the  cell  which 
makes  the  muscle.  According  to  Retzius,  the  essential  feature  of  the 
muscle  fibre  is  the  peculiar  and  characteristic  arrangement  of  this 
network,  by  which  the  striation  is  caused.  The  fibrillse  of  embryol- 
ogists  are  threads  of  protoplasm  and  are  not  the  same  as  the  "  primi- 
tive  fibrillse"  of  histologists,  but  are  characterized  by  staining  read- 
ily. That  the  fibrillation  was  developed  by  a  metamorphosis  of  the 
protoplasm  of  the  young  muscle  cell  has  long  been  the  conception  of 
embryologists,  see  for  exanrple  Max  Schultze's  article,  61.1,  pub- 
lished in  1861.  L.  Bremer,  83. 1,  was  the  first  to  place  this  concep- 
tion upon  a  firm  basis  of  observation,  by  tracing  out  further  than 
had  been  done  before  the  transformation  of  the  protoplasmatic  net- 
work of  the  developing  fibre.  Retzius'  results  were  extended  and 
made  the  basis  of  a  theory  of  the  structure  and  contraction  of  the 
muscle  fibre  by  B.  Melland,  85.1,  and  C.  F.  Marshall,  87.1,  90.1, 
both  working  in  A.  Milnes  Marshall's  laboratory  at  the  Owens  Col- 
lege ;  compare  also  Biitschli  und  Schewiakoff ,  91.1.  This  series  of 
investigations  render  it  necessary  to  accept  Retzius'  view — although 
Kolliker  in  the  sixth  edition  of  his  "  Gewebelehre  "  throws  the  weight 
of  his  great  authority  against  it.  As  it  now  stands  Retzius'  view 

*  Gerlach,  "Das  Verhaltniss  der  Nervenzu  denMuskeln  der  Wirbelthiere,"  Leipzig,  1874.     See 
also  Arch.  f.  Microsc.  Anat.,  xiii.,  1877,  397. 


474  THE   FCETTJS. 

may  be  summarized  thus:  fibrillae  and  sarcous  elements  are  post- 
mortem effects  due  to  the  cleavage  of  the  matrix ;  the  muscle  fibre 
really  consists  of  a  homogeneous  matrix  which  is  traversed  by  a  very 
regular  reticulum,  made  of  longitudinal  threads  connected  at  regu- 
lar intervals  by  transverse  threads,  corresponding  in  position  to 
Krause's  membrane;  at  the  nodes,  where  the  cross  and  long  threads 
unite,  there  are  little  thickenings.  The  thickenings  correspond  to 
the  balls  of  Schafer's  dumb-bells,  the  handles  of  which  are  the  long 
threads,  compare  Schafer,  Philosophical  Transactions,  1873.  Be- 
tween every  two  Krause's  membranes  are  numerous  fine  cross 
threads,  which  cause  the  appearance  of  the  dark  bands  and  therefore 
of  the  transverse  striation. 

The  transformation  of  the  reticulum  of  the  multinucleate  cell  of 
the  myotome  into  the  network  of  the  adult  muscle  fibre  has  been 
most  carefully  described  by  L.  Bremer,  83.1,  whose  results  may  be 
summarized  as  follows :  The  nucleus  of  the  muscle  fibre,  together 
with  the  protoplasm  surrounding  it,  constitutes  the  so-called  muscle 
corpuscle;  the  corpuscle  is  much  more  prominent  in  young  than  in 
old  muscle,  for  its  protoplasm  is  gradually  differentiated  into  muscu- 
lar substance ;  a  small  number  of  corpuscles  enter  into  the  formation 
of  each  fibre ;  the  substance  of  the  muscle  forms  a  network,  which 
was  first  partially  recognized  by  Heitzmann  (Wien.  Sitzungsber. , 
XVII. ,  Abth.  3,  1873);  the  meshes  of  this  network  appear  polyg- 
onal in  transverse — rectangular  in  longitudinal,  sections;  the  net- 
work is  a  modification  of  the  protoplasmatic  network  of  the  cor- 
puscles, and  is  so  arranged  that  there  are  alternating  rows,  both 
transverse  and  longitudinal,  of  fine  knots  and  large  knots  (corre- 
sponding to  the  fine  and  broad  striae) ;  the  fine  knots  are  connected 
by  fine  threads,  and  the  large  knots  by  coarse  threads ;  hence  there 
is  a  fine  and  a  coarse  net. 

MULTIPLICATION  OF  MUSCLE  FIBRES. — That  the  muscle  fibres 
multiply  during  embryonic  life  can  hardly  be  questioned  at  present. 
Two  methods  of  accounting  for  the  multiplication  have  been  advo- 
cated, the  first  that  it  is  effected  by  the  intervention  of  sarcoplasts 
(Margo) ,  the  second  that  it  is  by  a  direct  longitudinal  fusion  of  the 
fibre  (Weismann,  61.1).  I  consider  the  latter  view  the  correct  one. 

1.  Margo' s  Theory. — Bremer's  results,  83.1,  on  this  question  are 
as  follows :  The  post-embryonic  multiplication  of  fibres  takes  place 
by  means  of  the  structures  described  by  Margo  (59.1,  229)  under 
the  name  of  sarcoplasten;  these  are  lines  or  chains  of  muscle  cor- 
puscles, united  by  the  protoplasm  net,  and  derived  by  proliferation 
from  the  corpuscles  of  the  original  fibres ;  the  sarcoplast  gradually 
separates  from  the  parent  fibre,  undergoing  muscular  differentiation 
meanwhile,  and  also  becoming  connected  with  the  nerve.  The 
growth  of  the  fibre  is  initiated  by  a  multiplication  of  the  corpuscles ; 
the  sarcolemma  is  not  present  at  first,  but  appears  later,  being  prob- 
ably formed  by  the  fused  cell  membranes  of  the  corpuscles,  to  which 
appears  to  be  added  a  coat  of  connective  tissue,  and  also  around  the 
motor  plate  between  the  two  sarcolemmic  coats  appears  an  extension 
of  Henle's  sheath  of  the  nerve.  Paneth  has  recently  renewed,  85.1, 
Margo's  observations,  59.1,  giving  a  careful  description  of  the  sar- 
coplasts and  maintaining  that  they  are  the  agents  of  fibre  multipli- 


THE   MESOTHELIAL   MUSCLES.  475 

cation.  Sigmund  Mayer,  86.1,  attacked  Paneth,  because  he  found 
muscle  corpuscles  abundant  in  the  fibres  of  the  tail  in  tadpoles  during 
tin-  proems  of  resorption,  and  hence  concluded  thatt  he  OOTpuaolfiB  are 
muscle  destroyers  (sarcolytea).  This  opinion  has  been  accepted  l>y 
Uarfurth,  87.1,  but  the  mere  presence  of  the  corpuscles,  while  the 
muscle  fibres  are  becoming  destroyed,  is,  as  Paneth  justly  replied, 
87.1,  no  evidence  whatever  that  they  have  a  sarcolytic  function. 
There  remains,  however,  another  hypothesis  which  has  been  advanced 
l>y  Felix,  89. 1,  253,  namely,  that  the  so-called  sarcoplasts  represent 
muscle  fibres  partly  degenerated.  Felix's  interpretation  is  the  one 
which  has  most  commended  itself  to  me. 

-*.  lfY/.x-///'f////".s  Tlu'ury. — Felix's  conclusions  are,  that  from  the 
middle  «>t  the  third  month  until  the  end  of  foetal  life  there  are,  in 
the  museles,  fibres  with  multiplied  nuclei,  which  are  arranged  in 
rows.  These  fibres  with  multiple  nuclei  are  of  two  kinds,  those 
with  a  single  and  those  with  several  rows  of  nuclei.  In  the  first 
kind  the  nuclei  are  central,  color  deeply,  lie  transversely,  and  differ 
but  little  from  one  another ;  fibres  of  this  kind  do  not  divide  though 
they  may  grow;  some  of  them  degenerate  and  form  Margo's  sarco- 
plasts.  The  second  kind  of  fibres  have  several  rows  of  nuclei  in  the 
mantle  or  fibrillar  layer;  in  the  middle  part  of  the  rows  the  nuclei 
are  closely  crowded  and  compressed  into  all  possible  forms;  this 
crowding  probably  marks  the  centre  of  proliferation.  The  fibre 
divides  into  daughter  fibres,  one  for  each  row  of  nuclei.  The  fibre 
1><  comes  enveloped  in  a  sheath,  rich  in  nuclei  and  vessels,  and  this 
sheath  persists  while  the  fibre  is  dividing ;  afterwards  it  disappears. 
The  daughter  fibres  may  also  divide,  but  apparently  usually  into 
two  only. 

The  areas,  in  which  the  nuclei  are  crowded  together,  have  long 
been  known,  though  imperfectly  described.  They  are  usually 
termed  Muskelknospen  or  Muskelspindel  by  German  writers, 
and  they  mark  the  point  where  the  union  with  the  nerve  is 
established.  They  were  known  to  Weismann  in  1861,  61.1,  and 
were  shortly  after  described  by  Kolliker,  62.1,  in  amphibia.  Von 
Franque,  90.1,  records  some  observations  upon  them  and  gives 
references  to  the  scattered  observations  upon  them  made  by  a  number 
of  writers. 

The  Muscle  Plates. — The  development  of  the  muscle  plates  has 
already  been  described.  There  is  unfortunately  little  to  be  added 
at  present  concerning  their  later  history.  When  the  outer  leaf 
of  the  myotome  is  changed  into  the  mesenchyma  of  the  dermis, 
the  cells  nearest  the  muscle  plate  on  all  sides  retain  for  a  con- 
siderable period  their  epithelial  arrangement,  and  appear  to  act  as 
a  growing  layer,  and  presumably  contribute  both  to  the  mesen- 
chyma  on  one  side  (compare  Fig.  257)  and  the  muscle  plate  on 
the  other.  Certainly  the  muscle  plate  continues  to  grow  in  all 
directions,  but  most  rapidly  dorsalward  over  the  medullary  canal 
and  ventral  ward  into  the  somatopleure.  At  the  same  time  the 
muscle  plates  not  only  lengthen  out  as  the  whole  trunk  lengthens, 
but  each  one  grows  forward  under  the  one  in  front,  and  thus 
is  produced  the  stage  so  characteristic  of  fishes,  with  the  mus- 
cular segments  oblique  and  the  hind  border  of  each  overlapping  the 


476 


THE   FOETUS. 


segment  next  behind.  That  the  imbrication  is  produced  as  stated 
seems  to  me  clear  from  the  study  of  shark  embryos,  in  which  the 
original  position  of  the  segment  is  indicated  by  the  nerves,  ganglia, 
and  \nter-segmental  arteries ;  the  hind  edge  of  the  muscle  plate  coin- 


mes 


insth 


Cu 


ones 


FIG.  257.— Chick  Embryo,  Transverse  Section  of  the  Upper  Part  of  a  Myotome.  mes<  mcs', 
Mesenchyma;  msth,  mesothelium;  EC,  ectoderm;  CM,  cutis;  Mu,  muscle-plate;  Ep,  epithelioid 
layer,  x  296  diams. 

cides  with  posterior  limit  of  its  segment  thus  determined,  while  the 
anterior  edge  is  clearly  within  the  territory  of  the  next  segment  in 
front. 

In  the  region  of  the  limbs  the  muscle  plates  send  in  elasmobranchs 
buds  into  the  limbs  to  produce  their  muscles,  as  discovered  by  Bal- 
four,  "Comp.  Embryology,"  II.,  673.  According  to  Dohrn,  84. 1, 
163,  this  budding  takes  place  after  all  the  gill-clefts  have  become 
open,  and  the  cartilage  is  just  begining  to  appear  in  the  branchial 
arches ;  each  myotome  produces  an  anterior  and  posterior  bud,  which 
both  point  outward  and  downward ;  the  buds  have  at  first  a  spherical 
form,  but  soon  separate  from  the  parent  muscle  plate,  and  elongate, 
and  later  divide  each  into  two,  a  dorsal  and  ventral  secondary  bud, 
so  that  from  each  myotome  there  are  produced  four  buds.  The  main 
muscle  plate  continues  its  growth  into  the  abdominal  somatopleure. 
The  number  of  myotomes  which  contribute  to  the  limbs  is  uncertain, 
but  there  are  several.  It  is  probable  that  in  all  amniota  the  myo- 
tomes also  send  buds  to  form  the  muscles  of  the  limbs.  Van  Bem- 
melen,  89.1,  242,  has  shown  that  in  snake  embryos  with  the  fifth 
gill-cleft  just  formed,  the  myotome  of  the  second  to  tenth  post-occip- 
ital segments  send  downgrowths  into  the  limbs,  and  continue  on  in 


THi:    MI-X»THELIAL   MUSCLES. 


the  somatopleure  ventralward.  Of  the  eight  segments  tin- 
tlmv  have  their  outgrowths  oblique  to  enter  the  limbs.  Paters<>n, 
87.1,  has  exj.ressly  dmicd  this  origin  for  the  chick,  but  as  he  was 
able  t<>  distinguish  only  a  confused  mass  of  mesoderm  in  the  young 
limbs,  his  opinion  cannot  carry  weight,  but  must  be  considered 
based  upon  imperfect  observation. 

Anoin  ION.  —  A  certain  number  of  muscle  plates  disappear  during 
early  embryonic  life.  Thus  Froriep  has  shown,  86.  1,  that  in  the 
»••  .\v  embryo  there  are  four  rudimentary  muscle  plates  in  the  region 
of  the  occiput  or  hypoglossus,  which,  however,  all  disappear  very 
early.  It  is  probable  that  there  were  once  other  muscle  plates  in  the 
head  which  have  n«>\v  disappeared,  compare  p.  200.  Further,  it  is 
probable  that  in  man  there  are  rudimentary  muscle  plates  in  the  em- 
bryonir  tail,  which  h/is  been  shown  by  Fol  to  contain  at  least  nine 
rudimentary  segments,  some  of  which  may  advance  into  the  muscle- 
plate  sta.uv. 

Myotomic  Muscles.  —  There  is  no  part  of  embryology  so  obscure 
at  i  •  i  v  s«  •  1  1  1  as  the  development  of  the  muscular  system  .  Scarcely  the 
most  elementary  questions  have  been  answered.  Not  only  has  the 
development  of  the  single  muscles  from  the  mesothelial  plates  scarcely 
been  studied,  but  also  the  very  significance  and  the  arrangement  of 
l>lates  in  the  head  is  wrapped  in  uncertainty,  see  p.  2uo. 

The  following  points  in  regard  to  the  cephalic  myotomes  have  been 
ascertained.  Of  Van  Wijhe's  nine  myotomes,  seen  in  elasmobranchs, 
the  ///•*/  comes  to  lie  against  the  optic  vesicle  and  gives  rise  to  the 
rectus  superior,  rectus  inferior,  and  obliquus  inferior  of  the  eye;  the 
>nd  produces  the  obliquus  superior,  and  the  third  the  rectus  ex- 
ternus;  a  good  figure  of  the  three  myotomes  which  form  the  eye 
muscles,  as  observed  in  an  elasmobranch  embryo,  is  given  by  A. 
Froriep,  Anat.  Anzeiger,  N.  50,  see  also  Miss  Platt's  figures  91.2; 
the  J'OH,  -tli,  fifth,  and  .s  /./•///  disappear;  the  eighth,  ninth,  andtcnffi 
produce  muscles  running  from  the  skull  to  the  shoulder  girdle. 
Froriep,  86  .  1  ,  has  shown  that  there  are  four  myotomes  in  the  occipital 
1  ly  poglossal  region  of  mammals,  which  early  become  rudimentary,  but 
Van  Bemmelen  has  observed,  89.1,  241,  that  these  four  myotomes 
together  with  that  of  the  first  cervical  (atlas)  segment  grow  obliquely 
ventrahvanl,  so  as  to  meet  and  unite  into  a  single  cord  which  de- 
scends behind  the  last  (in  reptiles  the  fifth)  gill-cleft,  accompanied 
by  the  hypoglossal  nerves,  and  then  curving  forward  grows  into  the 
tongue  and  there  produces  the  lingual  musculature.  This  explana- 
tion of  the  origin  of  the  muscles  of  the  tongue  is  probably  correct, 
but  it  differs  from  that  offered  by  Froriep,  85.  1,  and  still  more  from 
that  of  His  (''Anat.  menschl.  Embryonen,"  III.,  92).  According 
to  Van  Wijhe,  82.  1,  the  coracohyoid  muscle  of  sharks  arises,  like  the 
mammalian  lingual  muscles,  from  the  downgrowth  of  the  posterior 
cephalic  and  anterior  cervical  myotomes. 

As  regards  the  development  of  the  muscles  of  the  rump  and  limbs 
we  possess,  so  far  as  I  am  aware,  practically  no  information  beyond 
the  little  which  has  been  noticed  in  connection  with  the  history  of 
the  muscle  plates,  p.  200. 

Muscles  of  the  Branchial  Arches.  —  That  these  muscles  all 
arise  from  the  mesothelium  of  the  arches  is  now  generally  believed 


478 


THE   FCETUS. 


roth 


cart 


FIG. 258.— Transverse  Section  of  a  Branch- 
ial Arch  of  a  Selachian  Embryo  /,  Branch- 
ial filament :  msth,  mesothelium ;  A,  artery ; 
V,  anterior  vein ;  e,  connecting  vein ;  cart, 
cartilage;  N,  nerve. 


although  by  no  means  rigidly  demonstrated,  except  for  elasmobranchs ; 
Van  Wijhe,  82.1,  states  that  the  corocobranchialis  and  coraco- 
mandibularis  muscles  of  sharks  are  developed  from  the  pericardia! 
mesothelium.  Anton  Dohrn,  84. 1,  109-114,  finds  in  selachians  that 
the  mesothelial  tube  lengthens  with  the  whole  arch  and  by  expand- 
ing in  the  transverse  plane  becomes  a  plate,  Fig.  258,  msth,  which 
stretches  across  the  arch  between  the  nerve  in  front  and  the  an! age  of 
the  cartilage  behind ;  the  coelomatic  cavity  is  obliterated  except  on  the 
outer  side  of  the  arch ;  the  plate  then  divides  close  to  the  nerve.  The 

further  history  is  complicated  and 
need  not  be  presented  here,  as  noth- 
ing definite  is  known  as  to  the  ho- 
mologies  of  the  branchial  muscles  of 
sharks  with  those  of  amniota. 

His  ("  Anat.  menschlicher  Embry- 
onen,"  Heft  III.,  92)  has  endeavored 
to  indicate  to  which  arches  certain 
muscles  belong,  but  has  not  worked 
out  the  actual  development.  He  as- 
signs the  palatoglossus,  styloglossus 
and  levator  palati  mollis  to  the  second 
arch  (counting  the  mandibular  as 
the  first) ;  the  stylopharyngeus  and  perhaps  both  the  palato-pharyn- 
geus  and  hypoglossus  to  the  third  arch.  -  Of  the  constrictors  of  the 
pharynx  the  upper  probably  belongs  to  the  third  arch,  but  the  mid- 
dle and  lower  to  the  fourth  arch. 

C.  Rabl,  87.1,  maintains  that  the  myothelium  of  the  hyoid  arch 
forms  the  embryonic  platysma,  which  spreads  out  in  front  of  and 
behind  the  external  ear  (hyoid  cleft)  and  breaks  up  into  the  individ- 
ual superficial  muscles  of  the  face  and  epicranium.  The  stapedius 
muscle  also  belongs  to  the  hyoid,  according  to  Rabl. 

Mandibular  Muscles. — Their  development  in  the  chick  has  been 
studied  by  Kaczander,  85. 1.  The  muscles  form  at  first  a  continuous 
mass,  which  grows  by  multiplication  of  the  fibres.  The  mass  is 
divided  into  separate  muscles  by  the  ingrowth  of  fibrillar  connective- 
tissue  partitions,  and  by  the  development  of  the  osseous  mandible, 
which  separates  the  muscles  attached  to  the  connective  tissue  from 
those  having  an  insertion  on  the  Meckel's  cartilage.  The  change  in 
the  direction  of  the  course  of  the  fibres  results  from  the  muscles 
adapting  themselves  to  changes  in  the  form  of  the  jaw.  The  inser- 
tion into  the  mesenchymal  anlage  of  the  mandible  remains  unaltered 
when  the  anlage  ossifies.  There  is  no  migration  of  the  insertions. 

Dohrn,  84.1,  113,  states  that  in  sharks  the  developmental  history 
of  the  mandibular  muscles  is  quite  different  from  that  of  the  muscles 
of  the  succeeding  arches. 

Muscles  of  the  Heart. — The  exact  history  of  the  genesis  of  the 
cardiac  muscle  fibre  has  still  to  be  worked  out.  In  the  rabbit  (Kolliker, 
"Grundriss,"  2te  Aufl.,  383)  the  musculature  of  the  heart  appears 
the  ninth  day,  and  by  the  tenth  or  eleventh  day  is  developed  over 
the  entire  organ,  including  the  bulbus  aorta3.  The  muscles  soon 
arrange  themselves  into  a  spongy  structure,  each  web  of  the  sponge- 
work  being  covered  by  endothelium,  Fig,  290,  but  during  the  third 


THE   MESOTHELIAL   MUSCLES.  479 

and  fourth  month  the  musculature  gradually  becomes  more  compact, 
so  that  at  the  beginning  of  the  fifth  month  the  spongy  structure  is 
confined  to  the  innermost  layers  of  the  muscular  wall.  The  stria - 
timis  appear,  according  to  A. C.  Bernays,  76.1,  487,  upon  one  side 
of  the  branching,  protoplasmatic  muscle  cells  (embryo  calf  of  12-16 
mm.)  and  later  around  the  periphery  of  the  cells  somewhat  as  in  the 
myotomic  muscle  cell. 


CHAPTER   XXII. 


THE  SPLANCHNOCCELE  AND    SEPTUM  TRANSVERSUM. 

RENAL  CAPSULES. 


THE  SUPRA  - 


THE  history  of  the  splanchnocoele  of  the  head  has  already  been 
given  as  fully  as  our  present  knowledge  permits,  see  p.  201,  and  ex- 
cept of  that  part  which  forms  the  pericardium.  In  this  chapter  the 
subdivision  of  the  main  ventral  coelom  or  splanchnocoele  into  the 
pericardial,  pleural,  and  abdominal  cavities,  and  in  connection  there- 
with the  development  of  the  diaphragm,  is  described.  It  is  to  be 
remembered  that  the  coelomatic  cavities  of  the  gill  arches  are  possi- 
bly part  of  the  splanchnoccele. 

Development  of  the  Septum  Transversum. — The  term  sep- 
tum transversum  was  first  introduced  by  Wilhelm  His  in  his 
description  of  his  embryo  M  ("  Anatomie  menschlicher  Embryonen," 
Heft  I.).  Our  present  knowledge  of  its  formation  and  metamor- 
phoses rests  chiefly  upon  the  investigations  of  His,  I.e.,  Heft  III., 
also  81.1,  and  of  Ed.  Kavn,  89.2. 

The  septum  transversum  is  the  primary  partition  across  the  body, 
the  heart  being  on  the  cranial  side,  the  abdomen  on  the  caudal  side 

of  the  partition ;  while  above  it  the 
coelom  forms  a  passage  on  each 
side  of  the  median  plane;  these 
passages  become  the  pleural  cavi- 
ties. The  septum  is  quite  thick ; 
it  includes  the  anlage  of  liver ;  by 
it  all  the  veins  make  their  entrance 
into  the  heart ;  later  on  the  anlages 
of  the  supra-renal  capsules  also  ap- 
pear in  it;  it  is  itself  the  anlage 
of  both  the  diaphragm  and  of  the 
membrane  separating  the  pericar- 
dial and  pleural  cavities  of  the 
adult.  The  character  and  general 
relations  of  the  septum  can  be  un- 
derstood from  Fig.  259.  The  sep- 
tum divides  the  pericardial  cav- 
ity, p.c,  from  the  main  abdominal 
cavity,  Ab.  c;  the  heart  is  sup- 
posed to  be  in  great  part  removed. 
The  septum  appears  to  be  much  enlarged  by  the  growth  of  the  liver, 
which  at  the  stage  represented  has  become,  as  it  were,  an  appendage, 
Li,  of  the  septum  proper,  which  may  be  conveniently  defined  as  the 
layer  of  connective  tissue  next  the  pericardial  sac.  Above  the  sep- 
tum is  the  small  passage,  PI,  into  which  the  lung,  Pul,  is  beginning 
to  project,  and  which  becomes  the  pleural  cavity  of  the  adult;  at  this 


FIG.  259. — His'  Embryo  R,  5  mm.  Recon- 
struction to  show  the  Septum  Transversum. 
p.  c,  Pericardial  cavity;  Md,  mandible:  Ao, 
aorta;  Au,  auricular  end  of  heart;  v.j,  vena 
jugularis;  Z>,  (7,  ductus  Cuvieri;  Put,  lung; 
PI,  pleural  cavity;  Li,  liver;  Ab.c,  abdominal 
cavity;  In,  intestine;  Um.v,  umbilical  vein; 
vi.  v,  vitelline  vein. 


BPLANCHNOCCELE     \M>    M:riTM    TRANSVKRsr  M. 


481 


Mage  it  is  termed  tin-  reCSSSUS paHetoltS  dorx'ilix  by  His.  It  will 
be  recalled,  that  the  ccelom  forms  originally  two  splanchmxxelic 
cavities;  in  the  region  «»f  the  heart  the  partition  disappears,  leaving 
a  >ingle  prriranli.il  cavity;  in  the  abdomen  the  partition  (ventral  mes- 
entery) below  the  intestine  disappears,  so  that  the  two  cavities  are 
brought  into  communication,  while  on  the  dorsal  side  of  the  septum 
the  partition  remain-;  but  the  two  pleura  1  >planchnocceles  alwa\> 
are  distinct  and  never  communicate  directly  with  one  another.  The 
arrangement  of  thr  veins  is  important.  The  jugular,  /•../,  coming 
from  the  head,  and  the  cardinal,  coming  from  the  rump  (Wolffian 
IM  >dy ) ,  unite  on  the  dorsal  side  of  the  cephalic  end  of  the  pleural  cavity 
into  a  single  Mem,  the  ductus  Cuvieri  (future  I'CIHI  runt  SHJK  ri»r}. 
which  passes  in  1 1 1 e  somatopleure. around  the  outside  of  the  pleural 
cavity  to  join  thr  other  veins  in  the  dorsal  part  of  the  septum.  The 
durtus  Cuvieri,  D.C,  is  just  at  the  boundary  of  the  pericardial  and 
pleural  cavitio,  and  its  growth  is  the  essential  factor  in  shutting 
the  opening.  The  umbilical  vein,  Um.  v,  joins  the  ductus  just  as  it 
enters  the  septum.  The  vitelline  or  omphalo-mesaraic  veins  enter 
the  septum  nearer  the  median  line;  the  four  veins  which  are  thus 
united  form  the  large  sinus  reunions  (see  Chapter  XXIV.)  from 
which  the  Mood  is  poured  into  the  heart. 

The  origin  of  the  septum  in  mammals  *  has  been  studied  as  yet 
only  in  the  rabbit  by  Uskow,  83.1,  His,  and  Ravn.  The  following 
description  is  based  on  Ravn's  89.2, 
1  -.'1  I  :'>'.».  The  head,  H,  of  the  em- 
bryo early  grows  forward,  Fig.  260, 
so  as  to  intrude  upon  the  region  of 
the  proamnion.  /V»>.  .Land  hence,  as 
will  he  evident  by  an  inspection  of 
the  figure,  the  head  is  bounded  in 
front  and  at  the  sides  by  the  proam- 
nion, and  therefore  the  coelom  of  the 
brad  cannot  communicate  with  the 
extra  -  embryonic  ccelom  directly. 
Around  the  edge  of  the  proamnion 
runs  the  omphalo-mesaraic  vein, 
oni.r,  the  continuation  of  which  is 
the  nnlage  of  the  heart,  Hi;  on  the 
double  origin  of  the  rabbit's  heart, 
see  p.  'l'l\.  The  vein  projects  con- 
siderably above  the  level  of  the 
splaiichnopleure  in  which  it  runs,  and 
this  projection  gradually  increases 
until  the  wall  of  the  vein  reaches  to 
and  unites  with  the  overlying  soma- 
topleure, and  thus  divides  the  coelom  into  two  parts,  the  recessus  pari- 
etalis  dorsalis,  r./>.(L,  and  the  recessus  parietalis  ventralis,  r.p.v. 
This  division  is  confined  to  the  region  of  communication  between  the 
pericardial  cavity,  P,  and  the  remaining  ccelom,  as  indicated  by  the 
position  of  the  reference  letters  r.p.d,  and  r.p.v.  A  cross-section 


ProA 


Fio.  260.— Head  of  a  Rabbit  Embryo, 
with  Segments,  seen  from  the  under  Side. 
/'/"..«.  Outline  of  proamnion;  H,  head; 
orn.v,  omphalo-mesaraic  vein;  /ff,  anlage 
of  heart;  «,  margin  of  the  opening  of  the 
Vorderdarin;  «,  primitive  segments;  Mil, 
medullary  canal;  r.p.v,  recessus  parictji- 
lis  \t-iitralis;  ,\)>.a,  recessus  parietalis 
dorsalis;  P,  pericardial  cavity.  x  25 
diams.  After  Ed.  Ravn. 


'On  the  development  of  the  diaphragm  in  the  chick  see  Lockwood.88. 1 ;  in  lizards  see  Ravn, 
31 


482 


THE    FCETUS. 


Pro.a 


rp.v. 


FIG.  xJ61.—  Rabbit  Embryo,  Eight  and  a 
Half  Days,  with  Eleven  or  Twelve  Som- 
ites: Cross  Section.  Pro.a,  Pro-amnion; 
v.  car,  cardinal  vein;  r.p.d,  recessus  pa- 
rietalis  dorsalis  (pleural  cavity) ;  Ht, 
anlage'of  the  heart;  r.p.v,  recessus  pari 
etalis  ventralis;  P/i,  pharynx.  x 
diams.  After  Ravn. 


of  a  little  older  stage  with  the  vorderdarm  just  closing  is  represented 
in  Fig.  2(51,  and  will  help  to  elucidate  the  disposition  of  the  parts. 

The  ventral  recessus  early  becomes 
closed  at  its  hinder  extremity  and  is 
thereby  converted  into  a  third  pocket 
of  the  pericardial  ccelom,  which  His 
has  described  under  the  name  of  the 
bursa  parietalis.  The  bursa3  sub- 
sequently become  merged  with  the 
pericardial  cavity.  The  dorsal  reces- 
sus, Fig.  2G1,  r.p.d,  is  the  anlage  of 
the  pleural  cavity  and  persists  for  some 
time  open  at  both  ends.  The  par- 
tition dividing  the  two  recessi  from 
one  another,  and  containing  the  om- 
phalo-mesaraic  veins,  is  the  anlage  of 
40  the  lateral  portions  of  septum  trans- 
versum  (Cadiat's  cloison  mesoder- 
mique,  Kolliker's  mesocardium  laterale,  Uskow's  Verwachsungs- 
briicke) . 

By  the  further  growth  of  the  embryo  the  head  lengthens  and  with 
it  the  median  heart  formed  by  the  union  of  the  two  heart  anlages. 
The  splanchnopleuric  wall  at  /o,  Fig.  146,  bounds  not  only  the  open- 
ing of  the  vorderdarm  into  the  yolk-sac,  but  also  the  posterior  wall 
of  the  pericardial  cavity,  and  is  the  anlage  of  the  median  portion  of 
the  septum  transversum.  As  the  liver  is  developed  at  the  hind  end 
of  the  vorderdarm  it  has  to  grow  out  into  this  wall,  /o,  and  conse- 
quently contributes  to  the  thickening  of  the  septum  transversum. 
The  septum  is  further  expanded  by  the  development  of  the  remain- 
ing veins,  (jugulars,  cardinals,  and  umbilicals),  which  are  all  ulti- 
mately united  with  the  omphalo-mesaraics  to  constitute  the  great 
sinus  reuniens. 

In  brief :  the  septum  transversum  includes  the  median  part  of  the 
splanchnopleuric  wall  separating  the  pericardial  cavity  from  the  neck 
of  the  yolk-sac,  and  the  two.  lateral  parts  resulting  from  the  two  up- 
growths of  the  splanchnopleure  to  carry  the  omphalo-mesaraic  or 
vitelline  veins  to  the  heart.  It  is,  therefore,  entirely  a  product  of 
the  splanchnopleure. 

Separation  of  the  Pleural  and  Pericardial  Cavities. — The 
septum  transversum  separates  the  two  cavities  as  soon  as  it  is  formed, 
and  in  the  adult  the  primitive  arrangement  is  easily  traced  in  part, 
despite  the  great  expansion  of  the  pulmonary  coelom.  The  septum 
leaves,  however,  a  direct  communication  open  as  shown  in  Fig.  259, 
where  the  ducts  of  Cuvier,  D.  C,  descend  from  the  dorsal  to  the 
ventral  side.  The  figure  further  shows  that  the  septum  is  oblique, 
so  that  the  pericardial  cavity  in  part  underlies  the  pleural  cavity. 
As  development  progresses,  the  three  cavities  all  expand,  and  more 
and  more  of  the  pericardial  cavity  cornes  to  lie  on  the  ventral  side 
of  the  pleural  cavities,  leaving  a  part  of  the  septum  transversum 
as  a  partition,  which,  of  course,  runs  as  far  headward  as  the  duc- 
tus  Cuvieri,  D.C.  This  partition  early  becomes  thin,  and  is  the 
membrana  pleuro-pericardiaca  which  was  partly  described  by  F. 


BPLANCHNOCOEL1   AND  ^HI-TIM  TRANSN  Ki;sr M.  -ts:j 

T.  Schmidt,  70. 1,, -UK!  t'skow,  83. 1,  and  more  fully  by  His,  81. 1, 
:5i:;,  and  Ravn,  89.2,  ]:><;.  The  anterior  passage  is  clos.-d  hy  the 
vth  of  the  ductus  Cuvieri,  which,  like  all  the  chief  veins  of  the 
emhryo,  lias  an  enormous  size;  it  causes,  therefore,  a  projection  which 
ultimately  shuts  the  passage  to  the  pericardium  completely.  Exactly 
at  what  time  the  shutting  off  occurs  is  not  stated,  hut  probably  dur- 
ing the  filth  week  in  the  human  embryo,  and  in  the  rabbit  by  the 
fifteenth  day.  The  separation  of  the  pleural  from  the  abdominal 
cavity  takes  place  much  later. 

Expansion  of  the  Pleural  Cavities. — Concerning  the  gradual 
enlargement  of  the  pleural  cavities  very  little  is  known  beyond  the 
fact  that  they  enlarge  at  the  same  rate  as  the  lungs.  In  the  rabbit 
at  fifteen  days  they  are  together  about  half  as  large  as  the  pericardial 
cavity. 

As  stated  above,  the  primitive  pleural  cavity  is  on  the  dorsal  side 
of  the  septum,  and  the  cephalic  limit  of  the  septum  is  marked  by  the 
ductus  Cuvieri,  or  future  vena  cava  superior.  Part  of  the  septum 
is  used  to  develop  the  pleuro-pericardial  membrane,  while  the  re- 
mainder, which  includes  the  hepatic  attachment,  develops  into  the 
diaphragm;  beyond  the  caudal  boundary  of  the  septum  the  lungs 
never  project.  These  considerations  show  that  the  pleural  cavities 
lie  entirely  within  the  territory  of  the  septum,  and  that  their  expan- 
sion takes  place  within  the  septum.  This  conception  renders  it 
necessary  to  regard  the  thorax  of  the  adult  as  chiefly  occupied  by 
the  distended  septum  transversum,  and  involves  important  changes  in 
our  morphological  notions  concerning  the  adult  condition. 

Mesocardium,  Mediastinum,  and  Mesentery. — These  mem- 
branes are  the  remains  of  the  tissue  which  originally  divides  the 
coelom  of  one  side  from  that  of  the  other.  The  tissue  disappears  for 
the  most  part  around  the  heart,  so  that  the  pericardial  cavity  is  con- 
tinuous  on  both  the  dorsal  and  ventral  sides  of  the  heart.  In  the 
ahdomen  this  continuity  is  established  only  on  the  ventral,  not  on 
the  dorsal,  side  of  the  intestinal  canal,  and  the  tissue  between  the 
two  lateral  coeloms  remains  to  form  a  very  thin  membrane,  the  mes- 
entery, by  which  the  intestine  is  attached  to  the  median  dorsal  wall 
( >f  the  abdomen.  Between  the  two  pleural  cavities  the  tissue  remains, 
forming  a  thick  partition,  the  mediastinum.  Concerning  the  gene- 
sis of  these  membranes  little  is  known. 

Sac  of  the  Omentum  and  Foramen  of  Winslow.*— In  the 
chick  soon  after  the  lungs  have  grown  out  from  the  oesophagus,  and 
just  when  the  first  forking  has  begun,  the  abdominal  coelom  is  found 
to  form  two  blind  diverticula  lined  by  well-marked  mesothelium  and 
extending  until  they  come  into  direct  contact  with  the  pulmonary 
entoderm.  Of  these  diverticula  I  have  found  no  mention.  Similar 
ones  have  been  observed  in  the  rabbit  by  Ravn,  89.2,  139;  their 
formation  is  connected  with  the  prolongation  of  the  ridge  of  meso- 
derm  on  the  side  of  the  oesophagus.  The  ridge  on  the  left  side,  and 
with  it  the  diverticulum,  disappears  very  early,  but  that  on  the  right 
^ide  persists  and  enlarges,  the  vena  cava  inferior  being  developed 
within  it,  on  which  account  Ravn  terms  it  the  ''vena  cava  Falte." 
This  fold  extends  down  into  the  abdomen ;  the  coelomatic  diverticu- 

*  Compare  also  chapter  xxix. 


484 


THE    FCETrS. 


Tci 


Som 


FIG.  262.  —Model  of  Part  of  the  Pleural  and 
Abdominal  Cavities  of  a  Rat  Embryo  at  a 
Stage  Corresponding  to  a  Rabbit  at  fifteen 
Days,  oe.  Oesophagus ;  L,  lung ;  J>,  anlage 
of  fold  to  form  the  diaphragm;  OHI,  omen- 
turn;  Som,  somatopleure ;  V.om,  omphalo- 
mesaraic  vein ;  F.  W,  foramen  of  Winslow ; 
v.c.t,  vein  in  the  plica  venae  cavee  of  Ravn. 
After  Ravn. 


lum  between  it  and  the  intestinal  canal,  Fig.  262,  is  the  "  re- 
cessus  superior  sacci  omenti"  of  W.  His  ("  Anat.  menschl.  Em- 
bryonen,"  Heft  I. ,  p.  65).  While  this  growth  of  the  "  vena  cava  Falte" 
is  taking  place  the  stomach  has  been  developing  its  great  bend  to 

the  left  as  indicated  by  the  dotted 
lines  in  Fig.  262,  carrying  with  it, 
of  course,  the  mesogastrium  and 
mesentery  of  the  duodenum,  and 
thus  forming  a  sac,  the  entrance  to 
which  is  partially  closed  by  the 
vena  cava  Falte.  The  sac  is  tho 
saccus  omenti,  the  entrance  to  it  is 
the  foramen  of  Winslow,  F.  W. 
The  saccus  is  bounded  on  the  dorsal 
side  and  on  the  left  by  the  meso- 
gastrium; on  the  ventral  side  by 
the  stomach,  the  position  of  which 
at  a  level  nearer  the  observer  may 
be  easily  imagined  from  the  figure ; 
and  on  the  right  by  the  vena  cava 
Falte,  v.c.i. 

In  the  rabbit,  according  to  Ravn, 
89.2,  146-147,  the  anterior  end  of 
the  recessus,  Fig.  262,  becomes  sep- 
arated, as  a  closed  sac,  about  the 
seventeenth  day,  and  forms  a  cavity 
between  the  ossophagus  and  the  so- 
called  lobus  inferior  medialis  of  the  right  lung,  and  persists  in  the 
adult.  A  similar  cavity  (Schleimxcheiae)  is  found  also  in  rats  and 
mice,  and  is  presumably  developed  in  the  same  way.  Ravn  thinks 
it  probable  that  an  homologous  cavity  is  present  in  the  human  em- 
bryo, but  aborts. 

Separation  of  the  Pleural  and  Abdominal  Cavities.— 
This  takes  place  much  later  than  the  separation  of  the  pleural  and 
pericardial  cavities,  for  it  is  not  effected  in  the  rabbit  until  the  seven- 
teenth day,  and  Kolliker  records  that  it  had  not  taken  place  in  a  two- 
months'  human  embryo.  This  agrees  with  the  fact  that  the  separa- 
tion takes  place  only  in  the  mammals,  not  in  other  vertebrates. 
Ravn,  89.2,  147,  is  the  only  investigator  who  has  attempted  to  fol- 
low out  the  process  accurately.  A  fold  is  formed,  Fig.  262,  which 
lies  obliquely  between  the  lungs  and  the  Wolffian  body  on  each 
side,  and  which  in  the  rabbit  at  fifteen  days  is  found  to  somewhat 
contract  the  opening  between  the  pleural  and  abdominal  cavities ;  the 
fold  extends  almost  if  not  completely  around  the  opening,  making  as 
it  were  a  circular  shelf.  Another  factor,  as  pointed  out  by  His,  is 
the  expansion  of  the  liver.  I  have  observed  also,  in  studying  Pro- 
fessor His'  embryo  Zw,*  that  the  anlage  of  the  supra-renal  capsule 
had  appeared  in  the  septum  transversum  on  the  ventral  side  of,  and 
close  to,  the  peritoneal  opening  of  the  pleural  cavity,  so  that  the  con- 

*  My  grateful  acknowledgments  are  due  to  Professor  His  for  the  very  generous  manner  in 
which  he  placed  his  material  at  my  disposal,  during  a  few  weeks  I  had  the  pleasure  to  spend  in 
Leipzig  in  1887. 


SUPRA-RENAL  r.M'sri.ES. 

elusion  was  inevitable  that    the  linal  factor  in  completing  the  do- 
of  the  opening  was  the  t-Towth  of  the  supra-renal  capsule. 

Diaphragm. — The  diaphragm  (Zircrchfcll}  is  developed  f  n  >m 
tliat  portion  of  the  septum  traiisversinn  which  interveiie>  between  the 
pericardial  and  ahdominal  cavities,  and  from  the  fold  which  shut^ 
off  the  connection  of  the  plenral  cavities  with  the  ahdominal.  The 
veins  pass  through  the  diaphragm  to  the  heart,  and  to  the  area 
around  the  veins  the  liver  is  permanently  attached  ;  it  is  out  of  th-> 
remainder  of  the  diaphragm  that  the  muscular  part  and  the  centrum 
tendineum  are  de\  <•!<  >ped,  but  concerning  their  development  no  ob 
rations  whatever  are  known  to  m  >. 

Lining  Membranes  of  the  Splanchnoccele. — These  mem- 
branes are  the  pericardial,  pleural,  and  peritoneal.  They  each  con- 
sist  of  a  layer  of  spe.-iali/.ed  connective  tissue  and  the  mesothelium, 
which  is  found  in  '.In-  adult  to  have  lost  its  primitive  character  of  a 
cuhoidal  epithelium  and  to  have  become  a  thin  layer  or  endothelium. 
Concerning  the  manner  in  which  the  transformation  is  effected,  there 
are  few  reliable  observations — compare  Chapter  XXIX. 

THE  SUPRA-RENAL  CAPSULES. 

It  is  only  with  considerable  hesitation  that  I  have  decided  to  treat 
the  supra-renal  capsules  as  organs  developed  in  the  septum  trans  ver 
sum  on  the  ventral  side  of  the  pleuro- peritoneal  opening.  I  have  made 
observations  which  lead  me  to  think  this  view  neces-ary  from  the  facts 
of  development,  but  I  have  not  been  able  hitherto  to  continue  the  r<  - 
search  to  a  satisfactory  conclusion.  As  the  kidney  grows  forward  until 
it  reaches  the  dorsal  pillars  of  the  diaphragm,  the  supra-renals  would 
coino  in  juxtaposition  with  the  upper  end  of  the  kidneys,  whether  the 
capsules  began  their  development  on  the  dorsal  side  of  the  pleuro-peri- 
toneal  opening  or  on  the  ventral  side,  for  in  the  latter  case  the  closure 
of  the  opening  would  bring  the  capsules  near  the  kidneys.  At  presei  1 1 
1  am  inclined  to  the  belief  thatthe  mesenchymal  portion  of  the  supra- 
renals  arises  on  the  ventral  side  of  the  opening  and  the  sympathetic 
portion  on  the  dorsal  side.  That  this  view  is  right  is  confirmed  by 
the  observation  that  the  capsule  lies  entirely  on  the  ventral  side  of 
the  kidney  in  the  human  embryo  at  two  months  and  at  three. 

Mesenchymal  Anlage.— The  mesothelium  in  the  region  of  the 
vena  cava  inferior  and  septum  transversum  throws  off  cells  to  con- 
tribute to  the  mesenchyma.  Janosik,  83.1,  who  observed  this  pro- 
0688  at  the  point  where  the  supra-renals  develop  in  mammals, 
concluded  that  it  was  a  special  process  and  that  the  supra-renal 
capsules  must,  therefore,  be  considered  as  derivatives  of  the  perito- 
neum. The  recognition  since  then  of  the  genetic  relation  of  the  whole 
mesendiyma  to  mesothelium  renders  it  unnecessary  to  assume  a 
special  relation  for  a  single  mesenchymal  organ.  The  same  criticism 
applies  also  to  Weldon,  85.1,  who,  having  observed  the  production 
of  mesenclwma  from  the  mesothelium  of  the  nephrotomes,  or  seg- 
mental  vesicles,  in  lizards  and  sharks,  concludes  that  there  is  a 
special  genetic  relation  between  the  supra-renals  and  the  segmental 
organs.  In  reptiles,  soon  after  the  vena  cava  is  formed,  there  appears 
on  each  side  of  that  vein  a  small  cluster  of  crowded  mesenchvmal 


•4  Slj  THE    FCETUS. 

cells  (Max  Braun,  82.1),  which  increases  quite  rapidly;  the  cells  of 
the  cluster  gradually  arrange  themselves  in  cords  which  become 
more  and  more  twisted  and  united ;  numerous  blood-vessels  are  early 
developed  between  the  cells,  probably  by  ingrowth  from  the  adjacent 
Wolffian  bodies.  The  nearness  of  the  first  trace  of  the  supra-renals 
to  the  vena  cava  has  also  been  noted  by  Gottschau,  83. 1,  by  Mitsu- 
kuri,  82. 1,  and  Weldon,  85. 1.  In  the  rabbit  the  first  distinct  trace 
of  crowding  of  the  cells  and  of  their  enlargement  to  form  the  anlage 
of  the  rupra-renals  may  be  seen  on  the  twelfth  day ;  on  the  four- 
teenth day  the  anlages  are  well  marked  (Mitsukuri,  82. 1) ;  by  the 
sixteenth  day  the  sympathetic  anlage  is  surrounded  by  the  meseii- 
chymal.  In  the  sheep  (Gottschau,  83.1,  449)  the  anlage  can  be  rec- 
ognized in  embryos  9  mm.  long ;  it  is  in  contact  with  the  sympathetic 
ganglion  tissue  in  embryos  of  11  mm.  and  in  those  of  13  mm.  has 
become  quite  sharply  defined  against  the  surrounding  mesenchyma. 
In  the  pig  the  first  trace  is  seen  in  9  mm.  embryos  according  to  Gott- 
schau, 83.1,  452. 

Balfour,  81.3,  homologies  the  mesenchymal  anlage  with  the  so- 
called  inter-renal  bodies  of  elasmobranchs. 

Sympathetic  Anlage. — On  the  dorsal  side  and  somewhat 
toward  the  median  line  of  each  mesenchymal  anlage  appear  a  cluster 
of  small  cells,  which  are  stained  brown  by  bichromate  of  potassium, 
as  first  observed  by  M.  Braun,  82. 1,  25,  in  reptile  embryos.  These 
cells  are  derived  from  the  chain  of  sympathetic  ganglia,  and  are 
characterized  by  being  smaller  and  more  granular  and  by  having 
smaller  nuclei  than  the  cells  of  the  mesenchymal  anlage.  I  have 
noticed  that  in  specimens  colored  with  alum-cochineal  they  stand  out 
conspicuously  owing  to  their  deeper  staining.  In  rabbits  of  fourteen 
days,  the  sympathetic  anlage  has  become  very  distinct  and  has 
increased  in  size,  and  in  those  of  sixteen  days  it  is  found  surround- 
ing the  mesenchymal  supra-renal  and  more  or  less  separated  from  the 
ganglion  proper.  At  this  time  traces  of  young  ganglion  cells  and  of 
nerve-fibres  are  said  to  be  clearly  recognized.  F.  M.  Balfour,  81.3, 
and  in  his  "  Comparative  Embryology,"  II., 664,  homologizes  the  sym- 
pathetic anlage  with  the  so-called  "true  supra-renals"  of  elasmo- 
branchs, bodies  which  develop  from  the  sympathetic  ganglia.  Ac- 
cording to  Balfour  (monograph  of  Elasmobranchs,  "  Works,"  I.,  472), 
who  greatly  extended  Semper's  observations,  75.2,  in  shark  embryos 
in  Balfour's  stage  L  the  ganglia  of  the  sympathetic  chain  are  par- 
tially divided  into  two  parts :  one  the  future  ganglion  proper,  the 
other  the  anlage  of  the  supra-renal,  which  receives  a  direct  artery 
from  the  aorta.  By  stage  O  these  supra-renal  anlages  have  acquired 
a  distinct  mesenchymal  investment,  which  penetrates  into  their 
interior  and  divides  it,  especially  in  the  case  of  the  anterior  anlages, 
into  a  number  of  distinct  alveoli.  By  stage  Q,  the  cells  are  differen- 
tiated into  larger  (ganglionic?)  cells  and  smaller  ones,  which  Balfour 
holds  to  form  the  true  supra-renal  tissue. 

The  observations  thus  far  made  indicate  that  the  sympathetic 
anlage  is  derived  from  a  series  of  spinal  ganglia,  which  give  off  a 
series  of  supra-renal  parts;  these  parts  remain  distinct  in  elasmo- 
branchs, but  fuse  into  one  mass  on  each  side  in  amniota.  Rudimen- 
tary ganglion  cells  arise,  but  soon  abort. 


\ 


SUPRA-RENAL  <  A  TM  LKS. 

Union    and    Ultimate    Fate  of  the   Two   Anlages. — The 

ir  -seucliymal  and  sympathetic  portions  very  soon  come  into  contact 
(sheep  of  1  1  mm.,  ra  I  thits  of  the  fourteenth  day).  At  first,  in  amniota 
at  least,  the  sympathetic  anlage  grows  most  rapidly  and  partially 
surrounds  the  mesenchymal  portion,  but  so<»n  the  relations  are  n- 

-«•«!   and  gradually  tiie  mesenchymal   portion   completely   in\> 
the  sympathetic  part,  hut  tor  some  time  there  remains  a  hilus  on  tin- 
inner  side  toward  the  posterior  end  of  the  organ.     Fig.  •y,:;  shov, 

transverse  section  of  the  left  supra- 
renal taken  about  the  middle  of 
the  body  from  a  rabbit  embryo  of 
twent\->i\  days.  The  cortex  is 
already  made  up  of  distinct  cell- 
groups,  which,  however,  are  not 
yet  differentiated  into  the  adult 
cortical  and  medullary  zones. 
Capillaries  are  well  formed  be- 
^  tween  the  adjacent  cell-group>. 

The  sympathetic  portion,  syin^  is 

-  V  divided    into  irregular   groups   of 

^    J^^-'  '  cells,  which  stain  readily  and  are 

V^v^-*  -^  mstli  situated    exclusively   in    the  cen- 

1 1  region ;  between  these  n TOU  i  .s 
«*?££?%&&'%  SS&  Hivivlatm.lv  large  blood-vessels. ,-. 

"•ir.;1..1,1:,1::  "•:r,:S    The  connective  tissue  has  formed 

gheaxh;  m*M,  meeothelium  lining  tiu>  i><xiy.    a    sheath,    .s,    around   the   organ. 

Mitsukuri  states  that  the  masses 

•  >f  nervous  origin  are  now  full  of  "  distinct  ganglion  cells,  supported 
in  a  connective-tissue  framework ;  scattered  among  the  larger  cells 
are  smaller  cells."  This  may  be  regarded  as  perhaps  the  Sauropsidan 
condition,  since  according  to  HansRabl,  91.1,  the  two  supra-renal 
tissues  persist  in  birds  throughout  life,  interlaced  with  one  an- 
other. 

Mitsukuri  believed  that  the  medulla  of  the  adult  capsules  ari.-e-* 
from  the  sympathetic  anlage,  but  Gottschau,  83.1,  showed  that  this 
was  not  the  case,  though  he  failed  to  ascertain  what  became  of  the 
sympathetic  masses.  By  a  considerable  series  of  observations  on  the 
supra-renal  capsules  of  human  embryos,  I  have  ascertained  that  there 
are  groups  of  cells  which  gradually  disappear  and  take  no  part  in 
the  production  of  the  adult  organ.  The  cells  are  in  clusters  in  the 
central  portion  of  the  organ  and  stain  very  readily,  so  that  they 
stand  out  conspicuously  in  the  sections.  In  appearance  they  resemble 
the  cells  assigned  to  a  sympathetic  origin  in  the  rabbit,  and  I  should 
feel  no  doubt  that  they  are  the  same  were  it  not  that  I  fail  to  find 
them  in  embryos  of  the  second  month,  so  that  if  they  are  really  of 
sympathetic  origin  then  the  union  of  the  two  anlages  must  take  place 
at  a  considerably  later  stage  in  man  than  in  other  animals.  These 
gr<  )ups  of  cells  are  readily  seen  in  the  three-months'  embf  yo,  but  in 
the  four-months'  embryo  they  are  disappearing  and  many  of  the  clus- 
ters are  hollow,  their  cavities  being  filled  with  what  is  apparently  a 
coagulum;  by  the  seventh  month  the  clusters  have,  so  far  as  I  have 
hitherto  observed,  entirely  disappeared.  That  both  the  cortex  and 


488  THE   FCETUS. 

medulla  of  the  adult  organ  are  formed  in  man  from  the  mesenchymal 
cells,  as  Gottschau,  83.1,  showed  was  the  casein  several  mammals, 
is,  I  think,  beyond  question.  The  cords  of  cells  are  at  first  uniform 
throughout,  but  I  find  that  toward  the  end  of  the  second  month  the 
cells  of  the  cords  multiply  and  become  smaller,  while  at  the  same 
time  the  cords  assume  a  more  radial  position  and  regular  arrange- 
ment around  the  periphery;  there  is  thus  developed  a  cortex,  char- 
acterized by  radiating,  small-celled  cords  and  a  medulla,  characterized 
by  irregular,  large-celled  cords.  In  the  cortex  the  cords  are  wide 
and  contain  numerous  cells ;  toward  the  interior  the  cords  break  up 
into  small  ones,  which  pursue  the  same  radial  course  and  consist  of 
cells  which  gradually  increase  in  size  toward  the  centre  of  the  organ. 
The  cords  are  marked  off  by  wide  capillaries,  with  distinct  endothe- 
lial  walls,  between  which  and  the  supra-renal  cords  there  appears 
to  be  no  connective  tissue,  although  in  the  medulla  there  is  more  or 


I  • 


p 

*'.  • 


FIG.  264. —Supra-renal  Capsule  of  a  four-months1  Human  Embryo.     Minot  Collection,  No.  35. 
Cross  section  of  the  medulla,     x  about  500  diams. 

less  connective  tissue  developed  early  around  the  vessels,  Fig.  264. 
It  seems  to  me  that  the  cortex  grows  at  the  expense  of  the  medulla, 
the  deep-lying  large  cells  dividing  into  smaller  ones.  The  medulla 
of  a  four-months'  embryo  is  represented  in  Fig.  264. 

The  cords  of  supra-renal  cells  are  very  irregular  and  often  con- 
nected together,  but  are  readily  seen  to  be  directly  continuous 
with  the  cortical  cords.  The  medullary  cords  are  much  more  widely 
separated  than  those  of  the  cortex  from  one  another,  the  spaces  be- 
tween them  being  filled  with  connective  tissue  and  blood-vessels, 
none  of  which  have  any  adventitial  or  muscular  walls.  The  great 
variety  of  appearances  presented  by  the  cells  of  the  cords  is  indi- 


3UPRA-BENAL   CAPSULES,  489 

cated  in  tin*  figure;  large  and  -mall,  regularly  and  irregularly 
shaped,  uninueleate  and  multinucleate,  light-stained  and  darkly- 
r-tained  cells  Liejumhled  together  without  obvious  law  of  arrangement. 
The  significance  of  this  M  range  picture  is  unknown.  It  should  be 
noted  that  i  he  nuclei  of  the  conl-<vils  are  all,  or  nearly  all,  decidedly 
larger  than  those  of  the  atljacent  connective  tissue.  As  develop- 
ment proceeds  the  cells  heroine  gradually  more  uniform  in  appear- 
ance, and  offer  by  the  seventh  month  little  variety;  also  the  conti- 
nuity <>t'  the  eords  is  lessened  in  the  medulla  and  the  blood-vessels 
become  larger.  It  is  evident  that  there  is  no  fundamental  difference 
between  cortex  and  medulla — in  the  former  the  cords  have  a  radial 
trend,  in  the  latter  they  run  irregularly:  the  medulla  is  also  char- 
acieri/.'-d  by  ha\  in L;' larger  supra-renal  cells  and  a  richer  blood  supply. 
Form  and  Size  of  the  Supra-renals. — The  supra- renal 
capsules  have  at  first  a  rounded  torm  and  lie  on  the  ventral  side  of 
the  cephalic  end  of  the  kidney.  Probably  about  the  third  month 
tli-.-y  begin  to  spread  on  in  the  dorsal  side  of  the  kidney,  the  head  end 
of  which  they  invest  like  a  cap.  The  capsules  grow  at  first  very 
rapidly,  afterward  more  slowly,  and  as  the  kidneys  grow  more 
steadily  the  relative  size  of  the  capsules  compared  with  the  kidneys 
•  s  through  striking  changes. 


CHAPTER  XXIII. 


raes 


Ov.T 


THE  UROGENITAL  SYSTEM. 

THE  early  history  of  the  urogenital  system  has  already  been  given, 
Chapter  XL,  p.  230.  We  have  now  to  consider  the  differentiation 
of  the  male  and  female  type  from  the  indifferent  condition.  In  or- 
der to  render  the  complex  changes  clear,  it  has  seemed  to  me  advisa- 
ble to  give  first  a  general  history  of  the  metamorphoses,  so  as  to 
bring  out  first  the  homologies  in  the  two  sexes,  next  to  present  the 
special  histories  of  the  single  parts,  and  finally  to  append  an  account 
of  the  external  genitalia. 

I.  GENERAL  HISTORY. 

The  Indifferent  Stage.  I. — The  early  history  of  the  urogenital 
system  has  been  given  in  Chapter  XI. ;  nevertheless  it  will  be  con- 
venient to  present  here  a  generalized  diagram  of  the 
indifferent  stage,  for  comparison  with  diagrams  of 
the  differentiated  system  male  and  female.  The 
indifferent  stage  is  characterized  by  all  tho  organs 
being  contained  in  two  longitudinal  urogenital  ridges 
Fig.  265,  one  on  each  side  of  the  body  and  projecting 
from  the  dorsal  wall  into  the  peritoneal  cavity.  At 
the  caudal  end  of  the  abdomen  the  two  ridges  draw 
closer  together  and  finally  come  into  contact  with 
the  anal  region  of  the  intestinal  canal.  The  ridge 
is  constituted  chiefly  by  the  Wolfnan  body,  w.b,  and 
it  therefore  contains  the  Wolffian  tubules  and  the 
Wolffian  duct,  W.D,  which  is  situated  on  the  side 
of  the  ridge  farthest  from  the  mesentery,  mes.  Close 
alongside  the  Wolffian  duct  lies  the  Miillerian  duct, 
M.D.  Both  ducts  open  into  the  cloaca,  O/,  or  ter- 
minal division  of  the  intestine. 

Changes  in  Both  Sexes. — The  essential  or 
fundamental  difference  between  the  two  sexes  is  the 
change  of  the  genital  ridge  into  an  ovary  or  testis 
according  to  the  sex.  The  secondary  differences 
are  chiefly  in  the  modifications  of  the  ducts,  and 
as  regards  these  the  most  important  changes  are 
that  in  the  male  the  Wolffian  duct  becomes  the  gen- 
ital duct  (duct  of  the  epididymis,  vas  deferens,  and  ductus  ejacula- 
torius) ,  while  in  the  female  the  Miillerian  duct  becomes  the  genital 
duct  (Fallopian  tube  or  oviduct,  uterus,  and  vagina) .  Before  con- 
sidering the  changes  more  in  detail  it  will  be  convenient  to  divide 
them  into  two  groups;  1,  common  to  both  sexes;  2,  characteristic  of 
one  sex. 


FIG.  265. —Diagram 
of  the  Indifferent 
Stage  of  the  Urogen- 
ital System  of  Am- 
niota.  Explanation 
in  text. 


GENERAL    HISTORY. 


491 


1.  CHAN  >MM<>.\  TO  BOTH    SEXES.  —  There    are  three    im- 

portant ehanges  from  the  indifferent  stage  to  be  noted  under  this 
head:  A,  the  amonof  the,  caudal  ends  of  the  urogenital  ridges  to  form 

a  single  median  r/r//  /"////  mrd;  B,  the  anterior  end  of  the  Wolffian 
body  persists  ana  undergoes  modification  in  connection  with  the 
genital  glands,  by  which  two  separate  organs  are  produced  in  each 
sex;  C,  in  the  course  of  development  the  genital  organs  become 
restricted  to  the  lower  (or  caudal)  end  of  the  abdomen,  and  do  not 
continue  to  stretch  the  whole  length  of  the  abdomen  as  at  first. 
Another  important  series  of  changes  is  that  by  which  the  cephalic 
portion  of  the  urogenital  ridge  acquires  in  the  female  a  transverse 
position,  in  consequence  of  which  the  upper  or  cephalic  end  of  the 
.Miillerian  duet,  or  in  other  words  the  future  Fallopian  tube,  runs 
transversely.  This  change  occurs  in  the  male  also,  but  is  less 
noticeable  and  is,  to  a  certain  extent,  masked  by  the  migration  of  the 
te>tis  from  the  abdomen  through  the  inguinal  ring  into  the  scrotum. 
-.*.  CHANGES  CHARACTERISTIC  OF  ONE  SEX  ONLY.  —  A.  MALE.  — 
The  general  plan  of  the  urogenital  ridge  in  the  male  is  indicated  in 
tlu-  diagram  Kig.  .''.«».  In  the  male,  as  stated  above,  the  sexual  gland 
becomes  a  testis  by  the  development  of  seminiferous  tubules,  and 
the  \V<  .Irfian  duct  becomes  the  genital  duct.  The  connection  between 
the  Wolffiaii  duct  and  the  seminiferous  tubules  is  established  by 
nn  >ans  of  the  anterior  tubules  of  the  Wolffian  body.  There  are  special 
ex  tensions  of  these  tubules  into  the 
testis,  which  unite  with  the  semi- 
niferous tubules  and  form  a  series 
of  anastomoses  with  one  another 
within  the  testis  (compare  Fig. 
••Mr,),  constituting  the  rete  testis, 
while  the  tubules  proper  of  the 
anterior  part  of  the  Wolffian  body 
remain  to  serve  as  the  channels  of 
connection  (vasa  efferentia)  be- 
tween the  rete  testis  and  the  Wolf- 
lian  duct,  which  is  thus  enabled 
to  serve  as  the  spermiduct.  A 
portion  of  the  anterior  Wolffian 
tubules  persist  as  a  separate  group, 
which  is  known  as  the  organ  of 
(iiraldes,  or  paradidymis  of  Wal- 
deyer.  The,  spermiduct  becomes 
differentiated  into  three  principal 
divisions:  1,  the  coiled  portion 

,v  4.1  IG.       .  —  o      us 

nearest  the  testis  constituting  the  giesof  t'.»-  s«-xuni  Appar;ims. 

rhir»t  nf  flip  pnirliilvmi^  •    -)    flip  Inner  r-  e/'  vasa  efferentia;   Epd,  duct  of  epididy- 

tneepl(im\miS,    «,  U         )ng  mij.    jr.Z).W.,ima!i  .luet;3f.  Z>,  MQller's  duct; 

deferens  running    through   the  6'.c,  senital  cord;    Ut.rn,  uterus  masculinus; 

-     /.   ij  °i  Je,  testis:  A'.-/..  n-t«-   Halleri  :  Fnrad.  paradi- 

lOld,  tO    Wnere    me  tWO  dj-niis;  /,  flmbria;  parr,  parovarium  or  eo- 


k.Male 


B.  Female 


FIG.  266.  —Diagram  to  niustrate  the  Homolo- 

////</. 


t,  ,1,1*  unite  to  constitute  the  genital 

cord;   3,  the  ductus  ejaculatorius, 

developed  below  the  point  where  the  seminal  vesicles  are  formed  and 

within  the  genital  cord.     The  Miillerian  ducts  remain  rudimentary 

and  their  middle  portions  usually  abort,  leaving  the  upper  fimbriate 


402  THE    FCKTTS. 

ends  to  develop  into  the  so-called  hvdatids  of  Morgagni,  and  the 
lower  or  caudal  ends  to  unite  within  the  genital  cord  to  form  the 
so-called  uterus  masculmus  (prostatic  vesicle),  a  rudimentary  repre- 
sentative of  the  female  uterus  and  vagina. 

B.  FEMALE. — The  general  plan  of  the  urogenital  ridge  in  the  fe- 
male is  indicated  by  the  diagram,  Fig.  '200.  In  the  female,  as  stated 
above,  the  sexual  gland  becomes  an  ovary,  by  the  development  of 
ovic  follicles,  and  the  Miillerian  duct  becomes  the  genital  duct.  The 
Wolffian  duct  remains  rudimentary  and  in  part  disappears ;  it  persists 
in  the  genital  cord  as  the  duct  of  Gartner,  but  does  not,  so  far  as 
known,  unite  with  its  fellow;  it  persists  also  at  its  upper  or  cephalic 
end  as  the  duct  of  the  parovarium  (epoophoron,  organ  of  Rosen muller) , 
which  comprises  the  group  of  Wolffian  tubules  in  the  female  homolo- 
gous with  the  vasa  efferentia  of  the  male.  There  also  persists  a  group 
of  Wolffian  tubules,  which  has  been  named  the  paroophoron  by  Wal- 
deyer  and  is  homologous  with  the  male  organ  of  Giraldes.  The  Miil- 
lerian ductus  unite  within  the  genital  cord  to  a  single  median  duct, 
which  enlarges  greatly  and  is  differentiated  into  the  uterus  and 
vagina ;  the  upper  or  cephalic  portions  remain  separate  and  form  the 
Fallopian  tubes  or  oviducts  proper ;  the  Miillerian  funnel  becomes  the 
fimbriate  opening  of  the  Fallopian  tube. 

Homologies  between  the  Sexes.— These  may  be  readily  fol- 
lowed by  means  of  the  accompanying  diagrams,  Fig.  266,  A  and  B, 
and  the  table  given  below.  The  diagrams  call  for  no  further  expla- 
nation than  is  given  on  the  figures. 

TABULAR  VIEW  OF  THE  HOMOLOGIES  OF  THE  HUMAN  UROGENITAL 
APPARATUS  IN  THE  Two  SEXES. 

INDIFFERENT  STAGE.  MALE.  FEMALE. 

Genital  ridge.  Testis.  Ovary. 

Wolffian  tubules.                     1.  Epididyinis.  1.  Epoophoron. 

2.  Paradidymis.  2.  Parovarium. 

Wolffian  ducts.                        3.  Vas  deferens.  3.  Duct  of  Gartner. 

(Vesicula  seminalis. ) 

4.  Ductus  ejaculatorius.  4.  (Usually  aborts.) 

5.  Hydatid  of  Morgagni.  5.  Fimbriate  opening. 
Mullerian  duct.                       6.  (Usually  aborts. )  6.  Fallopian  tube. 

7.  Uterus  masculinus.         7.  Uterus. 

8.  (Usually  undeveloped.   8.  Vagina. 

9.  Verum  montanum.          0.   Hymen. 
Urogenital  sinus.                   10.   Urethra.  10.   Urethra  and  vestibule. 

11.   Cowper's  glands.  11.   Bartholini's  glands. 

Genital  eminence.  12.   Penis.  12.  Clitoris  and  nymphse. 

External  labia.  13.  Scrotum.  13.  Labia  majora. 

II.  SPECIAL  HISTORIES  OF  THE  UROGENITAL  ORGANS. 

Sexual  Glands. — A.  MALE. — The  testis  becomes  recognizable  by 
its  histological  character  in  the  human  embryo  according  to  W.  Nagel, 
89.3,  309,  at  five  weeks;  according  to  Benda,  89. 1,  at  six  weeks.  It 
can  be  distinguished  from  the  ovaries  by  its  external  form  in  the 
three  months'  embryo.  By  the  abortion  of  the  Wolffian  body  and  by 
the  growth  of  the  testis  the  latter  becomes  the  principal  organ  of 
the  urogenital  fold  in  the  male.  The  Wolffian  part  of  the  fold  re- 
mains to  form  the  mesorchium,  the  lower  or  caudal  portion  of  the 


SPECIAL 


S    (>F    THK    IKodKMTAL    <>!;<, ANS. 


493 


fold  lemainiiiL;-  as  the  ^ubernaculum.  By  the  fourth  month  the 
test  is  has  a-sumed  its  permanent  fi.nii,  l>ut  its  growth  continue-. 

The  role  <>f  the  \VollHan  tubules  in  the  genesis  of  the  testis  is 
described  below,  p.  nun. 

IlistiHit'itcsix. — The  subsequent  account  of  the  development  of  the 
tcstis  follow-  Nau'el,  89.3,  clox-ly,  \vhose  results  I  accept,  both  as 
regards  his  i>h>ervat  imis  and  his  criticism  of  previous  investigations, 
although  they  require  niodiiicatioii  oxvin-'  to  what  has  become  known 
concern  in--  the  genetic  relation  of  the  mesenchyma  to  the  mesothe- 
liuin  of  the  -vnital  rid^e,  seep.  248.  As  described  in  Chapter  XII.  the 
-vnital  mesothelium  throws  off  cells,  which  at  first  assume  entirely 
the  character  of  loose  mesenchyma,  and  later  remain  cr.»\vded  to- 
ier  with  scarcely  a  trace  of  division  from  the  ]>arent  epithelium; 
in  this  denser  tissue  appear  lai'^c  cells,  the  so-called  "  Ureier." 
( )ut  i.f  this  anlai;v  a.i-e  developed  ejiithelioid  cords,  the  sexual  cords, 
whi<  h  include  some  of  the  uivier,  and  l>ecome  more  and  more 
separated  tr<.m  one  another  by  the  development  of  loose  mesen- 
chyma or  embryonic  connective,  tissue  between  them.  Nagel  finds 
that  the  male  sexual  ^land,  Fig.  2r,;.  may  be  recognized  even  in 


conn 


IfT 

-*J7. — Section  of  the  Testis  of  a  Human  Embryo  of  sixty  three  to  sixty-eight  Days.     6'.c, 
Sexual  cords;  wr,  ureier;  conn,  mesenchyma. 

an  embryo  of  13  mm.  by  the  small  number  of  the  ureier  as  com- 
pared with  the  ovary  of  corresponding  age.  In  the  testis  at  this 
Mage  (human  embryo  of  13  mm.),  the  sexual  cords  are  not  yet  very 
distinct  and  are  connected  with  the  superficial  epithelium.  In  an 
embryo  of  nine  weeks,  Fig.  267,  the  sexual  gland  *  is  covered  by  a 
regular  cuboidal  epithelium,  distinctly  bounded  against  the  under- 


*In  accordance  with  Knllikcr's  description  and  figure  < "Ornndriss."  Fipr.  388),  this  gland 
would  !>«•  an  ovary,  l.ut  V-.n  Ack.-r«-n  stat.-s  that  K ."'Hiker  has  lM-coine  doubtful  in  regard  to  his 
Fig.  i288,  and  1  think  it  must  be  regarded  as  the  section  of  a  testK 


404  THE   FOETUS. 

lying  tissue,  which  is  composed  of  wiesenchyma  with  sexual  cords, 
/S.c,  which  are  not  connected  with  the  mesothelium ;  the  submeso- 
thelial  layer  is  the  anlage  of  the  tunic<(  (ilhiujin^ct;  as  no  corre- 
sponding layer  exists  in  the  ovary,  its  presence  in  the  male  gland  at 
this  stage  establishes  one  of  the  most  characteristic  features  of  the 
testis ;  in  the  albuginea,  connective-tissue  fibrilkB  are  just  beginning 
to  appear.  The  central  portion  of  the  testis  is  occupied  by  sharply 
defined  sexual  cords,  which  frequently  anastomose  with  one  another 
and  contain  here  and  there  an  "  Urei,"  or  sexual  cell  of  Mihalko- 
vics ;  the  sexual  cells  are  clearer  and  larger  than  the  other  cells  of 
the  cords,  measuring  ll/j.  with  nuclei  of  8/*  diameter.  In  an  embryo 
of  35  mm.  the  general  structure  is  much  the  same,  but  the  albuginea 
is  thicker  and  more  fibrillar,  and  the  cords  are  more  regular  in  their 
arrangement;  the  cords  are  about  32/>.  thick  and  their  cells  show  a 
somewhat  epithelioid  arrangement ;  the  few  sexual  cells  they  con- 
tain now  measure  14-1G/*.  In  an  embryo  of  ten  centimetres  a  new 
feature  is  found  in  the  presence  of  the  interstitial  cells.  These  are 
large  cells  which  lie  between  the  sexual  cords,  and  are  probably 
developed  by  the  enlargement  of  the  connective-tissue  cells  between 
the  cords :  they  are  spindle-shaped  or  polyhedral,  with  several  pro- 
cesses each;  their  protoplasm  offers  a  peculiar  mat  appearance;  their 
nuclei  are  large,  with  one  or  two  nucleoli  and  a  distinct  intranuclear 
network. 

The  corols  are  the  solid  anlages  of  the  seminiferous  tubules.  The 
question  has  been  debated  at  great  length  whether  they  are  differ- 
entiated from  the  stroma  or  the  epithelium  of  the  genital  ridge — 
compare  the  synopsis  of  opinions  given  by  Nagel — but  as  the  epi- 
thelium (mesothelium)  produces  the  mesenchyma  or  stroma,  the 
question  appears  to  me  insignificant.  The  further  history  of  the 
sexual  cords  (future  seminiferous  tubules)  has  been  most  fully  stud- 
ied by  C.  Benda,  89.1,  compare  also  Prenant,  89.1,  90.1.  The 
cords  remain  solid  throughout  foetal  life,  the  smaller  cells  having  a 
radial  position  and  epithelioid  arrangement,  but  the  nuclei  are  irreg- 
ularly placed,  so  that  it  is  difficult  to  decide  whether  the  cells  are  in 
a  single  row  or  not  around  the  centre  of  the  cord.  The  large  ureier 
are  irregularly  distributed — less  irregularly  in  man  than  in  other 
animals — but  they  are  always  completely  imbedded  in  the  other  cells 
and  show  a  tendency  to  lie  near  the  periphery  of  the  cord  in  man, 
rodents,  dogs,  and  cats,  near  the  centre  in  ruminants  (ox).  As  to 
their  number,  few  ureier  are  found  in  the  cords  of  man,  while  in 
rodents  they  are  very  numerous ;  dogs  and  cats  occupying  an  inter- 
mediate position  as  to  number.  The  condition  described  is  attained 
in  man  about  the  sixth  week,  in  the  rabbit  the  seventeenth  day,  and 
persists  with  little  change  not  only  throughout  the  foetal  period,  but 
until  the  time  of  puberty,  when  the  cords  change  to  seminiferous 
tubules. 

The  conversion  of  the  male  sexual  cords  into  the  seminiferous 
tubules,  being  post-foetal,  does  not  fall  within  the  scope  of  this  work. 
The  reader  is  referred  to  the  investigations  of  Prenant,  89. 1,  Benda, 
89. 1,  and  F.  Hermann,  89.2.  According  to  Benda  the  epithelioid 
cells  give  rise  to  the  columns  of  Sertoli  (Benda's  Fusszellen)  and 
the  ureier  to  the  spermatocytes  (Benda's  Samenstammzellen) .  This 


8PK(  [AL    HISTORIES    <>K    IHK    [JROGENITAL   ORGANS, 


is  in  accordance  with  Hernia's  hypothesis  that  the  spermatocvtes 
have  no  genetic  relation  with  SertoliV  eolumns.  ;in  hypothesis  \\hich 
is  not  yet  established  firmly — comj>aiv  Chapter  III. 

B.  FEMALE, — The  o/v//-//  heroines  histolo^-ii-ally  recognizable 
about  tin*  same  time  as  the  testi>.  i.e.  *\\  weeks;  it  can  be  readily 
distinguished  ti-oni  tin-  testis  in  tin-  three  months'  human  embryo  by 
its  external  t'<»rm.  Inconsequence  of  the  abortion  of  the  Wolffian 
body  and  of  it >  own  growth  the  ovary  is  already  the  principal  on; an 
of  the  urogenital  fold  at  three  months.  As  the  greater  part  of  the 
fold  has  thinned  out  to  constitute  the  broad  ligament,  the  relations 
found  in  the  adult  are  established  at  the  age  under  consideration. 

Il/xtogenesis  of  the  Ovary. — According  to  Nagel  the  ovary  may 
be  distinguished  from  the  testis  in  human  embryos  of  only  12  i:> 
mm.  by  the  greater  abundance  of  the  developed  and  developing 
ureier.  In  an  embryo  of 

L2mm.Nagel,89.3,305,  JtaJJhajaBJgSffite-^  ,„  ,1, 

describes  the  ovary  as 
eoiisUtini;  of  thr prolifer- 
ated germinal  epitheli- 
um; the  proliferated  cells 
are  of  t  \\<  >  kinds,  the  more 
numerous  are  smaller, 
and  have  more  darkly 
stained  nuclei;  the  less 
numerous  are  the  young 
ureier  with  lighter 
stained  granular  nuclei. 
In  an  embryo  of  20  mm. 
the  ovary  projects  a  little 
from  the  surface  of  the 
urogenital  ridge,  and  is 
tilled  with  the  cells  from 
the  opit helium,  the  two 

kinds  being  present,  and,        FIO.  268. -Section  of  the  Ovary  of  a  Human  Embryo  of  7 
•i<  lw>fr»rt>    with  rmmprrm«    cm-     Math,  Mesothelium;  Ue,  ureier;  cc,  proliferated  small 
re,  Wltn  numerous    cH,s.  N//.  stroma  Or  connective  tissue.     Aft,-,-  \V.  Nagel. 

transitional  stages  be- 
tween them;  the  ureier  measure  10-16^  with  a  nucleus  of  8/^t  diame- 
ter— the  smaller  cells  8/*  with  a  nucleus  of  5/* ;  in  the  centre,  spindle- 
shaped  connective-tissue  cells  are  appearing.  In  an  embryo  of  30 
mm.  the  ovary  projects  still  more  from  the  Wolffian  body;  the  ureier 
are  larger,  Hi/*,  and  the  connective  tissue  or  stroma  is  more  devel- 
oped and  has  capillaries.  NageL  has  studied  also  embryos  between 
o  and  7  cm.  in  length,  but  we  may  pass  at  once  to  the  latter.  In 
embryos  of  7  cm.  the  ovary  is  triangular  in  section,  the  apex  of  the 
triangle  corresponding  to  the  attachment  to  the  Wolffian  body  or 
future  broad  ligament.  The  connective  tissue  now  forms  partitions, 
which  divide  the  remaining  cells  into  groups,  Fig.  2G8,  but  the  par- 
tit  ions  fade  out  toward  the  surface,  which  is  covered  by  a  single 
layer  of  cells,  which  has  begun  to  assume  the  character  of  an  epi- 
thelium entirely  distinct  from  the  underlying  cells.  In  an  embryo 
of  11  cm.  the  covering  epithelium  of  the  ovary  has  become  more 
sharply  bounded  and  the  development  of  the  stroma  has  extended 


496  THE   FCETUS. 

quite  to  the  surface,  dividing,  the  proliferated  cells  into  rounded 
groups  of  small  cells  and  ureier,  which  are  at  this  stage  very  nu- 
merous, and  indeed  outnumber  the  small  cells  in  the  balls.  These 
balls  are  a  highly  characteristic  feature  of  the  young  mammalian 
ovary,  but  their  arrangement  and  connections  with  one  another  have 
been  as  yet  only  very  imperfectly  studied ;  nevertheless  it  seems  safe 
to  say  that  they  are  not  separate  masses,  but,  as  seen  under  the 
microscope,  sections  of  contorted  and  anastomosing  cords.  If  this 
view  is  correct  then  there  is  an  evident  resemblance  between  the 
testis  and  ovary,  there  being  in  both  cords  derived  from  the  ger- 
minal epithelium,  containing  ureier  and  separated  from  one  another 
by  vascular  connective  tissue.  The  ovary  differs  from  the  testis  in 
having  larger  cords  and  a  much  larger  absolute  and  proportionate 
number  of  ureier.  That  we  have  to  do  with  sexual  cords  is  evident 
in  later  stages,  where  the  cords  are  very  distinct  and  are  found  still 
connected  with  the  covering  mesothelium  (Waldeyer's  Keimepithel) ; 
in  their  later  stage,  the  ovarian  sexual  cords  are  known  as  Pfliiger's 
cords  (Prliiger'schen  Schlauche),  being  named  after  their  discov- 
erer, and  they  differ  considerably  from  their  earlier  stage  in  that 
they  include  a  large  number  of  small  or  follicular  cells,  which  com- 
pletely surround  the  ureier  and  separate  them  from  one  another,  1  >y 
constituting  an  epithelioid  layer  or  follicle  around  each  urei.  The 
transition  from  the  stage  of  the  balls,  as  we  may  call  it,  to  the  stage 
of  Pfliiger's  cords  has  not  been  clearly  ascertained,  because  the 
origin  of  the  small  or  follicular  cells  is  still  uncertain  but  I  agreewith 
O.  Hertwig  ("Lehrbuch  d.  Entwickelungsgesch.,"  3te  AufL,  321) 
that  they  are  cells  of  the  original  cords  derived  from  the  mesothelium 
of  the  ovary,  although  Rouget  and  so  eminent  an  authority  as  Kol- 
liker  ("  Grundriss,"  423)  have  maintained  that  the  medullary  cords 
grow  around  the  ureier  and  produce  the  follicles ;  Kolliker  seems  to 
me  not  to  have  offered  sufficient  evidence  to  render  his  view  probable. 
Another  view  is  that  advocated  by  Foulis,  76.1,  who  believes  that 
the  ureier  becomes  entirely  free  and  that  the  follicles  are  developed 
from  the  stroma  cells — a  conception  which  cannot  be  maintained.  If 
we  assume,  as  we  apparently  must,  that  the  follicular  cells  arise  from 
the  sexual  cords,  the  question  would  still  remain,  whether  they  are 
derived  from  some  of  the  original  small  cells  or  from  the  ureier; 
that  the  latter  derivation  is  the  actual  one  is  to  my  mind  probable, 
because  there  appears  to  be  a  stage  in  the  development  of  the  sexual 
cords  of  the  mammalian  ovary  in  which  all  the  cells  are  converted 
into  ureier ;  but  until  further  investigations  shall  have  decided  it, 
the  question  of  the  origin  of  the  follicular  cells  must  be  considered 
an  open  one.  Mihalkovics,  85. 1,  449,  discusses  carefully  the  origin 
of  these  cells,  but  owing  to  the  distinction  he  draws  between  the 
sexual  cords  and  the  proliferation  of  the  germinal  epithelium  to  form 
the  ureier,  it  is  impossible  to  follow  his  own  account:  Mihalkovics 
also  gives  an  admirable  review,  pp.  423-428,  of  the  literature  upon 
the  development  of  the  ovary. 

Gubernaculum,  Processus  Vaginalis,  and  Descent  of 
the  Testis. — The  descent  of  the  testis  begins  very  early,  the  change 
in  position  being  evident  by  the  tenth  week,  but  the  passage  into 
the  scrotum  does  not  begin  until  the  seventh  month.  The  testis 


SPECIAL   HISTORIES   OF   THE  UROGENITAL  ORGANS.  497 

makes  three  movements:  1,  backward  to  near  the  inguinal  ring;  :>, 
forward  a  short  distance,  during  the  period  of  the  formation  of  the 
muscular  gubernaculurn;  3,  downward  into  the  processus  vaginalis. 
The  processus  does  not  extend  completely  into  the  scrotum  during 
foetal  life,  hence  the  foetal  scrotum  has  no  cavity  and  never  contains 
the  testis,  but  on  the  contrary  is  filled  by  a  very  vascular  connective 
tissue  like  the  labia  of  the  female.  At  birth  the  processus  lies 
partially  in  the  scrotum. 

The  cause  of  the  descent  of  the  testis  has  been  much  discussed 
and  many  fanciful  explanations  have  been  propounded.  There  is  no 
ivason  for  supposing  that  these  movements  are  in  any  wise  different 
I'mm  the  numerous  other  movements  of  organs  and  changes  of  form 
occurring  during  the  course  of  development.  These  changes  are  all 
due  to  inequalities  of  growth  in  the  tissues,  but  the  causes  of  these 
inequalities  are  not  yet  ascertained.  A  long-prevalent  tendency  has 
tainted  the  study  of  the  generative  organs  with  mysticism,  and  it 
must  be  attributed  to  this  tendency  that  so  many  far-fetched  expla- 
nations of  the  descensus  testiculorum  have  been  published.  The 
changes  in  the  gubernaculum  are  probably  the  immediate  causes  of 
a  part  of  the  changes  in  the  position  of  the  testis;  the  growth  of  the 
gubernacahun  accounts  for  the  forward  movement,  and  its  atrophy 
t'« •!•  the  passage  along  the  wall  of  the  processus  vaginalis;  it  must  be 
added  here  that  the  testis  also  accompanies  the  downgrowth  of  the 
processus,  and  is  not  dragged  down  merely  by  the  shortening  of  the 
gubernaculum.  Some  writers  have  supposed  that  the  muscles  of  the 
gubernaculum  effect  the  descent  by  their  contraction,  but  this  view 
lacks  foundation. 

The  descensus  has  been  carefully  studied  in  the  human  embryo  by 
Bramann,  84.1.  The  details  of  the  process  are  as  follows:  The 
u  r.  genital  fold  is  a  long  structure  reaching  to  the  posterior  or  caudal 
end  of  the  abdomen.  The  greater  part  forms  the  Wolffian  body,  and 
when  this  atrophies  the  fold  is  much  reduced ;  toward  the  head  end 
it  contains  the  testis  and  the  remnant  of  the  Wolffian  body  (epididy- 
mis),  the  portion  of  the  fold  dorsal  of  these  acting  as  a  suspensory 
membrane  to  which  the  name  of  mesorchium  has  been  given,  and 
which  is  comparable  to  the  mesentery;  it  is  quite  thick,  but  finally 
disappears.  The  part  of  the  urogenital  fold  tailward  of  the  testis 
contains  the  Wolffian  duct  (vas  deferens)  and  runs  to  the  point  of 
the  abdomen,  where  the  inguinal  ring  is  developed.  A  portion  of 
this  region  of  the  fold  is  converted  into  the  gubernaculum  Hunteri,  by 
an  ingrowth  of  muscular  fibres  from  the  obliquus  internus  and  obi. 
transversus.  The  mesorchium,  together  with  the  posterior  part  of 
the  fold,  including  the  gubernaculum,  is  the  homologue  in  the  male 
of  the  broad  ligament  of  the  female.  To  complete  the  statement  of 
the  homologies,  it  may  be  added  that  the  gubernaculum  becomes  the 
cremaster,  and  is  said  to  be  the  equivalent  of  the  round  ligament  of 
the  uterus  in  the  other  sex ;  the  latter  identification  needs  confirma- 
tion. 

The  first  change  which  occurs  is  the  nearly  complete  disappear- 
ance of  the  long  piece  of  the  urogenital  fold,  which  lies  tailward  of 
the  testis.  Accordingly  we  find  the  male  gland  at  the  end  of  the 
second  month  has  moved  into  the  immediate  neighborhood  of  the 
32 


498 


THE    FOETUS. 


inguinal  ring,  with  which  it  is  connected  by  the  short  remnant  of 
the  fold,  Fig.  269,  A.  The  vas  deferens  has  a  nearly  horizontal 
transverse  course.  The  second  change  is  the  conversion  of  this  hind 
remnant  of  the  urogenital  fold  into  the  gubernaculum,  a  process 
which  begins  with  the  fourth  and  ends  with  the  sixth  month,  it 
requiring  about  two  months  for  the  gubernaculum  to  attain  its  maxi- 
mum size.  To  form  the  structure  in  question,  the  fold  behind  the 
testis  enlarges  both  longitudinally  and  transversely  until  it  measures 
8-9  mm.  by  3-4  mm. ;  the  testis  moves  forward  meanwhile  a  corre- 
sponding distance.  At  first  the  gubernaculum  consists  only  of  the 


v.u 


FIG.  269  (To  Illustrate  the  Descensus  Testiculorum).— A,  Foetus  of  Fourteen  to  Fifteen 
Weeks,  x  2  diams.  B,  Foetus  of  the  first  half  of  the  seventh  month  with  the  processus  vagi- 
nal is  opened.  Te,  Testis;  epd,  epididymis;  v.d,  vas  deferens;  p.v,  processus  vaginalis;  r,  rec- 
tum; v. u,  bladder;  v.sp,  vasa  spermatica.  After  Bramann. 

peritoneum  and  the  inclosed  connective  tissue,  but  soon  muscular 
fibres  appear  in  its  caudal  portion ;  these  fibres  can  be  traced  to  a 
connection  with  the  obliquus  internus  and  obliquus  transversus; 
they  form  a  sheath  or  mantle  underneath  the  peritoneum  and  around 
a  central  core  of  connective  tissue ;  at  first  they  do  not  reach  to  the 
testis,  but  stop  at  that  point  where  the  gubernaculum  is  crossed  by 
the  vas  deferens.  They  appear  to  extend  farther  forward  later. 
The  fibres  are  not  parallel,  but  quite  irregular  in  their  courses.  At 
the  lower  end  of  the  muscle  a  bundle  of  connective  fibres  extends 
beyond  the  gubernaculum  into  the  side  of  the  processus  (see  below) . 
The  gubernaculum  is  now  completely  differentiated.  This  stage  of 
the  organ  is  permanent  in  some  rodents  and  other  mammals,  low  in 
the  series,  and  must  be  considered  of  great  phylogenetic  significance. 
Attention  is  directed  to  the  fact  that  its  muscular  fibres  are  striped, 
and  to  its  shape  shown  in  the  figure,  because  both  points  accentuate 
the  resemblance  to  the  rodent  cremaster.  While  the  gubernaculum 
is  being  formed  there  appears  at  its  caudal  end  a  little  pouch  made 
'by  an  evagination  of  the  peritoneum  at  the  inguinal  ring.  This  is 
the  anlage  of  the  processus  vaginalis;  it  lies  laterally  and  ventrally 
of  the  end  of  the  gubernaculum ;  it  enlarges  very  slowly  up  to  the 
end  of  the  sixth  month,  but  after  that  more  rapidly.  The  third 
change  is  the  true  descent  of  the  testis ;  the  evagination  of  the  pro- 
cessus includes  not  only  a  considerable  peritoneal  surface,  but  also 
the  gubernaculum,  and  later  the  testis ;  in  other  words  the  urogen- 
ital fold  extends  down  the  processus  and  forms,  indeed,  the  dorso- 
medial  wall  of  the  sac ;  as  the  sac  grows  down,  the  fold  (gubernacu- 


SPECIAL   HISTOIMKS    OF   THE    UROGENITAL   ORGANS.  409 

lum  and  t«-sti>)  grows  with  it,  Fi^.  IW,  B.  In  a  transverse  section 
tin-  lumen  of  the  prcxvssiis  vaginalis  appears  somewhat  crescent-like, 
the  concave  wall  c. ^responding  to  the  protuberance  of  the  urogeuital 
fold,  the  convex  wall  to  the  peritoneal  covering.  The  inesoivhium 
disappears  during  the  descent  into  the  processus.  Early  in  the  sev- 
enth month,  the  testis  is  drawn  into  the  mouth  of  the  sac,  Fig. 
B,  and  shortly  after  lies  wholly  in  the  interior  thereof.  But  the 
t,-Mi-  descends  to  the  bottom  of  the  processus;  this  translation  is 
accomplished  during  the  seventh  and  eighth  mouths,  first  by  a 
s hoi-ten ini;-  of  the  gubernaculum,  second  by  a  slipping  down  of  the 
te>tis  over  the  muscles;  the  portion  of  the  gubernaculum  between 
the  test  is  and  the  base  of  the  processus  is  reduced  to  an  inconspicuous 
baud  of  connective  tissue.  The  muscle  now  lies  between  the  testis 
and  the  base  of  the  penis  and  is  developed  in  that  position  into  the 
(•remaster.  At  last  the  processus  enters  the  scrotum,  and  an  entirely 
new  relation  of  parts  is  established;  the  transportation  of  the  testis 
into  the  scrotum  represents  a  very  advanced  stage,  since  it  takes 
place  only  in  the  higher  mammals,  and  accordingly  we  find  it  to 
occur  very  late  in  the  development  of  man. 

The  Broad  Ligament. — The  broad  ligament  of  human  anat- 
omy is  the  persistent  urogenital  fold,  reduced  to  a  relatively  thin 

pensory  membrane  for  the  ovary 
and  uterus  by  the  abortion  of  the  Parx 
Wolffian  tubules.  The  fundamental 
relations  here  involved  may  be  readily 
understood  from  Fig.  270,  which  rep- 
r.-ents  a  section  through  the  urogen- 
ital  fold  of  a  human  embryo  of  the 
third  month;  the  fold  is  suspended 
from  the  dorsal  wall  of  the  abdomen; 
the  \Volffian  body  is  considerably 

abort.-.!  and  divided  into  two  parts,  ^  -*„.  ^'-section  „,  the  Ovary 
one  Of  WhlCh,  near  the  baseot  the  told,  ami  \Volfflan  Body  of  a  Human  Embryo 

is  the  ;mla-o  of  the  parovarium,  Par,   ^^^^^Z\^S^SKi 
the, other,   near    Miiller's  duct,  Md,  £^fian  tube;  Oy'  ovary-    Afu>r  u 
is  the  anlage  of  the  epoophoron,  Epo. 

The  ovary,  Or,  projects  from  what  was  originally  the  medial  side  of 
the  Wolftian  body,  with  which  it  is  connected  by  a  thin  mesovarium. 
A<  in  the  adult  the  broad  ligament  contains  the  parovarium  and 
t'poophoron,  it  is  evident  that  it  is  really  the  Wolffian  body,  con- 
verted into  a  suspensory  membrane,  most  of  the  Wolffian  tubules 
beiii.u'  aborted. 

The  development  of  the  broad  ligament  is  accompanied  by  a  change 
of  jxxsition,  first  of  the  whole  genital  fold,  second  of  that  part  of  the 
fold  which  forms  the  ligament.  It  will  be  remembered  that  the 
two  folds  unite  in  part  to  form  the  genital  cord,  out  of  which  the 
uterus  and  vagina  are  developed ;  the  remainder  of  each  urogenital 
rid.u-e  is  transformed  in  the  female  into  the  broad  ligament  and  ovary. 
As  the  foetus  grows,  the  urogenital  ridge  fails  to  grow  proportion- 
ately, and  after  the  second  month  becomes  more  and  more  restricted 
to  the  caudal  or  pelvic  end  of  the  abdomen.  Its  relative  position  is 
s. .  rapidly  shifted  that  by  the  end  of  the  third  month  it  already  occu- 


500  THE   FOETUS. 

pies  its  permanent  situation.  While  this  modification  is  being- 
established  the  Wolffian  body  in  large  part  aborts,  and  the  portion  of 
the  fold  in  front  of  the  genital  cords  comes  to  occupy  an  oblique  and 
finally  a  nearly  transverse  position,  which  is  permanently  retained, 
so  that  the  broad  ligament  is  always  obliquely  transverse.  At  three 
months  the  ovary  is  as  long  (3  mm.)  as  the  Fallopian  tube,  and 
stretches  in  an  obliquely  transverse  direction  from  the  upper  or 
cephalic  end  of  the  genital  cord  (future  uterus)  to  the  Miillerian 
funnel  or  fimbriate  opening  of  the  Fallopian  tube.  By  the  fourth 
month  the  transverse  position  is  more  marked,  and  since  the  ovary  is 
originally  on  the  medial  side  of  the  urogenital  ridge,  it  remains  on 
that  side,  and  accordingly  is  situated  on  what  is  known  in  human 
anatomy  as  the  dorsal  side  of  the  ligament. 

The  development  of  the  round  ligament  and  of  the  ligament  of  the 
ovary  have  been  but  little  studied  by  modern  methods ;  Mihalkovics, 
85. 1,  418,  and  G.  Wieger,  85. 1,  have  shown  that  they  are  parts  of 
the  same  cord  of  tissues,  and  that  by  the  assumption  by  the  ovary 
of  its  transverse  position  this  cord  of  tissue  is  subdivided  into  the 
two  ligaments  by  becoming  bent  at  the  summit  of  the  uterus.  The 
primitive  ligament  is  usually  regarded  as  the  homologue  of  the 
gubernaculum  of  the  male. 

Epididymis  and  Epoophoron. — It  is  desirable  to  treat  this 
organ,  which  is  known  under  different  names  in  the  two  sexes,  as  a 
single  organ,  not,  as  is  often  done,  as  a  distinct  organ  in  each  sex. 
In  both  sexes  there  is  a  small  number  of  permanently  preserved  and 
considerably  modified  Wolffian  tubules  from  the  anterior  part  of  the 
urogenital  ridge,  which  remain  permanently  connected  with  the  ceph- 
alic or  upper  end  of  Wolffian  duct.  The  organ  thus  formed  becomes 
in  both  sexes  very  closely  associated  with,  indeed  we  might  better 
say  incorporated  in,  the  sexual  gland.  In  the  female  the  organ 
is  rudimentary  and  has  been  variously  named ;  as  it  was  first  accu- 
rately described  by  Rosenmuller,  02.1,  it  has  been  widely  known 
as  the  "organ  of  Rosenmuller;"  Kobelt,  who  demonstrated,  47.1, 
that  it  was  a  remnant  of  the  primitive  kidney,  introduced  the  term 
"parovarium."  Waldeyer  has  proposed,  70.1,  142,  "epoophoron" 
to  be  comparable  with  the  epididymis,  with  which  he  recognized 
the  parovarium  to  be  homologous.  In  the  male  the  organ  has  great 
functional  importance,  for  its  tubules  serve  to  convey  the  sperma- 
tozoa from  the  seminiferous  tubules  to  the  Wolffian  duct,  and 
accordingly  it  is  in  the  male  that  the  full  development  of  the  organ 
is  attained. 

A.  EPIDIDYMIS. — In  the  male  human  embryo  of  the  third  month 
there  are  found  from  ten  to  twenty  tubules  in  the  anterior  part  of 
the  Wolffian  body,  which  have  become  connected  with  sexual  cords 
or  future  seminiferous  tubules  of  the  testis,  and  have  retained  also 
their  connection  with  the  Wolffian  duct.  These  tubules  constitute 
the  epididymis,  and  the  portion  of  the  Wolffian  duct  which  follows 
immediately  below  them,  by  becoming  very  much  convoluted  gives 
rise  to  the  so-called  head  of  the  epididymis.  At  three  months  (Kol- 
liker,  "Grundriss,"  2te  Aufl.,  426)  traces  of  glomeruli  can  be  still 
found  in  the  primitive  kidney,  and  the  epithelial  tubules  anastomose 
with  one  another  in  the  region  between  the  Wolffian  body  proper 


SPECIAL   HISTORIES    OF   THE    UROGEXITAL   ORGANS.  501 

and  the  testis  proper.  These  anastomoses  constitute  the  rete  Hallt-ri. 
while  the  \Voltlian  tubules  become  the  vaxa  efferent  in  of  the  adult. 
According  to  Kolliker,  /.<•.,  the  vasa  become  convoluted  during  the 
fourth  and  fifth  month,  and  thereby  develop  the  so-called  coni 
vasculosi. 

The  early  development  of  the  epididymis  is  known  chiefly  through 
Bramfs  observations,  77.4,  14t»,  on  reptile  embryos.  Solid  out- 
growths appear  early  from  the  walls  of  the  Malpighian  corpuscles  of 
the  \Voltfiaii  body,  and  these  penetrate  toward  the  testis  as  cords, 
which  subsequently  acquire  a  lumen.  The  primitive  connection  is 
between  the  tubules  of  the  testis  and  the  mesonephric  glomeruli — 
a  disposition  which  is  permanent  in  some  of  the  amphibia  (see  J.  W. 
Spengel),  but  in  all  amniota  the  glomeruli  disappear.  C.  K.  Hof- 
inanii  (Bronn's  "  Thierreich,"  VI.,  III.  Abth.,  p.  2002)  asserts,  in  op- 
jH.sitinii  to  Braun,  that  the  glomeruli  persist  in  Lacerta  agilis  at 
least « >ne  year  after  hatching.  In  mammals,  Mihalkovics,  85. 1,  1 1  \', 
found  the  outgrowths  from  the  glomeruli  in  cat,  dog,  and  rabbit  em- 
bryos of  5-6  cm.,  but  the  Malpighian  corpuscles  disappear  early 
during  embryonic  life. 

!!.  Ki'oni'iioKoN  (or  organ  of  Rosenmiiller.) — Beyond  tracing 
out  the  general  hi>t<>ry  far  enough  to  establish  the  homology  with 
the  epididymis  (Waldeyer,  70. 1,  142),  little  has  been  done  to  eluci- 
date the  development  of  the  organ  in  the  embryo.  It  has  been 
already  pointed  out  that  the  medullary  cords  of  the  ovary  are  pre- 
sumably parts  of  the  epoophoron.  The  epoophoron  is  formed  from 
perhaps  ten  to  fifteen  Wolfnan  tubules,  and  the  outgrowths  from  the 
Malpighian  corpuscles  remain,  in  part  at  least,  solid  cellular  cords; 
tiie  .Malpighian  corpuscles  of  the  organ  disappear  very  early  in  the 
human  embryo  (?  third  month).  F.  Tourneux,  88.3,  has  described 
the  epoophoron  in  various  mammals  and  in  the  human  species  at 
birth  and  in  the  adult,  and  has  shown  that  its  structure  entirely  con- 
firms its  homology  with  the  epididymis. 

Paradidymis  and  Paroophoron. — By  these  names  is  desig- 
nated, in  males  and  females  respectively,  the  organ  constituted  by 
the  persistent  tubules  of  the  posterior  part  of  the  Wolffian  body. 
The  organ  was  first  described  in  the  male  by  Giraldes,  61.1,  under 
the  name  of  the  "  corps  innomine, "  and  was  first  described  in  the 
female  by  Waldeyer,  70. 1,  142.  The  persistent  rudimentary  meso- 
nephros  of  the  human  embryo  has  a  yellowish  color ;  the  tubules  are 
wide,  their  cells  pale  with  indistinct  nuclei,  and  have  not  only  no 
connection  with  the  sexual  gland,  but  have  lost  their  original  con- 
nection with  the  Wolffian  duct.  The  position  of  the  organ  in  the 
male  and  female  human  embryo  of  about  three  months  has  been 
figured  by  Waldeyer.  The  interesting  post-foetal  changes  have  been 
made  the  subject  of  an  excellent  paper  by  Czerny,  89. 1. 

Genital  Cord. — That  the  posterior  (lower  or  caudal)  ends  of  the 
two  urogenital  ridges  unite  into  a  single  median  mass,  the  genital 
cord,  has  been  pointed  out  above,  p.  491.  The  genital  cord  is  a 
structure  peculiarly  characteristic  of  the  placental  mammalia,  being 
found  only  in  them  and  in  certain  marsupials.  It  does  not  occur  in 
monotremes  or  Sauropsida.  The  genital  cord  and  its  significance 
were  first  recognized  by  Thiersch,  52.1.  The  fullest  history  of  the 


502 


THE   FCETUS. 


cord  yet  published  is  that  given  by  Mihalkovics,   85.1,   324-347, 
upon  which  this  section  is  based. 

The  pelvic  portions  of  the  two  urogenital  ridges  unite  so  as  to 
form  a  transverse  partition  (rabbit  embryo  of  about  14  mm.,  pig 
embryo  of  about  30  mm.).  This  partition  is  the  genital  cord  (Geni- 
talstrang)  of  Thiersch.  It  stretches  across  between  the  rectum, 
which  is  on  the  dorsal  side,  and  the  allantois  on  the  ventral  side, 
compare  Fig.  271 ;  it  is  thick  and  the  mesenchyma  of  which  it  is 
chiefly  composed  is  a  dense  tissue.  At  the  time  the  two  ridges 
unite,  their  pelvic  ends  contain  only  the  Wolffian  ducts,  hence  the 
Miillerian  ducts,  as  they  develop,  grow  into  the  already  formed  gen- 
ital cord,  and  in  the  female  (human  embryo  of  3  cm.)  begin  to  unite 
almost  immediately  after  they  appear  in  the  cord.  The  formation 
and  position  of  the  partition  is  well  illustrated  by  Mihalkovics, 
85.1,  Figs.  114  and  115.  After  the  genital  cord  is  once  formed,  it 
is  drawn  more  and  more  into  the  pelvis,  and  as  the  coelom  extends 
farther  into  the  pelvis  on  the  dorsal  than  on  the  ventral  side  of  the 


COP 


All  - 


FIG.  271.— Cross  Section  of  the  Rectum,  Genital  Cord,  and  Allantois  of  a  Male  Human  Embryo 
of  about  two  Months.  R,  Rectum ;  Coe,  coelom ;  <rc,  genital  cord,  with  the  two  Wolffian  ducts, 
and  the  median  united  Mullerian  ducts  between  them;  All,  allantois. 

cord,  we  obtain  in  sections  the  picture  reproducd  in  Fig.  271,  which 
shows  the  typical  relations  of  the  genital  cord  in  the  indifferent 
stage ;  the  cord  consists  chiefly  of  a  very  dense  mesenchyma,  and  is 
quite  sharply  bounded,  except  against  the  allantois,  and  it  contains 
three  longitudinal  epithelial  tubes,  of  which  the  median  represents  the 
united  Mullerian  ducts,  the  two  lateral  the  Wolffian  ducts,  W.d. 

Wolfiian  Duct. — The  cephalic  end  of  the  duct  remains,  as  we  have 
seen,  in  connection  with  the  anterior  Wolffian  tubules  as  the  duct  of 
the  epididymis  of  the  male,  of  the  epoophoron  of  the  female.  In  the 
male  it  also  forms  the  adult  spermiduct  and  the  vesicula3  seminales, 


BPBCIAL    HIST<»IMKS    OF   THE   UROGENITAL   ORGANS.  ;>u:> 

in  the  female  the  rudimentary  duct  of  Gartner.  It  is  important  to 
nete  that  in  both  sexes  the  Wolffian  duct  contributes  t<>  tin-  formation 
of  the  utero-vai;-inal  canal  (fused  Miillerian  ducts)  according  to  the 
observations  of  Mihalkovics,  85.1,  and  Tourneux,  87.2,  upon  t he- 
male,  and  of  Van  Ackeren,  89.1,  upon  the  female. 

A.  Si'KiiMihUCT   OP  THE  MALE. — Since  the  demonstration,    l»y 
Johannes  Miiller,  that  the  Wolth'an  duct  becomes  the  sperm  id  net, 
little  has  been  done  upon,  the  history  of  the  male  canal.     Thiei^ch  in 
1  >."»•.'  dre\v  attention  to  the  union  of  the  caudal  parts  of  theurogenital 
fold  a>  the  genital  cord,  see  above.     This  cordexi>ts  temporarily  in 
the  embryo  of  man,   ami  while  it  lasts  the  two  spermiducts  run 
through  it,  together  with  the  two  Miillerian  ducts,  which   partially 
abort  later.     This  stage  is  described  and  figured  by  Koliiker  (**  Ent- 
wickelungsgeschichte,"  1879,  p.  985  and  Fig.  598).     Later  the  geni- 
tal oord  divides,  and  its  dense  tissue  forms  a  thick  wall  around  each 
epithelial  Wolffian  duct. 

The  veftirn/n  xcmhialis  arises  as  a  lateral  evagination  of  the 
WoUfian  duct  At  five  months  the  evagination  is  a  simple  sac  about 
1  nun.  IOIILC,  and  is  situated  entirely  within  the  genital  cord.  The 

;ination  passes  at  first  out  horizontally,  and  then  bends  upward 

lialkovics,  85.1,  :j;y). 

B.  (i  AKTNKK'S  CANALS  OP  THE  FEMALE,   so  named  after  their 
discoverer,  are  epithelial  tubes  which  are  sometimes  found  in  the 
walls  of  the  uterus,  and  even  of  the  vagina,  one  on  each  side.     Their 

nificanoe  is  >aid  to  have  been  first  recognized  by  Jacobson  in  1830 
and  to  have  been  clearly  demonstrated  by  Kobelt  in  1847.  It  is 
known  that  the-  Wolffian  duets  always  run  through  the  genital 
cord,  Fiu.  871,  and  can  be  usually  seen  in  cross  sections  of  the 
uterine  portion  of  the  genital  cord  of  the  female  human  embryo  of 
four  to  five  months,  and  occasionally  in  older  specimens,  and  even 
in  the  adult.  On  the  disappearance  of  the  Wolffian  duct  see  Van 
Ackeren,  89. 1,  :>4.  In  the  foetus  the  Wolffian  ducts  open  into  the 
vagina  during  the  fourth  month;  their  ends  dilate  and  the  dilated 
cavities  fuse  with  the  lumen  of  the  vagina. 

Miillerian  Duct. — The  history  of  this  duct  is  the  reverse  of  that 
of  the  Wolffian  duct,  since  it  becomes  rudimentary  in  the  male,  and 
the  functional  sexual  duct  in  the  female. 

A.  MALE. — HYDATID  OP  MORGAGNI  AND  UTERUS  MASCULINUS. 
—In  the  male  the  middle  part  of  the  Miillerian  duct  usually  aborts, 
leaving  the  upper  part  with  its  open  funnel  close  to  the  testis  and 
the  lower  part  within  the  genital  cord.  The  upper  part  gives  rise 
to  the  Injdutid  of  Morgagni  as  maintained  by  Kobelt,  47.1, 
and  later  by  Waldeyer,  70.1,  127.  This  explanation  of  the  origin 
of  the  hydatid  has  by  no  means  been  put  beyond  question  by  strict 
observations — but  we  need  no  additional  evidence  to  set  aside  the 
notion  of  Fleischl  and  Krause  that  the  hydatid  is  the  homologue  of 
the  ovary  (!).  The  lower  part  of  the  Miillerian  duct  is  contained 
within  the  genital  cord,  where  it  unites  with  its  fellow  to  form  a 
single  median  canal  between  the  two  Wolffian  ducts — compare  Kol- 
iiker, "Entwickelungsgeschichte,"  1879,  Fig.  598,  m.  This  canal 
corresponds  to  the  cavity  of  the  uterus  and  vagina  in  the  female. 
It  varies  greatly  in  its  degree  of  development  in  individuals,  but 


504  THE    PCETUS. 

usually  persists  in  the  adult  as  a  small  sac,  known  jas  the  uterus 
masculinus  or  vesicula  prostatica.  According  to  E.  H.  Weber, 
the  vesicula,  which  is  rudimentary  in  man,  is  well  developed  in 
various  mammals.  For  references  to  the  literature  of  the  subject, 
and  an  account  of  the  organ  in  the  rabbit,  see  V.  von  Mihalkovics, 
85.1,  364-378.  The  Wolffian  duct  contributes  to  the  formation  of 
the  uterus  masculinus,  as  it  does  to  the  formation  of  the  vagina  in 
the  female,  see  p.  506. 

B.  FEMALE.  —  FALLOPIAN  TUBE,  UTERUS,  AND  VAGINA.  —  These 
are  developed  from  the  Miillerian  ducts,  but  it  is  to  be  remembered 
that,  strictly  speaking,  the  epithelial  Miillerian  ducts  produce  only 
the  epithelial  lining  of  the  adult  tuba,  uterus,  and  vagina,  arid  that 
the  connective  tissue,  which  forms  the  thickest  part  of  the  walls  in 
the  adult,  is  developed  from  the  mesenchyma  of  the  urogenital  fold. 
Fallopian  Tube.  —  The  fullest  account  is  that  given  by  Mihalko- 
vics, 85.1,  304-306.  The  tube  is  developed  from  that  part  of  the 
Miillerian  duct  which  runs  along  the  Wolffian  body  and  is  not  included 
in  the  genital  cord.  The  epithelium  becomes  much  thinner  except 
in  the  funnel,  where  it  retains  its  cylindrical  character.  Later  — 
in  chicks  about  the  eighth  to  tenth  day  —  the  mesenchyma  begins  to 
condense  around  the  duct,  thus  initiating  the  development  of  the  con- 

nective-tissue 
coats  of  the 
tube  ;  shortly 

.  -  _  __  after  the  mes- 
enchyma wall 
begins  to  de- 

TT'.a.  i          Ji        TIT-I 

velop  the  Mul- 

lerian  funnel  becomes  larger,  and 
F.  T.     its  surface  thrown  into  folds  —  the 

/  anlages  of  the  fimbria3.     As  the 

Wolffian     body     atrophies     and 

I  changes  into  the  transverse  broad 

_     .      „  _,     _  _.         ,    .        ligament,  the  Fallopian  tube  ap- 

FIG.  272.—  Section  of  Broad  Ligament  of  a 

Female  Human  Embryo  of  four  Months,      pears  more  and  more  at  the  edge 


Minot  Collection,  No.  35.     To  show  the  Fal-  f  i/u      nT-oo-am'+al  f<\\r\ 

lopian  tube,  F.T*  and  Wolfflan  duct,  W.d.  me  urogenitai  lOlCt, 

its  primitive  longitudinal   course 

to  a  transverse  one  —  the  primitive  course  being  retained  until  the  end 
of  the  third  month.  After  the  third  month  the  tube  elongates  faster 
than  the  broad  ligament  and  consequently  assumes  a  sinuous  course. 
By  the  sixth  or  seventh  month,  the  definite  transverse  position  is 
attained.  By  the  fourth  month,  Fig.  272,  the  folds  at  the  ovarial  end 
of  the  tube,  F.  T,  are  well  developed,  but  the  thick  dense  mesenchy- 
mal  coat  is  not  yet  divided  into  muscular  and  adventitial  layers  ;  at 
this  time  the  small  Wolffian  duct,  W.d,  still  persists,  though  later 
it  usually  disappears. 

Uterus  and  Vagina.  —  As  stated  above,  p.  502,  the  genital  cord 
contains  four  ducts,  compare  Fig.  271  :  the  two  laterally  placed 
Wolffian  ducts,  and  the  two  Miillerian  ducts,  which  lie  nearer  the 
median  line  and  more  dorsally.  In  man  the  genital  cord  is  the  an- 
lage  of  both  the  uterus  and  the  vagina  ;  within  the  cord  the  two 
Miillerian  ducts  unite  in  the  median  line,  forming  a  single  canal  ; 


SPECIAL   HISTORIES   OF   THE   UROGENITAL   ORGANS.  505 

tho  rephalad  portion  of  this  canal  becomes  dilated  into  the  uter- 
ine cavity,  and  its  epithelium  becomes  the  lining  of  the  uteru>: 
the  caudad  portion  develops  into  the  vagina;  the  mesodermic  tissue 
<>f  the  cord  is  converted  into  the  muscular  and  connective  tissue 
layers  of  the  adult  passages;  finalty  the  Wolffian  ducts  atrophy, 
ususally  completely,  but  they  sometimes  persist  to  a  greater  or  less 
extent  as  rudiments,  known  as  Gartner's  canals,  which  lie  on  one  or 
both  sides  in  the  walls  of  the  uterus. 

Our  knowledge  of  the  development  of  the  uterus  and  vagina  is 
based  upon  numerous  investigations.  The  fusion  of  the  Miillerian 
ducts  was  known  to  Johannes  Muller,  30.1,  in  1830,  but  he  failed 
to  ascertain  that  the  fusion  produced  not  only  the  uterus,  but  also 
the  vagina;  the  latter  was  regarded  for  some  time  as  a  derivative  of 
the  urogenital  sinus.  This  error  was  corrected  by  Bischoff  in  his 
manual  of  "Embryology."  Important  advances  were  made  by 
Thiersch,  52.1,  and  by  Leuckart  in  his  important  article  "  Zeugung," 
in  Wagner's  "  Handworterbuch,"  1853.  We  pass  to  the  modern 
period  of  investigations  based  chiefly  on  the  microscopic  study  of 
sections.  L.  Fiirst  described  in  18G7  the  fusion  of  the  Miillerian 
ducts  very  accurately.  H.  Dohrn's  researches  ("  Schriften  Ges.  Nat .- 
arias.,"  Marburg,  1809,  No.  3,  also  Bd.  IX.,  1871,  p.  255)  confirmed 
Fiirst's  observations  and  did  much  to  elucidate  the  history  of  the 
uterus.  Finally  may  be  mentioned  the  very  admirable  monographs 
of  Tourneux  and  Legay,  84.1,  of  Mihalkovics,  85.1,  332  and  347, 
and  W.  Nagel,  91.2,  which  have  been  my  chief  guides  in  the  pre- 
paration of  the  following  account,  and  to  which  I  refer  the  reader  for 
a  fuller  index  of  the  literature.  Interesting  additional  details  have 
been  recorded  by  Van  Ackeren,  89.1. 

The  genital  cord  extends  by  the  fourth  month  from  the  insertion 
of  the  Hunterian  or  round  ligaments  to  the  urogenital  sinus.  The 
Midler's  ducts  fuse  in  the  median  line  between  these  two  points,  ex- 
cept at  the  upper  end ;  that  is  to  say,  the  ducts  diverge,  after  the 
complete  fusion,  a  little  below  the  round  ligaments  and  these  diver- 
-••nt  portions  become  the  horns  of  the  uterus.  The  fusion  commences 
at  the  end  of  the  eighth  week  about  two-thirds  of  the  way  down 
troin  the  cephalad  end  of  the  cord  to  the  urogenital  sinus,  and  pro- 
gresses from  that  point  both  upward  and  downward,  but  the  upper 
two-thirds  are  united  before  the  lower  extremities.  The  process  is 
completed  according  to  Fiirst  by  the  end  of  the  third  month.  In  the 
pig,  mouse,  and  rabbit,  the  fusion  commences  at  the  same  relative 
point,  but  in  the  sheep  it  begins  higher  up.  The  single  canal  thus 
produced  is  known  as  the  genital  canal,  or  better  as  the  utero- 
r< i< i null  canal.  A  failure  of  the  lower  ends  to  fuse  leaves  two 
openings  (double  or  bi perforate  hymen). 

W.  Nagel,  91.1,  has  pointed  out  that  the  genital  cord  becomes 
bent  very  early  in  the  human  embryo,  so  as  to  divide  the  cord  into 
an  upper  or  uterine  limb,  which  is  inclined  ventralward  over  the 
bladder,  and  lower  or  vaginal  limb,  which  runs  longitudinally  be- 
tween the  bladder  and  rectum.  At  the  end  of  the  third  month,  the 
simple  epithelium  lining  the  cavity  of  the  canal  changes  its  charac- 
ter in  its  lower  third,  becoming  there  a  stratified  pavement  epithe- 
lium, which  passes  over  very  gradually  into  the  cylindrical  epithelium 


506  THE   FCETUS. 

of  the  upper  portion.  The  change  progresses  upward,  and  as  it  ad- 
vances, the  demarcation  between  the  two  kinds  of  epithelium  becomes 
sharper.  By  the  eighth  month  the  passage  is  abrupt  and  occurs  at 
the  middle  of  the  canal.  The  stratified  epithelium  lines  the  vaginal 
limb,  which  occupies  half  the  genital  cord  at  birth.  After  birth  the 
uterine  limb  enlarges  more  rapidly  than  the  vaginal. 

VAGINA. — During  the  fourth  month  the  vaginal  limb  expands  lat- 
erally and  becomes  flattened  dorso-ventrally.  Its  two  epithelial 
surfaces  meet  and  grow  together,  closing  the  lumen  of  the  vagina 
and  forming  an  epithelial  lamina,  the  cells  of  which  now  commence 
a  rapid  proliferation  which  thickens  the  vagina  and  forces  down  its 
lower  end,  thus  forming  the  hymen  because  the  actual  diameter  of 
the  vagina,  where  it  is  connected  with  the  sinus,  does  not  share  in 
the  general  dilatation.  The  epithelial  plate  of  the  vagina  has  two 
features  requiring  special  mention :  1 .  A  short  distance  above  the 
sinus  it  is  T-shaped  in  transverse  section ;  the  two  side  portions  are 
probably  remnants  of  the  Wolffian  ducts  which  unite  with  the  vagina 
at  this  point.  In  this  connection  it  is  significant  to  observe  that  in 
the  cow  the  persistent  ducts  of  Gartner  (Wolff)  open  into  the  vagina ; 
the  question  arises  whether  this  connection  is  not  general  in  the 
Placentalia.  2.  At  its  upper  end  the  lamina  forms  a  cup-shaped 
outgrowth,  which  embraces  the  lower  end  of  the  uterus.  Every- 
where between  the  two  points  thus  specialized  the  lamina  is  crescen- 
tic  in  section,  the  concavity  facing  the  back.  The  anlages  of  the  rugae 
of  the  vagina  appear  during  the  end  of  the  fourth  month  as  budding 
ridges  on  the  outside  of  the  lamina.  Finally  the  permanent  lumen 
of  the  vagina  begins  to  appear  during  the  sixth  month  and  is  formed 
by  the  breaking  down  of  the  central  cells  of  the  lamina.  This  pro- 
cess penetrates  the  cup-shaped  outgrowth  just  described,  so  that  the 
lower  end  of  the  uterus  protrudes  into  the  vagina,  in  consequence, 
be  it  remarked,  of  the  vagina  growing  up  around  the  extremity 
of  the  uterus.  The  stratified  epithelium  often  extends  a  short  dis- 
tance inside  the  os  uteri. 

UTERUS. — The  cavity  of  the  uterine  limb  is  always  open,  and  its 
epithelium  composed  of  a  single  layer  of  cells,  which  diminish  in 
height  from  50/i  (third  month)  to  25,u  (eighth  month).  A  short 
time  before  birth  the  epithelium  of  the  cervix  develops  into  beaker 
cells.  This  transformation  has  been  well  described  by  Moricke, 
82. 1.  The  cells  increase  in  length  and  the  nuclei  move  toward  the 
base  of  the  layer;  the  upper  portion  becomes  clear  and  no  longer 
stain  with  picrocarmine  owing  to  the  formation  of  mucus.  These 
cells  secrete  the  mucous  plug  which  fills  the  cervix  at  birth.  As  far 
as  ascertained  there  are  no  cilia  in  the  foetal  uterus.  The  develop- 
ment of  the  arbor  vitse  of  the  uterus  commences  at  the  end  of  the 
fourth  month  with  the  appearance  of  the  main  stems  (rachis),  which 
extend  from  a  little  above  the  future  os  nearly  to  the  fundus.  Their 
disposition  is  asymmetrical,  the  two  stems  of  the  posterior  wall  lying 
to  the  left,  of  the  anterior  wall  to  the  right ;  hence  the  cavity  of  the 
uterus  is  somewhat  S-shaped  in  section.  The  arbor  vitse  is  merely 
a  set  of  folds  of  the  uterine  mucosa. 

The  mesoderm  of  the  genital  cord  differentiates  very  slowly.  The 
first  noticeable  change  is  the  increased  vascularity  of  the  part  next 


SPECIAL    HISTORIES   OF  THE   UROGENITAL  OR(.\N>> 


507 


tin1  epithelium;  this  vascular  layer  becomes  the  niurosa,  and  the 
tissue  outside  it  the  inuseularis.  The  latter  does  not  become  distinct 
bistologioally  until  the  close  of  the  fifth  month.  The  muscular  fibres 
are  very  irregularly  disposed;  however,  the  trend  of  the  inner  ones 
is  circular,  of  the  outer  longitudinal. 

The  glands  of  the  uterus  and  vagina  do  not  appear  until  after 
Mirth,  except  in  the  cervix  uteri,  the  glands  of  which  arise  the 
middle  of  the  fifth  month  (Van  Ackeren,  89.1).  Cadiat,  84.1, 
maintains  that  those  of  the  corpus  uterus  arise  during  foetal  life. 
This  opinion  I  consider  erroneous;  has  not  Cadiat  mistaken  folds 
of  the  arbor  vitee  for  the  anlages  of  glands? 

The  following  table  indicates  the  growth  of  the  uterus  and  vagina : 


Fo-tus     from    VIT- 
t.-x  ; 

Supposed  Age. 

Canal,  Length. 

Vagina,  Length. 

rt'-nis. 

in 

~t    nun 

2.2  mm 

nun 

It  0 

»;  :> 

3.5 

13.0 

1.5 

c,  B 

21. 

10. 

11. 

80, 

29. 

16. 

i;;. 

Child 

Kij^lit  <!;!  \  - 

35. 

10. 

•j:, 

child  

l-<Mir  months. 

N 

30. 

M 

Child 

Tlnve  years 

40 

•J.') 

A  few  words  must  be  added  concerning  the  comparative  morphol- 
ogy of  the  uterus.  The  round  or  Hunter's  ligaments  mark  in  all 
mammals  the  division  between  the  Fallopian  and  the  uterine  por- 
tion of  Miiller's  ducts.  In  man  the  whole  of  the  uterine  division  is 
included  in  the  genital  cord  and  participates  in  the  formation  of  the 
single  median  uterus. 

1 1  N  MKN. — The  hymen  is  said  to  be  the  homologue  of  the  verum 
montanum  of  the  male  urethra.  It  appears  about  the  beginning  of 
the  fifth  month  as  a  transverse  ridge  situated  on  the  ventral  side  of 
the  vestibular  end  of  the  vagina,  and  projecting  into  the  urogenital 
sinus  (vestibulum).  At  this  time  the  vagina  begins  its  dilatation, 
and  as  it  widens  it  appears  to  force  down  the  hymen,  which  is  thereby 
rendered  more  protuberant.  The  hymen  is  a  thin  non -muscular  fold 
covered  on  one  surface  by  the  epithelium  of  the  sinus,  and  on  the 
other  by  the  epithelium  of  the  vagina,  the  latter  being  much  thicker 
than  the  former.  The  hymen  grows  rapidly  after  its  first  appear- 
ance. \Vhen,  as  may  happen  by  an  arrest  of  development,  the 
lo\ver  ends  of  the  Miillerian  ducts  do  not  fuse,  the  hymen  presents 
t\vo  orifices  leading  into  a  single  vagina  (H.  Dohrn,  75.1;  Tour- 
neux  and  Legay,  84.1,  345).  The  development  of  the  hymen  has 
been  studied  by  H.  Dohrn,  75.1,  78.1,.Tourneux  et  Legay,  84.1, 
Mihalkovics,  85.1,  :J-l'.i,  and  Van  Ackeren,  89.1,  30. 

Development  of  the  Kidney.* — The  true  or  permanent  ma- 
niote  kidney  has  no  homologue,  so  far  as  known,  in  the  arnniota^ 
the  so-called  kidneys  of  the  latter  being  Wolffian  bodies  (mesonephri). 
We  are  entirely  unable  at  present  to  trace  the  probable  evolution  of 
the  kidney,  for  the  view  advanced  by  Semper,  75.2,  that  it  is  a  modi- 
fication of  the  hind  end  of  the  Wolffian  body  is  negatived  by  the 

*For  furthfi-.l  '     I  H;i:nl>iuvr.  «JO.  1. 


508 


THE   FCETUS. 


mes 


embryonic  development  of  the  kidney.  Nor  do  we  possess  any  light 
as  to  the  factors  by  which  the  development  of  the  kidneys  is  initiated 
in  embryos.  In  short,  we  are  compelled  to  confine  ourselves  to  a 
bare  narration  of  the  actual  development,  as  known  at  present. 

THE  RENAL  ANLAGE. — The  renal  anlage  consists  of  three  parts, 
the  epithelial  evagination  of  the  Wolffian  duct,  the  condensed  mesen- 
chyma, and  Braun's  cords,  which  appear  in  the  order  named. 

The  epithelial  evagination  appears  on  the  dorsal  side  of  the 
Wolffian  duct  near  the  opening  of  the  duct  into  the  intestinal  canal 
(cloaca) .  The  evagination  appears  in  the  chick  at  the  end  of  the  fourth 
day,  in  crocodiles  of  12  mm.,  inLacerta  agilis  about  eight  days  after 
the  eggs  are  laid,  in  the  rabbit  the  eleventh  da}',  in  sheep  embryos 
of  8  mm.,  in  human  embryos  of  5  mm.  The  evagination  rapidly 
changes  in  character :  first,  by  elongating  forward  and  by  the  en- 
largement of  its  cephalic  end,  Fig.  444,  N ;  second,  by  acquiring  (in 
the  chick  by  the  sixth  day)  a  direct  opening  into  the  urogenital 
sinus  or  hind  end  of  the  intestinal  canal.  The  enlarged  blind  end  is 
the  anlage  of  the  epithelial  portions  of  the  kidnay,  that  is  to  say,  of 
the  lining  of  the  pelvis  and  of  the  renal  tubules ;  the  remainder  of 
the  evagination  becomes  a  long  narrow  tube,  which  may  be  at  once 
designated  as  the  ureter,  although  it  corresponds,  of  course,  only  to 
the  epithelial  lining  of  the  adult  ureter.  The  way  in  which  the 
evagination  grows  is  well  illustrated  in  Fig.  444,  B,D,C. 

The  blind  end  of  the  renal  evagination  grows  forward  on  the  dor- 
sal side  of  the  Wolffian  body  and 
continues  this  growth  while  it  is 
developing  into  the  kidney,  so  that 
the  more  advanced  the  kidney  in 
its  differentiation,  the  more  .of  the 
Wolffian  body  is  covered  dorsally 
by  it.  The  mesenchyma  around 
the  blind  end  very  soon  becomes 
condensed,  but  the  condensation, 
at  least  in  crocodiles,  occurs  chiefly 
on  the  medial  side  of  the  renal 
tube.  The  relations  just  described 
are  well  illustrated  in  Fig.  2715. 
The  condensed  mesenchyma  can 
be  followed  some  distance  along 
FIG.  273. -cross  Section  through'  the  Hind  the  UTe^Qr  and  there  gradually  be- 

End  of  the  Left  Wolffian  Body  of  a  Crocodile    COmeS    lOOSer,     and     its    Concentric 

mJeSteryjf  i?i,°iSestinafSnai;^?'t? wSm^n  arrangement  disappears  (Wieders- 

tubules;  W.d,  Wolffian  duct;  msth,  mesothe-  V,pim     Qf)  3     4-iP^         Thp  rvrimiHvp 

lium;    car,  cardinal   vein;   fcf,  evagination  to  l       m'   yu-°?  4 

f orm  the  kidney ;  mes,  condensed  mesenchyma.  anlage  of  the  kidnev,  therefore, 
After  Wiedersheim.  ,  v  j  •  -,  ,  -,  -,  £ 

comprises   the   dilated   end  of  an 

epithelial  tube  and  condensed  mesenchyma.  It  is  convenient  to 
consider  the  history  of  the  two  separately. 

MESENCHYMA. — The  histogenesis  of  the  mesenchymal  portions  of 
the  kidney  is  almost  unknown.  It  seems  to  me  particularly  desira- 
ble that  the  history  of  the  blood-vessels  should  be  ascertained.  Golgi, 
89.1,  341,  observed  that  in  the  foetal  kidney  the  arteries  subdivide 
and  form  an  irregular  network  of  capillaries  in  the  peripheral  portion 


mslK 


SPECIAL 


op   THE    UROGENITAL  OK<  ,  \  \  - 


509 


of  the  organ,  and  when  the  glomerulus  begins  to  form  it  contains  a 
single  l<>op  of  this  network,  and  later  from  this  primary  loop  second- 
ary loops  bud  forth  until  the  circulation  of  the  glomerulus  is  com- 
pleted. 

It  is  important  to  note  that  the  fibrous  capsule  is  developed  very 
early.  1  ><  -tore  there  are  any  glomeruli  —  for  instance,  it  is  present  in 
tin-  kidney  of  an  embryo  rabbit  of  fifteen  days,  and  at  sixteen  days 
i>  tigured  as  quite  thick  by  Kolliker  ("  Entwickelungsgesch.,"  1 
Fig.  581).  The  capsule  is  definitely  present  in  human  embryos  of 
.'.">  mm.  length  and  is  formed  of  spindle-shaped  anastomosing 
cells  (  \V.  Nagel,  89.3,  3G7).  My  observations  have  led  me  to  be- 
lieve that  the  capsule  is  the  essential  mechanical  condition  for  the 
development  of  the  glomeruli,  compare  below. 

TUBULES  AND  MALPIGHIAN  CORPUSCULES.  —  The  tubules  arise 
as  branches  of  the  blind  end  of  the  renal  evagination  and  the  blind 
ends  of  the  branches  form  the  so-called  Malpi^hian  corpuscles.  The 
branching  begins  very  early,  compare  Fig.  444,  Z>,(7,  and  gradually 
a  distinction  becomes  recognizable  between  the  enlarged  end  of  the 
ureter,  destined  to  form  the  pelvis  and  the 
tubules  proper  —  a  distinction  which  be- 
comes more  and  more  marked  as  develop- 
ment progresses.  The  branches  are  at  first 
short  l)i  it  wide,  and  form  wide  angles  with 
one  another;  their  walls  are  a  rather  high 
cylinder  epithelium.  At  an  early  period— 
in  the  rabbit  by  the  fourteenth  day  —  the 
branches  reach  the  capsule,  which  has  mean- 
while been  differentiated  from  the  surround- 
ing mesenchyma.  The  capsule  seems  to 
prevent  the  further  elongation  of  the  branch 
in  its  line  of  growth,  and  to  force  the  end 
of  the  branch  to  curl  over,  thus  by  a  simple 
mechanical  condition  causing  the  formation 
of  the  anluge  of  the  Malpighian  corpuscle. 
This  role  of  the  capsule  has  not  been  noticed 
hitherto,  so  far  as  I  am  aware.  My  atten- 
tion was  called  to  it  by  observing  that  in 
older  kidneys  (human  embryos  of  three, 
four,  and  five  months)  the  formation  of  the 
Mai  pi  u;hian  corpuscles  always  goes  on  close 
against  the  capsule,  Fig.  274;  one  sees  a 
straight  collecting  tubule,  which  runs  to 
the  capsule  and  there  bends  over  into  the 
anlasre  of  the  convoluted  tubule  and  Malpi-  FIG.  274.  -section  of  a  Kidney, 

,    .  .,  i       Human   Embryo    of    about   five 

ghian  corpuscle;  the  younger  the  corpuscle  Months.  Minot  collection,  NO.  M. 


the  nearer  is  it  to  the  capsule.     To  explain 

the  difference  in  position,  we  must  assume  tubule. 

that  the  corpuscles  remain  approximately 

where  they  arise  and  that  the  capsule  enlarges,  and  thereby  gives 

opportunity  for  new  Malpighian   corpuscles  to  be  developed  out- 

side  of   the   older  ones  —  examination  of  the  carefully  drawn    Fig. 

•.'I'',  will  make  the  distribution  of  the  corpuscles  clear.     The  collect- 


510 


THE    FCETUS. 


ing  tubules  appear  to  all  arise  as  branches,  at  first  from  the  end  of 
the  ureter,  after  that  from  the  collecting  tubules  already  formed — 
the  details  of  their  development  have  still  to  be  ascertained ;  at  first 
the  branches  devaricate  at  wide  angles,  but  later  they  show  the 
characteristic  U-shaped  fork  of  the  adult,  compare  Fig.  276,  col. 
The  convoluted  tubules  and  Malpighian  corpuscles  develop  accord- 
ing to  Golgi,  89.1,  as  follows:  The  end  of  the  tubules  bend  over, 
Fig.  275,  into  an  S-shape;  in  Golgi's  diagrams  each  main  tubule  is 
represented  as  forming  two  convoluted  tubules  at  once ;  whether  this 
is  the  case  is  not  quite  clear  from  his  text,  but  it  is  probably  true, 
I  think,  of  the  first-formed  Malpighian  corpuscles,  but  later  each 
straight  tubule  forms,  so  far  as  I  can  observe,  only  one  corpuscle  and 
convoluted  tubule.  The  different  parts  of  the  S-shaped  tubule  have 
each  their  fixed  destiny.  The  end  of  the  S  (in  the  diagrams  the 
lower  part)  receives  the  vascular  loop,  which  gives  rise  to  the  blood- 
vessels of  the  future  glomerulus,  gl;  the  lower  limb  of  the  S,  «, 
elongates  enormously  and  forms  the  first  division  of  the  convoluted 
tubule  including  the  loop  of  Henle,  H\  the  upper  limb,  6,  of  the  S 
also  elongates  very  much — though  less  than  the  lower  limb — and  is 


FIG.  275.—  Semidiagrammatic  Figures  of  Developing  Renal  Tubules  of  a  Mammal.  1,  2,  3, 
4,  5,6,  Successive  stages;  gl,  blood-vessels  of  glomerulus;  a,  first,  6,  second  portion  of  convo- 
luted tubule;  H,  Henle's  loop.  After  Golgi. 

the  anlage  of  the  second  division  of  the  convoluted  tubule ;  where 
the  two  limbs  join  the  tubule  passes  close  to  the  Malpighian  cor- 
puscle and  seems  to  be  intimately  attached  to  it.  This  attachment 
is  preserved,  according  to  Golgi,  in  the  adult  kidney.  During  de- 
velopment it  acts  as  a  fixed  point,  which  parts  the  convoluted  tubule 
into  two  primary  divisions,  which,  as  is  well  known,  are  persistent. 
Henle 's  loop  rapidly  elongates  in  the  direction  parallel  to  the  straight 
or  collecting  tubule  and  toward  the  medulla,  its  elongation  perhaps 
explaining  why  it  increases  in  diameter  less  rapidly  than  the  remain- 
ing parts  of  the  tubules.  The  development  of  the  corpuscles  has  been 
described  quite  fully  by  Toldt,  74.1,  and  also  by  Kolliker  in  his 
"  Entwickelungsgeschichte,  1879,"  949,  but  it  is  to  be  noted  that  the 
S-shaped  tubule  is  not  merely  the  anlage  of  the  Malpighian  corpuscles, 
as  supposed  by  these  authors,  but  also  of  the  convoluted  tubule. 
The  blind  end  alone  forms  the  corpuscle ;  the  wall  of  this  end  is  pushed 
in  by  the  very  formation  of  the  S,  and  the  end  assumes  somewhat 
the  shape  of  a  rubber  ball  with  one  side  pushed  in  (Toldt) ,  in  the 
concavity  of  which  a  network  of  capillaries  appears,  Fig.  275,  gl. 
In  older  kidneys  of  the  human  embryo,  the  concave  side  is  always 


SI'KCIAL   HISTORIES   OF   THE    UROGENITAL 


511 


turned  away  from  tin-  straight  collecting  tubule  with  which  the  cor- 
puscle is  connected,  Fig.  -.'iiJ.     Tbe  epithelium  upon  the  convex  sid. 
is  much  thinner  than  that  of  the  concave  side,  and  as   development 
progresses  this  difference  becomes  more  marked;  the  space  of  the 


Cap* 


col 


FIG.  276.— Section  Parallel  to  the  Medullary  Rays  of  the  Kidney  of  a  Human  Foetus  of  about 
five  Months.     Minot  Collection,  No.  34.    Explanation  in  text. 

tubule  is  the  cavity  of  the  corpuscle ;  the  thin  epithelium  is  the  lining 
of  Bowman's  capsule;  the  thicker  epithelium  covers  the  glomerulus. 
The  further  differentiation  depends  chiefly  upon  the  assumption  of 
the  spherical  form  and  upon  the  growth  of  the  glomerulus  and  its 
-els.  The  original  area,  by  which  the  vessels  enter  the  glomer- 


512 


THE   FCETUS. 


ulus,  remains  about  the  same,  or  perhaps  even  diminishes  in  size, 
but  the  Malpighian  corpuscle  grows,  and  hence  the  neck  by  which 
the  vessels  enter  becomes  relatively  much  smaller.  The  corpuscles 
continue  their  growth  for  a  long  period,  and  are  smaller  in  the  foetus 
than  in  the  adult,  therefore  they  must  continue  to  grow  after  birth. 

Some  authors  have  maintained  that  there  is  an  atrophy  of  some 
of  the  tubules  of  the  fcetal  kidney,  but  I  agree  with  Golgi,  89.1,  in 
believing  that  of  this  there  is  no  valid  evidence. 

I  present  figures  of  two  typical  sections  of  human  fcetal  kidneys, 
Figs.  276,  277.  The  first,  Fig.  276,  represents  a  radial  section  of  a 


JVri°  BftSSsM 10 UJB  •*  «  • •  «M   <$$*      *>S» 


FIG.  277.  —Cross  Sections  of  the  Medullary  Tubules  of  the  Kidney  of  a  Human  Embryo  of  about 
Five  Months.     Minot  Collection,  No.  34.     Explanation  in  text. 

kidney  at  about  five  months.  The  capsule,  Cap,  is  fibrous  and  thick. 
The  separation  of  the  cortical,  (7,  and  medullary  zones,  3f,  is  given 
by  the  distribution  of  the  Malpighian  corpuscles,  of  which  the 
youngest  stages  are  found  near,  the  oldest  farthest  from,  the  capsule ; 
between  the  two  zones  are  situated  the  main  blood-vessels,  vv, 
drawn  dark  in  the  figure ;  the  medullary  rays,  R,  are  distinct,  but 
consist  each  of  only  a  few  tubules :  the  convoluted  tubules,  cc,  are  very 
pale  and  not  all  of  them  are  represented ;  to  render  the  figure  clearer 
they  are  drawn  without  nuclei ;  Henle's  loops,  H,  are  found  at  all 
levels,  and  show,  as  yet,  no  very  distinctive  histological  features ; 


BPB<  IAL    EOSTOBIBS    OF   THE   UROGENITAL   OBGAJT8,  513 


the  collecting  tubule>.  ro//,  are  large  and  show  the  typical  branching 
with  great  perfection.  Especially  characteristic  of  the  fo-tal  kidney 
is  the  l,-;rgr  proportion  of  connective  tissue,  and  the  consequent  wide 
-eparation  of  the  t  ubules.  The  second  section  is  through  the  medulla 
at  right  angle^  to  the  direction  of  the  tubules,  Fig.  277.  Here  the 
wide  separation  ,,f  the  tubules  by  connective  tissue  is  more  apparent 
than  in  the  previous  figure.  The  collecting  tubules  are  large 
and  have  a  cylinder  epithelium  with  evenly  placed  nuclei;  the 
Ih-nle's  tubules.  H,  are  much  smaller,  but  vary  greatly  in  size;  as 
Golgi  has  pointed  out,  it  is  sometimes  the  ascending,  sometimes  the 
dexvndin.u-,  limb  which  is  small.  Every  collecting  tubule  is  sur- 
rounded by  a  space,  which  at  first  I  thought  artificial,  but  as  I  find 
it  in  all  specimens,  including  the  freshest  and  best  preserved,  I  con- 
clude that  it  exists  during  life,  and  regard  it  as  probably  a  lymph 
space. 

UKATN'S  CORDS.—  Max  Braun,  77.4,  1<M)-201,  described  cords  of 
cells,  which  extend  in  very  early  stages  of  the  embryos  of  lizards 
through  the  renal  anlage.  These  cords  differ  but  little  from  the  rest 
of  the  mesenchyma,  except  in  having  their  cells  more  closely  crowded 
together,  and  that  they  can  be  traced  to  a  direct  connection  with  the 
mesothelium.  This  observation  has  since  been  confirmed  —  on  chicks 
by  A.  Sed-wick,  80.1,  on  crocodile  and  turtle  embryos  by  R. 
\Viedersheini,  90.3.  The  cords  I  must  regard  from  Braun's  own 
descriptions  as  merely  the  beginning  of  the  condensed  mesenchyma 
of  the  renal  anlage.  The  three  authors  who  have  observed  the  cords 
id  thriu  as  the  anlages  of  the  convoluted  tubules,  though  they 
brin-  no  direct  proof  in  support  of  this  view,  and  since  it  has  been 
positively  demonstrated  tiiat  the  convoluted  tubules  arise  from  the 
collecting  tubules,  the  view  in  question  must  be  abandoned. 

SHAPE.  —  The  kidney  is  early  marked  out  definitely  by  the  devel- 
opment of  its  capsule,  and  in  its  first  form  is  already  "kidney- 
shaped,"  and  has  a  smooth  surface.  When  the  development  of  the 
Malpighian  corpuscles  begins,  the  surface  of  the  kidney  changes, 
and  at  ten  weeks  (Burdach)  is  already  divided  into  lobes,  separated 
from  one  another  by  shallow  but  sharply  defined  depressions.  The 
n  umber  of  lobes  is  usually  about  eighteen  in  the  human  embryo,  but 
Burdach  ("  Physiologic,"  Bd.  II.,  1828)  describes  eight  lobes  at  ten 
werks.  The  lobate  stage  is  found  in  all  amniota  and  is  permanent 
in  Sauropsida  and  cetaceans,  but  in  most  mammalia  is  confined  to 
the  foetal  period.  In  man  the  lobes  disappear  soon  after  birth  and 
the  surface  of  the  kidney  again  becomes  smooth.  Each  lobe  corre- 
spomls  to  the  base  of  a  Malpighian  pyramid. 

The  appearance  of  the  foetal  kidney  is  also  affected  by  its  upper 
end  being  covered  by  the  relatively  large  hood  formed  by  the  supra- 
renal capsule. 

HITMAN  KIDNEY.  —  The  following  dates  as  to  the  development  in 
man  are  taken  chiefly  from  Kolliker's  "  Entwickelungsgeschichte,  " 
1  s;o,  p.  95*2.  In  an  embryo  six  or  seven  weeks  old  the  kidney  meas- 
ured If  mm.,  was  flattened,  bean-shaped,  and  overlaid  the  Wolffian 
body.  In  the  eighth  week,  it  measured  2.5  mm.  long,  and  lay  far 
behind  the  large  supra-renal  capsule,  with  which  it  comes  in  contact 
during  the  third  month.  The  lobules,  as  first  fully  described  by 
33 


514  THE    FOETUS. 

Toldt,  74.1,  appear  during  the  second  month  and  remain  marked 
upon  the  external  renal  surface  throughout  foetal  life.  The  Malpi- 
ghian.  corpuscles  begin  to  form  toward  the  end  of  the  second  month, 
and  continue  forming  until  a  few  weeks  after  birth.  The  Henle's 
loops,  as  shown  by  Golgi,  89.1,  begin  their  development  immedi- 
ately after  the  corpuscles  appear,  and  may  be  recognized  in  three- 
months'  embryos,  as  I  have  observed,  but  are  not  well  developed 
until  the  fourth  month. 

URETER. — Concerning  the  embryonic  history  of  the  ureter  little  is 
known.  Kupffer,  65.1,  66.1,  has  shown  that  the  stretch  of  the 
Wolffian  duct  between  the  original  evaginatlon  and  the  urogenital 
sinus  elongates  somewhat,  but  as  development  proceeds  this  part 
becomes  included  more  and  more  in  the  sinus,  with  the  result  that 
the  two  canals  open  separately.  During  these  changes,  the  ureter 
becomes  twisted  so  that  its  opening  is  situated  in  front  of  that  of 
the  Wolffian  duct. 

As  to  the  histogenesis  of  the  ureter  I  know  of  no  observations. 

HISTORICAL  NOTE. — Remak,  50.1,  was  the  first  to  describe  cor- 
rectly the  development  of  the  kidney ;  he  observed  the  forward  growth 
of  the  ureter  from  the  cloaca,  the  enlargement  of  the  end  of  the 
ureter,  and  the  outgrowth  from  it  to  produce  the  collecting  and  con- 
voluted tubules.  Kupffer,  65.1,  66.1,  showed  that  the  ureter  was 
an  evagination  of  the  Wolffian  duct  near  the  cloaca,  and  this  has 
since  been  confirmed  by  numerous  observations  on  all  classes  of  am- 
niota ;  Kupffer  added  also  the  erroneous  notion  that  the  urinif erous 
tubules  do  not  all  arise  as  products  of  the  ureter.  Unfortunately 
Kupffer's  error  was  upheld  by  Bornhaupt,  67. 1,  Colberg  (Cbl.  Med. 
JFiss.,1863).  Goette,Thaysen,  73. 1,  Braun,  77.4,  Sedgwick,  80. 1, 
Balfour,  Riedel,  74.1,  and  Emery,  83.1,  and  even  Wiedersheim, 
90.3.  The  authors  since  Braun  have  been  largely  influenced  by 
theoretical  considerations,  especially  by  the  wish  to  demonstrate 
that  the  true  kidney  is  developed  similarly  to  the  Wolffian  body 
(mesonephros) ,  in  other  words  that  its  secretory  tubules  are  olifferent 
in  origin  from  its  ducts.  Remak's  original  view  found  few  uphold- 
ers, of  whom  Waldeyer,  70.1,  132,  Toldt,  74.1,  Kolliker  ("Ent- 
wickelungsgeschichte,"  1879),  and  Golgi,  89.1,  and  W.  Nagel, 
89.3,  365,  are  certainly  the  most  important.  Golgi  may  be  said  to 
have  put  the  matter  beyond  debate  so  far  as  mammals  are  concerned. 
My  own  observations  enable  me  to  affirm  with  confidence  that  the 
tubules  arise  as  evaginations  of  the  ureter,  and  that  in  man  the  con- 
voluted tubules  and  Malpighian  corpuscles  arise  as  branches  of  the 
collecting  tubules.  The  facts  are  so  clear  that  it  is  difficult  now  to 
understand  how  the  opinion  could  have  been  entertained  that  the 
convoluted  tubules  arose  from  the  blastema,  and  not  as  outgrowths 
of  the  collecting  tubules. 

Allantois  and  Bladder. — That  portion  of  the  allantois  which 
lies  within  the  body  of  the  embryo,  and  extends  from  the  anus  to  the 
umbilicus,  becomes  the  bladder.  It  has  been  mentioned  already  that 
the  ureters  very  early  separate  entirely  in  mammals  from  the  Wolffian 
ducts  and  come  to  open  into  the  neck  of  the  allantois.  The  dilatation 
of  the  embryonic  portion  of  the  allantois  to  a  fusiform  vesicle  begins 
in  man  during  the  second  month ;  one  end  of  the  vesicle  is  connected 


sl'K<   I  AL    lilvnUMKS    (»K    THK    I   K<  M  ,  KN  ITAL    ORGAN"-  515 


witli  the  anal  end  of  the  intestinal  canal,  while  tin*  other  end  tai)ers 
nut  and  is  pmlmi^vd  as  the  so-called  urachus,  into  which  the  cavity 
of  tlie  vesicle  is  prolonged,  hut  at  some  time  not  yet  definitely  ascer- 
tained the  cavity  of  the  urachus  disappears,  though  seldom  com- 
pletely, for  Lusehka  (Virchow's  1  1  /•<•//.,  XXIII.)  found  remnants  of  it 
even  in  the  adult.  The  urachus  is  transformed  into  the  ligamentum 
e  medium  (K«">lliker,  "  Entwickelungsgesch.,"  1879,  p.  953). 

The  main  vesicle  becomes  the  bladder.      The  entoderm  of  the 

allaiitois  becomes  the  epithelium,  and  the  mesenchyma  becomes  the 

connective  tissue  and  muscular  walls  of  the  bladder.     The  histogen- 

and  changes  in  shape  of  the  embryonic  bladder  have  still  to  be 

investigated. 

Recently  Ketterer,  90.1,  and  Keibel,  91.1,  have  revived  Rathke's 
conception,  32.1,  I.,  57,  that  the  bladder  is  an  outgrowth  of  the 
cloaca,  which  Incomes  early  divided  into  a  dorsal  or  intestinal  and 
ventral  or  allantoie  division.  The  distinction  seems  to  me  arbitrary 
between  this  notion  and  the  view  adopted  above,  since  the  allantois 
is  in  any  case  a  prolongation  of  the  entodermal  canal,  and  neither 
Retterer  nor  Keibel  show  that  there  is  a  true  division  of  the  cloaca. 

Urogenital  Sinus.  —  I  base  this  section  upon  Mihalkovics'  mon- 
ograph, 85.1,  307-324.  As  shown  in  Fig.  444,  the  allantois  is  the 
direct  continuation  of  the  intestinal  canal,  and  the  urogenital  ducts 
open  into  the  allantoie  portion  or  the  part  of  the  canal  on  the  ventral 
side  of  the  future  anus.  After  the  anus  is  formed,  there  is  a  terminal 
portion,  the  so-called  cloaca,  into  which  both  the  intestinal  canal 
proper  and  the  allantoie  canal  open.  The  greater  part  of  the  allan- 
tois dilates  into  the  bladder,  but  between  the  bladder  and  the  cloaca 
the  allantoie  canal  remains  narrower;  it  is  into  this  narrower  portion 
that  the  Miillerian  ducts  open;  the  stretch  between  the  bladder 
proper  and  the  opening  of  Muller's  duct  is  the  urethra  in  the  strict 
sense,  while  the  part  below  received  from  Johannes  Muller  the  name 
of  6'//>".s  urogenitalis.  The  female  adult  urethra  corresponds  to  the 
urethra  as  .here  defined,*  but  the  male  urethra  includes  both  the 
urethra  and  the  sinus.  This  maybe  called  the  monotreme  stage, 
and  is  characterized  by  there  being  merely  a  single  or  cloacal  opening, 
through  which  the  excrement,  urine,  and  genital  products  are  dis- 
charged ;  the  stage  is  the  permanent  one  in  non-mammalian  verte- 
brates and  in  monotremata.  An  important  advance  is  made  in  pla- 
cental  mammals  by  the  subdivision  of  the  cloacal  opening  into  the 
ventral  urogenital  opening  and  the  dorsal  anal  opening,  which  takes 
place  in  the  human  embryo  about  the  fourteenth  week,  and  involves 
the  complete  separation  of  the  urogenital  sinus  from  the  intestinal 
canal.  In  the  male  the  closure  of  the  raphe  penis  converts  the  sinus 
into  the  prolongation  of  the  urethra  proper,  as  we  may  term  the  neck 
of  the  allantois  or  bladder  above  the  opening  of  the  fused  Miillerian 
ducts  (uterus  masculinus)  .  In  the  female  the  sinus  persists  as  the 
vestibulum  into  which  the  urethra  and  vagina  both  open.  The  sepa- 
rated urogenital  and  anal  openings  lie  at  first  in  a  shallow  fossa  or 
recess,  the  raised  edges  of  which  represent  the  anlagesof  the  external 
genitalia  :  see  the  following  section. 

PROSTATIC    GLAND.  —  This   gland  is  present  during  the  fourth 

*  The  relations  an-  w.-ll  shown  by  KOlliker,  "Grundriss,"  Fig.  295. 


516  THE   FCETUS. 

(Kolliker)  or  fifth  month  (Mihalkovics)  as  a  series  of  branching 
evaginations  of  the  epithelium  of  the  upper  end  of  the  urogenital 
sinus,  which  expand  into  wide  irregular  cavities.  The  muscular 
tissue  is  developed  much  later  from  the  mesenchyma  of  the  walls  of 
the  sinus  (Kolliker,  "Entwickelungsges.,"  1879,  p.  1000,  and  Mihal- 
kovics, 85.1,  378).  The  evaginations  make  their  first  appearance, 
according  to  Tourneux,  89.1,  257,  about  the  twelfth  or  thirteenth 
week,  and  persist  in  the  female,  though  more  or  less  in  a  rudimen- 
tary condition  (Tourneux,  Soc.  Biol.,  Paris,  Jan.,  1888). 

COWPER'S  AND  BARTHOLINI'S  GLANDS.* — These  names  are  ap- 
plied to  the  same  glands  in  the  male  and  female  respectively ;  they 
arise  as  paired  evaginations  of  the  lower  part  of  the  urogenital  sinus. 
According  to  Van  Ackeren,  89.1,  44,  the  glands  of  Bartholini  be- 
gin their  development  in  man  toward  the  end  of  the  fourth  month ; 
during  the  fifth  month  the  branches  (acini)  increase  in  number  and 
are  found  separated  from  one  another  by  considerable  mesenclwmal 
tissues ;  by  the  sixth  month,  as  already  described  by  R.  Geigel,  83. 1, 
they  form  considerable  organs  1X1.8  mm.,  of  rounded  form,  but  the 
left  gland  is  a  little  smaller  than  the  right ;  the  acini  now  lie  close 
together. 

III.  EXTERNAL  GENITALIA. 

The  main  facts  in  the  development  of  the  external  genitalia,  and 
the  homologies  between  the  two  sexes,  were  worked  out  by  Tiede- 
mann — see  his  "Anatomie  der  kopflosen  Misgeburten,"  Landshut, 
1813,  p.  84.  A  very  good  description  of  the  foetal  penis  and  clitoris 
is  given  by  Joh.  Fr.  Meckel  in  his  "  Handbuch  der  menschlichen 
Anatomie,"  1815-1820,  so  that  Johannes  Muller  in  1830,  30.1, 
could  add  but  little.  Some  further  details  have  been  given  by  H. 
Rathke,  32.2,  and  by  Kolliker.  Ecker  in  his  "  Icones  Physiologica3n 
has  given  a  series  of  figures,  which  have  been  extensively  copied  in 
text-books,  and  have  been  reproduced  in  the  well-known  and  some- 
what inaccurate  wax  models  of  Ziegler.  In  1888-89  appeared 
Tourneux's  admirable  monographs,  88.1,  89.1,  upon  which  the 
following  account  is  based.  We  have  to  consider  the  history,  1,  of 
the  genital  tubercle  (penis-clitoris) ,  2,  of  the  genital  labia  (scrotum- 
labia  majora).  The  external  genitals  are  homologous  in  the  two 
sexes,  but  in  the  male  they  are  more  specialized  than  in  the  female ; 
the  condition  in  the  adult  female  corresponds  to  that  of  the  foetal  male. 

Genital  Tubercle. — The  anal  plate  becomes  very  much  thick- 
ened until  it  constitutes — sheep  embryos  13-25  mm.,  pig  embryos 
14-20  mm. — a  thick  plug  of  epithelium,  on  the  dorsal  side  of  which 
appears  an  external  invagination,  Fig.  278,  the  vestibule  anolc 
of  Tourneux,  which  gradually  penetrates  until  it  leaves  only  a  thin 
epithelial  membrane  to  close  the  rectum,  while  the  main  plug 
(bouchon  cloacal)  closes  the  urogenital  sinus  or  pedicle  of  the  allan- 
tois,  S.  ug.  The  accumulation  of  mesenchyma  on  the  ventral  side, 
of  the  epithelial  plug,  p/,  of  the  cloaca  is  indicated  by  an  external 
prominence,  which  may  be  already  designated  as  the  genital  tuber- 
cle, t.  g.  As  development  progresses  the  genital  tubercle  rapidly 

*The  paper  by  Swiecicki  in  Gerlach's  "Beitrage,"  1883,  I  have  not  seen. 


EXTERNAL    (JKMTALIA. 


517 


Coe 


lengthens  and  the  epithelium  upon  its  dorsal  side  is  reduced  I'mm  a 
-[•••at  plug  to  a  thin  layer,  and  l»y  the  disappearance  of  the  plug  l>«-th 
rectum  and  urogenital  sinus  become  open  to  the  exterior ;  the  ma  — 
of  tissue  between  tlie  t\v<>  oj>enings  is  termed  l»y  Tourneux  the  frjili 
or  rfH'run  JH'TIIH'-H/.  The  genital  tubercle  owes  its  origin  to  the 
thickening  of  the  anal  plate,  it  gives  rise  to  the  penis  in  the  male,  to 
the  clitoris  and  nyniplue  in  the  female.  The  tubercle  is  quite  prom- 
inent-  iiu-aMiring  l.:>mm.  in  length — in  the  human  embrj'o  by  the 
tenth  week  and  is  then  found  to  have 
it-  end  knob-like,  indicating  the  future 
glans,  and  its  d<>rsal  «>r  anal  side  with  a 
shall«»\v  groove,  wliich  directly  prolongs 
the  channel  of  the  urogenital  sinus,  but 
only  as  far  as  the  knob  of  the  glans. 
About  the  tenth  week  there  appear  two 
eminences  alongside  the  genital  tubercle 
and  the  urogenital  opening,  which  we 
may  call  the  genital  labia — compare  Fig. 
Inh.  The  labia  are  solid  hillocks  of 
•nchyma  with  a  covering  of  epitheli- 
um, see  Fig.  MM. 

In  the  female  they  persist  as  the  labia 
ma  jura  and  in  the  male  as  the  scrotum. 

1*1-: MS. — In  the  male  the  genital  tuber- 
cle continues  elongating  as  follows: 

Foetus in.  60.         lo.-).     nun. 

Peni>  3.  :{.-"">  nun. 

and  its  dorsal  groove  not  only  deepens 
while  it  remains  very  narrow,  but  also 
closes,  beginning  at  the  base ;  the  line  of  closure  remains  permanent- 
ly marked  by  the  raphe  of  the  penis;  the  effect  of  the  closure  is 
to  form  an  epithelial  canal  which  prolongs  the  urogenital  sinus  (or 
urethra)  into  the  penis;  the  epithelial  canal  separates  from  the 
epithelium  covering  the  penis,  except  just  below  the  glans,  where 
the  permanent  opening  is  established.  During  the  third  month 
there  appears  fh>t  an  epithelial  ridge  upon  the  glans,  as  in  Fig. 

,  this  ridge  lies  in  the  prolongation  of  the  groove;  it  soon 
disappears  and  the  groove  extends  gradually  on  to  the  glans.  It  is 
at  this  stage  (end  of  the  third  month)  that  the  thick  prepuce  of  the 
glans  begins  to  develop,  but  the  groove  on  the  anal  side  prevents  its 
forming  a  complete  ring  around  the  organ.  The  prepuce  appears  as 
a  slight  ridge  which  overgrows  the  glans,  the  epithelium  of  the  inner 
surface  uniting,  as  the  fold  extends,  with  the  epithelium  of  the 
glans — the  two  epithelia  fusing  into  one  solid  plate,  Fig.  297, 
cj>.  Later  the  groove  becomes  closed  to  a  canal,  and  the  terminal 
opening  of  the  canal  is  shut  by  the  growth  of  the  epithelium,  Fig. 

.  «,  which  plugs  up  the  orifice.  This  fact  is  important  from  its 
bearing  on  the  question  of  the  origin  of  the  amniotic  fluid,  p.  :>4n. 
The  two  epithelial  layers  of  the  prepuce  are  separated  by  mesoderm. 
The  relations  which  now  exist  can  be  better  explained  by  reference 
to  Fig.  MT(.>,  which  shows  the  glans  in  longitudinal  section;  observe 
the  thickened  epithelium,  a,  closing  the  orifice  of  the  urethra;  also 


Fio.  278.— Longitudinal  Median  Sec- 
tion of  the  Cloaca  of  a  She*>i>  Ktnhryo 
of  18  mm.  Coe,  Coelom ;  Al,  allantofs; 
EC,  ectoderm  ;  N.  »</.  sinus  nru^i-nita- 
^fiiital  tulx-ivl*-:  ill.  rpithr- 
liom;  CNt,  anal  j»lat.-:  /-.//.  i-ntoderm. 
/  ^' iliauis.  After  TourntMix. 


518 


THE   FCETTS. 


G 


that  the  epithelial  plate — and  consequently  the  prepuce  also— extends 
further  on  the  ventral  side,  ep,  than  on  the  dorsal,  and  that  though 

_  the  glans,  G,  is  very  vascu- 
lar, the  corpus  cavernosum  is 
remarkable  for  the  absence  of 
blood-vessels.     Finally  there 
may  be  noticed  in  the  epithe- 
lial  plate,  ep,  several  places 
where  the  cells  are  arranged 
more  or  less  concentrically; 
^     the    appearances  have  been 
BL     thought  by  Schweigger-Sei- 

(f     -lf  \  B    del,   66.1,  to   be   connected 

v    !  with  the  cleavage  of  the  epi- 

thelial plate  to  form  the  epi- 
thelium of  the  prepuce  and 
of  the  glans.  This  cleavage 
does  not  take  place  until  after 
birth,  but  just  when  is  not 
known. 

The  corpus  cavernosum 
develops  slowly;  it  is  first 
marked  out  as  a  dense  mes- 
enchyma,  in  which  the  blood 
capillaries  develop  more  and 
more,  beginning  in  the  third 
month ;  but  it  is  not  until  the 
sixth  month  that  the  capil- 
laries begin  to  show  any 
marked  dilatation.  The  cor- 
pus spongiosum  develops  also  chiefly  by  the  expansion  of  capillaries, 
but  considerably  later  than  the  cavernosum.  Retterer  (Soc.  Biol., 
Paris,  1889,  p.  399)  states  that  in  various  mammals  the  corpus  caver- 
nosum is  very  dense  and  fibrous  before  any  blood-vessels  appear  in  it. 
CLITORIS  AND  NYMPH^E. — The  development  of  the  genital  tubercle 
in  the  female  is  in  all  respects  similar  to  that  of  the  male,  but  it  does 
not  pass  beyond  the  stage  in  which  there  is  an 
open  urethral  groove.  The  glans  and  prepuce 
are  formed,  as  in  the  male,  to  constitute  the 
clitoris,  but  the  borders  of  the  urethral  groove 
do  not  unite,  as  they  do  in  the  male  to  form 
the  stalk  of  the  penis,  but  remain  as  elevated 
ridges  known  as  the  labia  minora  or  nympha3 
in  the  adult.  During  the  third  month  the  dif- 
ference between  the  male  and  female  tubercle 
becomes  more  and  more  clearly  marked,  and 
the  distance  between  the  urogenital  and  anal 
openings  increases.  By  the  end  of  the  third 
month,  Fig.  280,  the  glans  measures  about  1 
mm.,  while  the  lips  (anlages  of  the  nymphs) 
of  the  urethral  groove  measure  about  3  mm.  in  length ;  around  the 
base  of  the  glans  can  be  seen  the  commencing  fold  of  the  preputium> 


FIG.  279.  —Longitudinal  Section  of  the  fenis  of  a 
Human  Embryo  of  about  five  Months.  Minot  Coll. 
No.  34.  a,  Epithelial  plug ;  Pp,  prepuce ;  ep,  epithe- 
lial lamina  between  prepuce  and  glans;  G,  glans; 
U,  urethra. 


V 


Jab 


FIG.  280.— External  Genital- 
ia,  Female  Embryo  of  105  mm. 

Kl.  Glans:   ny,  nymphae;   lab, 
ibia  Tiiajora;   an,  anus;    ?«/, 
urogenital     opening.       After 
Tourneux.     X  about  5  diams. 


EXTERNAL   GENITALIA. 


519 


and  upon  tin-  glans  can  be  seen  tlw  median  epithelial  crest,  which 
subsequently  disappears,  the  urethral  groove  extending  on  to  the 
glans  (luring  the  fourth  month. 

The  groove  persists,  so  that  in  the  adult  the  prepuce  does  not  ex- 
tend, as  in  the  male,  completely  around  the  glans,  but  is  cleft  on  the 


Fio.  281.— Section  of  the  Clitoris  and  Labia  Majora  of  a  Human  Embryo  of  about  four  and  one- 
half  Months.    (Minot  Collection  No.  49. ) 

anal  side.  Tourneux,  89.1,  254,  observed  in  two  cases  epithelial 
ingrowths  from  the  epithelium  of  the  groove  of  the  glans;  these  in- 
growths he  regards  as  the  anlages  of  the  "glande  clitoridienne"  of 
\\ ".  rthheimer  (Journ.  <1e  VAnat.  et  Physiol.,  1883),  the  homologue 
of  the  mucous  glands  of  the  sinus  of  Guerin  in  the  male.  The  mes- 


r~- -AIU 


FIG.  282.  —External    Genitalia  of  the  Female  Human  Foetus  at  about  four  Months.     Minot   Col- 
lection No.  57.     A,  Ventral,  B,  peri  nee  1  view,    cl,  Clitoris;   la,  labia  majora;  An,  anus. 

enchyma  of  the  glans  persists  at  a  stage  corresponding  approxi- 
mately to  that  of  the  homologous  tissue  in  the  male  at  eight  months, 
hut  the  corpora  cavernosa  develop  as  in  the  male  into  true  erectile 
tissue.  The  accompanying  Fig.  281  represents  a  section  of  the 
clitoris  and  labia  majores  of  a  foetus  of  the  fifth  month,  the  urethra 


520  THE   FCETUS. 

extends  into  the  glans,  which  is  covered  by  the  prepuce ;  the  glans 
is  almost  buried  between  the  large  labia  majora. 

Scrotum  and  Labia  Majora. — There  appear  two  prominences 
during  the  tenth  week,  one  on  each  side  of  the  genital  tubercle.  These 
prominences,  which  are  merely  hillocks,  so  to  speak,  of  mesodenn 
covered  by  foetal  epidermis,  are  the  anlages  of  the  male  scrotum  and 
the  female  labia  majora.  Their  relations  are  well  shown  in  Fig. 
280.  In  both  sexes  the  genital  labia  attain  considerable  size ;  in  the 
female  the  fcetal  type,  Fig.  282,  is  but  slightly  modified,  but  in  the 
male  the  two  labia  meet  and  unite  during  the  fourth  month  between 
the  base  of  the  penis  and  the  anus  to  form  the  scrotum ;  the  raphe 
marks  in  the  adult  the  line  of  junction ;  as  stated  above,  p.  498,  the 
vaginal  processes  grow  into  the  scrotum  and  later  the  testis  descends 
into  it,  p.  499. 


CHAPTER  XXIV. 
TRANSFORMATIONS  OF  THE  HEART  AND  BLOOD-VESSELS. 

WE  have  already  considered,  Chapter  X.,  the  origin  and  early 
history  of  the  heart  and  blood-vessels,  and  have  now  to  consider  the 
metamorphoses  of  the  foetal  organs  of  circulation  to  the  time  of  birth. 
We  shall  take  up,  1,  the  heart;  2,  the  arteries:  :>,  the  veins. 

I.  TRANSI  OKMATION  OP  THE  HEART. 

We  left  the  heart,  p.  288,  as  a  median  longitudinal  tube,  with 
double  walls,  the  inner  endothelium,  and  the  outer  mesothelial  or 
muscular:  the  double  tube  was  free  except  at  its  ends,  which  were 
attarln-d  to  the  walls  of  the  pericardium;  the  anterior  end  communi- 
1  with  the  aortic  vessels,  the  posterior  (caudal)  end  with  the 
veins  united  in  the  septum  transversum.  To  develop  the  adult  heart 
out  of  this  simple  tube,  five  principal  sets  of  changes  occur:  1,  the 
bending  of  the  tubular  heart;  2,  the  outgrowth  of  the  auricles;  3, 
r.ian^rs  in  the  thickness  and  histological  constitution  of  the  walls; 
•*.  the  development  of  valves;  5,  the  appearance  of  secondary  parti- 
tions dividing  the  right  heart  from  the  left. 

The  literature  upon  the  heart  is  very  extensive,  but  the  history 
given  by  His,  "Anat.  menschl.  Embryonen,"  Heft  III.,  129-184, 
of  the  human  heart,  and  that  given  by  Born  of  the  rabbit's  heart, 
89. 1,  are  so  thorough  that  I  have  relied  chiefly  on  these  two  authors. 
Special  mention  ought  also  to  be  made  of  Carl  Rose's  dissertation, 
88.1.  Besides  the  special  papers  on  the  heart,  there  are  numer- 
ous observations  scattered  in  general  works.  The  general  develop- 
ment of  the  chick's  heart  is  described  in  the  text-books  of  Kolliker 
and  of  Foster  and  Balfour,  and  in  J.  Masius'  excellent  article,  89.2, 
based  upon  models  constructed  by  Bern's  method.  Of  earlier  papers 
that  of  Lindes,  65.1,  is  specially  noteworthy,  and,  as  pointed  out  by 
His  and  Born,  far  in  advance  of  his  time. 

Bending  of  the  Heart  and  Formation  of  the  Auricles.— 
After  the  disappearance  of  the  mesocardium  on  both  the  dorsal  and 
ventral  side  of  the  primitive  heart,  the  heart  is  attached  only  by  its 
aortic  and  venous  ends.  The  early  enlargement  of  the  pericardial 
cavity  has  l>een  already  described;  its  size  is  important  as  affording 
the  heart  room  to  elongate,  bend,  and  enlarge.  The  straight  median 
heart  grows  rapidly,  and  to  find  room  bends  to  the  right;  in  the 
chick  the  bending  begins  at  the  close  of  the  first  day  and  increases 
very  rapidly  during  the  second  day,  Fig.  283,  and  becoming  at  the 
same  time  more  complicated  by  the  assumption  of  an  irregular 
S-shape.  In  mammals  the  same  form  is  assumed,  and  is  found  in 
the  rabbit  at  nine  days,  in  human  embryos  of  2.15  mm.,  Fig.  284. 


522 


THE    FCETUS. 


Pro.  A 


The  venous  end  of  the  heart  Fig.  284,  V.  I,  lies  somewhat  to  the  left 
and  extends  for  a  short  distance  toward  the  head  and  then  pa 
into  the  ventricular  portion  of  the  tube,  which  curves,  as  shown  in 

the  cut,  V.  I,  on  to  the  ventral  side,  where  it 
crosses  obliquely  to  the  right  side,  and  then 
bending  dorsalward  finally  runs  toward  the 
head  and  becoming  narrower  passes  into  the 
bulbus  aortse,  A.b,  or  division  of  the  heart 
tube,  which  passes  in  the  median  line  into 
the  trunk  of  the  aorta.  At  this  stage  there 
is  a  very  short  venous  or  auricular  division 
of  the  heart,  a  very  long,  thick,  and  much 
bent  ventricular  division^  and  a  bulbous  di- 
vision of  intermediate  dimensions.  The  dif- 
ferentiation of  these  divisions  comes  out  more 
clearly  from  the  study  of  the  endothelial 
heart  (or  heart  cavities)  at  this  stage,  Fig. 
285.  The  general  course  of  the  heart  may 

__  "  11  i   •      •  i    •       r* 

D6    best    Understood  DV  Combining  thlS  figure 

w{^  ^  preceding?  remembering  that  Fig. 

284  shows  the  muscular  heart,  which  at  this 

stage  is  still  separated  by  a  considerable  space  from  the  endothelial 
heart,  and  is  much  larger  than  the  inner  tube.  In  Fig.  285  the  divi- 
sions of  the  heart  are  clearly  marked;  the  auricular  division,  V.h,  i» 
very  short  and  receives  the  omphalo-mesaraic,  v.o.m,  and  umbilical, 
v.u,  veins  and  the  ducti  Oiivieri  on  each  side;  it  is  continued 


^•°^)ticvesi" 

cics  ,  j  /  o.  ./i,  prociiiiinon  ,  ct.c.  v.  •, 
amnio  -  cardial      vesicle;      Ht, 

heart.        3  diams.   After  DU- 


FIG.  284.  —Reconstruction  of  the  Heart  and 
Veins  of  a  Human  Embryo  of  2.15  mm.  (His1 
embryo  Lg.).  Mb,  Mouth;  A.b,  bulbus  aorta-; 
V.m,  left  ventricle;  F.c,  vena  cava  superior; 
S.r,  sinus  reuniens;  i\u,  vena  umbilicalis ; 
V.I,  venous  limb  of  heart;  Ho,  anlage  of  auri- 
cle; -D,  septum  transversum;  Lb,  liver;  Lby, 
hepatic  duct.  X  40  diams  After  W.  His. 


F.  A.     V.  o.  m.     V.  u.     V.  c. 


FIG.  285.— Endothelial  Heart  of  a  Human 
Embryo  of  2.15  mm.,  seen  from  the  Left  Side 
(His'' Embryo  Lg).  A.b,  Bulbus  aorta-:  /•'/-, 
f return  Halleri;  F,  ventricle;  V.h.  auricle; 
r. o.Ht.  vena  omphalo-mesaraica ;  V. i<,  vena 
umbilicalis;  V.c,  vena  cava;  C.a,  auricular 
canal.  X  40  diams.  After  W.  His. 


headward  by  a  portion,  C.a,  the  auricular  canal,  which  connects  the 
auricle  with  the  ventricle,  V.  The  ventricle,  F,  is  the  widest  and 
longest  division  of  the  heart ;  it  describes  a  somewhat  complex  curve 


:  R  LNSFi  >RM  \  ri<>\    OF    THK    H: 


from  left  to  right,  and  is  tlu'ii  continued  headward  on  the  right  side 
nf  the  embryo  hy  a  verv  narrow  division,  the  fretum  Halleri,  /•'/•, 
\vhicli  leads  into  the  somewhat  wider  and  curving  bulbu>  ;i«>rt,-r.  In  a 
slightly  older  stage,  Fig.  286,  tin-  lateral  outgrowths  of  the  auricular 
division  have  appeared.  I"//,  and  are  the  anlagvs  of  the  true  auric- 
ular ea\  ities:,  the  two  limbs  of  the  ventricle  are  now  nearer  together 
and  where  they  j<»in  have  a -distinct  apex,  which,  owing  to  the  in- 
en-a-<-d  hendinu '  "!  the  heart,  lies  a  little  below  (in  the  Hgniv)  the 
level  of  tlie  auricle>.  I'lt..  The  irregularly  S-like  course  of  the  h< 

ry  evident  in  this  figure;  one  loop  of  the  S  is  constituted  by  the 
auricular  division,  I '//,  the  auricular  canal,  r.rf,  and  part  of  the  ven- 
tricle I";  the  second  loop  of  the  8  is  constituted  by  the  whole  of  the 
ventricle,  by  the  fretuin  Halleri,  7«V,  and  the  bulbils,  A.h.  As  the 
auricular  division  coinpris. ^  only  ahout  a  sixth  of  the  length  of  the 
heart  tube  and  consequently  «,nly  ahout  a  third  of  one  loop  of  the  S, 
we  cannot  say  that  the  heart  consists  of  a  venous  and  an  arterial 


r.u. 

•JH6.  —Reconstructed   Side  View  of  the 

thelial  Heart  <>f  a  Human  Embryo  of  4.2  mm.  <  His 

KuiKr  \urii -ul.ir  canal ;  A.b.  bulbus  a«.r- 

•MIIII    Halleri:    I",  vntri.-l.-;    /'.  wall  of 

pericardium;    r. »/.   \.-na   umbilical!*;   P.O.  m,  vena 

omphalu  -  M  i.c'/.      cardinal    vein;     }'.j 

jiiu'iilar   v.-in;   I' It,  auricle.      >'40<liams.     After  W. 


.Ulil 

FIG.  •>>?.— M<  Kiel  of  tlie  Muscular  Heart 
of  a  Kabiiit  Kmhryo  <>f  nine  to  nine  and 
tlf  Days,  seen  from  the  left  side. 
'  .  Auricular  canal;  B.n.  bulbus  aortce; 
A1,  first,  ,4a,  second  aortic  arch;    !>..[, 
dorsal  aorta;  DC  ductus  Cuvieri :  r.mn, 
vena  umbilical  is:    r.om.  vena  omphalo- 
ii.-a:     .1".    aurirl,-:    I',    ventricle. 
X  fiOdiams  Horn. 

loop.     This  mode  of  description  has  unfortunately  been  often  used, 
and  has  led  to  much  unnecessary  confusion. 

The  heart  of  the  rabbit  agrees  closely  with  that  of  man.  Fig.  287 
18  a  side  view  of  the  rabbit's  heart  at  nine  to  nine  and  one-half  days, 
witli  the  first  aortic  arch,  A\  fully  developed,  and  the  second,  ^ia, 
just  forming.  The  model  shows  especially  well  the  union  of  the  three 
venous  trunks  of  each  side  in  the  large  median  sinus  reuniens,  which 
opens  into  the  auricle;  later  the  sinus  merges  with  the  right 
auricle  and  so  disappears  as  a  separate  division.  It  will  be  noticed 
upon  comparison  of  Figs.  288  and  287  that  the  muscular  heart 
-hows  the  division  of  the  cardiac  tube  far  less  distinctly  than  does 
the  endothelial  heart,  nevertheless  the  auricles,  Au,  auricular  canal, 
C,  and  ventricle,  T,  are  perfectly  distinguishable  in  the  model  of 
the  muscular  heart,  Fig.  287.  The  most  conspicuous  of  the  changes 
which  now  follow  are,  first,  the  descent  of  the  ventricle  and  second 


524  THE    FGGTUS. 

the  enlargement  of  the  two  diverticula  of  the  auricles.  Both  changes 
are  well  illustrated  by  the  heart  of  a  rabbit  at  twelve  and  one-half 
days — see  Born,  /.  c.,  Fig.  19.  Comparison  of  this  figure  with  the 
preceding  renders  evident  that  the  ventricle  has  descended  so  as  to 
lie  below,  i.e,  farther  tail  ward,  nevertheless  the  arterial  exit  of  the 
heart  tube  (or  the  transit  to  the  aorta)  lies,  as  before,  above  or  head- 
ward  of  the  auricles,  so  that  the  descent  of  the  ventricle  has  depended 
upon  or  been  accompanied  by — we  may  express  it  either  way — the 
lengthening  of  the  bulbus  aortse,  B. a.  Fig.  288  represents  the  endo- 
thelial heart  of  a  human  embryo  at  about  the  stage  wre  are  now  con- 
sidering, and  illustrates  how  the  auricles,  Ho,  enlarge  on  each  side 
and  embrace  the  bulbus  aorta3  between  them,  and  also  how  between 
the  two  sides  of  the  ventricle  the  heart  tube  is  somewhat  constricted, 
forming,  as  it  were,  a  narrow  passage.  Into  the  space  left  by  this 


LY 


/  c.s.. 

FIG.  289.  —Inner  Surface   of  the  Heart  of  a 

FIG.  288. —Endothelial  Heart  of  a  Human  Human  Embryo  of  10  mm.  (His' Embryo  Pr). 
Embryo  of  5  mm.  (His'  Embryo  R).  S.v,  Si-  V.cs,  Vena  cava  superior;  S.sp,  septum  spu- 
nus  venosus  ;  Ho,  auricle;  C.a,  auricular  riumofHis;  Ss,  septum  superius;  L. Au,  auri- 
canal ;  V.I,  ventricle;  p,  passage  between  the  cle  of  left  side;  Auc,  auricular  canal;  L.  V, 
two  sides  of  the  ventricle;  C.s,  conus  arterio-  ventricle;  S.i,  septum  inferius,  V.E,  Eusta- 
sus.  X  40  diams.  After  W.  His.  chian  valve :  Ai,  area  interposita.  x  24  diams. 

After  W.  His. 

constriction  grows  tissue  from  the  wall  of  the  muscular  heart,  which 
tissue  gives  rise  to  the  septum  inferius,  that  plays  the  chief  part  in 
the  ultimate  division  of  the  right  from  the  left  ventricle. 

We  turn  now  to  the  consideration  of  the  interior  of  the  heart  at  a 
slightly  more  advanced  stage,  in  which  the  muscular  and  endothelial 
hearts  are  closely  conjoined,  owing  chiefly  to  the  growth  of  the  muscu- 
lar heart  having  obliterated  the  space  between  it  and  the  endothelium. 
Fig.  289  exhibits  a  view  of  the  inside  of  the  heart  of  a  human  em- 
bryo of  ten  millimetres.  The  two  auricles  have  expanded  so  as  to 
meet  above,  leaving,  however,  a  partition,  which  is  known  as  the 
septum  superius,  8.8;  between  the  two  sides  of  the  ventricle  is 
another  partly  developed  partition,  produced  as  just  described  and 
known  as  the  septum  inferius ;  in  the  auricular  canal  there  is  also  a 
projecting  cushion,  which  in  conjunction  with  its  fellow  tends 
to  divide  the  right  from  the  left  side  of  the  auricular  canal.  We 


TRANSFORMATION    OF   THE    HEART.  525 

thus  encounter  at  three  points  the  commencements  of  the  ultimate 
division  of  the  heart  into  right  and  left  sides.  The  opening  of 
tin-  venous  sinus  ivuniejis  is  no  longer  in  the  median  line,  but  upon 
the  right  side  of  the  heart,  or  in  other  words,  into  the  right  auricle. 
The  opening  itself  is  bordered  by  two  thin  folds  or  rudimentary 
valves,  of  which  the  lateral  one,  I'.H,  is  the  anlage  of  both  the 
valvula  Eustachii  and  the  valvula  Thebesii,  while  the  medial  fold 
ultimately  disappears;  as  it  exists  in  the  embryo  it  has  been  named 
by  His  the  nilnilii  rr.sf ihuJi  xim'xtrd.  Above  the  venous  orifice  is 
a  small  septum,  *S'..S7>,  which  disappears  early  in  foetal  life  and  is 
then-fore  known  as  the  septum  spurium.  The  septum  spurium  may 
be  regarded  as  the  prolongation  upward  of  the  united  right  and  left 
venous  valves.  The  space  between  the  spurious  and  the  superior 
septa  is  named  by  Born  the  sjuitinm  intrrxt'jtttile;  it  is  indicated  for 
a  time  by  a  bulge  upon  the  exterior;  it  merges  with  the  general 
eavity  of  the  right  auricle,  when  the  spurious  septum  disappears. 
Below  the  valvula  sinistra,  and  between  it  and  the  septum  superius, 
is  the  */>////  /v.s7 //;////.  .Li,  of  His;  it  is  identical  with  the  so-called 
area  interposita  of  earlier  stages.  The  area  interposita  is  com- 
I  M  •>»  (1  of  connective  tissue  and  contains  no  muscle  fibres ;  it  is  wedge- 
shaped,  and  as  seen  in  the  interior  of  the  heart,  Fig.  289,  presents  a 
triangular  outline.  It  belongs,  strictly  speaking,  to  the  septum 
transversum,  and  corresponds  to  part  of  the  area  by  which  the 
venous  end  of  the  heart  is  permanently  attached  to  the  septum  trans- 
\-er>um  or  diaphragm.  The  septum  superius  or  interauricular  par- 
tition extends  on  to  the  area  interposita,  and  there  fades  out. 

The  Primitive  Ventricle. — The  ventricle  is  at  first  simply  a 
bent  tube;  it  may  therefore  be  described  as  consisting  of  two  limbs, 
which  pass  into  one  another  at  the  apex  of  the  ventricle,  Figs.  286 
and  288.  The  connection  between  the  two  limbs  is  originally  very 
narrow,  but  it  early  widens  out  so  much  that  the  two  limbs  may 
be  said  to  fuse  info  one  general  cavity.  This  may  be  called  the 
stage  of  the  primitive  ventricle,  since  it  is  characteristic  of  the 
iehthyopsida.  While  the  two  limbs  are  fusing,  the  junction  with 
the  ventricle  of  the  aorta  (f return  Halleri,  as  the  narrow  part  of  the 
aorta  is  called)  moves  toward  the  median  line  and  takes  up  its  per- 
manent position  just  in  front  of  the  auricular  canal.  The  change 
in  position  of  the  beginning  or  ventricular  end  of  the  aorta  allows 
the  aorta,  Fig.  288,  C.s,  to  take  a  nearly  straight  course  between 
the  auricles.  The  apex  of  the  primitive  ventricle  is  rounded,  and 
it  is  not  until  some  time  after  the  heart  is  completely  divided  that 
the  ventricle  assumes  the  adult  pointed  shape. 

Changes  in  the  Walls  of  the  Heart.— We  consider  here, 
1.  the  histogenesis  of  the  heart;  2,  thickness  of  the  walls;  3,  the 
special  connective  tissue  or  non-muscular  areas. 

1.  HISTC  x ;  KN i .sis. — For  what  little  is  known  concerning  the  devel- 
<  >])inent  of  muscle  fibres  see  p.  478.  The  heart  consists  originally  of 
the  endothelial  tube  and  the  outer  muscular  tube.  The  endothelium, 
Fig.  :J'.»n,  cndo,  retains  its  primitive  character  as  a  thin  layer  lining 
the  cavity  of  the  heart,  but  the  exact  appearance  of  the  cells  at  suc- 
ive  stages  has  still  to  be  observed;  so  far  as  known  the  endo- 
thelium  does  not  give  off  any  cells  to  fill  up  the  space  between  it  and 


526 


THE   FCETCTS. 


the  muscular  heart.  As  soon  as  the  inner  surface  of  the  ventricle 
becomes  irregular,  Fig.  290,  Ven,  we  find  the  endothelium  close 
against  the  muscular  wall  and  following  it  exactly. 

The  muscular  heart  or  outer  heart  tube  produces  the  pericardial 
covering  (mesothelium)  of  the  heart,  the  muscle  fibres,  and  the  con- 
nective tissue ;  Fig.  290  illustrates  the  general  course  of  these  modi- 
fications. The  muscular  heart  tube  begins  to  thicken,  and  throws 
off  a  certain  number  of  cells  which  assume  a  mesenchymal  charac- 
ter, and  stretch  across  the  space  between  the  outer  and  inner  heart, 


eudo 


Von 


FIG.  290.— Section  of  the  Heart  and  Pericardial  Cavity  of  a  Rabbit  Embryo  of  ten  and  one- 
half  Days.  EC,  Ectoderm;  P,  pericardial  cavity;  Msth,  mesothelium;  B. a,  bulbus  aortae; 
endo,  endothelium;  Fen,  ventricle;  J.o,  aorta. 

as  is  shown  in  the  bulbus  aorta3,  Ba,  of  Fig.  290.  Gradually  the 
number  of  these  cells  increases  until  the  entire  space  is  occupied  and 
the  muscular  heart  is  a  compact  wall  reaching  to  the  endothelium. 
During  its  growth  we  see  the  muscular  heart  acquire  a  more  and 
more  clearly  differentiated  external  layer  of  endothelioid  cells;  as 
indicated  in  the  figure  the  layer  is  particularly  distinct  in  the  ten 
and  a  half  days'  rabbit  over  the  ventricle.  The  remaining  cells  be- 
come for  the  most  part  muscle  fibres,  but  others  retain  the  mesen- 
chymal character  and  give  rise  to  the  connective  tissue,  and  perhaps 


TRANSFORMATION   OF  THK   HKART.  527 

al.-o  to  the  blood-vessels  of  the  heart — when  the  blood-vessels  first 
appear  in  the  cardiac  walls  I  do  not  know. 

•».  Tm<  KNKSS  OF  TUP.  WALLS. — From  what  has  been  said  it  is 
evident  that  the  thickness  of  the  walls  depends  upon  t lie  growth  of 
the  muscular  heart,  which  takes  place  so  that  each  division  of  the 
bent  heart  has  its  characteristic  thickness  of  wall.  In  the  auricles 
the  walls  never  become  very  thick,  and  are  always  of  about  the  same 
diameter  throughout,  excepting  where  the  veins  enter  and  the  heart 
is  attached  to  the  septum  transversum.  In  the  auricular  canal  the 
walls  become  considerably  thicker  than  in  the  auricles,  and  much 
less  thick  than  in  the  ventricles,  where  the  walls  are  most  developed, 
and  form  many  irregular  projections-into  the  interior  of  the  heart  so 
that  the  t i ssue  assumes  a  spongy  appearance,  Fig.  290,  Ven,  which 
early  becomes  one  of  the  most  marked  characteristics  of  the  ventri- 
cles. G.  A.  Gibson,  91. 1,  found  that  during  foetal  life  the  walls  of 
both  ventricles  are  approximately  equal  in  thickness,  therefore  the 
thickness  of  the  adult  left  ventricle  is  acquired  after  birth.  In  the 
bulhus  aortse  the  walls  become  a  little  thicker  than  in  the  auricles. 

3.  NON-MUSCULAR  AREAS  OF  THE  HEART. — There  are  several 
spots  where  connective  tissue  is  developed  to  the  complete  or  partial 
exclusion  of  the  muscle-fibres  of  the  outer  heart.  These  spots  have 
great  importance  in  the  differentiation  of  the  heart.  They  are : 
the  area  interposita;  the  thickened  edge  of  the  septum  superius;  the 
bolsters  of  the  auricular  canal ;  and  the  ridges  in  the  bulbus  aortae. 

Sinus  Venosus. — A  venous  sinus,  more  or  less  distinct  from  the 
auricles  and  formed  by  the  union  of  the  large  veins  entering  the 
heart,  is  found  temporarily  in  mammalian  embryos,  and  represents 
the  adult  condition  of  reptiles.  At  first  the  sinus,  for  which  His 
uses  the  name  sinus  reuniens,  is  symmetrically  placed  in  the  septum 
transversum  at  the  venous  end  of  the  heart.  As  soon  as  the  heart 
has  become  bent  and  the  descent  of  the  ventricles  has  occurred,  the 
sinus  necessarily  lies  on  the  dorsal  side  of  the  auricular  division  of 
the  heart  and  appears  partly  free  from  the  septum  transversum 
as  a  short  piece,  Fig.  287,  between  the  septum  and  the  auricle. 
The  sinus  is  long  in  the  transverse  direction,  narrow  in  the  longi- 
tudinal and  dorso-ventral  direction  of  the  embryo.  But  as  the  lat- 
eral outgrowths  forming  the  auricles  are  developed,  the  lateral  ends 
of  the  sinus  are  bent  headward,  so  that  it  becomes  somewhat  horse- 
shoe-shaped— the  convexity  being  toward  the  apex  of  the  heart.  At 
the  same  time  the  sinus  grows  much  less  rapidly  than  the  auricles ; 
thus  it  becomes  proportionately  smaller  in  later  stages — in  a  rabbit 
of  fourteen  days  its  length  is  equal  to  only  half  the  width  of  the 
auricles.  Into  the  ends  of  the  sinus  open  the  ducts  of  Cuvier,  Fig. 
287,  D.Cy.,  and  on  each  side  but  nearer  the  median  line  the  omphalo- 
mesaraic  and  umbilical  veins  (rabbits  of  eleven  days).  The  two 
inesaraic  and  umbilical  openings  are,  however,  soon  replaced  by  a 
single  vein,  the  vena  cava  inferior,  which  opens  on  the  right  side  of 
the  sinus,  Fig.  287.  The  cava  inferior  is  present  in  rabbits  of  twelve 
and  one-half  days,  and  its  development  is  described  later  in  this 
chapter.  By  the  time  the  cava  inferior  is  developed,  the  sinus  is  no 
longer  found  opening  into  the  heart  in  the  median  line,  but  upon 
the  right  side,  Fig.  287 ;  this  change  Born  attributes  to  the  manner 


528  THE    FCETUS. 

in  which  the  partial  separation  of  the  sinus  from  the  septum  trans- 
versum  is  effected;  the  furrow  or  groove,  which  produces  the  sepa- 
ration, cutting  in  deeper  on  the  left  than  on  the  right  side,  thus 
forcing  the  veins  from  the  left  side  over  to  the  right.  The  actual 
opening  of  the  sinus  into  the  right  auricle  is  elongated  and  oblique, 
as  shown  in  Fig.  280,  and  is  bordered  by  two  valves,  which  unite  at 
the  upper  end  of  the  heart  and  are  continued  as  the  septum  spurium. 
The  history  of  the  valves  is  given  below,  p.  532.  The  sinus  as  a 
whole  bulges  somewhat  into  the  interior  of  the  auricle.  The 
stage  to  which  we  have  now  traced  the  sinus  venosus  is  found  in  the 
human  embryo  of  10  mm. 

In  the  course  of  its  further  development  the  mammalian  sinus 
merges  into  the  right  auricle  and  entirely  disappears  as  a  distinct 
division.  The  modification  is  accomplished  very  gradually,  by  the 
expansion  of  the  right  auricle  backward  and  downward  ;*  it  thus 
embraces  the  whole  of  the  right  horn  of  the  sinus,  converting  the 
right  horn  into  a  part  of  the  auricular  cavity,  and  the  dorsal  or 
posterior  wall  of  the  horn  into  an  integral  part  of  the  auricular  wall, 
consequently  the  valves  of  the  venous  opening  appear  to  spring  from 
the  posterior  wall  of  the  heart.  The  three  permanent  body  veins  open 
as  before  with  a  common  oblique  mouth ;  compare  Fig.  289.  The  upper 
end  of  this  orifice  corresponds  to  the  opening  of  the  vena  cava  superior 
dextra,  the  lower  end  to  the  opening  of  the  vena  cava  inferior,  the 
middle  to  the  opening  of  vena  cava  superior  sinistra.  The  sinus  is 
found  almost  completely  merged  in  the  auricle  in  rabbit  embryos  of 
about  twenty  days,  and  its  limits  can  be  traced  in  considerably  older 
stages,  and  according  to  His  even  in  the  adult  human  heart. 

The  left  horn  of  the  sinus  remains  outside  the  auricle  and  becomes 
the  coronary  sinus  of  the  adult. 

Division  into  Right  and  Left  Hearts. — The  developmental 
conditions  which  result  in  the  complete  division  of  the  heart  are 
established  by  the  primitive  bending  of  the  heart  and  the  outgrowth 
of  the  auricles;  the  former  initiates  the  division  of  the  ventricle, 
the  latter  the  final  separation  of  the  two  auricles.  The  division  is 
supplemented  by  that  of  the  auricular  canal  and  aorta.  Accordingly 
we  may  consider  the  division  of  the  heart  under  four  heads :  division 
of,  1,  the  auricles;  2,  the  auricular  canals;  3,  the  ventricle;  4,  the 
aorta. 

1.  DIVISION  OF  THE  AURICLES. — The  histories  of  the  process: 
given  by  His  and  Born,  differ  in  several  essential  points.  I  follow  the 
latter  authority,  I.  c.,  pp.  308-312,  as  giving  the  presumably  correct 
account.  When  the  two  auricles  grow  forth,  they  expand  upward, 
but  there  remains  between  them  a  partition,  Fig.  291,  to  which  His 
applies  the  name  of  septum  super  i  us,  Born  the  name  septum 
primum.  As  the  auricles  continue  to  expand,  the  septum  of  course 
increases  by  the  continued  meeting  of  the  auricles,  and  it  also  in- 
creases, without  doubt,  by  its  own  growth.  The  septum  early 
acquires  a  very  characteristic  appearance  by  the  thickening  of  its 
lower  edge,  Fig.  292,  just  above  the  auricular  canal;  the  thin 
part  of  the  partition  contains  muscle  fibres,  but  the  thickened  edge 

*  I  follow  Born.  89. 1,  326,  but  not  intelligently,  for  I  have  been  unable  to  understand  fully 
his  account  of  the  merging  of  the  sinus  in  the  auricles. 


TRANSFORMATION  OF  THE   HEART. 


529 


Peri- 


R.V. 


FIG.  291. — Section  in  the  Frontal  Plane  through  the  Heart  of 
a  Rabbit  Embryo  of  thirteen  Days,  c.aw,  canal  is  auricularis; 
s.sp,  septum  spurium;  «*,  septum  superius;  R.  P".,  right  ven- 
tricle; Peri,  pericardial  cavity.  X  11*  diams. 


consists  of  embryonic  connective  tissue;  the  septum  is,  of  course, 
covered  with  endothelium.  Seen  from  the  side,  the  edge  of  the  sep- 
tum presents  a  curved  outline,  being  concave  toward  the  ventricle. 
The  only  connection 
between  the  auricle  is 
now  under  the  edge 
of  the  septum.  This 
communication  has 
been  homologized 
with  the  foramen 
ovale  of  the  fcetal 
heart  in  later  stages. 
Born  has  shown  that 
this  homology ,  which 
was  maintained  by 
his  predecessors,  is 
incorrect,  and  that 
the  septum  grows 
down  to  the  auricular 
canal,  Cy,  and  by  unit- 
ing with  the  partition 
developed  in  the  canal 
closes  permanently 
the  primary  commu- 
nication (ostiumpri- 
in  n m  of  Born). 

The  true  foramen  ovale  is  developed  as  a  perforation  of  the  upper 
part  of  the  septum  superius.  This  perforation  is  termed  by  Born 
the  oxtiiun  secundum,  1.  c.,  p.  311.  It  appears  in  rabbit  embryos  of 
1  \  mm.  (about  fifteen  days) ;  it  is  small  at  first  and  situated  close  to 
the  wall  of  the  auricle ;  as  to  how  it  is  developed,  Born  gives  no  in- 
formation. It  soon  enlarges,  and  in  rabbits  of  7.3  mm.  or  nearly 
thirteen  days  is  about  the  same  size  as  the  earlier  communication 
(oxfimit  prim  n  in),  which;  from  this  stage  on,  gradually  contracts 
until  in  rabbits  of  10-12  mm.  it  has  closed.  In  rabbits  of  ten  milli- 
metres (thirteen  and  one-fourth  days)  a  new  septum  appears  above 
the  foramen  ovale;  it  is  crescentic  in  shape,  and  belongs  to  the  right 
auricle,  since  it  springs  a  little  to  the  right  of  the  insertion  of  the 
septum  superius.  This  new  partition  (septum  secundum)  was  first 
recognized  by  Born,  and  can  be  followed  a  little  way  alongside  the 
septum  superius :  it  is  also  distinguished  by  being  thicker  than  the 
septum  superius;  its  edge  forms  part  (liinbus  Vieusenii)  of  the 
boundary  of  the  foramen  ovale.  The  foramen  ovale  remains  open 
during  fcetal  life,  and  in  man  is  not  completely  closed  until  some 
time  after  birth. 

On  the  posterior  wall  of  the  auricle  the  septum  superius  runs  on  to 
the  area  interposita  of  His,  see  p.  525,  and  can  for  part  of  its  extent 
be  regarded  as  an  upgrowth  of  that  area.  Born,  in  opposition  to 
His,  attributes  little  special  importance  to  this  relation.  The  closure 
of  the  primary  communication  between  the  auricles  is  better  de- 
scribed in  connection  with  the  division  of  the  auricular  canal. 

2.  DIVISION  OF  THE  AURICULAR  CANAL.— The  auricular  canal 
34 


530 


THE   FCRTUS. 


in  human  embryos  of  8  mm.  is  found,  as  it  were,  invaginated  into 
the  ventricle.  Fig.  292,  c,  c'.  There  appear  also  two  prominences  of 
connective  tissue,  one  on  the  posterior,  one  on  the  anterior  wall  of 
the  canal.  These  prominences  are  the  Endothelkissen  of  F.  F. 
Schmidt,  70. 1 ,  the  Endocardkissen  of  Born,  I.  c. ,  320.  They  increase 
in  height  until  they  meet  and  unite  (rabbit  embryos  of  about  thirteen 

days)  so  as  to  divide  the  passage 
of  the  auricular  canal  into  two 
channels,  c  and  c'.  The  prom- 
inences are  wide;  hence,  when 
they  meet,  the  greater  part  of 
the  canal  is  closed  and  the 
channels  are  relatively  small. 
Each  channel  maintains  the 
direct  connection  between  the 
auricle  and  ventricle  of  its  own 
side,,  and  is  triangular  in  sec- 
tion. The  triangular  section  is 
a  necessary  consequence  of  the 
mode  of  formation ;  each  prom- 
inence forms  a  side,  and  the 
original  wall  of  the  canal  makes 
the  third  side.  While  the  prom- 
inences are  joining  one  another, 

FIG.  292. -Oblique  Section  of  the  Heart  of  a  ^e  edS6  °f  the  Septum  SUperiuS 
Human  Embryo  of  8.5  mm.  (His'  Embryo  I.).  also  Unites  With  them,  SO  that, 
CE,  Oesophagus ;  Lg,  lung ;  svs,  sinus  venosus  sin- 
ister; L.Au,  left  auricle;  R.Au,  right  auricle; 
c,  c',  channels  of  auricular  canal;  x,  connective 
tissue;  Sp.f,  septum  inferius;  Sao,  anlages  of 
aortic  septum ;  V.E,  Eustachian  valve;  svd,  sinus 
venosus  dexter,  x  24  diams.  After  W.  His. 


Sao 


except  for  the  open  foramen 
ovale,  both  auricles  and  the 
auricular  canal  are  divided  be- 
fore the  ventricles.  His  has 
proposed  so  designate  the  septum  thus  formed  by  the  term  septum 
intermedium,  but  a  special  new  term  seems  to  me  superfluous.  As 
shown  in  Fig.  292,  the  septum  consists  of  a  thinner  part  between 
the  auricles  and  a  thicker  part  in  the  auricular  canal.  His  attrib- 
utes considerable  importance  to  the  area  interposita,  as  contributing 
to  unite  the  septum  superius  with  the  prominences  of  the  auricular 
canal;  compare  p.  525. 

The  auricular  canal  soon  ceases  to  be  a  recognizably  distinct  part 
of  the  heart,  and  is  represented  only  by  the  openings  between  the 
auricles  and  ventricles  (ostia  atrio-ventriculares) ,  and  by  the  atrio- 
ventricular  valves. 

3.  DIVISION  OF  THE  VENTRICLES. — The  two  limbs  of  the  ventricle 
are,  it  will  be  remembered,  at  first  entirely  distinct,  Fig.  286,  and 
«ven  after  the  ventricle  has  grown  considerably  and  the  connection 
between  the  two  limbs  has  widened  so  much  that  they  form  essen- 
tially one  continuous  cavity,  the  original  division  between  the  left 
limb  and  the  right  limb  is  marked  by  a  groove  on  the  external  sur- 
face. This  groove  corresponds  to  a  fold  of  the  cardiac  wall,  and 
hence  is  represented  in  the  interior  of  the  heart  by  a  projection  which 
grows,  as  development  proceeds,  although  the  external  groove  is 
gradually  obliterated.  The  growth  of  the  projection  establishes  the 
partition  between  the  ventricles,'  which  is  known  as  the  septum 


TRANSFORMATION   OF    THE   HEART.  531 

i,  Fig.  2'.f.\  .s/>./.  This  septum  is  thick,  and  consists  chiefly 
of  muscle  fibres ;  it  has  a  partially  trabecular  structure,  and  certain 
of  its  trabecuL-B  are  ultimately  transformed  into  chordae  of  the  atrio- 
ventrifiilar  valves.  In  a  side  view  the  upper  edge  of  the  septum  is 
seen  to  be  curving,  the  septum  as  a  whole  being  crescent-shaped  ;  it 
is  situated  somewhat  to  the  right  side  of  the  median  line,  Fig.  291. 
Alter  it  is  fully  developed  (rabbit  embryos  of  10  mm.)  the  septum 
reaches  nearly  to  the  auricular  canal  and  if  it  were  prolonged  it 
w<  >uld  join  the  right-hand  side  of  the  partition  in  the  auricular  canal ; 
on  the  posterior  side  of  the  heart  the  septum  does  actually  join  the 
auricular  canal,  but  on  the  anterior  side  it  fades  out  toward  the 
a«  >rta.  In  brief,  the  broad  communication  between  the  two  ventricles 
becomes  an  interventricular  foramen  bounded  by  the  partition  of  the 
auricular  canal  and  by  the  edge  of  the  septum  inferius;  it  repeats 
for  the  ventricles  the  role  of  the  foramen  ovale  for  the  auricles,  p. 
I  >ut  were  it  to  close  over,  as  does  the  foramen  ovale,  the  left 
ventricle  would  have  no  exit,  because,  as  already  described  (compare 
Fig.  288),  the  aorta  is  the  prolongation  of  the  right  limb  of  the  ventri- 
cle. In  order  to  furnish  the  necessary  outlet  the  aorta  is  divided 
into  two  vessels,  and  one  of  these  (aorta  vera)  becomes  connected 
through  the  interventricular  foramen  exclusively  with  the  left  ven- 
tricle, thereby  rendering  the  separation  of  the  ventricles  complete. 
Accordingly,  to  fully  understand  this  separation  we  must  follow  the 
history  of  the  division  of  the  aorta. 

4.  DIVISION  OF  THE  AORTA. — The  cardiac  aorta  comprises  the 
f return  Halleri  and  bulbus  aortse,  which  at  an  early  stage  differ  in 
the  width  of  their  cavities,  Fig.  286.  This  difference  is  soon  lost, 
and  the  cavity  (endothelial  aorta)  becomes  flattened  except  in  the 
truncus  aortse  or  upper  part  of  the  bulbus,  where  the  cylindrical  form 
is  retained.  The  plane  of  the  flattened  cavity  changes;  it  is  sagittal 
where  the  aorta  arises  from  the  conus  arteriosus  of  the  right  ventri- 
cle, and  as  we  ascend  along  the  aorta  we  find  the  anterior  edge  of 
the  cavity  moving  toward  the  left  until  the  plane  of  the  flattened 
cavity  becomes  transverse.  Meanwhile  the  muscular  wall  of  the 
aortic  heart  has  developed,  partly  into  muscle,  partly  into  connective 
tissue,  and  this  connective  tissue  develops  into  a  ridge  on  each  side 
<  -f  the  flattened  cavity.  The  ridges  increase  and  unite,  thus  dividing 
the  aorta  into  two  channels,  the  anterior  or  left  channel  becoming 


Fio.  293.— Sections  at  Different  Levels  through  the  Cardiac  Aorta  of  a  Human  Embryo  of 
11. 5  nun.  i  His1  Embryo  Rg).  The  lowest  section  is  on  the  left,  the  highest  on  the  right,  a, 
Aorta;  P,  pulmonary  division.  X  15  diams.  After  W.  His. 

that  of  the  pulmonary  artery,  the  posterior  or  right  channel  becoming 
the  permanent  true  aortic  cavity.  The  union  of  the  ridges  begins 
just  where  the  aorta  divides  to  form  the  aortic  arches,  and  the  parti- 
tion at  this  point  is  sagittal,  cf.  Fig.  293.  The  formation  of  the 
partition  progresses  downward  toward  the  ventricle,  the  plane  of 


532  THE   FCETUS. 

the  partition  gradually  changing  to  transverse.  The  two  ridges 
are  found  to  extend  into  the  ventricle,  and  participate  in  the  closure 
of  the  interventricular  foramen,  by  developing  an  oblique  partition 
which  grows  down  to  the  edge  of  the  septum  inferius,  and  thus  con- 
verts the  interventricular  foramen  into  the  orifice  of  the  true  aorta. 
The  blood  in  leaving  the  left  ventricle  must  now  pass  through  the 
foramen,  then  across  a  space  which  originally  belonged  to  the  right 
ventricle,  but  which  has  been  shut  off  by  the  down-growth  of  the 
septum  aorticum.  The  ventricular  extension  of  the  aortic  partition 
is  effected  chiefly  by  the  left  or  anterior  ridge,  the  right  or  posterior 
ridge  passing  out  more  on  to  the  lateral  wall  of  the  ventricle  where 
it  fades  out;  the  left  ridge  (rabbit  embryos  of  14  mm.)  runs  on  to 
the  edge  of  the  septum  inferius.  The  division  of  the  aorta  and 
ventricle  is  completed  in  rabbit  embryos  of  about  sixteen  days. 

At  the  upper  end  of  the  aorta  the  partition  extends  so  that  the 
fifth  aortic  arches  are  connected  only  with  the  pulmonary  aorta, 
while  the  remaining  arches  are  connected  with  the  true  aorta  only. 
Soon  after  the  internal  partition  is  formed,  the  external  division 
commences  as  two  grooves  on  the  outside  of  the  aorta,  beginning 
just  between  the  fourth  and  fifth  aortic  arches.  The  two  grooves 
extend  to  the  ventricle  and  gradually  deepen,  until  the  aorta  is  com- 
pletely divided  into  two  vessels  (Born,  /.o.,  337),  which  have,  as  soon 
as  they  are  separated,  both  their  connections  with  the  heart  and 
their  relative  positions  to  one  another  essentially  as  in  the  adult. 

The  heart  is  now  completely  divided. 

Valves  of  the  Heart.  — The  entrances  of  the  pulmonary  veins, 
have  no  valves ;  the  entrances  of  the  body  veins  have  two  valves  in 
the  embryo,  of  which  the  left  disappears  and  the  right  persists  as 
the  Eustachian  valve  and  Thebesian  valve ;  the  right  atrioventricular 
passage  has  the  tricuspid,  the  left  the  bicuspid  or  mitral  valve.  The 
entrances  of  the  pulmonary  or  right,  and  true  or  left  aorta  are  each 
guarded  by  three  semilunar  valves.  As  is  well  known,  all  these 
valves  are  set  so  as  to  favor  the  flow  of  blood  toward  the  arteries  and 
prevent  its  flow  toward  the  veins.  We  shall  consider,  1,  the  venous 
valves :  2,  the  atrioventricular  valves :  3,  the  aortic  valves. 

1.  THE  VENOUS  VALVES. — In  a  human  embryo  of  10  mm.,  the 
opening  of  the  body  veins  or  sinus  venosus  into  the  right  auricle  is 
guarded,  as  shown  in  Fig.  289,  by  two  valves  or  thin  flaps  of  the 
heart  walls ;  at  the  upper  side  of  the  oblique  opening  the  two  valves 
unite  and  are  continued  as  the  septum  spurium,  S.sp;  the  left  valve 
lies  near  the  septum  superius  and  merges  into  the  area  inter posita ; 
the  right  valve  is  from  the  start  much  larger  than  the  left,  and  de- 
velops into  the  valvula  Eustachii  and  valvula  Thebesii.  The  venous 
valves  owe  their  origin  to  the  sinus  venosus  being  pushed  into  the 
right  auricle  and  in  consequence  forming  a  fold  which  projects  around 
the  venous  orifice  into  the  cavity  of  the  heart.  The  edge  of  this 
fold  grows  considerably  and  becomes  the  anlage  of  the  venous  valves. 

The  left  valve  gradually  disappears — probably  completely  or  nearly 
so.  But  His  thought  it  contributed  to  form  part  of  the  edge  of  the 
foramen  ovale.  Born's  later  observations,  /.c.,  331,  suggest  rather 
that  it  never  unites  with  the  septum  superius  (inter-auricular  parti- 
tion) but  simply  aborts,  and  for  a  time  (embryos  of  the  fourth  month) 


TRANSFORMATION   OF   THE   HEART.  533 

can  be  recognized    as  a  slight  ridge  on  the  posterior  wall  of  the 
auricle. 

The  right  f«/rt%  which  is  always  larger  than  the  left,  persists  in 
greater  part.  Karly  in  its  development  it  begins  to  grow  unequally, 
so  that  tlu-rc  is  a  larger  upper  tlap  bounding  the  main  venous  open- 
ings, and  a  smaller  lower  tlap  bounding  the  mouth  of  the  coronary 
vein;  the  two  flaps  are,  of  course,  continuous  with  one  another 
though  separated  by  a  notch;  the  upper  flap  is  the  anla^e  of  the 
KuMachian,  the  lower  of  the  Thebesian  valve.  The  Eustachian 
valve  does  not  include  the  whole  upper  division  of  the  primitive 
valve,  for  the  uppermost  part  aborts,  though  it  can  still  be  traced 
in  the  human  embryo  of  four  months  and  even  at  seven  months  (Born, 
]..  :;:)->).  * 

The  septum  xpurium  is  to  be  regarded  as  the  prolongation  of 
the  united  right  and  left  venous  valves.  As  it  contains  muscular 
fibres,  its  probable  function  is,  as  suggested  by  Born,  to  draw  the 
two  valves  together  and  prevent  the  back  flow  of  the  blood,  a  func- 
tiun  of  great  importance  in  the  embryonic  heart  before  the  atrioven- 
trieular  valves  are  developed.  In  a  human  embryo  of  34  mm. 
(beginning  of  the  third  month),  the  septum  is  so  much  reduced  that 
it  would  not  be  recognized  without  knowledge  of  the  preceding 
^es,  and  at  this  time  we  find  the  tricuspid  and  mitral  valves  in 
action. 

2.  THE  ATRIOVENTRICULAR  VALVES. — Their  development  has 
been  studied  by  Bernays.  76.1,  whose  results  have  been  confirmed 
by  Born,  89.1,  340.  W.  His*  observations  ("  Anat.  menschl.  Em- 
bryonen,"  Heft  III.,  152-1GO)  also  are  important.  The  valves 
proper — in  distinction  to  the  muscles  and  tendons,  which  belong  to 
the  ventricle — are  to  be  regarded  as  morphologically  modifications 
<  »t  the  walls  of  the  auricular  canal,  the  canal  being  to  a  certain  extent 
in va^inated  into  the  ventricles  (W.  His,  /.c.,  Fig.  105).  Theinvag- 
inated  portions  of  the  canal  become  the  anlages  of  the  atrioventricular 
valves,  on  the  left  side  the  mitral,  and  on  the  right  the  tricus- 
pid. When  the  auricular  canal  divides  into  the  two  atrioventricular 
channels,  each  channel  or  ostium  is  triangular  in  section,  and  as  this 
form  is  preserved  on  the  right  side  of  the  heart,  there  are  three 
valves  developed,  one  as  the  prolongation  of  each  of  the  three  walls 
of  the  ostium,  but  on  the  left  side,  in  consequence  of  as  yet  unde- 
termined conditions,  there  are  developed  only  two,  the  mitral  valves. 
In  each  case  the  lateral  valves  are  developed  from  a  fold  of  the  heart 
wall,  which,  as  indicated  at  x  in  Fig.  292,  is  formed  partly  by  the 
Avail  of  the  auricular  canal,  partly  by  the  wall  of  the  ventricle,  and 
partly  by  connective  tissue  in  the  interior  of  the  fold.  The  medial 
valves — one  on  the  left  side,  two  on  the  right— may  be  described  as 
prolongations  of  the  septum  intermedium,  Fig.  292.  The  mus- 
cular trabecula3  of  the  ventricle  are,  almost  from  the  start,  con- 
nected with  the  ventricular  surfaces  of  the  atrioventricular  valves ; 
out  of  these  trabecula3  are  developed  the  chords  of  the  valves,  known 
in  the  adult  as  the  papillary  muscles  and  chordae  tendinese.  The 
trabeculse  are  original!}'  very  irregular  in  their  arrangement,  but  as 
development  progresses  those  which  are  connected  with  the  valves 
become  longer  and  slenderer,  and  descend  in  main  lines  directly  from 


534  THE   FCETUS. 

the  valves  to  the  ventricular  walls,  but  preserve  the  network  charac- 
ter. A  little  later  (pig  and  calf  embryos  of  45-60  mm.)  the  valvular 
trabeculae  become  very  slender  though  still  muscular,  in  the  neigh- 
borhood of  the  valves,  but  toward  the  apex  of  the  heart  the  fine 
trabeculaB  unite  into  plump  bundles,  the  papillary  muscles  (Bernays, 
76. 1,  495.  At  this  stage  each  papillary  muscle  breaks  up  into  some 
six  or  eight  muscular  cords,  which  are  inserted  into  the  valves.  In 
older  embryos  (in  man  during  the  fifth  month)  the  muscular  cords 
change  into  tendinous  chords ;  the  muscular  tissue  in  them  disap- 
pears and  is  replaced  by  mesenchyma,  which  becomes  fibrillar ;  hence 
each  papillary  muscle  is  connected  by  several  filamentous  tendons 
with  its  valve.  The  slender  tendons  are  the  chordae  tendinea?  of 
the  human  anatomy. 

3.  THE  AORTIC  OR  SEMILUNAR  VALVES. — Before  the  bulbus 
aortas  completely  divides  into  the  true  and  the  pulmonary  aortaB, 
there  appear  four  small  protuberances  at  its  ventricular  orifice. 
Each  protuberance  is  a  mass  of  connective  tissue  covered  by  endo- 
thelium ;  two  of  them  are  merely  the  ends  of  the  ridges,  described  p. 
531,  by  which  the  aorta  is  divided.  When  the  division  is  completed, 
the  ends  of  the  two  ridges  are  also  divided,  making  four  protuber- 
ances, or  in  all  six — three  for  each  aortic  trunk.  These  protuber- 
ances are  the  anlages  of  the  semilunar  valves,  and  may  be  seen  in  a 
human  embryo  of  seven  weeks.  They  grow  until  they  meet  so  as  to 
close  the  aortic  entrances,  and  assume  the  adult  form  by  becoming 
concave.  Their  exact  history  has  still  to  be  worked  out ;  compare 
Tonge,  70. 1,  387,  on  the  semilunar  valves  of  the  embryo  chick. 

II.  THE  ARTERIAL  SYSTEM. 

We  left  the  arterial  system  consisting  of  the  cardiac  aorta,  the 
five  aortic  arches,  and  four  carotids,  the  dorsal  aorta,  vitelline  or  om- 
phalo-mesaraic  arteries,  and  allantoic  arteries  (see  p.  274-276)  and 
have  now  to  trace  the  changes  which  result  in  the  adult  system 
of  arteries — changes  which  are  very  numerous. 

Aortic  Arches. — The  general  scheme  of  the  metamorphosis  of 
the  great  arteries  of  the  five  gill-arches  is  indicated  by  the  diagrams, 
Fig.  294.  A  is  the  primitive  condition:  The  wide  pharynx,  Ph,  is 
shaded  to  suggest  its  rounded  form ;  the  four  gill-clefts  of  the  left 
side,  are  also  indicated,  1,  2,  3,  4.  From  the  heart,  Ht,  runs  out  the 
aorta,  which  soon  forks;  each  fork  gives  off  five  branches,  I,  II, 
III,  IV,  V,  one  in  front  of  each  cleft  and  a  fifth  behind  the  fourth 
cleft.  On  the  dorsal  side  the  five  arches  unite  into  a  common  trunk, 
which  joins  the  corresponding  trunk  from  the  opposite  side  to  form 
the  median  dorsal  aorta,  Ao.  Now,  as  the  clefts  develop  from  in 
front  backward,  so  the  first  branchial  arch  arises  first,  the  second 
next,  and  so  on,  until  the  series  is  completed ;  shortly  after  each  arch 
is  formed  the  aortic  vessel  appears  in  it. 

The  disposition  in  the  human  embryo  corresponds  entirely  to  the 
diagram,  for  the  relations  are  all  the  same,  although,  owing  to  the 
rolling  up  of  the  embryo,  the  primitive  topography  is  disturbed :  thus 
in  Fig.  300,  we  at  once  recognize  the  four  clefts  and  the  five  arches. 

The  homologies  of  this  complicated  aortic  system  with  that  of  the 


THE    AKTKKIAL    SVSTKM. 


535 


adult  mammal  arc  shown  in  the  diagram  Fig.  294,  B.  The  shaded 
parts  are  preserved  in  the  adult;  the  others  disappear.  The  parts 
lost  are  the  h'rst  and  second  arches;  the  dorsal  connection  between 
the  third  and  fourth  left  arches;  the  upper  part  of  the  left  fifth  arch; 
there  disappear  on  the  right  side  the  upper  part  of  the  fourth  and 
the  whole  of  the  fifth  arch,  and  also  the  dorsal  connection  of  the 
arches  with  the  median  dorsal  aorta,  Ao.  There  remain  parts  as 
follows:  1.  The  heart  aorta,  which  by  an  internal  septum  is  divided 
into  two  aortse  (p.  531),  one  of  which  maintains  a  communication 
with  the  right  ventricle  and  is  continuous  headward  with  the  fifth 
arch  of  the  left  side;  from  the  middle  of  this  arch  springs  a  vessel 
which  soon  forks  to  make  the  two  pulmonary  arteries,  P;  during 


Fio.  294.— A,  Diagram  of  Pharynx  of  an  Amniote  Vertebrate.  1,  2,  8,  4,  Gill  pouches  (clefts) 
ofthf  pharynx.  /'// :  <  ><•.  (jesophagus :  I.  II.  Ill,  IV,  V,  aortic  arches  springing  from  the  fork  of 
the  uortii  nf  the  heart,  Ht;  on  the  dorsal  side  the  five  arches  again  unite  into  a  single  trunk, 
\s  hi«  h  joins  its  opposite  fellow  to  form  the  median  dorsal  aorta,  Ao;  Af,  invagination  of  the  ec- 
t.  «lfnn  tu  t'<inn  the  mouth;  Elr.e,  external  carotid  springing  from  the  ventral  side  of  the  first 
iiortii-  ar«-ii :  ln.<  •.  internal  carotid,  springing  from  the  dorsal  side  of  the  first  aortic  arch :  om, 
omi>halo-mesaraie  veins  emptying  into  the  heart.  The  arrows  indicate  the  direction  of  the 
blood-currents.  B,  diagram  of  gill-arches  as  preserved  in  mammalia;  the  shaded  portions  are 


those  retained.  th««  unshaded  vessels  are  lost;  aa,  ductus  arteriosus;  P,  pulmonary  artery. 
other  letters  are  the  same  as  above. 


The 


foetal  life  the  upper  part  of  this  arch,  da,  persists  as  the  well-known 
ductus  arteriosus,  so  that  there  is  a  direct  communication  between 
the  pulmonary  and  the  body  aorta.  Soon  after  birth  the  lumen  of 
the  ductus  is  obliterated.  2.  The  left  fourth  arch,  which  is  very 
much  enlarged,  to  constitute  the  permanent  aortic  arch ;  as  shown  in 
the  diagram,  the  obliteration  of  parts  is  such  that  the  left  fourth 
arch  is  the  only  permanent  channel  of  communication  between  the 
heart  and  the  dorsal  aorta,  Ao;  hence  the  aorta  of  the  adult  springs 
from  the  heart,  and  gives  off  to  the  right  a  branch,  then  makes  itself 
a  great  arch  on  the  left  side  up  to  the  back,  where  it  is  continued 
down,  i.e.  tailward.  3.  The  third  arches  on  both  sides,  appearing, 
as  the  figure  clearly  shows  they  must,  as  portions  of  the  internal 
carotid,  In.c;  the  ventral  stem  between  the  third  and  fourth  arches 
is  the  common  carotid  of  the  adult  on  each  side,  while  the  continua- 


536 


THE   FCETUS. 


tion  of  that  stem  head  ward  becomes  part  of  the  external  carotid. 
4.  The  right  fork  of  the  aorta  becomes  the  arteria  innominata,  a; 
part  of  the  right  fourth  arch  remains  as  the  right  subclavian  artery, 
b;  the  corresponding  left  subclavian  being  given  off  from  the  corre- 
sponding left  arch,  that  is  to  say,  by  the  great  arch  of  the  aorta. 
The  ventral  stem  between  the  right  third  and  fourth  arches  becomes 
the  common  carotid  of  the  right  side. 

But,  though  the  connections  and  metamorphoses  of  the  aortic 
arches  are  sufficiently  illustrated  by  Fig.  294  to  elucidate  the  homol- 
ogies,  yet  the  actual  course  of  the  arches  is  somewhat  different,  Fig. 
295,  the  branching  taking  place  as  described  p.  306.  The  cardiac 
aorta  at  first  opens  under  the  pharynx  between  the  bases  of  the  man- 
dibular  and  hyoid  arches,  but  by  the  time  the  five  aortic  arches  are 
developed  it  has  moved  tailward ;  finally  when  during  the  second 


Oe 


coe. 


FIG.  296. — Aortic  System  of  His1  Embryo 
Bl,  4.25  mm.  I-V,  Aortic  arches;  t7fc,  mandi- 
ble; /Sd,  thyroid  gland;  K,  main  aorta;  P, 
pulmonary  artery ;  Lg,  lung;  Oe,  resophagus; 
X  36  diams.  After  W.  His. 


FIG.  295.  —Anterior  Wall  of  the  Pharynx  of  a 
Human  Embryo  of  3.2  mm.  length.  1  to  4, 
Gill-pouches ;  the  ectodermal  pouches  are  sep- 
arated by  thin  walls  from  the  entodermal ;  the 
gill-arches  show  the  aortic  arches  drawn  in 
dotted  lines  and  arising  from  the  heart  aorta, 
Ao;  M,  mouth;  Oe,  oesophagus;  Coe,  body 
cavity.  X  50  diams.  After  His. 

month  the  head  is  bent  back  or  raised,  compare  Chapter  XVIII. , 
Figs.  223,  226,  and  the  front  of  the  neck  elongates,  the  heart  remains 
on  the  level  with  the  thorax,  and  the  position  of  the  aorta  is  relatively 
lowered.  The  five  aortic  arches  are  found  in  human  embryos  of 
2.6-3.2  mm.,  and  all  persist  for  a  short  time,  but  as  soon  as  the  neck 
bend  begins  to  develop  (embryos  of  4  mm.)  the  disappearance  of  the 
first  aortic  arch  occurs,  Fig. 296,  to  be  very  soon  followed  by  the  dis- 
appearance of  the  third  arch,  but  the  dorsal  part  of  these  arches  per- 
sists, as  already  explained,  as  the  internal  carotid,  while  the  ventral 
part  persists  as  the  stem  of  the  external  carotid,  which  gives  off  in 
the  region  of  the  hyoid  arch  a  branch,  and  in  the  region  of  the  man- 
dibular  arch  a  second  branch.  The  branches  are  designated  by 
His("Anat.  menschl.  Embryonen,"  Heft  III.,  187)  as  the  arteria 
lingualis  and  arteria  maxillaris  communis  respectively.  The  ar- 
rangement with  three  arches  open,  the  first  and  second  closed,  is 
shown  in  Fig.  298.  As  both  the  third  arches  and  the  left  fourth  per- 


THE    ARTERIAL   SYSTEM.  58? 

sist,  we  have  next  to  consider  the  modification  of  the  right  fourth 
arch(--lor/f(  tlt'.sct'mlcnx  (fcx£ra),which  in  embryos  of  3  mm.,  and  even 
less,  is  smaller  in  diameter  than  the  corresponding  left  arch,  a  differ- 
ence which  His  is  inclined  to  attribute  to  the  oblique  insertion  of  the 
cardiac  aorta  rendering  the  left  arch  the  more  direct  continuation  of 
the  cardiac  aorta.  Curiously  enough  the  difference  is  lost  tempora- 
rily (embryos of  7-10  mm.),  but  becomes  very 
marked  again  in  those  of  11-12  mm.,  Fig. 
&97,  so  that  it  now  is  hardly  more  than  a 
branch  of  the  aorta,  supplying  the  carotid 
and  vertebral  arteries,  v,  of  the  right  side. 
In  an  embryo  of  13.8  mm.  the  right  fifth 
arch  has  disappeared,  and  with  it  the  piece 
connecting  it  with  aorta  descendens  dextra. 
The  disposition  of  the  main  stems  persists  at 
this  sta^e,  with  little  change  except  in  their 
diameters  until  after  birth.  The  cardiac 
aorta  (<n>rl«  <ix<-cti(l<'n.s)  divides  into,  1,  the 
smaller  left  arch  (urfrrin  a/«>ti t/in<i)  which 
is  continued  as  the  artcn'n  m&cfomdaod 
ui\  .-s  off  as  a  branch  the  stem  leading  to  the 
first,  second,  and  third  arches  of  the  right  FIO.  297.— Aortic  System  of 
side;  this  stem  is  the  right  carotis  commu-  SoflS^S»L1Vl!Krt<SS3 

ii  Iff :    and  into.    •>,   the  larger   left   arch,  arCUS    artery;  Ao,  aorta:    N,/.  tliyn, 

fi»rt(t\  which  is  homologous  with  the  anony-  T&£  ItSauS^S^oSSSM^  ,^ 
ma  and  like  it  .^ives  off  the  carotis  communis  ySRFKf**'  xa4diams 
and  subclavia  of  its  side,  and  is  then  contin- 
ue* I  on  to  the  permanent  dorsal  aorta.  The  connection  of  the  ano- 
nyma  (right  fourth  arch)  with  the  dorsal  aorta  is  preserved  for  some 
time. 

The  history  of  the  fifth  arches  is  given  in  the  section  on  the  pul- 
monary arteries,  p.  538. 

DEVELOPMENT  OF  THE  AORTIC  WALL. — The  aortae,  like  all  other 
blood-vessels,  consist  at  first  of  a  simple  endothelium,  to  which  are 
added  the  muscular  and  adventitial  walls  by  differentiation  of  the 
surrounding  mesenchyma,  which  begins  to  condense  around  the 
aortee  by  the  end  of  the  second  month,  and  during  the  second  month 
the  separation  of  the  mesenchymal  coat  into  tunica  media  and  tunica 
adventitia  becomes  apparent  (see  His,  "  Anat.  menschl.  Embryonen," 
Heft  III.,  198,  also  Morpurgo,  85.1).  Erik  Muller's  paper,  88.1, 
describes,  strictly  speaking,  not  the  origin  of  the  muscular  tissue  of 
the  aorta,  but  of  the  primitive  mesenchyma  from  the  inner  wall  of 
the  primitive  segments. 

AORTIC  ARCHES  IN  BRANCHIATE  VERTEBRATES. — In  aquatic 
vertebrates  the  aortic  arches  do  not  remain  as  large  vessels,  but 
they  break  up  into  smaller  vessels  and  capillaries,  which  are  distrib- 
uted through  the  branchial  filaments,  or  respiratory  outgrowths  of 
the  gill-arches.  When  this  modification  occurs  the  ventral  end  of 
each  aortic  arch  acts  as  the  afferent  stem  (branchial  artery)  and  the 
dorsal  end  as  the  efferent  stem  (branchial  vein)  of  the  gill.  It  is  evi- 
dent that  the  branchial  veins  are  morphologically  distinct  from  the 
true  veins,  and  belong  not  to  the  venous,  but  to  the  arterial,  system. 


538  THE    FCETUS. 

EVOLUTION  OF  THE  AORTIC  ARCHES. — That  there  were  in  the  early 
vertebrates  more  gill-arches  than  are  preserved  in  the  amniota,  has 
been  stated  already.  But  the  exact  number  is  uncertain,  and  as  there 
must  have  been  one  aortic  trunk  in  each  gill-arch  the  number  of  the 
aortic  arches  is  uncertain.  It  seems,  however,  probable  that  there  were 
at  least  nine  as  indicated  by  the  structure  of  marsipobranchs  (Julin, 
87.3)  and  Chlamydoselachus  (Howard  Ayers,  89.1).  Indeed,  von 
Boas,  87. 1,  has  adduced  weighty  evidence  to  support  his  belief  that 
even  in  amniota  the  number  of  aortic  arches  is  six,  a  belief  which 
Zimmerman,  89.1,  has  supported.  To  settle  this  question  must  be 
left  to  research  based  upon  very  extended  comparative  anatomical 
and  embryological  observations. 

It  is  improbable,  as  Ayers,  89. 1,  has  demonstrated,  that  the  united 
dorsal  ends  of  the  aortic  arches,  which  form  two  stems,  represent 
the  forward  continuation  of  the  median  aorta,  but  that  rather  there 
was  primitively  a  median  dorsal  aorta  extending  over  the  pharynx 
to  the  hypophysis,  and  that  there  were  lateral  anastomoses  which 
have  been  preserved  while  the  cephalic  median  aorta  has  disappeared. 
Ayer's  hypothesis,  which  seems  to  me  well  justified,  is  incompatible 
with  the  current  notion  that  the  dorsal  aorta  represents  two  stems 
fused  in  the  median  line — a  notion  which  has  been  specially  advo- 
cated by  Macalister  (Jour.  Anat.and  Physiol.,XX..,  193,  188G). 
The  special  importance  of  the  question  at  present  is  its  bearing  on 
the  comparison  of  the  arterial  systems  of  vertebrates  and  annelids. 

The  abortion  of  the  aortic  arches  is  attributed  by  general  consent  to 
the  head-bend,  and  consequent  cramping  of  the  branchial  region, 
but  the  factors  which  have  caused  the  modification  of  the  five  partially 
preserved  arches  of  mammals  have  still  to  be  ascertained.  Hoch- 
stetter,  90. 1,  577,  suggests  that  the  development  of  a  new  trunk  of 
supply — the  internal  mammary — for  the  anterior  intercostal  arteries 
may  have  been  concerned  in  the  abortion  of  the  right  aortic  root  and 
the  changed  position  of  the  left  aortic  root,  but  leaves  his  thought 
unexplained. 

Internal  Carotids. — As  indicated  in  the  diagram,  Fig.  294, 
the  internal  carotids  are  developed  out  of  the  first,  second,  and  third 
aortic  arches;  the  third  arch  loses  its  connection  on  the  dorsal  side 
with  the  fourth  arch,  but  keeps  its  connection  with  the  second  and 
first ;  there  "is  thus  a  direct  blood-channel  from  the  cardiac  aorta  to 
the  vessel,  which  runs  from  the  dorsal  end  of  the  first  arch  to  the 
head  and  brain,  Fig.  294,  In.  c.  The  internal  carotid  of  the  adult 
comprises  the  third  aortic  arch,  the  dorsal  part  of  the  second  arch, 
the  dorsal  part  of  the  first  arch,  and  the  whole  of  the  true  internal 
carotid  of  the  embryo.  His  ("Anat.  mensch.  Embryonen,"  Heft 
III.,  192)  states  that  in  man  the  dorsal  connection  between  the 
fourth  and  fifth  arches  is  lost  during  the  fifth  week ;  and  points  out 
that,  asv  the  heart  and  cardiac  aorta  descend,  the  position  of  the  third 
arch  becomes  more  and  more  oblique,  compare  Fig.  298. 

Pulmonary  Aorta  and  Arteries. — In  Sauropsida  the  fifth 
aortic  arches  are  preserved  on  both  sides,  in  reptiles  completely,  in 
birds  partially,  but  in  mammals  the  fifth  arch  entirely  disappears  on 
the  right  side  and  partially  on  the  left,  as  established  by  the  classic 
investigations  of  Heinrich  Rathke,  57. 1.  In  all  amniota  the  lungs 


THE    ARTERIAL   SYSTEM. 


539 


' 


$t\ 


Fio.  296.  —Aortic  System  of  W.  His'  Embryo  Rg,  11.5 
mm.  Uk,  Mandible;  Zg,  tongue;  I-V,  aorti.-  un-h.-,; 
Av,  vertebral  artery;  P,  pulmonary  artery;  Lg,  lung; 
Oe,  oesophagus;  T,  truncus  pulmoualls;  Ao,  aorta,  x 
is, Hums.  After  W.  His. 


are  supplied  by  arterial  branches  springing  from  the  middle  of  the 
fifth  aortic  arch,  in  Sauropsida  on  both  sides,  in  mammals  on  the 
left  side  only,  Fig.  X)(.'N  /'.  The  right  fifth  arch  disappears  in  man 
very  early,  but  the  left  persists  throughout  fcetal  life.  Concerning 
the  development  of  thepul- 
monarv  artery  proper,  i.e. 
the  branch  from  the  arch 
to  the  lungs,  Fig.  298, 
J\  little  is  known.  His 
(•"Anat.  mensch.  Embry- 
onen,"  Heft  II.,  180)  finds 
the  reptilian  condition  — 
the  ri-ht  and  left  fifth 
arches,  each  producing  a 
branch  to  the  lungs — in  an 
embryn  of  4.2  mm.  and 
more  distinctly  developed 
in  embryos  of  5-6  mm. 
but  later  both  pulmonary 
arteries  are  found  to  spring 
b\  a  common  stem  from 
the  left  fifth  arch.  How 
the  change  comes  about  I 
do  not  know,  and  I  have 
found  no  explanation  of  it. 
The  arteries  have  a  special  relation  to  the  bronchi,  as  is  explained  in 
the  section  on  the  lungs  in  Chapter  XXIX.  Returning  now  to  Fig. 
.  it  will  be  observed  that  the  pulmonary  artery,  P,  divides  the 
fifth  aortic  arch  into  a  lower  part,  T,  connected  with  the  heart,  and 
an  upper  part,  I',  connected  with  the  dorsal  aorta.  The  lower  part 
is  the  future  trunk  of  the  pulmonary  aorta,  and  as  the  lungs  develop 
the  pulmonary  artery  increases  in  calibre  until  it  equals  the  trunk, 
7'.  in  diameter.  The  upper  part,  F,  is  known  as  the  ductus  arteri- 
M.S//.S  or  (hn-his  Botalli  (BotallischerGang)  and  it  remains  through- 
out the  foetal  period  as  an  open  channel,  so  that  blood  from  the  right 
ventricle  flows  in  part  to  the  lungs,  in  part  into  the  dorsal  aorta. 
As  stated  above,  the  lumen  of  the  ductus  arteriosus  disappears  soon 
after  birth. 

Dorsal  Aorta  and  Its  Branches. — There  are  many  valuable 
observations  on  the  fcetal  arteries  scattered  in  the  works  of  the  older 
embryologists,  in  the  descriptions  of  human  embryos  (Chapter 
XVIII.)  and  in  articles  dealing  with  the  development  of  special 
organs,  but  these  observations  have  never  been  collated,  nor  has  any 
attempt  l^een  made,  so  far  as  I  am  aware,  to  study  comprehensively 
the  morphology  of  the  dorsal  aorta  and  its  branches.  This  is  the 
more  singular  as  much  labor  as  been  expended  upon  the  aortic  arches 
and  veins.  An  exception  has  been  made  in  the  case  of  the  interseg- 
mental  and  vertebral  arteries,  see  below. 

That  the  dorsal  aorta  is  formed  very  early  by  the  ingrowth  of  the 
omphalo-mesaraic  arteries  and  that  these  arteries  are  the  primitive 
branches  of  the  aorta  has  been  already  explained.  The  next  branches 
to  be  formed  are  the  umbilical  or  allantoic,  which  very  early  acquire 


540 


THE    FCETUS. 


a  large  size  and  appear  as  the  main  branches  of  the  aorta,  but  the 
dorsal  aorta  is  prolonged  to  the  tail,  and  in  tailed  vertebrates  persists 
as  a  permanent  and  considerable  vessel  (arteria  caudalis)  but  in 
man  it  remains  only  as  a  small  vessel,  the  sacra  media.  From  the 
umbilical  arteries,  as  soon  as  the  anlages  of  the  legs  appear,  arise 
bra/iches,  the  iliac  arteries,  one  on  each  side  to  supply  the  corre- 
sponding limbs.  With  the  progress  of  development  the  iliacs  become 
the  main  branches,  and  the  allantoic  vessels  are  very  much  reduced, 
becoming  the  relatively  small  hypogastric  arteries  of  the  adult.  Of 
the  omphalo-mesaraic  or  vitelline  arteries  the  left  aborts  very  early, 
while  the  right  persists,  and  soon  develops  the  arteria  mesenterica 
superior  as  a  small  branch,  which  ultimately  becomes  the  principal 
continuation  of  the  main  stem. 

INTERSEGMENTAL  ARTERIES. — The  first  branches  of  the  aorta  to 
appear  in  the  embryo  are  a  series  of  small  vessels,  which  pass  upward 
and  outward  on  each  side  of  the  embryo.  One  of  these  vessels  is  to 
be  found  between  every  adjacent  pair  of  myotomes,  and  hence  they 
have  been  called  the  interprotovertebral  arteries.  In  the  region  of 
the  pharynx  where  the  aorta  is  double,  each  aorta  gives  rise  to  the 
intersegmental  arteries  of  its  own  side.  Farther  from  the  head  the 

vessels  arise  in  pairs  from  the  dorsal 
aorta.  In  longitudinal  horizontal  (i.e. 
frontal)  sections  of  the  primitive  seg- 
ments the  intersegmental  arteries  show 
very  well,  compare  Fig.  119,  Is.  The 
metamorphoses  of  the  vessels  under  con- 
sideration have  been  worked  out  for  the 
region  of  the  head  and  neck  by  Froriep, 
86.1  (pp.  89,  96,  103,  108,  139),  and 
Fr.  Hochstetter,  90.1,  90.3.  There 
are  six  intersegmental  arteries  between 
the  seven  cervical  segments ;  of  these  the 
sixth  gives  rise  to  the  arteria  subclavia  as 
a  branch.  There  are  also  two  segmental 
arteries  head  ward  of  the  cervical  ones; 
these  two  lie  respectively  between  the 
first  cervical  and  the  last  occipital  seg- 
ments, and  between  the  last  and  the 
penultimate  occipital  segments.  Of  these 
eight  arteries  the  first  very  early  aborts, 
the  second  gives  rise  to  a  vessel  which 
runs  forward  in  the  head  to  the  mid- 
brain  and  there  joins  the  internal  carotid, 

FiS-  299'  A  series  of  anastomes  are 
s,  intersegmental  now  developed  between  the  intersegmen- 

tal  arteries  of  the  neck  and  united  and 
t7rdFraeHochstettSerman'sarch>  Af"  enlarged  anastomosing  vessels,  Fig.  299, 

Av,  appear  as  a  prolongation  through  the 

neck  of  the  vertebral  artery.  The  intersegmental  branches  rapidly 
abort,  except  the  sixth  in  the  neck  which  persists  as  the  stem,  Fig.  299, 
s.  cl,  of  the  vertebral  artery,  and  as  soon  as  the  fore  limb  buds  out  (rab- 
bits of  eleven  days)  sends  a  branch  to  it,  which  becomes  the  subclavian 


THK  v KNOTS  SYSTEM.  :»i  i 

artery.  The  artery  1  ><  -t  \veen  the  sixth  and  seventh  cervical  vertebrae  is 
thus  seen  t«>  acquire  a  special  ini}>ortance,  as  it  becomes  the  stem  of  the 
sub-clavian  and  vertebral  arteries  of  the  adult.  We  also  learn  that  the 
vertebral  artery  is  the  earlier  developed,  and  that,  therefore,  the  sub- 
clavian  is  morphologically  a  branch  of  the  vertebral  artery,  instead 
;»f  the  vertebral  being  a  branch  of  the  subrlavian,  as  usually  described 
in  human  anatomy.  The  small  original  intersegmental  arteries 
persist  on  the  dorsal  side  of  the  vertebral  artery  in  the  neck,  and 
supply  in  the  adult  the  circulation  of  the  vertebral  column.  The 
next  following  intersegmental  arteries,  i.  e.  those  between  the  seventh 
cervical  and  first  thoracic,  and  between  the  first  four  or  the  thoracic 
segments,  undergo  a  similar  change,  a  secondary  longitudinal  vessel 
he  ing  developed  between  them  also  (rabbits  of  thirteen  davs),  and  as 
they  disappear,  this  vessel  becomes  a  branch — intercostalis  superior 
of  human  anatomy — of  the  common  stem  of  the  vertebral  and  sub- 
clavian  arteries.  Hochstetter  states,  90.1,  577,  that  the  internal 
mammary  arises  as  a  branch  of  the  subclavian  at  about  the  same 
time  as  the  superior  intercostal. 

The  subclavian  does  not  long  retain  its  original  position,  but  en- 
larges and  migrates  from  the  dorsal  to  the  ventral  side  of  the 
sympathetic  ganglion  chain  (Hochstetter,  90.1,  578-580). 

The  remaining  intersegmental  arteries  of  the  thorax  are  said  to 
give  rise  to  the  intercostal  arteries. 

The  vertebral  arteries  unite  in  the  occipital  region  (human  embryo 
of  10  mm.  according  to  W.  His,  Z.c.,  193)  to  form  the  arteria  basi- 
Itirix,  Fig.\!45,  while  further  forward  they  remain  distinct,  resulting 
in  the  development  of  the  circulus  Willisii. 

r.MKiLiCAL  ARTERIES. — These  acquire  a  large  size  in  the  human 
embryo  and  owing  to  the  reduction  of  the  caudal  artery  (sao" 
nn'tliti)  appear  as  the  terminal  forks  of  the  dorsal  aorta.  They  curve 
around  past  the  cloaca,  run  in  the  walls  of  the  allantois  or  anlage  of 
the  bladder,  to  the  umbilicus,  and  thence  through  the  umbilical  cord  to 
the  placenta.  They  develop  each  a  branch,  which  runs  to  the  hind 
limb  as  soon  as  it  buds  forth.  Until  birth  the  umbilical  artery  per- 
-  as  the  main  stem,  but  after  birth,  having  lost  its  main  function, 
it  ceases  to  develop  and  becomes  the  hypogastric  artery  of  the  adult. 
The  branch  to  the  leg  (the  common  iliac)  continues  to  enlarge  and 
after  birth  becomes  more  and  more  the  chief  vessel,  so  that  the  root 
of  the  umbilical  artery  is  converted  into  the  beginning  of  the  iliac 
artery  and  the  hypogastric  into  a  branch  of  the  iliac.  The  precise 
history  of  these  vessels  has  still  to  be  worked  out  thoroughly. 

III.     THE  VENOUS  SYSTEM. 

The  Primitive  Veins. — By  this  heading  I  mean  the  jugular, 
cardinal,  vitelline,  and  umbilical  veins,  or  main  venous  stems  of  the 
first  completed  embryonic  circulation.  The  initial  arrangement  of 
the  four  pairs  of  trunk  veins  can  be  studied  in  a  human  embryo  of 
4. 'I  mm.,  Fig.  300.  From  the  head,  where  it  extends  to  the  fore- 
brain  and  has  several  branches,  comes  the  jugular  vein,  Jg,  descend- 
ing nearly  to  the  level  of  the  septum  transversum.  From  the  tail 
comes  the  cardinal  vein — the  posterior  cardinal  of  comparative 


542 


THE   FCETUS. 


Ot 


anatomy — to  meet  the  jugular  vein.     Only  part  of  the  cardinal  vein 
is  drawn  in  the  figure ;  in  reality  it  extends  the  whole  length  of  the 

rump  and  ends  in  the  tail.  In  a 
cross  section  the  cardinal  vein  is 
seen  to  be  situated  originally  in 
the  splanchnopleure  of  the  embryo, 
just  at  the  level  of  the  nephrotomes 
(or  intermediate  cell  masses). 
This  position  being  kept  brings 
the  vein,  as  soon  as  the  Wolffian 
tubules  are  developed,  to  lie  just 
above  the  Wolffian  body,  and  late- 
ral of  the  aorta,  compare  Figs. 
301,  135,  and  137.  The  jugular 
vein  occupies  the  corresponding 
situation  in  the  neck,  but  at  the 
level  of  the  segments,  which  in  the 
chick  shows  an  open  connection 
with  the  splanchnoccele  (S.  Dexter, 
91.1),  crosses  from  the  splanchno- 
pleure between  the  myotomes  and 
splanchnocoele  to  the  somatopleure 
and  runs  forward  to  the  head. 

The  jugular  and  cardinal  veins 
unite  forming  a  common  trunk, 
Fig.  300,  D.C— the  ductus  Cu- 
vieri— which  passes  in  an  oblique, 
transverse  direction  in  the  somato- 
FIG.  3oo. -His1  Embryo Lr  (4. 2  mm.).  Recon-  pleure  to  the  anterior  edge  of  the 

uuJuua  septum    transversum,    and    there 
r,'  aiiantoic  bends   toward  the  median  ventral 
line    to  empty   into    the    venous 
end  of   the  heart  by  way  of  the 
sinus  venosus. 

From  the  yolk-sac  come  up  the  two  vitelline  (omphalo-mesaraic) 
veins,  one  on  each  side,  om,  and  from  the  allantois 
stalk  pass  up  through  the  somatopleure  the  two  al- 
lantoic veins,  also  one  on  each  side,  Fig.  300,  Al.v. 
A  cross  section  through  the  rump  shows  the  differ- 
ence in  situation  of  the  cardinal  vein,  Fig.  301,  (7, 
in  the  splanchnopleure  above  the  Wolffian  body,  and 
the  umbilical  vein,  Uv ,  in  the  somatopleure.  The 
umbilical  vein  empties  into  the  ductus  Cuvieri ;  the 
vitelline  vein  into  the  sinus  venosus.  For  good  fig- 
ures of  the  relation  of  the  primitive  veins  to  the 
rabbit's  heart,  see  Born,  89.1,  Taf.  XX.,  Fig.  15. 
The  veins,  as  they  approach  the  heart,  pass  by  the 
anlage  of  the  liver,  and  as  this  organ  develops  it  en- 
ters into  intimate  relations  with  the  vessels,  which 
undergo  numerous  modifications.  It  will  be  conve- 
nient to  consider  the  changes  in  the  hepatic  veins 
collectively,  and  therefore  we  take  up  first  those  After  \v  ms 


cuvieri:  Am,  edge  of  amnion; 
vein;  car,  internal  carotid i;  J,  first  aortic  arch; 
Au,  auricle;  Ven,  ventricle;  Li,  liver;  om., 
vitelline  vein;  Al,  allantoic  diverticulum ; 
Art,  allantoic  artery.  After  W.  His. 


Uv 


Coe 


FIG. 301.— Cross  Sec- 
tion through  the 
Hinder  Part  of  His' 
Embryo  R  (5  mm.). 
My,  Myotome ;  C,  car- 
dinal vefn;  Uv,  um- 
bilical vein ;  Coe,  coe- 
lom.  X  20  diams. 


THE   VENOUS    sYsTKM. 


543 


changes  iii  the  primary  veins  which  ;nv  not  associated  with  tin-  de- 
velopment  of  the  liver.  But  to  do  this  we  must  present  tin-  early 
hiMory  of  the  n>nu  rura  inferior. 

Vena  Cava  Inferior. — This  is  a  large  unpaired  vessel,  which 
is  developed  somewat  later — in  rabbits  not  until  the  txvel ft h  day- 
than  the  four  pairs  of  primary  veins.  Our  present  knowledge  of  its 
development  rests  < -hit -fly  upon  F.  Hochstetter's  admirable  invoti- 
gatinns,  87.2,  88.1,  88.3.  It  arises  as  a  small  vessel  from  the 
ductus  venosus  of  tin*  liver  and  running  through  the  hepatic  sub- 
stance is  continued  on  the  right  side  ventrad  of  the  aorta  in  the 
tissue  between  the  two  primitive  kidneys,  Fig.  302,  A,  ci,  to  a  point  a 
little  beyond  the  aortic  origin  of  the  superior  mesenteric  artery.  It 
gradually  enlarges  and  forms  two  fine  branches,  which  pass  around 
the  aorta  and  anastomose  with  the  cardinal  veins,  the  communication 


.— Three   Diagrams  to  illustrate  the  Transformation  of  the  Venous  System.    After  O. 
Hertwig.     (Explanations  in  the  text.) 

heing  established  about  at  the  origin  of  the  renal  vein,  Fig.  302,  A,  r. 
By  the  thirteenth  day  the  anterior  portion  of  the  cardinal  vein  is 
nearly  aborted.  The  lower  part  of  the  right  cardinal  appears  now 
as  the  direct  continuation  of  the  enlarged  vena  cava,  and  in  fact  is 
the  anlage  of  the  lower  part  of  the  adult  cava  inferior,  Fig.  302,  C. 
By  the  fourteenth  day  the  renal  veins  appear  as  branches  of  the 
cava.  and  the  caudal  ends  of  the  two  cardinals  are  united,  thus  con- 
verting the  lower  branches  of  both  these  veins  into  branches  of  the 
cava  inferior.  But  in  man  this  fusion  of  the  cardinal  veins  does 
not  take  place,  but  instead  there  is  developed  a  cross  anastomosis  by 
which  the  lower  ramifications  of  the  left  cardinal  become  branches 
of  the  cava,  Fig.  302.  C  in  Fig.  302  represents  diagrammatically 
the  permanent  condition.  The  true  vena  cava  inferior  extends  only 
to  the  renal  veins,  r,  which  are  persistent  segmental  branches  of  the 
cardinal  veins;  beyond  this  point  the  cava  is  really  the  persistent 
right  cardinal  vein;  a  cross  anastomosis,  ilcs,  becomes  the  left  com- 
mon iliac,  while  the  terminal  branches  of  the  cardinals  are  converted 
into  the  external  and  internal  iliacs  on  each  side,  and  empty  their 


544  THE    FCETUS. 

blood  into  the  right  cardinal,  or  lower  segment  of  the  adult  cava 
inferior. 

Metamorphoses  of  the  Primitive  Veins. — By  a  series  of 
changes  beginning  very  early  indeed  in  the  embryo  the  four  pairs  of 
symmetrically  placed  veins  take  on  an  asymmetrical  arrangement. 
The  chief  factors  of  the  change  are,  1,  the  development  of  new  cross 
trunks,  which  become  main  stems ;  2,  the  abortion  of  parts  of  the 
primitive  veins ;  3,  migration  of  the  vessels. 

The  changes  which  occur,  in  the  venous  sinus  have  been  already 
indicated ;  those  which  occur  in  the  liver  are  described  in  a  separate 
section  below. 

Changes  of  the  Ductus  Cuvieri  and  their  Connections. — We 
have  already  noticed  the  relations  of  the  ductus  to  the  horns  of  the 
sinus  venosus,  p.  527,  and  the  role  of  the  ductus  in  shutting  off  the 
pleural  from  the  pericardial  cavity,  p.  482.  The  transformation  of 
the  ductus  begins  with  a  change  in  their  position,  their  course  be- 
coming steeper,  in  consequence  of  the  descent  of  the  heart,  and  at 
the  same  time  they  project  across  the  opening  of  the  pleural 
cavity  into  the,  pericardial  cavity,  and  by  finally  closing  across 
this  opening  the  ductus  are  enabled  to  unite  with  the  medias- 
tinum, thus  bringing  the  two  veins  nearer  together.  Of  the  veins 
supplying  the  ductus  the  jugulars  continue  to  develop  and  with 
the  growth  of  the  head  to  acquire  an  increasing  importance,  while 
the  cardinal  veins  have  their  circulation  impeded  owing  to  the  com- 
petition of  the  vena  cava  inferior ;  the  preponderance  of  the  jugular 
is  further  increased  by  the  vein  of  the  fore  limb,  the  subclavian, 
Fig.  302,  A.  5,  emptying  into  it.  -The  two  sides  of  the  sinus  venosus 
early  become  asymmetrical,  and,  owing  to  the  migration  of  the  sinus 
toward  the  right  side  of  the  heart,  p.  527,  the  right  ductus  (the  future 
vena  cava  superior  dextrd)  has  a  shorter  and  more  direct  course  to 
the  heart  than  the  left  ductus,  which  has  to  bend  around  the  left 
auricle  toward  the  right.  The  left  ductus  runs  along  the  coronary 
groove  of  the  heart,  and  there  receives  the  coronary  vein,  concerning 
the  development  of  which  we  have  no  definite  information.  This 
may  be  called  the  Sauropsidan  stage,  since  it  is  permanent  in  all 
reptiles  and  birds ;  but  it  is  said  to  be  retained  in  certain  mammals. 
In  man,  however,  a  further  stage  is  reached  by  the  partial  abortion 
of  the  left  ductus  (vena  cava  superior  sinistra).  The  reduction 
begins  with  the  development  of  a  cross  anastomosis,  Fig.  302,  B,  as, 
between  the  two  jugulars.  The  anastomosing  vessel,  which  is  the 
future  vena  anonyma  sinistra,  runs  obliquely  from  the  left  to  the 
right  jugular,  where  the  conditions  for  the  return  of  blood  to  the 
heart  are  more  favorable ;  the  cross  vessel  enlarges  and  in  the  same 
measure  the  right  ductus  enlarges  also,  with  the  further  consequence 
that  the  right  cava  usurps  more  and  more  of  the  blood  from  the  left 
jugular.  This  leads  to  the  gradual  closure  of  the  left  ductus  Cuvieri 
(cava  sinistra)  except  of  the  end  next  the  heart,  which  persists  as 
the  vein  delivering  the  vena  coronaria  into  the  right  auricle.  We 
thus  learn  that  the  cardiac  orifice  of  the  coronary  vein  is  really  the 
mouth  of  the  vena  cava  superior  sinistra.  The  development  of  the 
valve  (valvula  Thebesii)  of  this  orifice  is  described  p.  532. 

The  cardinal  veins  undergo  a  similar  change  to  the  jugulars,  see 


THE   VENOUS   SYSTEM. 


545 


KILT.  :Jn-j,  (  '.  in  that  a  cross  vein  appears  which  takes  the  blood  of  the 
left  cardinal  into  the  right,  so  that  the  stream  of  both  cardinal:-  i- 
poured  into  the  right  ductus  Cuvieri  (cava  sup.  dextra).  In  the 
account  of  the  vena  cava  inferior  it  has  been  explained  how  the 
lower  parts  of  the  two  cardinals  are  changed,  and  only  the  upper 
parts  left.  A>  the  main  function  of  the  cardinals  appears  to  be  to 
maintain  the  circulation  of  the  Wolffian  bodies,  the  cardinals  lose 
their  importance  as  the  bodies  abort.  They  persist,  however,  in 
part  to  give  rise  to  the  r/;//>/o.s-  and  hon  nizijyox  veins  of  the  adult, 

illicit 'iit ly  indicated  by  Fig.  302,  C,  <iz,  hz\  hz. 
Veins  of  the  Hand'  and  Foot.— Fr.  Hochstetter,  91.1,  has 
shown  that  in  all  amniota  there  is  a  vein  (Randvene)  which  runs 
around  the  edge  of  the  hand  (or  foot)  but  when  the  digits  appear  this 
"  randvene"  is  divided  and  gradually  disappears.  The  veins  are  de- 
veloped as  a  network  of  capillaries,  connected  with  the  randvene  and 
the  venous  trunk  of  the  limb.  As  the  digits  grow  out,  the  rand- 
vene persi>ts  on  each  side  of  each  digit,  but  is  interrupted  at  the 
apex.  The  randvene  thus  gives  rise  to  the  digital  veins  and  proba- 
hly  also  is  continued  on  the  ulnar  side  as  the  permanent  vein  of  the 
arm,  and  correspondingly  on  the  leg. 

Hepatic  Veins. — The  following  account  is  an  abstract  of  His' 
researches  (u  Anat.  menschl.  Embryonen,"  Heft  III.,  200-210).  The 
liver  grows  out  into  the  septum 
transversum  and  by  its  enlarge- 
ment comes  very  soon  into  con- 
tact with  the  vitelline  and  um- 
bilical veins  on  their  way  to  the 
sinus  venosus.  The  hepatic  cy- 
linders grow  into  the  veins,  push- 
ing, however,  the  vascular  endo- 
t  helium  before  them,  and  dividing 
the  veins  into  numerous  channels, 
which  constitute  a  network  of 
fine  branches.  The  four  vessels 
are  thus  hroken  up  into  smaller 

«  Is,  but  for  a  while  they  per- 
sist iu  part  as  larger  stems  lead- 
ing from  the  liver  to  the  sinus 
venosus.  The  liver  is  now  sup- 
plied with  all  the  blood  from  the 
chorion  (placenta)  and  the  yolk- 
This  stage  is  found  in  a 
human  embryo  of  4.25mm.,  Fig. 

:><>:;.  The  United  umbilical  veins  FIG.  303.  -Reconstruction  of  a  Human  Embryo 
nf  thp  nllantoip-st'ilk  Ml  na«ra  (His*  Bl.)  of  4.25mm.  Front  view.  V.j,  Jugular 

BcaiK,   ^  n ,  pass  vein .  card  cardinal  vein .  D  c  ductu'g  cgiirt . 

UptothellVerintheSOmatOpleiire    Li,  liver:    S.r.    sinus   venosus:   f.l.   fore  limb; 
£          v      •  i        £  Ai       i     j         xu      i    £±    v.u. «,  upper  part  of  left  umbilical  vein;  v.u".s, 
Of  each  Side  Of  the  body;    the  left    lower  part   of   same;    All.  .allantoic   stalk:  in. 

'.5, is  already 


an! 


|_     intestine;  vu.d.  ritfht  umbilical  vein;    Vi,  vitt'l- 
line  vein,     x  18  diams.     After  W.  His. 

edly  larger  than  the  right;  both 

veins  break  up  within  or  near  the  liver  into  small  vessels.     The  two 
vitelline  veins,  TV,  run  in  the  splanchnopleure  or  wall  of  the  intestine 
and  unite  just  before  they  attain  the  liver,  then  separate  and  pass 
35 


546 


THE   FCETUS. 


,V.Ar 


vu 


V.US. 


around  the  entodermal  intestinal  canal  to  unite  again  on  its  dorsal 
side,  making  a  complete  venous  ring  ;  they  then  again  separate  and 
pass  back  again  around  the  intestine,  forming  a  second  complete  ring 
before  they  break  up  into  small  hepatic  vessels.  On  the  right  side 
the  umbilical  and  vitelline  trunks  remain  separate  as  they  leave  the 
liver,  and  open  separately  into  the  sinus  venosus,  but  on  the  left  side 
the  two  trunks  unite,  as  shown  in  the  figure,  and  empty  by  a  com- 
mon stem  into  the  venous  sinus,  S.  r. 

In  the  next  stage  the  lower  part  of  the  right  umbilical  has  no  longer 
any  connection  with  the  upper  part  of  the  same  vessel  ,  and,  therefore, 

since  it  continues  to  act  as  a 
venous  path,  its  stream  is  directed 
downward.  The  left  umbilical 
vein,  on  the  contrary,  has  in- 
creased in  size,  Fig.  304,  V.us, 
and  is  prolonged  within  the  liver 
by  a  large  stem,  which  joins  the 
left  side  of  the  upper  venous  ring 
formed  by  the  vitelline  veins,  vi. 
The  upper  ring  is  connected  by  a 
newly  developed  large  trunk, 
V.ar.y  the  vena  ascendens,  or 
vena  Aranti  —  as  to  the  origin  of 
which,  we  possess  as  yet  no  satis- 
.  304  -Reconstruction  of  the  Venous  factory  data.  Remnants  of  the 

Trunks  and   Liver   of   His1   Embryo   R,  5mm.  ..    J          c  ,-,  ,.,.       ,          -, 

F.p,  Portal    vein;    v.u,  right    umbilical   vein:  portions    of  the  Umbilical  and  VI- 

V.Ar,  vena  Arantii;  V.us,  vena  umbilicalis  sin-  4-^11^^  VPITI<S     wTnVVi    in     tViP     r»rp- 

istra;    vi,    vitelline  veins;     V.u.d,    vena    urn-  TOlline  Veins,  WniCn    in    1 

bilicalis    dextra.      The  vessels   left   white   are  vioUS  Stage  took    the    blood    from 

aborted,      x  40  diams.      After  W.  His.  ,  ,         -,. 

the  liver  to  the  sinus  venosus, 

still  persists.  It  will  be  seen  that  the  essential  difference  between 
this  stage  and  the  preceding  is,  that  whereas  previously  all  the 
blood  passed  the  liver  through  small  vessels,  now  only  part  of  it 
flows  through  small  vessels,  the  rest  through  large  trunks  directly 
to  the  heart. 

The  third  stage  is  established  by  developing  the  single  portal  vein 
out  of  the  two  vitelline  veins.  This  is  accomplished  as  indicated  by 
the  diagram,  Fig.  304,  which  is  to  be  compared  with  the  previous 
figure.  The  left  side  of  the  upper  ring  formed  by  the  vitelline  veins, 
Fig.  303,  vi,  and  the  right  side  of  the  lower  ring  persist,  leaving  parts 
of  each  ring  to  form  a  single  continuous  vessel,  the  venapor  tee,  which 
from  its  mode  of  origin  necessarily  makes  one  complete  spiral  turn 
around  the  intestine.  Herewith  the  condition  is  reached  which  per- 
sists throughout  foetal  life,  Fig.  305.  The  portal  vein  and  left 
umbilical  vein  supply  the  liver  with  venous  blood,  and  also  form 
within  the  liver  near  its  lower  surface  two  large  stems  which  unite 
and  are  continued  forward  by  the  single  vena  Arantii.  These  three 
great  veins  after  the  third  month  are  found  to  lie  near  the  median 
plane,  and  to  follow  straighter  courses  than  in  Fig.  305. 

The  final  stage  is  not  reached  until  after  birth,  when  the  umbilical 
vein  rapidly  aborts.  A  little  later  the  large  channel  formed  within 
the  liver  by  the  vena3  portse  and  Arantii  also  disappears,  except 
that  the  part  between  the  union  of  the  vena  cava  inferior  with  the  vena 


THE    V  KNOTS 


547 


Arantii  (ductus  venosus)  and  the  heart  is  retained  and  functions  as 
the  cardiac  end  of  the  adult  cava  inferior.     In  the  fourth  <»r  adult 
>tage,  the  liver  is  supplied  by  the  portal  vein,  the  representative  of 
the    vitelliue    or    omphalo-mesaraic 
veins  of  the  embryo,  and  all  the  por- 
tal 1  flood  passes  through  the  liver  in 
small  vessels  (capillaries),  though,  of 
course,  larger  venous  branches  per- 
sist to  distribute  the  blood  to,  and 
collect  it  from,  the  capillaries  of  the 
hepatic  lol uiles. 

Pulmonary  Veins. — It  was  first 
shown  by  Fr.  Schmidt,  70.1,  that 
the  pulmonary  veins  are  four  vessels, 
which  unite  into  a  short  common 
stem  emptying  into  the  left  auricle. 
Their  history  has  been  further  eluci- 
dated by  ll'i>.  87.3,  103,  and  G. 
Born,  89. 1,  Ul  :$,  :'>:J4.  The  common 
stem  appears  first  as  a  capillary  ves- 
-«  1  arising  from  the  left  auricle  near 
the  interauricular  septum  (twelve 
days'  rabbits) ;  the  small  vessel  runs 
through  the  mesocardium  posterius 
directly  toward  the  anlage  of  the 
lungs;  by  enlarging  and  branching 
this  vessel  forms  the  system  of  the 
pulmonary  veins,  but  for  some  time 
after  its  appearance  it  remains  small. 
The  development  is  not  the  same  in 
the  rabbit  and  in  man;  in  the  lat- 
ter the  common  stem  enlarges  and 
merges  into  the  auricular  cavity,  at 
first  as  a  recess,  later  without  demar- 
cation; hence  the  four  pulmonary 

veins  open  into  the  heart  by  two  orifices,  the  two  veins  on  each  side 
uniting  before  they  empty.  Still  later  (two  months'  embryo)  the 
four  veins  each  open  separately,  more  of  the  vein  being  annexed  by 
the  heart.  In  the  rabbit  the  primitive  condition  is  permanent,  and 
the  four  pulmonary  veins  unite  before  joining  the  heart. 

The  course  of  the  four  veins  in  the  lungs  has  been  described  by 
His,  87.3,  103.  They  run  from  the  central  stem  one  to  each  lobe 
of  the  lung ;  in  other  words,  from  the  start  there  is  an  upper  and  a 
lower  vein  in  each  lung;  the  pulmonary  veins  are  situated  below 
the  forking  of  the  trachea,  and  this  relative  position  the  main  stems 
retain  throughout  life, — compare  Fig.  459. 


FIG.  806. —Reconstruction   of   the  Venous 
System  of  His*  Embryo  Rg,  11.5  mm. 


CHAPTER   XXV. 


THE  EPIDERMAL  SYSTEM. 


THAT  portion  of  the  ectoderm  which  remains  upon  the  surface  of  the 
embryo  is  called  the  epidermis;  it  constitutes  the  outer  skin;  for 
convenience  the  inner  skin  (cutis  or  dermis)  is  treated  in  connection 
with  the  history  of  the  true  skin  in  this  chapter.  We  have  also  to 
consider  the  development  of  the  following  epidermal  appendages: 
nails,  hairs,  and  glands. 

I.     THE  SKIN. 

Epidermis. — The  ectoderm  of  all  amniote  vertebrates  is  at  first 
a  single  layer  of  cells,  which  presents  considerable  variations  in  ap- 
pearance not  only  in  different  classes,  but  also  at  different  stages  of 
the  same  species,  and  even  in  different  parts  of  the  same  embryo. 
Since  in  all  invertebrates  the  ectoderm  consists  of  a  single  epithelial 
layer,  we  may  call  the  first  stage  of  the  vertebrate  epidermis  the 
invertebrate  stage.  The  appearance  of  the  ectoderm  while  in  this 
stage  has  been  indicated  by  the  figures  and  descriptions  scattered 
through  Chapters  V.-XV.,  and  until  a  comprehensive  study  of  the 
ectoderm  of  amniota  in  the  one-layered  condition  shall  have  been 
made,  it  is  impossible  to  give  a  minute  description  of  it  possessing 
much  value  or  any  interest.  The  epidermis  of  Amphioxus  and  the 
ectoderm  of  the  amnion  never  pass  beyond  the  one-layered  stage, 
p.  335. 

In  its  second  stage  the  epidermis  becomes  two-layered.  The  cells 
of  the  single  layer  become  irregularly  placed ;  some  have  their  nuclei 

nearer  the  outer,  others 
nearer  the  inner,  surface 
of  the  ectoderm.  The 
difference  rapidly  in- 
creases, and  though  for 
a  time  the  cells  stretch 
through  the  whole  thick- 
ness of  the  layer,  yet  they 
gradually  draw  away, 
some  from  the  upper, 
others  from  the  lower, 
surface,  until  they  have 
definitely  arranged  them- 
selves in  two  distinct 
layers,  Fig.  306.  This 
stage  is  established  in  the 
human  embryo  by  the  end  of  the  first  month,  and  persists  over  part 
certainly  of  the  embryo,  at  least  until  the  close  of  the  second  month. 


FIG.  306.— Section  of  the  Skin  of  a  Human  Embryo  of 
sixty -three  to  sixty-eight  Days.  Minot  Collection,  No.  138. 
a,  Outer  layer  of  epidermis;  6,  inner  layer  of  epidermis; 
c,  cutis. 


THK    >K  IN. 

In  stained  sections  the  outer  layer,  Fig.  30G,  a,  is  composed  of  some- 
what flattened  eells.  \\-\\\\  irregularly  shaped,  slightly  granular  nu- 
clei, and  a  re  darker  ami  thicker  walls  than  the  cells  of  the  inner  layer. 
These  latter,  Fig.  :><><;,  6,  are  larger  and  clearer,  and  have  larger, 
more  granular  nuclei  of  round  shapes.  The  appearance  of  the  outer 
« -el  Is  suggests  a  necrotic  change.  Bowen's  careful  researches,  88.1, 
render  it  probable  that  the  outer  layer  is  the  epitricliinm,  compare 
below. 

It  is  a  remarkable  fact  that  the  primitive  blastoderm  in  amphibia, 
teleosts,  and  ganoids  never  passes  from  the  several-layered  to  the 
one-layered  condition,  but  only  to  the  two-layered  condition.  For 
description  of  this  stage  in  Boinbinator,  see  A.  Goette,  75.1,  and 
in  tele«»ts  see  M'Intosh  and  Prince,  90.1,  739,  in  Lepidosteus, 
Hal f«»ur  and  Parkfr,  82.1.  The  development,  therefore,  in  this 
gr<  nip  of  forms,  offers  a  marked  difference  from  that  found  in  mar- 
sipohranrhs  and  amniota,  but  since  in  Petromyzon  we  encounter  the 
< -lie-layered  stage,  we  must  consider  the  succession  of  stages  adopted 
in  this  chapter  as  the  primitive  one,  and  conclude  that  the  precocious 
appearance  of  the  two-layered  stage  in  amphibians,  etc.,  is  a  second- 
ary modification,  the  cause  of  which  is  unknown.  That  the  two 
layers  of  the  epidermis  are  homologous  throughout  the  vertebrate 
series,  we  have  no  reason  to  doubt  (Balfour,  "Comp.  Embryol.," 
II.,  300).  Where  the  epidermis  has  an  initial  division  into  two 
layers,  the  inner  is  commonly  termed  the  nervous  layer,  and  it  has 
the  main  share  in  forming  all  the  organs  derived  from  the  epidermis ; 
the  outer  layer,  according  to  homologies  I  hold  to  be  probable,  must 
be  identified  with  amniota  epitrichium,  although  unlike  the  true 
epitrichium  it  disappears  as  a  distinct  layer,  its  cells  showing  them- 
selves between  those  of  the  inner  layer  (Goette,  75. 1,  158). 

The  ectoderm  of  the  chorion  and  umbilical  cord  never  advances 
beyond  this  stage,  unless  we  regard  the  formation  of  the  chorionic 
cellular  layer  as  such  an  advance. 

The  tfi  in  I  xtage  is  very  gradually  reached  by  the  increase  in  the 
number  of  layers  until  there  are  several.  I  consider  it  probable  that 
this  stage  is  established  in  two  ways — one,  the  more  primitive,  in- 
volves the  disappearance  of  a  distinct  outer  layer,  as  in  amphibia; 
the  other  depends  upon  the  preservation  of  the  outer  layer,  as  the 
epitrichium.  This  view  can  be  advanced,  at  present,  only  as  an 
hypothesis. 

1.  The  primitive  method  is  maintained  in  amniota  only  over  very 
limited  special  regions ;  as  such  I  venture  to  designate  the  cornea, 
the  nasal  pits,  the  mouth  cavity  and  lips,  and  the  anal  ectoderm. 
Over  these  parts  the  distinct  outer  layer  disappears  as  such,  and  we 
have  developed  a  stratified  epithelium,  which  never  produces  a  true 
horny  layer,  but  consists  of  a  basal  row  of  protoplasmatic  cells,  and 
several  layers  of  cells  above,  which  are  clear  in  appearance  and  have 
thickened  walls.     The  details  of  the  process  of  differentiation  have 
not  yet  been  worked  out. 

2.  The  secondary  method  of  forming  the  several-layered  epider- 
mis is  established  over  the  skin  proper.     It  can  be  well  seen  in  the 
human  embryo  of  the  third  month.     In  an  embryo  of  two  and  one- 
half  months,  Fig.  307,  there  are  four  to  five  layers  of  cells.     The 


550  THE   FCETUS. 

basal  layer,  6,  is  composed  of  a  single  row  of  cuboidal  cells,  which 
are  rich  in,  protoplasm,  though  small  in  size,  and  which  have  round 
nuclei.  This  basal  layer  persists  throughout  life  in  all  amniota,  and 
is  one  of  the  most  characteristic  features  of  the  amniote  epidermis. 


FIG.  307.— Epidermis  from  the  Occiput  of  the  Human  Embryo  of  two  and  one-half  Months. 
Eptr,  Epitrichial  layer ;  m,  Malpighian  layer ;  6,  basal  layer.    After  Bowen. 

Above  the  basal  comes  the  middle  layer,  which  varies  from  two  to 
three  cells  in  thickness ;  its  cells  are  irregular  in  shape  and  size,  and 
are  so  large  that  the  nuclei  of  many  of  them  do  not  appear  in  the 
section.  The  outermost  layer,  Eptr,  is  the  epitrichium,  and  con- 
sists of  a  single  layer  of  large  dark  cells,  which  from  their  arching 
up  may  be  termed  dome  cells.  It  is  probable  that  the  epitrichium  is 
the  outer  layer  of  the  second  stage  preserved  and  modified,  and  that 
all  the  middle  cells  come  from  the  inner  layer  of  the  previous  stage, 
but  conclusive  proof  of  this  identification  is  still  required.  The  his- 
tory of  the  epitrichium  is  treated  in  the  next  section. 

The  fourth  stage  is  characterized  by  the  presence  of  a  horny  layer 
(stratum  corneum).  The  stratum  corneum  presents  marked  varia- 
tions in  structure,  and  it  is  probable  that,  as  explained  in  the  follow- 
ing paragraph,  at  least  two  morphologically  distinct  layers  have 
been  confused  under  a  common  name.  Unfortunately  almost  noth- 
ing is  known  concerning  the  genesis  of  the  horny  layer.  Bowen 's 
observations,  89.1,  render  it  probable  that  it  arises  from  the  epi- 
trichium, but  if  this  view  be  adopted  we  encounter  certain  difficul- 
ties which  our  present  knowledge  cannot  remove.  If  Bowen 's 
hypothesis  is  correct,  we  must  define  the  fourth  stage  as  characterized 
by  the  cornification  of  the  thickened  epitrichium.  Concerning  the 
process  of  cornification  we  possess  some  information,  which  is  re- 
ferred to  more  fully  under  the  head  of  nails,  p.  555.  When  the 
horny  layer  is  produced  the  skin  is  considerably  thickened  and  the 
number  of  layers  of  cells  which  it  comprises  is  much  increased. 
The  line  of  division  between  the  horny  layer  and  the  underlying 
mucous  or  Malpighian  layer  becomes  quite  sharp.  It  must  be  as- 
sumed that  cells  of  the  deep  layer  are  added  to  the  horny  layer. 

The  fifth  stage  is  established  by  the  development  of  the  stratum 
lucidum.  Bowen  has  made  the  important  discovery  that  the  stratum 
lucidum  of  the  human  embryo  lies  immediately  underneath  the  epi- 
trichium, and  is  directly  continuous  with  the  nail,  and  the  epitri- 
chium is  continuous  with  the  horny  layer  outside  the  stratum  luci- 
dum. Bowen  suggests,  89. 1,  449,  that,  where  there  is  no  epitrichial 
layer  nor  characteristic  stratum  lucidum  (Zander's  Typus  B,  88.1), 
the  stratum  really  extends  over  the  Malpighian  layer,  being  modified 
and  constituting  the  horny  layer  of  those  parts.  The  essential  char- 
acteristic of  the  stratum  lucidum  is  that  its  cells  are  solidly  cornified, 
their  nuclei  being  obliterated.  When  the  epitrichial  cells  cornify 
they  acquire  thickened  walls,  but  remain  hollow  (Zander's  Typus 
A,  86.1,  88.1).  The  histogenesis  of  the  stratum  lucidum  is  de- 


THE   SKIN.  551 

scribed  in  the  section  on  the  nails,  p.  555,  the  process  having  scarcely 
IM-.-H  studird  except  in  connection  with  the  investigation  of  the 
nails. 

The  ridges  (/r/r.s  <!'//,  n/i-)  on  the  under  or  dermal  side  of  the 
epidermis  lu'gin  to  appear  on  the  hairless  parts,  according  to 
Blaschko,  87.1,  about  the  fourth  month,  but  on  the  hairy  parts, 
win -IT  they  are  always  rudimentary,  they  do  not  appear  until  toward 
the  end  of  foetal  life.  There  are  primary  and  secondary  ridges. 
The  former  are  the  first  developed,  and  from  them  the  solid  out- 
growths to  form  the  sweat  glands  originate.  Fig.  308  represents 


Ep 
Ri 


Fio.  308.— Section  of  the  Skin  of  the  under  Side  of  the  Right  Second  Toe  of  four  months'  Em- 
bry...  Mi  not  Collection,  No.  123.  Ep,  Epidermis;  Ri,  primary  ridge  of  epidermis;  S,  sweat  gland ; 

His. 

a  section  across  the  primary  ridges :  the  epidermis  is  some  seven  or 
<  i-ht  cells  thick,  its  outer  surface  irregular,  but  not  thrown  into 
folds  or  ridges ;  the  structure  of  the  superficial  layer  is  indistinct 
but  the  epitrichium  seems  to  have  disappeared ;  the  dermal  surface 
is  thrown  up  into  regular  rounded  equidistant  ridges,  Ri,  from 
which  grow  out  here  and  there  the  solid  anlages  of  sweat-glands, 
S.  These  ridges  do  not  arise  all  at  the  same  time,  but  their  forma- 
tion spreads  from  sundry  centres,  nor  do  the  ridges  run  in  straight 
lines  altogether,  but  on  the  contrary  in  parallel  curves.  The  ridges 
under  the  nails  appear  first  (three  and  one-half  months)  under  their 
distal  and  lateral  borders,  later  under  their  central  and  proximal 
portions;  additional  ridges  appear  between  those  first  formed  (F. 
Curtis,  89.2,  179).  In  the  next  stage,  which  is  assumed  by  the 
epidermis  only  upon  the  palms  and  soles,  the  outer  surface  forms  a 
low  ridge  over  each  of  the  inner  ridges.  The  external  ridges  with 
the  openings  of  the  sweat  glands  upon  them  are  easily  seen  upon  the 
adult  hand.  When  the  external  ridges  are  developed  there  appear 
also  secondary  ridges  on  the  dermal  side,  between  the  primary  ridges. 
The  secondary  are  much  smaller  than  the  primary  ridges  and  under- 
lie the  grooves  separating  the  external  ridges. 


552 


THE   FOETUS. 


The  origin  of  epidermal  pigment  has  been  already  discussed, 
p.  419. 

Epitrichium. — The  external  layer  of  the  skin  is  known  to  be 
stratified  in  all  amniota,  but  the  homologies  of  the  strata  have  never 
been  satisfactorily  determined.  That  the  mucous  or  Malpighiaii 
layer  is  the  same  in  all  classes  is  evident,  but  that  the  horny  layer 
comprises  two  distinct  strata  is,  I  think,  extremely  probable,  as 
stated  above.  One  stratum  may  be  homologized  with  the  stratum 
lucidum,  the  other  with  the  epitrichium.  Where  the  epitrichium  is 
lost  (nails  and  hairy  skin)  the  stratum  lucidum  forms  the  superficial 
layer  of  the  epidermis,  but  when  the  epitrichium  is  preserved,  it 
forms  the  outer  layer  and  the  stratum  lucidum  underlies  it.  The 
history  of  the  epitrichium  is  the  key  to  the  morphology  of  the  am- 
niote  epidermis. 

The  epitrichium  was  discovered  by  Welcker,  64. 1,  in  the  embryos 
of  a  sloth  (Bradypus),  where  it  forms  a  continuous  membrane  over- 
lying the  hairs.  Welcker  found  the  layer  in  several  mammals, 
including  man,  and  demonstrated  that  it  belongs  to  the  epidermis, 
becoming  separated  from  the  rest  of  the  outer  skin,  when  the  hairs 
grow  forth.  In  the  sloth  it  forms,  so  to  speak,  an  extra  foetal  en- 
velope, which  we  find  mentioned  by  Eschricht  and  Ebsen  (Miiller's 
Arch.,  1837,  41)  and  and  by  Simon  (Miiller's  Arch.,  1841,  370-372), 
but  these  authors  did  not  ascertain  its  origin.  Kerbert,  77. 1,  dem- 
onstrated the  epitrichium  in  reptiles;  Jeffries,  83.1,  and  Gardiner, 
84.1,  in  birds — the  latter  author  adding  also  considerably  to  our 
knowledge  of  the  layer  in  mammals.  Kolliker  failed  to  recognize 
the  layer  in  man  (see  his  "  Entwickelungsges.,"  1879,  and  "  Gewebe- 
lehre,"  6te  Aufl.,  204).  Minot,  86,  showed  that  the  layer  is  present 
in  the  human  embryo  at  certain  stages  and  is  absolutely  distinct 
from  the  underlying  horny  layer.  The  history  of  the  human  epitri- 
chium has  been  quite  fully 
worked  out  by  J.  T.  Bow- 
en,  89.1. 

The  epitrichium  becomes 
well  marked  during  the 
third  month,  as  a  single 
layer  of  cells  of  large  size, 
and  each  arching  up  from 
the  surface,  Fig.  307,  Eptr. 
Over  the  hairy  parts  of  the 
skin  the  development  does 
not  seem  to  progress  beyond 
this  stage.  The  cells  of  the 
epitrichium  enlarge  and 
gradually  flatten  down,  but 
before  they  are  completely 
the  flattened  there  intervenes  a 
condition  in  which  the  ex- 
panded cells  are  flattened  ex- 
cept in  their  central  part,  which  forms  a  dome-like  projection  on  each 
cell ;  into  this  dome  the  nucleus  and  protoplasm  of  the  cell  are  found 
withdrawn  and  degenerating.  Later  the  cells  are  very  large,  Fig.  309, 


a 


Fia.  309.— Epitrichium  of  a  Human  Embryo  of 
Fifth  Month,    a,  6,  Cells  of  two  layers  of  the  underlying 
horny  layer  drawn  on  the  same  scale. 


THK    SKIN.  553 

three  to  six  times  the  diameter  of  the  underlying  epidermal  cvll>; 
there  are  no  transitional  forms,  as  Kolliker  has  erroneously  main- 
tained, between  the  epit^chia]  and  the  underlying  cells.  The  out- 
lines of  the  cells  are  polygonal  and  very  distinct;  in  the  middle  of 
each  cell  is  an  irregular  lump  of  degenerated  protoplasm,  in  which 
the  nucleus  can  sometimes  be  distinguished.  The  epitrichium  over- 
lies the  hairs;  those  hairs  which  project  from  their  follicles  lie  be- 
tween the  epitrichium  and  the  rest  of  the  epidermis. 

Over  the  hairless  parts  of  the  skin  the  epitrichium  probably  per- 
sists and  becomes  several-layered,  except  that  it  disappears  in  great 
part  over  the  nails,  see  p.  555.  Thus,  in  an  embryo  of  three  months, 
there  appear  on  the  palms  several  layers  of  cells,  all  of  which  have 
the  vesicular  character  and  dark  look  of  the  cells  of  the  single-layered 
stage.  It  is  unknown  how  this  growth  of  the  epitrichium  is  effected  ; 
the  primitive  epitricoial  cells  have  so  much  the  appearance  of  degen- 
erating tissue,  that  it  is  improbable  that  they  proliferate,  hence  we 
must  assume  that  the  growth  is  effected  by  the  addition  of  cells  from 
the  deeper  layers.  It  was  indicated  above  that  in  other  parts  the 
many-layered  epitrichium  probably  undergoes  cornification  accord- 
ing to  Zander's  Typus  A,  and  forms  the  stratum  corneum  of  authors, 
which  is  found  overlying  the  stratum  lucidum.  This  probability 
rhietly  upon  Bowen's  observation  that  the  epitrichium  over  the 
developing  nail  is  continuous  with  the  horny  layer.  If  we  accept 
this  interpretation,  we  must  say  that  the  epitrichial  cell  cornifies  so 
as  to  form  a  thick-walled  vesicle,  while  the  underlying  cells  cornify 
so  as  to  form  solid  scales  (Zander's  Typus  B,  88.1).  That  the 
epitrichium  in  birds  and  mammals  may  become  horny  was  demon- 
strated l»v  ( iardiner's  careful  researches,  84.1. 

Dermis. — Although  the  dermis  or  cutis  is  of  exclusively  mesen- 
chymal  origin,  it  is  convenient  to  consider  its  development  in  con- 
nection with  that  of  the  external  skin.  In  very  early  stages  the 
mesenchyma  extends  to  the  ectoderm,  but  shows  no  trace  of  a  special 
layer  under  the  epidermis.  This  layer  is,  however,  well  marked  in 
embrj-os  of  two  months  by  the  condensation  of  the  dermal  mesen- 
chyma, the  cells  becoming  flattened  in  a  plane  parallel  with  the 
surface,  and  hence  they  appear  somewhat  elongated  in  vertical  sec- 
tions of  the  skin,  Fig.  300,  c;  the  nuclei  are  granular,  the  protoplasm 
forms  a  rich  network  of  great  delicacy.  Later  the  protoplasm  is,  I 
find,  more  condensed  around  the  nuclei,  and  the  cells  have  more  indi- 
viduality; at  the  same  time  the  protoplasmatic  network  becomes 
coarser  and  simpler  in  character.  During  the  third  month  (Kolliker, 
"Entwickelungsges.,"  1879,  p.  774)  the  primitive  dermis  becomes 
differentiated  nito  two  layers,  the  true  dermal  (corium,  Lederh<tnf) 
and  the  subdermal  (Unterhautzellgewebe) ,  the  tissue  being  more 
condensed  in  the  former  and  more  fibrillar  in  the  latter.  During 
the  latter  half  of  the  fourth  month  fat  cells  arise  in  the  subdermal 
layer  and  steadily  increase  thereafter  in  both  number  and  size,  and 
by  the  end  of  the  fifth  month  the  whitish  fat  islands  are  conspicuous 
to  the  naked  eye.  The  skin  now  comprises,  Fig.  310,  the  epidermis, 
fc/><  the  dermis  or  cutis,  On,  and  the  fat-layer  F\  below  is  loose  con- 
nective tissue,  c.  The  hairs  grow  to  the  bottom  of  the  fatty  layer. 
The  origin  of  the  columnse  adipose  (J.  C.  Warren,  77.1),  calls  for 


554  THE   FCETUS. 


investigation.     The  papillae  of  the  dermis  can  be  first  seen  during 
the  sixth  month  (Kolliker,  I.  c. )  on  the  hand  and  feet,  forming  a  double 


FIG.  310.— Vertical  Section  of  the  Skin  of  a  Human  Embryo  of  the  fifth  Month.    Ep,  Epider- 
mis; CM,  cutis;  F,  fat  layer:  c,  loose  connective  tissue. 

row  between  every  pair  of  primary  ridges,  Fig.  308,  Ri.  The  elas- 
tic fibres  appear  during  the  seventh  month  (Kolliker,"  Entwickelungs- 
ges.,"  2te  Aufl.,  776). 

II.    NAILS  AND  HAIRS. 

Nails. — A  nail  is  a  modified  area  of  the  stratum  lucidum,  situ- 
ated upon  the  upper  side  of  the  terminal  joint  of  a  digit  and  laid 
bare  by  the  loss  of  the  overlying  epitrichium.  This  definition  is 
essentially  different  from  that  hitherto  current,  and  is  based  on 
Bowen's  discoveries,  89.1. 

The  first  indication  of  the  nails  may  be  seen  in  the  human  embryo 
at  the  beginning  of  the  third  month  as  a  thickening  of  the  epitrichium 
over  the  end  of  the  digit.  In  most  mammals  this  position  is  per- 
manent and  there  is  developed  a  terminal  claw,  but  in  man,  as  dis- 
covered by  Zander,  84.1,  the  terminal  position  is  transitory,  and 
the  ungual  area  migrates  on  to  the  dorsal  side  of  the  digit.  The 
change  of  position  is  attributed  by  Kolliker,  88.2,  25,  to  the  growth 
and  expansion  of  the  palmar  side  of  the  finger-tip.  A  secondary 
result  of  the  migration  of  the  nail  is  the  transfer  of  the  terminal 
branches  of  the  two  digital  nerves  of  the  palmar  surface  to  the  back 
of  the  finger  (toe)  tips,  Zander,  84. 1.  The  nail  area  is  marked  out 
quite  definitely  by  a  limiting  groove  or  depression  which  persists 
more  or  less  distinctly  throughout  life. 

As  soon  as  the  nail  area  has  reached  its  dorsal  permanent  position, 
there  appears  at  its  proximal  edge  an  oblique  ingrowth  of  the  Mal- 
pighian  layer  of  the  epidermis,  to  form  the  so-called  root  of  the  nail. 
The  epitrichium  over  the  nail  is  much  thickened — see  Bowen,  /.c., 
Fig.  3 — but  is  thickest  near  and  beyond  the  distal  edge  of  the  nail. 
The  primary  ridges  of  the  Malpighian  layer  now  appear,  but  only 
over '  the  palmar  surface  of  the  finger  or  toe  tip,  and  as  they  do  not 
appear  until  much  later  under  the  nail,  they  establish  a  marked 
difference  between  the  epidermis  surrounding  and  that  covering  the 


NAILS   AND    HAIRS.  555 

nail  area.  The  epitrichial  layer  over  the  area  has  received  the 
>|M-<-ial  name  of  <'/)<»< '/ch  tun  from  Unna,  76.1.  Until  the  fourth 
month  th« TO  is  little  change  except  that  the  anlage  of  the  root  of 
the  nail  grows  considerably,  and  at  the  same  time  becomes  more 
and  more  inclined  toward  a  horizontal  position,  a  change  which  pro- 
ves until  by  the  eighth  month  the  nail-root  is  horizontal,  i.e.,  in 
tin-  same  plane  with  the  nail-bed  proper — compare  Fig.  311. 

About  the  beginning  of  the  fourth  month  there  appear,  Kolliker, 
88.2,  1,  granules  in  the  uppermost  cells  of  the  Malpighian  layer. 
Tin-  granules  are  rounded  in  form,  variable  in  size  and  have  a  decided 
aHinity  for  coloring  matters,  especially  for  acid  fuchsin,  Zander, 
86.1,  285.  Very  soon  the  cells  form  a  stratum  lucidum,  which 
appears  first  in  the  distal  part  of  the  ungual  area  and  is  very  thin, 
thence  spreads  proximalward,  and,  last  of  all,  appears  in  the  nail- 
it  »<  ,t,  being  there  also  preceded  by  the  granular  cells.  By  the  middle  of 
tlie  fourth  month  the  stratum  lucidum  is  present  over  the  whole  nail 
and  also  extends  on  to  the  palmar  surface,  Fig.  311,  s .1.  The  gran- 
ules have  been  supposed  to  be  identical  with  eleidin,  but  on  this 
point  there  has  been  some  discussion,  which  is  summarized  by  Kol- 
liker ("Gewebelehre,"  6te  Aufl.,  216);  Ranvier  ("Traite  technique 
d'Histologie,"  88G)  was  the  first  to  observe  that  the  granules  differ 
somewhat  from  true  eleidin.  There  can  be  little  question,  if  any, 
that  the  granules  are  directly  connected  with  the  cornification  of  the 
i  •»  -11s  to  form  the  nail  proper.  The  granules  were  descri  bed  by  Brook  in 
1  ^:5,  in  a  paper  (Schenk's  "  Mitth.,"  II.,  159),  which  I  have  not  seen, 
and  their  relation  to  keratosis  was  more  fully  studied  by  Zander, 

86. 1,  whose  results  have  been  in  the  main  confirmed  by  Kolliker, 

88.2,  amlF.  Curtis,  89.2.     The  walls  of  the  granular  cells  gradually 
become  thickened  (marginal  keratinization  of  Curtis),  the  cell  be- 
eoines  flattened,  its  nucleus  disappears,  the  walls  unite,  and  there  is 
thus  produced  a  horny  scale  in  the  place  of  the  cell.     By  the  trans- 
it TI nation  of  additional  cells,  the  horny  stratum  lucidum  is  constantly 
thiekened  on  its  underside — compare  Fig.  163,  in Kolliker's " Gewe- 
belehre," Gte  Aufl.     During  the  fifth  month  the  development  of  the 
stratum  gradually  extends  beyond  the  nail  area  over  the  rest  of  the 
linger- tips,  and  more  slowly  into  the  nail-root. 

The  epitrichium  disappears  over  the  nail  at  about  five  months, 
first  in  the  centre,  then  toward  the  base,  sides,  and  distal  end,  but  a 
small  band  persists  as  theperionix  across  the  root  of  the  nail,  Fig. 
311,  Ep'i  and  a  large  mass,  Ep",  forms  a  conspicuous  ridge  after 
the  fifth  month,  across  the  distal  end  of  the  nail,  and  is  continued 
over  the  palmar  surface  of  the  digit,  as  a  considerable  horny  layer 
covering  the  stratum  lucidum,  s.l.  The  nail,  N,  although  the  direct 
continuation  of  the  stratum  lucidum,  has,  of  course,  its  surface  ex- 
posed. The  epitrichium  varies  greatly  in  appearance,  for  it  may 
either  preserve  more  or  less  the  vesicular  form  of  its  cells,  or  its  cells 
may  be  more  or  less  cornified  and  flattened.  It  is  probably  owing  to 
the  frequency  of  the  latter  modification  that  the  nature  of  the  layer 
has  been  overlooked.  The  cornification  of  the  epitrichium  is  pre- 
ceded by  the  appearance  of  eleidin  granules  in  its  cells,  Curtis, 
89.2,  17. 

The  final  step  in  the  development  of  the  nail  is  the  change  by 


556 


THE   FOETUS. 


which  its  distal  edge  becomes  free,  according  to  Kolliker,  88.2,  7, 
by  desquamation  of  the  stratum  lucidum  at  the  point  where  the  nail 
passes  distally  into  the  stratum  of  the  palmar  surface. 


Ep' 


FIG.  311.— Longitudinal  Section  of  the  Nail  of  the  Great  Toe  of  a  Human  Embryo  of  five 
Months.  Minot  Coll.  No.  95.  Ep,  Remnant  of  epitrichium;  Ep',  distal  ridge  of  epitrichium  :  .V, 
nail ;  s.  I,  stratum  lucidum ;  6,  bone.  From  a  section  by  Dr.  Bowen,  stained  with  acid  fuchsin. 

Morphology. — The  discovery  that  the  nails  are  modified  portions 
of  the  stratum  lucidum  gives  the  question  of  their  evolution  an  en- 
tirely new  aspect.  It  renders  it  probable  that  the  claws  and  hoofs 
are  also  derived  from  the  stratum  lucidum,  and  that  the  develop- 
ment and  changes  of  this  layer  of  the  epidermis  will  have  to  be  care- 
fully investigated  in  the  lowest  amniota  before  we  can  hope  to 
understand  the  origin  of  claws. 

It  may  be  safely  assumed  that  the  nail  is  a  modified  claw.  Zan- 
der, 84.1,  having  observed  the  primitive  terminal  position  of  the 
nail  area  (Nagelfeld)  in  the  human  embryo,  and  its  subsequent  mi- 
gration to  the  dorsal  side  of  the  digit,  concluded  that  the  human 
nail  represented  a  terminal  claw  flattened  out,  and  that  the  centre  of 
the  nail  must  correspond  to  the  point  of  the  claw.  Boas,  84.1, 
from  comparative  anatomical  studies  on  claws,  hoofs,  and  nails, 
established  a  distinction  between  the  volar  side  and  the  palmar  side 
of  claws  and  hoofs,  and  homologized  the  nail  with  the  volar  side  of 
a  claw,  which  may  therefore  be  termed  the  nail-plate  (Nagelplatte) ; 
Boas  further  maintained  that  the  palmar  side  (sole-plate,  Sohlen- 
lioni)  of  the  claw  becomes  rudimentary  in  man,  and  believed  that  its 
representative  is  the  small  area  of  epidermis  under  the  edge  of  the 
nail  in  the  adult;  tbis  area  probably  corresponds  to  that  which  in 
the  embryo  is  covered  by  the  epitrichial  ridge,  Fig.  311,  Ep" ,  at 
the  distal  edge  of  the  nail.  This  interpretation  has  been  adopted  by 
Gegenbaur,  85.1,  in  whose  laboratory  Boas'  researches  were  carried 
out.  In  view  of  our  present  knowledge  it  seems  to  me  that  Boas' 
conception  must  be  accepted,  with  the  modification,  however,  that 
the  stratum  lucidum  covered  by  epitrichium  over  the  end  of  the  digit 
must  be  considered  the  homologue  of  the  sole-plate  (Sohlenhorn), 
and  that  not  merely  the  epitrichial  ridge  at  the  edge  represents  the 


NAILS   AND    HAIRS.  557 

sole-plate.  To  decide  the  question,  we  must  acquire  exact  knowl- 
edge of  the  relation  of  the  sole-plate  to  the  stratum  lueidum  in  clawed 
and  hoofed  mammals. 

Hairs. — A  hair  is  a  long  downgrowth  of  the  mucous  layer  of  the 
epidermis  into  the  cutis,  Fig.  312,  A;  into  the  enlarged  end  of  the 
d«»\vn growth  extends  a  papilla,  p,  of  mesenchymal  tissue;  the  down- 
gro \vth  separates  into  two  parts,  the  axial  or  hair  proper,  H,  which 
gmws  upward  and  projects  above  the  surface,  and  a  peripheral  part  or 
follicle, /.  At  the  base  of  the  hair,  the  hair  itself  and  the  follicle 
unite. 

The  hairs  arise  in  man  as  solid  processes  of  the  epidermis,  the  ends 
of  which  very  soon  expand,  Fig.  313,  5,  ('»,  and  acquire  the  dermal 
papilla,  7.  In  other  cases,  as  has  been  observed  by  Alexander 
Goette  68.1,  and  also,  it  is  said,  by  Reissner  and  Feiertag,  the 
papilla  is  formed  first,  as  a  slight  projection  of  the  dermis  into  the 
Mfdpighian  layer  of  the  ectoderm;  the  overlying  epidermis  then 
forms  a  downgrowth,  which  carries  the  papilla  with  it;  in  other 
respects  the  hair  develops  as  in  man.  O.  Hertwig  ("Entwicke- 
lun^sgeschiGhte,"  J5te  Aufl.,436)  regards  the  type  of  development  in 
which  the  papilla  appears  first,  as  the  more  primitive;  this  view  is 
plausible,  and  enables  us  to  assume  that  the  hairs  were  evolved  by 
modifications  of  the  epidermis,  overlying  special  dermal  papillae. 
Hertwig  fortifies  his  hypothesis  by  comparison  with  the  teeth,  which 
in  the  lower  vertebrates  are  developed  from  dermal  papillae,  while 
in  the  higher  forms  there  is  a  deep  ingrowth  of  the  epidermis  before 
the  mesenchymal  papilla  of  the  dental  germ  appears. 

The  hair  anlages  appear  in  the  human  embryo  at  about  three 
months,  andean  be  first  seen  over  the  forehead  and  eyebrows,  but 
very  soon  (sixteenth  to  seventeenth  week)  are  developed  over  the  en- 
tire head,  and  a  little  later  the  rest  of  the  body,  so  far  as  it  is  ever 
hairy — on  the  limbs  the  hairs  appear  about  the  twentieth  week.  By 
the  end  of  the  fifth  month,  all  the  hairy  areas  are  marked  out.  From 
the  third  to  the  seventh  month  at  least — my  observations  do  not  go 
further — new  hair  anlages  continue  to  arise,  so  that  one  finds  various 
>t,iges  at  once.  It  is  thus  possible  to  study  in  one  preparation  the 
gradual  differentiation  of  the  hair.  In  embryos  of  five  to  seven 
months,  which  have  died  and  been  retained  in  utero,  the  epidermis 
is  usually  loosened  and  may  be  isolated.*  Such  a  piece  of  epidermis 
stained  with  alum  haBmatoxylin  and  viewed  from  the  under  side  is 
represented  in  Fig.  313.  I  distinguish  two  kinds  of  nuclei,  those 
which  are  more  darkly  stained  and  those  which  are  lighter.  Some 
of  the  light  nuclei  appear  dark  because  of  the  epitrichial  cells  un- 
derlying them.  The  darkly  stained  nuclei  all  belong  to  cells  which 
participate  in  the  formation  of  hairs.  At  first  the  dark  nuclei  make 
a  little  cluster,  as  at  1  and  2 ;  the  clusters  grow  in  size — one  a  little 
larger  is  seen  just  to  the  left  of  that  numbered  2,  one  a  good  deal 
larger  is  shown  at  3.  Sections  show  that  such  clusters  are  on  the 
under-side  of  the  epidermis  and  form  slight  protuberances  or  rudi- 
mentary papilla;  the  papillae  lengthen  out  and  acquire  rounded 
ends,  4 ;  they  grow  rapidly  down  into  the  cutis,  and  by  the  contrac- 

*  The  process  may  be  imitated  by  soaking  the  skin  of  a  foetus  for  several  days  in  a  0.75  per 
cent  salt  solution  to  which  a  little  thymol  has  been  added  to  render  it  aseptic. 


558 


THE    FCETUS. 


NAILS    AND    HAIRS. 


559 


tion  of  their  upper  part  become  club-shaped,  5  and  (i.  The  next  step 
is  tlu'  formation  of  the  dermal  papilla?  of  the  hair,  7;  a  little  notch 
arises  at  the  thick  end  of  the  epidermal  ingrowth,  and  the  tissue  fill- 
ing this  notch  is  the  so-called  dermal  papilla.  The  figure  presents 
also  a  well-developed  hair;  here  the  axial  portion  of  the  papilla  has 


FIG.  313.— Isolated  Epidermis  of  a  Human  Embryo  of  five  to  six  Months.  1-7,  Hair  anlages 
in  successive  stages;  h.  hair:  F.  follicle  from  which  the  hair  has  been  pulled  out;  Gl,  anlage  of 
sebaceous  gland ;  /.  wall  of  follicle;  fc',  bulb  of  hair. 

formed  the  hair,  /?,  while  the  cortical  portion  has  formed  the  follicle, 
/;  the  end  of  the  hair  is  thickened,  h',  as  the  so-called  hair-bulb ;  the 
sebaceous  gland,  &/,  has  begun  to  grow  out  from  the  follicular  walls. 
In  the  upper  part  of  the  follicle  the  hair  lies  quite  free,  hence  in  sev- 
eral places  where  the  hairs  have  been  forcibly  torn  off  the  upper  part 


560  THE   FCETUS. 

of  the  follicle,  F,  still  remains,  while  the  lower  part  attached  to  the 
hair  is  gone. 

The  differentiation  of  the  hair  in  the  axis  of  the  downgrowth  be- 
gins about  three  to  five  weeks  after  the  anlage  appears,  when  the 
anlages  are  from  0.25-0.40  mm.  long,  and  before  the  dermal  papilla 
is  recognizable.  Two  changes  mark  the  commencing  differentiation 
of  the  hair  and  the  follicle :  1 ,  the  axial  cells  elongate  in  the  direction 
of  the  future  hair :  2,  the  outermost  layer  of  cells  assumes  the  char- 
acter of  a  cuboidal  epithelium.  The  next  step  is  the  formation  of 
the  papilla,  Fig.  313,  7,  which  is  followed  by  the  separation,  in  an- 
lages of  0.6-0.7  mm.,  of  the  axial  mass  of  elongated  cells  into  a 
smaller  darker  central  portion,  the  hair  proper,  Fig.  312,  H,  and  a 
lighter  portion,  which  constitutes  the  inner  follicular  sheath,  s.  It 
is  at  this  stage  that  the  sebaceous  glands,  Fig.  313,  Gl,  and  Fig. 
312,  A,  gl,  buds  from  the  follicular  tissue.  At  the  enlarged  base  of 
the  hair  the  la}rers  all  merge  into  one  another.  The  hair  proper 
grows  in  length  very  much,  in  diameter  very  little,  and  by  its  elon- 
gation penetrates  the  epidermal  layers,  being  accompanied  by  the 
inner  follicular  sheath.  As  all  the  hair  anlages  descend  obliquely, 
the  hair  penetrates  the  epidermis  obliquely  and  within  the  epidermis 
is  bent  down.  By  its  continued  elongation  it  finally  reaches  the 
surface  of  the  skin,  and  its  tip  remains  covered  only  by  the  epitri- 
chium  (Minot,  83),  and  when  that  disappears  the  hair  is  free.  The 
detailed  history  of  the  hair  follicles  calls  for  much  further  study.  I 
have  observed  the  following  details :  In  a  longitudinal  section  of  a 
fully  developed  hair,  Fig.  312,  A,  the  upper  part  of  the  follicle,  F, 
is  seen  to  have  a  central  cavity,  which  is  partly  filled  by  the  frag- 
ments of  the  broken-down  inner  follicular  sheath;  on  the  lower  side 
of  the  hair,  and  at  the  end  of  the  hollow  division  of  the  follicle,  is 
the  anlage  of  the  sebaceous  gland,  gl;  from  this  point  down  there 
is  no  space  between  the  wall  of  the  follicle  and  the  hair ;  immediately 
below  the  gland  is  an  eminence,  m.i,  which  is  formed  by  a  thicken- 
ing of  the  follicle,  and  serves  for  the  insertion  of  the  slender  muscle, 
muse,  the  erector  pili.  How  this  muscle  arises  is  unknown.  The 
thickening  of  the  follicle  where  the  muscle  is  attached  is  not  men- 
tioned in  the  text-books  I  have  consulted.  From  repeated  observa- 
tions I  conclude  that  it  is  a  typical  feature  of  the  human  hair.  It 
has  been  described  and  figured  by  Unna,  76.1.  Below  the  muscular 
insertion  the  follicle  is  differentiated  into  three  layers,  which  are 
better  shown  under  a  higher  power,  Fig.  312,  D;  there  is  an  inner- 
most sharply  limited  horny  layer,  £,  with  no  trace  of  cellular  struct- 
ure, a  middle  layer  of  granular  cells,  c,  and  an  outermost  layer  of 
clear  epithelioid  cells,  Ep,  having  their  nuclei  in  their  bases  toward 
the  hair,  h.  The  follicle  is  incased  in  a  fibrous  mesenchymal  tunica 
propria,  tu.  Returning  to  Fig.  312,  A,  the  two  outer  layers  of  the 
follicle  are  seen  to  merge  into  one  another  toward  the  base  of  the  hair, 
and  to  thin  out  and  disappear ;  the  inner  sheath,  s,  on  the  contrary, 
thickens,  becomes  more  and  more  distinctly  cellular,  and  finally 
expands  as  the  hair  bulb  around  the  papilla.  The  hair  proper,  H, 
is  of  nearly  uniform  diameter  until  it  reaches  the  bulb,  where  it 
expands  to  embrace  the  papilla,  pa,  and  fuses  with  the  inner  follicu- 
lar sheath.  A  network  of  blood-vessels,  v,  in  the  tunica  propria 


NAILS    AND    HAIl;^.  501 

is  spun  around   the  hulb,  but  vessels  have  not,  in  the  stage  figured, 
penetrated  the  papilla  itself,  pa. 

/.<m  iit/u  is  the  term  applied  to  the  first  coat  of  hairs  in  the  embryo. 
This  coat  is  a  conspicuous  feature  at  seven  months.  It  is  to  be  re- 
garded as  the  embryonic  reproduction  in  man  of  an  ancestral  simian 
characteristic  (Darwin,  "Descent  of  Man,"  Chap.  I.).  The  hair- 
are  fine,  comj >ared  with  those  of  the  adult,  and  are  therefore  usually 
described  as  woolly  hairs;  they  are  lost  from  most  parts  of  the  body, 
and  replaced  by  larger  and  coarser  hairs.  Over  the  face  the  lanugo 
persists  throughout  life,  but  owing  to  its  fineness  and  loss  of  color  is 
not  usually  noticed. 

/.o.s.s  and  KriH'ini/  of  Hairs. — The  length  of  life  of  a  single  hair 
is  not  IOIILC,  for,  as  is  well  known,  the  hairs  are  continually  shed. 
In  many  mammals  the  shedding  is  an  annual  process,  but  in  man 
it  takes  place  constantly.  As  the  number  of  hairs,  except  in  cases 
of  baldness,  does  not  diminish  sensibly,  it  follows  that  new  hair- 
must  he  continually  formed. 

The  loss  of  hairs  begins  during  fo?tal  life.  The  hairs  shed  by  the 
fo-tus  fall  into  the  amniotic  fluid  and  are  sometimes  swallowed  by 
the  embryo  and  found  in  the  meconium,  see  Chap.  XXIX.  Imme- 
diately after  birth  the  shedding  of  the  lanugo  occurs,  its  place  being 
taken  in  certain  parts  by  coarser  hairs.  The  shedding  of  the  hair  is 
initiated  by  chanires  in  the  hair  bulb,  or  expanded  end  of  the  hair 
fitted  over  the  papilla;  the  multiplication  of  cells  in  the  bulb,  by 
means  of  which  the  growth  of  the  hair  is  maintained,  ceases,  and  the 
hulh  atrophies,  separates  from  the  papilla,  and  breaks  up  into  a  bun- 
dle of  fibres;  the  hairs  in  which  the  bulbs  have  become  fibrous  are 
the  KnUwnhnure  of  J.  Henle,  the  Jlwllmare  of  P.  Unna,  76.1. 
That  these  hairs  which  have  no  papilla  cannot  grow  has  been 
demonstrated  experimentally  by  L.  Ranvier.  For  a  time  the  hair 
is  still  retained  in  place  by  the  sheath  of  the  follicle  pressing  against 
it.  It  is  finally  either  pulled  out  by  some  outside  force,  or  pressed 
out  by  the  secondary  hair  (Ersatzhaare);  there  is  also  an  actual 
shortening  of  the  follicle  of  the  atrophying  hair,  a  fact  observed  by 
von  Ebner,  76.1,  and  confirmed  by  Kolliker  ("Gewebelehre,"  6te 
Ann1..  -Ml. 

As  to  the  development  of  the  secondary  or  replacement  hairs 
(Ersatzhaare),  authors  are  not  agreed.  That  there  is  a  long  contin- 
ued production  of  new  hair-germs  during  foetal  life  is  well  known, 
and  that  the  process  is  continued  after  birth  has  been  maintained 
by  several  writers,  but  such  hairs  cannot  be  regarded  as  secondary 
but  only  as  primary  hairs.  The  true  secondary  hairs  are  those 
which  arise  from  the  follicles  of  previous  hairs.  According  to  some 
authors,  the  old  papilla  is  preserved  and  the  new  hair  is  formed  over 
it,  but  this  opinion  does  not  appear  to  me  to  rest  upon  satisfactory 
observations.  Far  better  founded  is  the  view  of  Kolliker  ("  Gewe- 
belehre," 5te  Aufl.,  1867,  p.  137),  that  the  new  hairs  are  developed 
from  buds,  which  spring  from  the  base  of  the  old  follicles  soon  after 
the  old  hair  bulb  has  atrophied ;  the  buds  are  small  in  diameter,  and 
lengthen  out  the  old  follicle;  the  cells  show,  at  first,  no  differentia- 
tion, the  bud  resembling  closely  a  young  hair  germ ;  in  it  a  new  hair 
is  developed  in  the  same  way  as  in  the  primary  hair  germs.  The 
86 


562  THE   FCETUS. 

figure  showing  the  development  of  the  secondary  hairs  given  by 
Kolliker  in  his  "  Gewebelehre, "  5te  Aufl.,  have  been  reproduced  by 
him  in  the  sixth  edition,  Figs.  186  and  187,  also  in  his  "Entwicke- 
lungsgeschichte,"  1879,  Figs.  476,  477. 

Sebaceous  Glands. — As  the  sebaceous  glands  are  outgrowths 
of  the  hair  follicles,  they  are  appropriately  treated  here.  They  ap- 
pear as  thickenings  of  the  follicles  of  the  hair  germs,  about  the 
time  the  hair  proper  reaches  the  level  of  the  epidermis.  The  thick- 
enings are  solid,  and  as  they  enlarge  become  somewhat  lobulated, 
Fig.  312,  A,  gl;  they  usually  are  situated  on  the  under  side  of  the 
hair,  Fig.  312,  A,  C,  but  sometimes  spring  laterally.  Even  before 
the  lobulation  begins,  the  anlage  is  seen  to  be  differentiated  into  an 
outer  layer,  Fig.  312,  C,  cor,  in  which  the  cells  retain  their  original 
character,  and  are  small  and  granular :  and  a  central  mass  of  larger 
modified  cells,  Sb.  The  latter  increase  in  number  until  they  find  an 
exit  into  the  cavity  of  the  follicle,  Fig.  312,  A,  gl.  According  to 
Kolliker  ("  Entwickelungsgesch.,"  1879,  p.  797)  the  central  cells 
contain  fat  globules  and  are  discharged  into  the  follicle,  thereby 
becoming  the  secretion  of  the  gland ;  the  cortical  layer  persists  as 
the  germinating  bed  of  new  fatty  central  cells.  In  specimens  hard- 
ened in  alcohol,  stained  in  alum  cochineal,  and  cut  in  paraffine,  the 
central  cells  of  the  sebaceous  glands  of  the  foetus  present  a  highly 
characteristic  appearance;  they  are  rounded  or  oval,  and  much 
larger  than  the  cortical  cells,  Fig.  312,  C.  Under  a  high  power, 
Fig.  312,  B,  each  cell  is  seen  to  be  separated  from  its  fellows,  to 
have  a  distinct  outline,  a  coarse  intracellular  network  and  a  finely 
granular  rounded  nucleus,  lying  in  a  perinuclear  space,  which  is 
darker  than  the  rest  of  the  cells.  The  further  development  of  the 
gland  consists  principally  in  the  addition  of  lobules,  which  arise  as 
buds  of  the  cortical  layer,  the  fatty  central  cells  developing  later  in 
each  bud  (alveolus) ;  the  neck  connecting  the  lobules  with  the  hair 
follicle  becomes  the  duct  of  the  gland.  The  growing  gland  spreads 
around  its  hair  follicle,  but  the  position  of  its  duct  permanently  indi- 
cates its  origin  from  the  under  side  of  the  hair. 

As  the  development  of  the  sebaceous  glands  begins  at  a  definite 
stage  of  the  hairs,  and  as  the  hair  germs  continue  arising  throughout 
foatal  life,  so  we  encounter,  at  any  time  after  the  fifth  month,  glands 
in  various  stages.  The  first  glands,  according  to  Kolliker,  appear 
on  the  head  at  about  four  and  one-half  months,  on  the  body  at  about 
five  months. 

Vernix  Caseosa. — As  we  have  learned  from  their  development 
the  sebaceous  glands  begin  their  secretory  activity  by  the  end  of  the 
fifth  month.  Their  fatty  secretion  is  discharged  on  the  surface,  and, 
together  with  the  shed  portions  of  the  epidermis,  usually  forms  a 
more  or  less  extensive  coating  of  the  embryo.  Minot,  83,  has  sug- 
gested that  the  persistence  of  the  epitrichium  may  be  a  factor  in  the 
formation  of  the  coating,  which  is  known  as  the  vernix  caseosa 
(smegma  embryonum,  Kcisefirniss,  Fruchtschmiere) .  Simon 
("  Med.  Chemie, "  II. ,  486)  is  said  to  have  been  the  first  to  show  that  the 
vernix  consists  entirely  of  sebaceous  cells,  fat  globules  and  epidermal 
cells,  and  therefore  could  not  be  a  product,  as  some  of  the  older  writers 
imagined,  of  the  amniotic  fluid.  Quantitatively  the  epidermal  cells 


GLANDS    OF   THE   SKIN  563 

are  the  chief  components.  The  vernix  becomes  conspicuous  during 
the  sixth  nmnth  and  increases  until  birth.  It  is  extremely  variable 
in  amount.  Kolliker  states  that  Buck  (k'  De  vernice  caseosa,"  Halis, 
1M4)  found  it  might  increase  to  3.5  drachms  in  weight.  In  other 
cases  it  is  almost  entirely  absent.  Elsasser  (Schmidt's  Jahrbticher, 
Bd.  VII.,  1833)  found  that  about  half  the  children  of  both  sexes  are 
born  without  vernix  caseosa,  the  other  half  with  a  varying  amount. 
( )n  the  chemical  composition  of  the  vernix  see  Davy  (London 
Med.  Gazette <  1M4)  and  Buck  ("De  vernice  caseosa,"  Halis,  1844). 
The  vernix  contains  nine  to  ten  per  cent  fats  and  seventy -eight  to 
eighty-four  per  cent  water. 

III.    GLANDS  OF  THE  SKIN. 

The  development  of  the  sebaceous  glands  of  the  hair  has  been 
described,  p.  562 ;  concerning  the  development  of  the  other  sebaceous 
glands,  such  as  those  of  the  external  ear  and  of  the  prepuce,  little  is 
known;  the  glands  of  the  eyeballs  and  eyelids  are  treated  in  Chapter 
XXVIII. ;  there  remain  to  be  considered  here  the  sweat  glands  and 
the  mammary  glands. 

Sweat  Glands. — They  arise  as  solid  ingrowths  of  the  Malpighian 
layer  of  the  epidermis,  somewhat  similar  at  first  to  young  hair- 
germs.  They  may  be  distinguished  from  hair-germs  by  their 
descending  perpendicularly  instead  of  obliquely,  and  by  appearing 
in  the  fresh  state — not  whitish,  like  hair-germs,  but  yellowish. 
They  appear  on  the  hairless  parts  (soles  and  palms)  early  during 
the  fifth  month,  but  not  until  much  later  on  the  hairy  parts.  Kolli- 
ker, 88.2,  15,  has  observed  that  the  sweat  glands  are  developed 
earlier  on  the  under  than  on  the  upper  side  of  the  digits,  and 
earlier  on  the  third  digit  than  on  the  others.  The  ingrowths  arise 
on  the  soles  and  palms  from  the  primary  ridges,  Fig.  308,  S.  The 
lower  end  is  somewhat  thicker  than  the  upper  part  of  the  ingrowth, 
which  rapidly  elon- 
gates, passes  through 
the  dermis  proper, 
and  when  it  reaches 
the  fatty  layer  or  sub- 
dermal  tissue,  the 
anlage  of  the  gland 
begins  to  assume  a 
contorted  course,  the 
end  of  the  gland  roll- 
ing Over  toward  the  FIG.  314. -Section  of  the  Sole  of  the  Foot  of  a  Foetus  of  the 
prrir?prmi<3  TTicr  314.  fiftl1  Month,  to  show  the  Sweat  Glands,  which  arise  from  the  in- 
1S,  Tig.  ar*.  ner  or  Malpighian  layer  of  the  epidermis. 

The    lumen    of    the 

gland  can  be  readily  distinguished  at  this  time,  but  does  not  extend 
through  tlje  epidermis  until  later — on  the  foot,  not  until  the  seventh 
month  (Kolliker) .  This  fact  is  important,  because  it  sets  aside  the 
notion,  formerly  advanced,  that  the  sweat  glands  produce  the  liquor 
amnii.  At  the  time  of  birth,  the  glands  are  longer,  more  coiled, 
and  their  ducts  take  a  spiral  course,  but  the  spiral  turns  are  by  no 
means  so  close  together  or  so  numerous  as  in  the  adult. 


504 


THE   FCKTl'S. 


Epti 


Mammary  Glands. — The  milk  glands  vary  in  position.  It  is 
probable  that  there  were  typically  two  rows  of  glands,  and  that 
different  portions  of  these  rows  are  preserved  in  different  mammals, 
e.  g.,  the  head  ward  portions  in  primates,  the  tailward  portions  in 
ruminants. 

According  to  O.  Schultze,  92.1,  the  first  trace  of  the  mammae 
may  be  observed,  in  pig  embryos  of  15  mm.  and  rabbit  embryos  of 
thirteen  to  fourteen  days,  as  a  continuous  ridge-like  thickening 
(Milchlinie)  running  from  the  fore-limb  to  the  inguinal  fold.  In 
the  next  stage  (20  mm.)  the  ridge  is  specially  thickened — in  the  pig 
at  5-7  points,  at  each  of  which  a  mamma  is  developed ;  each  local 
thickening  becomes  separate  and  assumes  a  rounded  form.  The 
local  thickening  of  the  epidermis  is  the  anlage  of  a  milk  gland,  and 

this  anlage  has  been  long  known  and 
marks  the  site  of  the  future  nipple. 
In  man  the  thickening  may  be 
observed  toward  the  end  of  the  sec- 
ond month.  It  is  at  first  very  slight, 
though  it  causes  a  discernible  ex- 
ternal protuberance.  Later  it  pro- 
jects from  the  epidermis  into  the 
dermis.  The  thickening  commences 
when  the  epidermis  is  two-layered 
and  solely  at  the  expense  of  the  in- 
ner layer,  the  outer  layer  persisting 
for  a  time  as  a  distinct  epitrichium, 
Fig.  315,  A,  Eptr.  The  epithelial 
ingrowth,  Fig.  315,  B,  Ep.in,  en- 
larges, and  the  cells  in  its  central 
portion  gradually  cornify  and  fall 
out,  so  that  the  ingrowth  becomes 
hollow;  but  the  excavation  pro- 
gresses very  slowly  and  sometimes 
is  not  completed  until  after  birth. 
Soon  after  the  hollowing  has  begun 
the  ingrowth  sends  out  buds,  which 
resemble,  in  their  appearance  and 
early  development,  true  sweat 
glands.  The  organ  may  be  said  to 
be  now  in  the  monotreme  stage. 
C.  Gegenbaur  showed  in  1886  that 
m  Echidna  the  mamma  is  a  de- 

FiG?375.-Development  of  the  Mammary    pressed  area  of  the  skin,  from  which 

Giami  in  the  Rabbit,    A  Embryo  of  17  mm.   spring  a  number  of  lacteal  glands 

B,  embryo  of  29  mm.     C,  embryo  of  75-80  S  -, .  i        j         • 

mm.      Eptr,   epitrichium  ;     Bp,  epidermis  :  resembling  the     Sweat      glands     in 

Ep.in,  epidermal  ingrowth;  Cit,  dermis;  . «/,  annf»flranrv»          TVif>      rltmr^ee^rl     arpp 

milk  glands  proper;  Mac,  anlage  of  muscle;  appearance. 

rfr,  stroma  of  gland    (A  and  B  eye  much  Gegenbaur  terms   the   Driisenfeld 

more  highly  magnified  than  C.)    After  Rein.      /    ,fo     ,  x         T, 

(gland  area) .  It  seems  to  me  be- 
yond possible  question  that  the  thickening  of  the  outer  skin  to  form 
the  depressed  area  by  a  subsequent  loss  of  cells  in  no  wise  militates 
against  the  homology  here  maintained,  and  which  was  first  advanced 
by  Gegenbaur. 


Epin 


GLANDS    OF    THK    MX  IN.  505 

In  the  stage  of  Fig.  315,  C,  all  the  parts  of  the  adult  gland  may 
be  recognized.  The  tissue  around  the  epithelial  ingrowth,  /\/>.  in, 
is  destined  to  form  the  protuberant  nipple,  of  which  the  dermal  tis- 
sue is  clearly  differentiated  during  foetal  life,  although  the  nipple 
do«-s  not  usually  become  protuberant,  according  to  Rein,  82.1,  until 
after  birth.  The  boundary  of  the  dermal  tissue  of  the  nipple  is 
marked  br  a  distinct  layer  of  smooth  muscle  fibres,  msc.  Outside 
or  Mow  the  muscular  layer  is  the  fibrillar  connective-tissue  stroma, 
.s/r,  into  which  the  glands  grow,  and  within  which  they  are  differ- 
entiated. 

The  next  stage  of  development  is  reached  by  a  series  of  changes, 
of  which  the  most  important  are:  1,  the  obliteration  of  the  depres- 
sion, which  arose  by  the  hollowing  out  of  the  epithelial  ingrowth: 
-.'.  the  development  of  branches  from,  and  cavities  in,  the  milk 
glands  proper ;  13,  the  development  of  the  fat  layer  under  the  gland ; 
and,  4,  the  growth  of  the  nipple.  The  branching  of  the  glands  begins 
with  the  seventh  month,  and  even  at  the  time  of  birth  is  very  slightly 
advanced.  The  lumen  of  the  glands  appears  first  in  their  enlarged 
lower  ends,  not  long  before  birth,  and  then  extends  toward  the  mouth 
of  the  glands.  In  each  gland  we  can  distinguish:  1,  the  terminal 
branched  glandular  portion,  and,  2,  the  duct.  The  duct  consists  of 
a  wide  part,  .s///  //.s  lacfcns  of  authors,  next  the  secretory  portion,  and 
a  narrow  part,  which  extends  into  the  nipple  and  opens  there  on  the 
apex ;  the  orifice  of  the  duct  is  funnel-shaped,  and  hence  is  termed 
the  pars  iiij  ninlibularis.  The  fat  layer  is  a  continuation  of  that  of 
the  skin,  locally  thickened;  about  five  or  eight  years  after  birth  fat 
develops  also  in  the  stroma  of  the  mamma  between  the  gland  tubules. 

The  course  of  development  has  been  shown  by  Rein  to  be  essen- 
tially the  same  in  several  classes  of  mammals  as  in  man.  There  are, 
however,  noteworthy  secondary  differences,  particularly  in  rumi- 
nants ;  i  n  t  hem  the  nipple  is  precociously  developed  and  the  epithelial 
ingrowth  carried  up  on  to  its  apex  before  the  gland  buds  appear ; 
the  central  cells  of  the  ingrowth  disappear  as  in  man,  but  the  de- 
pression left  by  their  loss  is  not  obliterated,  but  is  permanent. 
Moreover,  there  is  only  a  single  gland  bud  developed,  which  grows 
out  to  a  considerable  length  to  attain  the  base  of  the  long  nipple  or 
teat,  where  it  branches.  Consequently  in  ruminants  there  is  but  a 
single  duct  through  the  nipple,  instead  of  several  as  in  man  and 
most  mammals.  In  the  horse  (Rein,  82.2,  685)  the  epithelial  in- 
growth forms  two  buds,  hence  there  are  two  ducts  in  the  adult. 

Mill:  cf  Ilirfli. — Although  the  mammary  gland  is  immaturely 
developed  at  birth,  yet,  as  is  well  known,  there  is  frequently  a  secre- 
tion discharged  from  it  for  a  few  days  after  birth.  Scanzoni,de 
Sinety,  and  Rein,  82.1,  4fi4,  have  shown  that  this  secretion  is  true 
milk.  It  is  known  in  German  as  Hexenmilch. 

M<mf(/unuTti\v  glands  have  been  shown  by  Rein,  82.1,  470,  to 
be  accessory  rudimentary  milk  glands. 

EVOLUTION  OF  THE  MAMMARY  GLAND. — That  the  mammary 
gland  arose  through  specialization  of  a  group  of  epidermal  glands, 
is  a  necessary  deduction  from  the  facts  of  comparative  anatomy  and 
embryology.  Several  authors  have  thought  that  the  milk  gland  was 
evolved  from  the  sebaceous  glands,  others  from  the  sweat  glands. 


566  THE   FCETUS. 

The  latter  opinion  rests  upon  strong  evidence,  the  former  principally 
upon  the  analogy  of  there  being  considerable  fat  in  both  the  seba- 
ceous and  lacteal  secretions.  Haidenhain  (Hermann's  "  Physiologic, " 
Bd.  V.,  380)  has  shown  that  in  the  milk  glands  there  is  no  fatty 
metamorphosis  of  the  central  cells,  as  in  sebaceous  glands,  but  a 
secretion  from  the  gland  walls,  as  in  the  sweat  glands,  so  that  there 
is  nothing  in  the  structure  or  function  of  the  adult  gland  to  justify 
a  comparison  with  the  sebaceous  type.  As  regards  the  embryolog- 
ical  development,  the  primary  epithelial  ingrowth,  Fig.  315,  A, 
Ep.  in,  is,  I  think,  to  be  regarded  merely  as  the  result  of  a  modified 
method  of  developing  the  depressed  glandular  area  (Drilsenfeld) ; 
the  glands,  sensu  strictu,  arise  as  solid,  long,  slender  ingrowths  of 
the  Malpighian  layer,  and  resemble  closely  the  true  sweat-gland 
anlages  and  not  the  sebaceous  glands.  Another  point  of  importance 
is  the  resemblance,  which  has  been  observed  by  Gegenbaur,  86.1, 
between  the  milk  glands  of  the  lowest  mammalia  and  the  sweat 
glands.  The  derivation  of  the  milk  glands  from  the  sweat  glands 
is  indicated  by  the  structure  and  mode  of  secretion  of  the  adult 
mamma,  by  the  development  of  the  gland,  and  by  the  structure  of 
the  gland  in  the  Echidna. 

Gegenbaur,  75.1,  86.1,  has  maintained  that  there  are  two 
types  of  milk  glands,  one  type  modified  sweat  glands,  the  other  type 
modified  sebaceous  glands ;  he  has  maintained,  also,  that  there  are 
two  types  of  nipple.  The  embryology  of  the  organ  shows  that  both 
the  nipple  and  the  gland  are  of  one  type,  certainly  in  most,  probably 
in  all,  mammalia.  Gegenbaur's  conception  that  there  are  two  mor- 
phologically distinct  forms  of  nipple  was  based  upon  Huss'  obser- 
vations, which  are  inaccurate  in  several  important  respects. 

LITERATURE. — Our  knowledge  of  the  development  of  the  mamma3 
was  derived  chiefly  from  the  observations  of  Langer,  52.1,  and  of 
K6lliker("Gewebelehre,"1867),  until  Huss  in  1873,  73.1,  greatly 
widened  our  acquaintance  with  the  early  stages  in  man  and  rumi- 
nants. Huss'  memoir  contained  important  errors,  especially  as  to  the 
origin  of  the  ruminant  teat,  and  these  errors  led  Gegenbaur,  73.1, 
75. 1,  to  his  notion  of  two  types  of  teats — a  notion  which  has  passed 
into  the  text-books,  although  shown  by  Rein  to  be  untenable.  H. 
Klaatsch,  84.1,  argues  against  Rein  in  favor  of  Gegenbaur,  but 
does  not,  it  seems  to  me,  invalidate  either  Rein's  observations  or 
conclusions.  Rein's  investigations,  82.1,  82.2,  easily  take  the 
first  place.  Creighton's  paper,  77.1,  added  but  little,  how  little 
may  be  judged  from  his  conclusion  that  the  glands  are  developed 
from  the  mesoderm. 


CHAPTER  XXVI. 

THE  MOUTH  CAVITY   AND   FACE. 

THE  face  may  be  said  to  be  a  charactn  i>iic  of  the  higher  verte- 
brates, and  to  acquire  increased  importance  as  we  ascend  the  series. 
In  the  marsipobranohs,  ganoids,  and  selachians,  the  face  does  not 
form  a  projecting  apparatus,  there  being  merely  an  area  on  the  ven- 
tral side  of  the  head,  which  is  distinguished  by  including  the  mouth 
and  the  nasal  pits.  The  primitive  Hrran-vnirnt  is  somewhat  masked 
in  the  marsipobranchs  by  the  modification  of  the  mouth  into  a 
large  sucking  apparatus,  but  in  ganoids  and  elasmobranchs  it  is 
preserved  throughout  lift*  with  little  alteration.  That  the  vertebrate 
mouth  belongs  primitively  on  the  under  side  of  the  head,  and  is  at 
first  a  simple  transversely  expanded  orifice,  is  clearly  established  by 
the  embryology  of  every  vertebrate  class.  Balfour  ("Comparative 
Embryology,"  II.,  :>1  T)  seems  to  have  been  the  first  to  definitely  for- 
mulate this  generalization.  The  evolution  of  the  face,  so  far  as  we 
could  judge  at  present,  depended,  Jirst,  upon  the  enlargement  and 
fusion  of  the  oral  and  nasal  cavities,  which  involved  a  change  of 
site  for  the  hypophysis;  second,  upon  the  partial  separation  of  the 
nasal  and  oral  cavities,  leaving  the  posterior  nares  open ;  fh  in  I,  upon 
the  growth  and  specialization  of  the  facial  region,  of  which  the 
elongation  of  the  jaws  is  the  most  conspicuous  indication;  fourfh. 
upon  the  development  of  a  prominent  external  nose.  At  the  same 
time  there  occur  modifications  of  position  in  the  face  in  relation  to 
the  brain  and  its  case  or  cranium,  which  it  will  be  well  to  mention 
briefly  in  order  to  render  the  following  sections  of  this  chapter 
dearer. 

The  position  of  the  face,  or  oral  region,  is  originally  determined 
by  the  head-bend,  as  is  more  fully  explained  in  the  following  section, 
see  also  Fig.  310.  If  we  imagine  a  median  longitudinal  section  of 
the  head  to  occupy  a  rectangular  area  divided  into  quarters,  then  we 
may  say  the  Imver  posterior  quarter  corresponds  to  the  mouth  region, 
the  other  three  quarters  to  the  brain.  As  development  progresses, 
the  oral  quarter  enlarges  out  of  proportion  to  the  rest  of  the  head  so 
as  to  project  forward  in  front  of  the  fore-brain ;  in  this  stage,  which 
is  represented  by  the  adult  amphibians,  the  bulk  of  the  facial  appa- 
ratus is  very  great  proportionately  to  the  cranium.  In  the  reptiles, 
the  oral  region  is  elongated  still  further  in  front  of  the  brain  case, 
resulting  in  the  long  snout.  In  mammals  a  third  stage  is  established 
by  the  great  increase  in  size  of  the  brain,  especially  of  the  cerebral 
hemispheres,  in  consequence  of  which  the  brain  comes  to  extend 
over  the  snout,  as  it  were;  in  man,,  whose  brain  has  the  maximum 
enlargement,  the  facial  apparatus  is  almost  entirely  covered  by  the 
brain.  The  modifications  involved  in  the  increase  of  the  brain  in 


THE    FCETUS. 

mammalia,  so  far  as  the  skull  is  concerned,  have  been  considered  p. 
467 ;  they  are  well  indicated  by  Wiedersheim  in  his  u  Grundriss  der 
vergleichenden  Anatomie,"  2te  AufL,  Fig.  84.  In  brief,  the  facial 
apparatus,  1,  underlies  the  hind  brain,  as  in  elasmobranchs ;  2,  pro- 
jects in  front  of  the  brain  (amphibia,  reptiles) ;  3,  is  covered  by  the 
cerebrum  (mammals). 

Formation  of  the  Oral  Cavity. — When  the  medullary  tube 
enlarges  to  form  the  brain — see  Chapter  XXVII. — the  end  of  the 
head  bends  over  to  make  room  for  that  enlargement.  The  bending 
of  the  head  carries  the  oral  plate  over  on  to  the  ventral  side  of  the 
freely  projecting  head,  compare  p.  262.  In  Fig.  100,  the  head-bend 
is  just  developing;  Ent,  indicates  the  anterior  extremity  of  the 
entodermal  canal,  and  the  reference  line  crosses  the  oral  plate,  or 
membrane  formed  by  the  union  of  the  entoderm  and  ectoderm ;  the 
oral  plate  occupies  the  entire  space  between  the  fore-brain,  /£>,  and 
heart,  ht,  and  there  is  as  yet,  properly  speaking,  no  oral  cavity,  but 
it  arises  by  the  next  changes  which  occur.  The  changes  which  de- 
velop the  mouth  cavity  are  the  growth  of  the  brain  and  of  the  peri- 
cardial  cavity,  both  of  which  expand  ventrally,  leaving  a  space — the 
mouth  cavity — between  them,  Fig.  170.  Laterally  the  cavity  is 
bounded  by  a  wall  or  sheet  of  tissue,  which  stretches  from  the  peri- 
cardial  somatopleure  to  the  head  and  is  the  anlage  of  the  cheek ;  it 
may  be  called  the  cheek  plate  ( Wangenplatte) .  The  mouth  cavity  i  s 
now  a  shallow  fossa  between  the  head  and  the  heart,  and  still  with- 
out connection  with  the  entodermal  canal  (human  embryo  of  2.15 
mm.  with  two  aortic  arches).  The  fossa  cannot,  strictly  speaking, 
be  regarded  as  an  invagination,  such  as  is  the  invertebrate  vorder- 
darm,  p.  261,  but  is  rather  the  result  of  the  growth  of  the  parts  sur- 
rounding the  oral  plate.  The  oral  pit  is  lined  by  ectoderm. 

While  the  oral  fossa  is  developing,  the  formation  of  the  gill  pouches 
begins.  About  the  time  the  third  branchial  arch  is  formed,  the  oral 
plate  ruptures  in  the  human  embryo,  and  the  oral  fossa  communi- 
cates widely  with  the  pharynx,  Fig.  320.  Upon  the  lateral  and 
ventral  sides  no  boundary  can  be  found  later,  but  upon  the  dorsal  or 
cranial  side  a  projection  persists,  Fig.  319,  in  front  of  which  appears 
an  evagination  of  the  oral  fossa,  to  constitute  the  anlage  of  the 
hypophysis  cerebri  or  pituitary  body  (see  below,  p.  571),  and  behind 
which  appears  a  second  evagination  from  the  pharynx  to  constitute 
the  so-oalled  Seesel's  pocket,  p.  268.  The  oral  cavity  proper  and 
the  pharynx  are  now  merged  into  a  single  wide  cavity,  Fig.  320,  for 
which  we  have  in  English  no  special  term — in  German  it  is  some- 
times called  the  Mundraclienraum  '(His,  "  Anat.  menschL  Embry- 
onen,"  Heft  III.,  26).  The  ectodermal  mouth  cavity,  or  oral  fossa, 
does  not  correspond  to  the  mouth  cavity  of  the  adult,  for  the  adult 
cavity  must  include  part  of  the  pharynx,  since  it  includes  the  tongue, 
which  is  developed  from  the  floor  of  the  pharynx,  and  in  fact  His 
has  shown  ("Anat.  menschl.  Embryonen,"  Heft  III.,  31)  that  the 
arcus  palato-glossi,  which  are  taken  as  the  boundary  between  mouth 
and  pharynx  in  the  adult,  are  derived  from  the  second  or  hyoid 
arches  of  the  pharynx.  Hence  the  adult  mouth  cavity  includes  the 
ectodermal  oral  fossa  plus  the  region  of  the  first  gill-arches  of  the 
pharynx 


TIIK  MOTTH  CAVITY   AND  PACE, 


N 
MX 

M 

Md 


br 


I 


v 


In  human  embryos  of  the  third  week   the  mouth   is  a  five->ided 
orifice,  and  1  observe  that  the  same  >hape  appears  in  other  mammalian 
embryo<.  and  also  in   lx)th   amphibian   and   elasmobranch  embryos, 
Kig.  :)!•;,  hence  it  is  probably  characteristic  of  all 
vertebrates  in  the  stage  with  live  unmodified  aortic 
arches.      The     month     is     bounded    (Hi-.    "Anat. 
menschl.  Kmbryonen,"  Heft  III.,  •'!")  anteriorly  by 
the  wall  (.s7/V///r///.s7)  of  the  head  covering  the  fore- 
brain    between  the  nasal  pits,  Kig.  'M>'<.  \.  laterally 
by  the  maxillary  processes,  3/a%  and   la tero- posteri- 
orly by  the  mandibular  processes,  Md;  the  latter 
an    the  first  branchial  arches,  and  unlike  the  fol- 
lowing arches,  br,  they  meet  in  the  median  ventral 
line. 

Another  important  factor  in  the  development  of 
the  oral  re-ion  is  the  descent  or  migration  of  the 
heart.  It  will  U»  remembered  that  the  aortic  end 
of  the  heart  moves  from  the  antei  ior  or  buccal  end 
of  the  pharynx,  tailward.  The  change  in  the 
heart's  position  leaves  the  greater  part  of  the 
pharynx  free  to  IK)  differentiated  in  intimate  asso- 
ciation with  the  oral  region,  and  the  change  also 
separates  the  mouth  and  the  heart,  so  that  very 
early  we  find  the  caudal  or  lower  boundary  of  the 
month  to  }*>  no  longer  the  pericardial  somatopleure, 
but  the  mandibalar  processes  or  arches,  the  ventral 
ends  of  which  are  developed  between  the  mouth  and 
heart. 

In  certain  teleosts,  some  time  after  the  first  pair 
^•ill-pouches  develop,  the  mouth  breaks  through 
in  the  ventral  region  of  these  po< -ket>  a>  a  bi-lateral  involution  of  the 
ectoderm,  fusing  with  the  entoderm  and  opening  on  each  side  of  a 
central  partition;  neither  involution  crosses  the  median  lino.  The 
double  oral  in  vagi  nation  was  discovered  by  A.  Dohrn,  82. 1,  and  the 
discovery  has  \>eeu  confirmed  by  J.  B.  Platt,  91.1,  202.  In  other 
toJeoMs  (Mclntoafa  and  Prince,  90.1,  773)  the  mouth  is  single  and 
median  in  origin,  as  in  the  remaining  vertebrates.  The  significance 
of  the  aberrant  double  origin  is  unknown,  though  Dohrn  interprets 
it  as  evidence  of  the  evolution  of  the  mouth  by  the  fusion  of  two 
gill-clefts. 

The  Evolution  of  the  Vertebrate  Mouth  is  still  one  of  the 
most  puzzling  of  the  unsettled  problems  of  morphology.  The  in- 
crease of  extent  of  the  mouth  cavity  in  the  higher  as  compared  with 
the  lower  vertebrates  is  discussed  in  the  next  section  on  the  hypo- 
physis. The  present  section  treats  only  of  the  origin  of  the  verte- 
brate mouth.  The  first  question  is,  necessarily,  whether  the  mouth 
of  vertebrates  is  homologous  with  the  mouth  of  invertebrates  or  is  a 
new  st met  ure.  The  formation  of  the  embryo  by  concrescence  enables 
us,  I  think,  to  decide  between  these  alternatives.  In  Peripatus,  the 
leeches,  and  the  annelids  with  well-marked  concrescence,  the  union 
of  the  ectental  lines  is  incomplete,  the  anterior  and  posterior  ends  not 
meeting,  but  leaving  the  two  ends  of  the  elongated  gastrula  mouth 


i —  J 

Fio.  SIC.— Ac-ant  hins 
Kml.ryo  <>t  i;  nun., 
ninli-r  sidf.  nih.  Mid 

iiniiii:    X.  IIHSH!  pit; 
MJC*     maxiHarv    pm- 

•  -.•ss;    M,  moiitii;   Mil. 
iiiHiiilihular    process; 
//r.  hranrhial   archt-s ; 
///.  positii.n  «: 
Y.     yolk    stalk    cut 


570 


THE   FCETUS. 


open,  to  form  the  mouth  and  anus  respectively ;  the  mouth  is  car- 
ried inward  by  the  invagination  of  the  vorderdarm,  and  the  primi- 
tive mouth  is  thereafter  merely  the  opening  of  the  vorderdarm  or 
oesophagus  into  the  archenteric  canal.*  In  those  invertebrates  in 
which  the  process  of  concrescence  is  plainly  marked,  the  mouth  is 
seen  to  be  the  anterior  extremity  of  the  gastrula  mouth,  and  to  be 
bounded  by  the  ectental  line;  the  site  of  the  invertebrate  mouth 
is  where  concrescence  begins,  and  it  is,  therefore  surrounded  by  the 
ectodermal  neural  plate, f  forming  the  brain  (Scheitelplatte),cesoph- 

a  g  e  a  1  commissures,  and 
ventral  nerve  chain  (Bauch- 
yanglienkette) .  The  corre- 
sponding point  in  the  verte- 
brate embryo  is  easily  found, 
being  between  the  optic 
evaginations  at  the  place 
marked  m,  in  Fig.  317,  and 
which  probably  corresponds 
to  the  future  infundibulum 
in  position.  So  far  as  I  am 
aware,  the  relations  at  this 
point  during  early  stages  in 
vertebrates  have  never  been 
thoroughly  studied  with  the 
intention  of  ascertaining 
whether  any  traces  of  a  com- 
munication with  the  archen- 

teron  could  be  found.  Until  this  is  done,  there  can  be,  in  my  judg- 
ment, little  hope  of  our  knowing  what  has  become  of  the  invertebrate 
mouth. 

The  above  determination  of  the  site  where  we  have  to  search  for 
the  original  mouth  may  be  accepted  with  considerable  confidence. 
If  it  is  correct,  it  sets  aside  two  hypotheses  which  have  attracted 
attention :  first,  the  hypothesis  that  the  vertebrate  mouth  is  identical 
with  that  of  the  invertebrate,  and,  second,  the  hypothesis  that  the 
old  mouth  is  represented  by  the  hypophysis,  \  for  neither  of  these 
structures  are  derived  from  any  part  of  the  gastrula  mouth.  That 
both  these  hypotheses  are  untenable  is  evidenced  by  the  deductions 
involved  in  their  adoption.  The  annelid  brain  lies  in  front  of  the 
mouth ;  if,  therefore,  either  the  hypophysis  or  the  mouth  of  vertebrates 
is  identical  with  the  annelid  mouth,  then  the  brain  and  spinal  cord 
must  correspond  to  the  ventral  nerve  chain  only,  and  the  annelid 
brain  must  have  entirely  disappeared.  The  vertebrate  brain  and 
eyes  thus  become  new  structures — a  conception  which  seems  to  me 

*  The  meaning:  of  the  double  origin  of  the  mouth  described  by  C.  Semper  in  budding  annelids 
and  by  Kleinenberg  in  Lopadorhynchus  has  not  been  explained.  That  it  has  the  significance 
attributed  to  it  by  Kleinenberg  can  hardly  be  admitted,  for  there  is  no  evidence  that  it  represents 
a  primitive  mode  of  development. 

t  It  seems  to  me  justifiable  to  speak  of  this  as  a  continuous  neural  plate,  although  there  is  a 
certain  independence  of  development  between  the  "Scheitelplatte  "  and  ventral  chain,  and  al- 
though the  commissures  develop  later. 

%  That  the  hypophysis  represents  the  annelid  resophagus  was  firsc  suggested  by  A.  Dohrn.  72.2, 
but  he  has  since  withdrawn  his  opinion.  Similar  was  Richard  Owen's  infelicitous  homology 
of  the  hypophysis,  infundibulum,  and  pineal  gland  with  the  old  oesophagus  (Proc.  Linn.  Soc. , 
London,  xvi).  Beard  has  revived  Dohrn 's  theory,  but  has  not  succeeded  in  rendering  it  more 
plausible,  to  my  judgment.  Compare  A.  Dohrn,  83.1. 


FIG.  317.— Blastoderm  of  a  Dog-Fish,  Acanthias, 
with  commencing  Concrescence.  I/,  Point  corre- 
sponding to  invertebrate  mouth;  R,  blastodermic 
rim. 


THE   MOUTH    CAVITY    AND    FACE.  51  1 

indefensible.  Another  deduction  involved  in  the  views  under  dis- 
cussinii  is  that  a  line  of  concrescence  runs  from  the  hypophysis  or 
mouth  to  the  i'oiv-hrain,  representing  the  closure  of  the  gastrulu 
along  that  distance-  yet  of  such  a  line  not  a  trace  can  be  detected. 

A>  tin-  infundibnlum  is  an  invagination  of  the  ectoderm  toward 
the  an -In  nt  (Ton  developed  at  or  near  the  point  where  the  invertebrate, 
mouth  lay,  it  is  (juite  possible  that  it  corresponds  to  the  oral  invagi- 
nation (vnrdenlarni)  of  annelids.  This  identification  has  been  more 
or  less  in  the  minds  of  morphologists  for  twenty  years  past,  but  no 
one  has  yet  brought  decisive  evidence  to  justify  it*  nevertheless,  its 
plausibility  must  be  admitted. 

Since  the  vertebrate  mouth  is  regarded  as  a  new  structure,  the 
•//'/  question  comes:  How  did  it  arise?  As  we  have  seen,  the 
first  trace  of  the  mouth  is  the  oral  plate,  p.  262,  formed  by  the  union 
of  t  he  ectoderm  and  entoderm  over  a  small  area  without  mesodenn  in 
f n  >nt  of  the  brain ;  by  the  development  of  the  head-bend  the  plate  is 
carried  over  on  to  the  ventral  side  and  the  oral  cavity  is  developed. 
Thnr  is  nothing  in  this  history  which  we  can  recognize  as  a  clew  to 
the  origin  of  the  mouth,  but,  on  the  other  hand,  there  is  nothing  in 
it  strictly  incompatible  with  Anton  Dohrn's  hypothesis  that  the 
month  of  vertebrates  represents  two  gill-slits  united  in  the  median 
ventral  line.  The  chief  facts  in  favor  of  Dohrn's  suggestion  seem  to 
me  to  be :  first,  that  the  trigeminal  nerve  shows  the  same  relation  to 
the  mouth  as  other  cranial  nerves  (facial,  glosso-pharyngeal,  and 
vagus)  to  the  gill-clefts:  second,  that  the  gill-clefts  approach  the 
median  line  anteriorly,  the  first  pair  being  nearest,  the  last  pair  furth- 
est from  the  middle  plane;  third,  that  the  oral  plate  is  formed  like 
the  membrane  across  a  gill-cleft  (Verschlussplaite) ,  p.  264,  of  ecto- 
derm and  entoderm  united  without  mesoderm.  Dohrn  has  recurred 
repeatedly  to  this  hypothesis  in  his  "  Studien." 

Two  other  theories  have  to  be  mentioned,  namely,  Semper's  and 
Hal  four's.  The  former,  76.3,  W><  observed  a  small  ectodennal  pit 
on  the  dorsal  side  of  the  head  of  a  leech,  which  suggested  the  possi- 
bility of  such  a  pit  deepening  and  becoming  connected  with  the 
archenteron,  and  so  creating  a  new  mouth.  Balfour  ("Works,"  I., 
894)  has  suggested  that  vertebrates  and  annelids  arose  from  an 
ancestor  with  lateral  nerve  cords,  and  that  in  annelids  the  cords 
united  to  form  a  median  ventral  chain,  in  vertebrates  to  form  a 
median  dorsal  chain,  so  that  in  the  former  there  is,  in  the  latter 
there  is  not,  an  oesophageal  ring.  The  development  of  both  types 
l»y  concrescence  proves  that  the  neural  sides  are  identical  in  annelids 
and  vertebrates.  Therefore  Balfour's  hypothesis  falls — and  with  it 
of  course,  Gegenbaur's — that  the  brain  is  the  same  in  both  tyj)es, 
but  that  the  vertebrate  spinal  cord  is  an  outgrowth  of  the  annelidan 
snpra-O2Sophageal  ganglion,  the  annelidan  ventral  chain  being  lost 
in  vertebrates. 

Hypophysis. — The  hypophysis  cerebri,  Rathke's  pocket  or  pitui- 
tary body  (Himanhang),iB  a  structure  of  very  problematical  sig- 
nificance, which  has  been  much  studied  and  speculated  upon  by 
embryologists.  It  arises  in  all  vertebrates  as  an  evagination  of  the 
ectoderm  near  the  dorsal  border  of  the  oral  plate,  but  is  separated 
from  the  plate  by  a  fold  of  the  ectoderm.  In  Petromyzon,  Fig.  318, 


572 


THE    FOETUS. 


"pin 


the  fold,  .F,  acquires  great  size,  and  is  shown  by  Dohrn,  83.1,  to 
develop  into  the  roof  of  the  mouth  and  the  upper  lip ;  accordingly 
the  hypophysal  invagination,  hy,  is  outside  the  oral  cavity  proper, 

and  more  intimately  associated 
with  the  olfactory  area  and 
nasal  pit,  N.  The  hypophysis 
runs  toward  the  end  of  the 
notochord,  nch,  and  is  nearly 
met  by  a  small  blind  diverti- 
culutn  of  the  archenteron, 
which  is  presumably  homo- 
logous with  Seesel's  pocket,  p. 
20S,  of  amniota;  in  the  lamp- 
rey the  hypophysis  early  gives 
rise  to  glandular  diverticula, 
and  itself  becomes  the  adult 


V 


N        by 


FIG.  318.- -Longitudinal  Median  Section  of  a  re- 
cently hatched  Larva  of  Petromyzon.  /&,  Fore- 
brain;  pin,  pineal  gland;  mb,  mid-brain;  Nch, 
notochord;  Ent,  entoderrn:  mes,  mesoderm;  M, 
mouth  cavity  ;  F,  fold  between  hypophysis  and 
mouth;  hy,  hypophysis;  JVt  nasal  pit.  After  C. 
Kupffer. 


authors.      In 
owing,    prob- 


nasal    duct    of 
elasmobranchs, 

ably,  to  the  increased  head- 
bend  and  size  of  the  fore-brain, 
the  region  between  the  HOM* 
and  oral  plate  is  turned  in  so 
as  to  be  almost  wholly  included  in  the  oral  cavity,  and  accordingly 
the  fold,  Fig.  319,  F,  and  hypophysis,  hy,  now  appear  as  appendages 
of  the  oral  cavity,  for  I  homologize  the  transverse  fold,  Fig.  319,  F, 
which  borders  the  hypophysis  in  shark  embryos,  with  the  fold,  Fig. 
318,  F,  which  forms  the  upper  lip  in  the  lamprey.  In  amphibians, 
according  to  A.  Goette's  observations,  75.1,  288,  317,  upon  Bom- 
binator,  the  hypophysis  arises  as  a  solid  ingrowth  from  the  nervous 
layer  (cf.  p.  549)  of  the  ectoderm,  in  front  of  the  mouth,  and,  as 
development  proceeds,  there  follows  the  inclusion  of  the  hypophysal 


FIG.  319.—  Longitudinal  Section  of  an  Acanthias  Embryo  of  13.2_mm.    f.b.  Fore-brain:    nt.h. 

ain ;  m.  d,  medulla  oblongata ;  hy,  I 
and  archenteron ;  Ph,  pharynx ;  ht,  heart 


mid-brain;  m. d,  medulla  oblongata ;  hy,  hypophysis  evagmation  :  F,  fold  separating  hypophysis 

;  Li,  anlage  of  liver;  Yk.s,  yolk-stalk. 


area  in  the  general  mouth  cavity ;  there  is  no  distinct  fold  between 
the  hypophysis  and  the  oral  plate.  In  amniota  nearly  the  whole  ecto- 
dermal  area  between  the  oral  plate  and  the  nasal  pits  is  turned  in 
and  incorporated  with  the  mouth  cavity  before  the  evagination  to 
form  the  hypophysis  appears ;  hence,  the  organ  develops  as  an  out- 


THK    M<U  TH    CAVITY    AND    FACE. 


573 


growth  of  the  oral  chamber.  The  comparative  embryology  of  the 
pituitary  body  teaches  us  that  the  mouth  cavity  increases,  as  we 
aM-end  the  vertebrate  series,  by  the  annexation  of  neighboring  terri- 
tory, and  that  the  primitive  upper  lip  of  vertebrates  disappears,  with 
the  further  consequence  that  in  cyclostomes  the  homologue  of  the 
maxillary  process  is  to  be  sought,  not  in  the  lip,  but  between  the 
hyi>opbysis  and  nasal  pits. 

///  manumit^  the  hypophysis  is  first  indicated  (Ko'lliker,  "Ent- 
wickelungsgesch.,"  1879,  p.  302)  by  a  slight  groove  a  little  in  front 
of  tlie  oral  plate,  but  it  does  not  have  the  form  of  a  distinct  evagina- 
tion  until  after  the  oral  plate  (Rachenhaut)  is  ruptured.  The  ecto- 
derm of  the  mouth  over  the  hypophysal  area  lies  against,  and  is 
apparently  intimately  soldered  to,  the  ectoderm  of  the  brain,  a  point 


b) 


Fin  *jn.  —Median  Section  of  the  Head  of  a  Rabbit  Embryo  of  thirteen  and  one-half  Days. 
/b,  Fore-brain;  mb,  mid-brain;  cbl,  cerebellum;  /i6,  hind-brain;  ncft,  notochord;  hy,  hypophy- 
sis: /.  t,.M  .-..nvspondintf  to  the  lip  of  Petromyzon ;  EC,  ectoderm;  P,  somatopleuric  wall  of 
pericardium;  A/d,  mandible;  Ao,  wall  of  the  aorta. 

which  has  been  generally  overlooked,  but  which  seems  to  me  of 
Li i « -at  importance.  It  is  commonly  stated,  e.  </.,  by  Kraushaar,  85. 1, 
87,  that,  when  the  oral  plate  ruptures,  a  portion  of  it  persists  upon 
the  dorsal  side,  and  is  the  beginning  of  the  fold  which  separates  the 
hypophysis  from  the  pharynx.  I  think  that  this  is  probably  not  the 
case,  but  that  all  trace  of  the  oral  plate  disappears  and  that  a  new 
fold  arises  as  a  duplicature  of  the  ectoderm  filled  with  mesoderm,  Fig. 
:;•>(),  p.  This  new  fold  I  homologize  with  the  lip  of  Petromyzon, 
Fig.  318,  F.  The  hypophysis  is  now,  Fig.  320,  hy,  a  diverticulum 
of  the  oral  cavity,  with  one  wall  attached  to  the  brain,  and  the  other 
formed  by  a  fold  dividing  the  hypophysis  from  the  mouth.  The 


574  THE    FCETUS. 

epithelium  of  the  mouth  is  one-layered,  and  not  thickened,  as  is  that 
of  the  hypophysis ;  the  cells  are  multiplying  rapidly  in  the  stage  fig- 
ured, there  being  numerous  karyokinetic  figures,  which,  so  far  as  I 
have  seen,  are  always  near  the  free  surface  of  the  epithelium.  The 
relations  of  the  notochord  to  the  hypophysal  wall  have  been  dis- 
cussed, p.  183 ;  in  the  specimen  figured  above,  there  is  a  connection 
between  the  chorda  and  the  lower  posterior  part  of  the  hypophysis. 
The  organ  in  the  stage  of  open  invagination  was  described  by  Rathke, 
hence  the  invagination  is  often  called  "  Rathke 's  pocket;"  Rathke 
supposed,  erroneously,  that  it  was  developed  from  the  archenteron 
(pharynx) . 

The  hypophysal  diverticulum  now  elongates  and  its  upper  end 
expands  to  a  considerable  vesicle,  the  lower  end  remaining  narrow 
as  the  pedicle.  At  the  same  time  the  floor  of  the  brain  forms  an 
outgrowth  behind  the  hypophysis,  which  is  the  anlage  of  the  infun- 
dibulum — compare  Chapter  XXVII.  The  two  diverticula  have 
their  walls  united.  It  is  probable  that  the  cementing  together  over 
the  hypophysal  area  of  the  buccal  and  cerebral  ectoderm  is  the 
mechanical  condition  causing  the  formation  of  the  two  diverticula. 
The  hypophysis  now  grows  rapidly;  the  pedicle  elongates  and  its 
lumen  is  obliterated ;  the  mesenchyma  meanwhile  condenses  to  form 
the  base  of  the  skull  (sphenoid) ;  the  pedicle  aborts  entirely  (in  the 
rabbit  by  the  sixteenth  day)  but  the  position  for  its  passage  through 
the  sphenoid  is  marked  a  little  longer,  but  is  ultimately  obliterated 
by  the  growth  of  the  sphenoidal  cartilage.  According  to  Miklucho- 
Maclay  (70.1,  40,  Anm.)  the  passage  persists  in  sharks.  Lanzert 
(see  Henle's  Jahresbericht,  1868,  p.  88)  found  traces  of  the  passage, 
named  by  him  canalis  cranio-pharyngeus,  in  children  at  birth  in 
ten  cases  out  of  one  hundred.  There  is  then  left  merely  the  upper 
end  of  the  hypophysis  as  a  closed  epithelial  vesicle  lying  in  the  fu- 
ture sella  turcica  close  to  the  infundibulum.  The  vesicle  becomes 
flattened  in  the  longitudinal  direction,  and  the  flattened  vesicle  soon 
acquires,  at  least  in  the  pig,  a  yoke  shape  in  section  by  becoming 
first  convex  toward  the  fore-brain,  then  concave  in  its  centre,  toward 
the  infundibulum,  as  may  be  observed  in  a  pig  embryo  of  18  mm. 
(Kolliker,  "  Entwickelungsges. , "  2te  Aufl.,  Fig.  329.)  The  vesicle 
completes  its  development  by  sending  out  hollow  buds  from  its  ante- 
rior wall  (rabbits,  20-30  mm.) ;  in  birds,  according  to  "W.  Miiller, 
71.4,  and  Mihalkovdcs,  77.1,  buds  arise  from  both  walls.  The 
buds  elongate  and  branch;  numerous  blood-vessels  are  developed 
between  them ;  the  buds  separate  from  the  parent  vesicle  (rabbits  of 
40  mm.),  but  continue  to  grow;  their  lumen  disappears,  and  they 
produce  a  highly  vascularized  complex  of  hypophysal  cords.  Kolliker 
thinks  ("Entwickelungsgeschichte,"  1870,  531)  that  the  main  vesi- 
cle persists  recognizably  in  man  into  adult  life. 

The  infundibulum  also  contributes  to  the  production  of  the  adult 
hypophysis  of  mammals,  although  in  lower  vertebrates  it  persists  as 
an  integral  portion  of  the  brain,  and  is  differentiated  into  ganglionic 
tissue.  As  first  shown  by  W.  Miiller,  71.4,  the  pointed  end  of  the 
infundibulum  undergoes  in  amniota  an  enlargement,  beginning  in 
sheep  embryos  of  35  mm.,  in  pig  embryos  of  32  mm.  (Kolliker, 
"Entwickelungsges.,"  1879,  531).  The  knob-like  enlargement  loses 


THE   MOUTH   CAVITY    AND    FACE.  575 

its  cavity,  and  although  tlu>  differentiation  of  nervous  tissue  begins 
in  it,  its  cells  early  acquire  an  indifferent  character,  and  it  is  pene- 
trated by  blood-vessels  and  connective  tissue;  the  connection  with 
the  brain  is  permanently  retained.  The  knob  is  designated  in  the 
adult  as  the  posterior  lobe  of  the  hypophysis,  although  it  can  in  no 
sense  be  regarded  as  part  of  the  true  hypophysis. 

Hixtnrirttl  \<>te. — The  following  memoranda  are  taken  from 
Mihalkovics,  77.1,  and  Kraushaar,  85.1.  The  older  authors  re- 
garded the  hypophysis  as  part  of  the  brain;  this  conception  was  held 
by  Von  Baer  "Entw.-Ges.,"  I.,  104,  103,  and  II.,  293,  and  found  as 
late  as  1802  a  defender  in  F.  Schmidt,  62. 1,  51,  although  Rathkehad 
discovered  the  hypophysal  evagination  in  1838,  38.1,  and  Rathke's 
discovery  had  been  confirmed  by  Kolliker  ("  Entwickelungsges. , " 
186 1 ,  p.  242) .  Rathke  subsequently,  6 1 . 1 , 100,  withdrew  his  opinion 
that  the  evagination  formed  the  hypophysis,  but  W.  Miiller,  71.1, 
demonstrated  that  it  was  unquestionably  correct,  but  retained  the 
erroneous  opinion  that  the  evagination  was  developed  from  the  arch- 
enteron.  That  the  evagination  belongs  to  the  oral  cavity  was  finally 
proven  for  amphibia  by  A.  Goette,  75.1,  and  for  mammals  by 
Mihalkovics,  whose  researches,  77. 1,  83-94,  are  the  most  important 
yet  made  on  the  organ.  Mihalkovics'  results  on  mammalia  have 
been  confirmed  by  Kolliker,  79.2,  Kraushaar,  85.1  (His,  "Anat. 
menschl.  Embryonen  ") ,  and  others.  The  development  of  the  hypo- 
physis in  the  lamprey  has  been  especially  studied  by  Dohrn,  83. 1, 
whose  results  have  been  confirmed  by  subsequent  investigators 
(Scott,  83.2,  Shipley,  88.1,  Kupffer,  90.1). 

That  the  notochord  had  some  connection  with  the  hypophysis  has 
been  held  by  several  authors.  C.  B.  Reichert,  40.1,  179,  regarded 
the  pituitary  body  as  the  end  of  the  notochord,  but  twice  later,  1861 
and  1878,  changed  his  opinion.  Dursy,  69.1,  maintained  that  the 
notochord  was  united  with  the  pocket  of  Rathke,  and  formed  part  of 
the  hypophysis;  see  also  J.  B.  Platt,  91.1. 

Nasal  Pits. — In  this  section  the  development  of  the  cavity  of  the 
nose  is  taken  up — for  the  history  of  the  olfactory  organ  proper,  see 
Chapter  XXVIII.  The  formation  of  the  nasal  pits  begins  with  the 
differentiation  of  the  olfactory  plates,  which  are  two  areas  of  thick- 
ened epidermis  situated  just  in  front  of  the  mouth  and  in  actual 
contact  with  the  wall  of  the  fore-brain.  The  plates  give  rise  to  the 
olfactory  epithelium  of  the  adult.  In  Petromyzon  instead  of  two 
plates  there  is  a  single  median  one,  which  extends  to  the  anlage  of 
the  hypophysis,  Fig.  318.  This  fact  renders  it  probable  that  primi- 
tively there  was  a  single  median  plate  in  vertebrates,  which  has 
become  divided ;  in  the  lamprey  such  division  is  established  later. 
H.  Ayers,  90. 1,  240,  however,  states  that  the  nasal  area  or  olfactory 
plate  of  the  larval  lamprey  is  divided  by  a  median  non-olfactive  raphe 
into  two  lateral  pockets,  right  and  left,  to  which  the  right  and  left 
olfactory  nerves  are  respectively  distributed.  It  is  possible  that  more 
exact  observation  will  show  that  in  all  vertebrates  there  is  at  first  a 
single  plate,  which  is  early  divided.  Balfour,  "Comp.  Embryol.," 
II.,  533,  regards  the  condition  in  Petromyzon  as  secondary,  but  gives 
no  evidence  to  support  his  opinion,  which  was,  perhaps,  really  due 
to  the  tradition  which  says  the  vertebrate  olfactory  organ  is  paired. 


576 


THK    P<ETU8. 


The  nasal  pits  proper  are  developed,  as  pointed  out  by  A.  Goette, 
75. 1,  not  by  the  invagination  of  the  olfactory  plate,  which  is  apposed 
to  the  brain  ab  initio,  but  by  the  upgrowth  of  the  ectoderm  and 

mesoderm  around  the  plate. 
The  upgrowth  takes  place 
on  the  medial,  upper,  and 
lateral  side  of  each  plat  , 
and  hence  forms  two  pus 
with  a  partition,  the  future 
septum  narii,  between 
them.  They  are  the  nasal 
pits  and  communicate  along 
their  whole  lower  side  di- 
rectly with  the  mouth  cav- 
ity, Fig.  322.  The  mode  of 
development  of  the  nasal 
pits  or  sacs  renders  it  highly 
probable,  it  seems  to  me, 
that  the  essential  mechani- 
cal condition  is,  as  with  the 
hypophysis,  the  union  of 
the  epidermal  plate  with  the 
brain  wall.  The  nasal  pit 
is  at  first  very  shallow,  Fig. 
321,  and  the  olfactory  plate 
is  exposed  laterally;  and 
there  can  be  seen  at  its  lower 
part  a  small  depression, 
the  anlage  of  the  organ  of 
Jacobson. 

The  growth  of  the  nasal  pits  in  man  has  been  described  by  His 
("Anat.  menschl.  Embryonen,"  Heft  III.,  45-55).  There  are  two 
principal  changes,  1,  the  growth  of  the  tissues  around  the  olfactory 
plate ;  2,  the  migration  of  the  pits  away  from  the  brain.  Fig.  322 
gives  a  view  of  an  early  stage  in  which  the  pits 
are  small  and  shallow  and  the  tissue  is  forming 
a  ridge  around  them,  which,  however,  does  not 
extend  on  to  the  oral  side,  so  that  the  pits  open 
freely  into  the  mouth  cavity.  The  nasal  pits  are 
widely  separated  by  a  projecting  mass  of  tissue, 
which  I  propose  to  call  the  nasal  process,  and 
which  is  the  Stirnfortsatz  of  German  embryo- 
logists.  Between  the  nasal  pit  on  each  side  and 
the  mouth  the  anlage  of  tl  e  nasal  process  is 
thickened  and  rounded,  making  a  protuberance 
—the  processus  globularis  of  His.  The  nasal 
process  includes  the  partition  between  the  two 
nasal  chambers,  the  anlage  of  the  future  nose  ofFlaG'  Hmna^Emb^yo1  of 
and  of  the  future  intermaxillary  region  of  the  ^mm:s?  fr°n$ ^ i|i'armsAfter 
upper  lip.  The  maxillary  process  extends  be- 
tween the  mouth  and  eye,  toward  the  nasal  pit,  and  later  by  joining 
the  processus  globularis  begins  the  separation  of  the  nasal  and  buccal 


FIG.  321.— His'  Embryo  A,  7.5mm. 


THE    MOUTH    CAVITY    AND    FACE. 


577 


re.ef. 


chambers  and  completes  the  permanent  upper  border  of  the  mouth — 
compare  Fig.  :5\!l,  /.,  M.r.  As  development  proceeds,  the  lateral 
ridge,  see  Fig.  321,  grows  forward  and  covers  in  the  nasal  pit  from 
the  side,  and  may  therefore  be  regarded  as  the  anlage  of  the  wing  of 
the  adult  nose.  We  now  have  the  two  external  nares.  Turning  to 
the  growth  of  the  nasal  ehamU-rs,  we  lindthat  they  enlarge 
as  the  whole  face  enlarges,  and  that  they  occupy  an  increas- 
ing space,  Fig.  o^J,  A7/,  opening  widely  into  the  mouth 
cavity  above  the  palate  shelf.  The  figure  shows  that  the 

palate  develops  from  the  walls  of 
the  mouth  cavity,  and  the  space 
above  it  is,  therefore,  oral,  not  na- 
sal; hence  the  nasal  cavity  of  the 
adult  includes  more  than  the  nasal 
pit  of  the  embryo.  It  is  from  the 
nasal  pits  proper  that  the  so-called 
labyrinth  of  the  nose  is  formed. 
The  development  of  the  lab}' ri  nth 
begins  with  the  appearance — in 
man  during  the  third  month — of 
three  projecting  folds  on  the  lateral 
wall  of  each  nasal  chamber,  Fig. 
326,  the  folds  are  the  upper,  middle, 
and  lower  turbinal  folds  (Xasen- 
muscheln)  and  consist  at  first  each 
of  a  duplication  of  the  ectoderm 
rilled  with  indifferent  mesenchyma, 
which,  however,  very  early  changes 
into  cartilage ;  the  turbinal  cartilage 
is  a  consequence,  not  a  cause,  as 

-,  £    ,£         ,          , 

often  stated,  of  the  development  of 
the  turbinal  fold.  The  formation 
of  the  labyrinth  advances  by  the  formation  of  outgrowths,  which 
become  the  ethmoidal  sinuses,  by  the  appearance — in  man  during 
the  sixth  month — of  the  antrum  Highmorii,  or  expansion  of  the  nasal 
cavity,  into  the  region  of  the  superior  maxilla,  and  finally  by  the 
evaginations  to  form  the  sphenoidal  and  frontal  sinus,  which,  how- 
ever, do  not  arise  in  man  until  after  birth.  Finally  we  consider  the 
separation  of  the  olfactory  plate  from  the  brain.  This  does  not  take 
place  until  the  olfactory  ganglion  develops  from  the  epithelium 
(ectoderm)  of  the  plate.  The  olfactory  nerve  fibres  are  developed 
very  early,  in  the  chick  during  the  third  day — compare  Chapter 
XXVII.  The  fibres  lengthen,  the  olfactory  and  neural  epithelia 
separate,  and  ultimately  the  osseous  cribriform  plate  is  developed 
between  them. 

For  observations  on  the  development  of  the  posterior  nares,  see  Fr. 
Hochstetter,  91.2. 

JACOBSON'S  ORGAN. — The  organ  of  Jacobson  arises  very  early  as 
a  small  distinct  invagination,  on  the  medial  wall  of  the  nasal  pit, 
as  first  stated  by  Dursy,  69.1.  Our  knowledge  of  its  development 
is  due  chiefly  to  Kolliker,  77.2,  79.2,  766,  and  Fleischer,  78.1. 
At  four  months  it  is  a  cylindrical  blind  canal,  running  from  its 
37 


-•3.  —Reconstruction  of  the  Face  of 
Us'   Kmhryo  Sch.     JV.of,   Olfactory  nerve; 
,//,    nasal    <avity;    R.T     Rathke's  pocket; 
notochord;    T.   tonsil;    P.g,    processus 
'./,  ]>alate  anlage;    UK,  mandi- 
Qe.     A:;.-:-  v. 


578  THE   FCETUS. 

original  orifice  backward  in  the  septum  narii.  It  is  surrounded  by 
a  small  cartilage(Jacobson's  cartilage)  near  its  orifice;  this  separate 
cartilage  is  derived  from  a  growth  of  the  main  cartilage  of  the  sep- 
tum. The  canal  is  innervated  by  the  olfactory  nerves,  and  in  certain 
mammals  it  is  much  more  developed  than  in  man. 

THE  EXTERNAL  NOSE  is  developed  toward  the  end  of  the  second 
month  by  a  growth  of  the  nasal  process  (His/'  Anat.  menschl.  Embry- 
onen, "  III. ,  35) .  It  is  at  first  short  and  broad,  having  at  three  months 
very  nearly  the  shape  which  is  permanent  in  certain  negro  races. 
The  external  nares  and  wings  of  the  nose  are  carried  forward  with 
the  general  nasal  upgrowth.  At  three  months  the  external  nares 
are  usually  completely  closed  by  the  growth  of  their  epithelium, 
which  forms  a  plug  of  gelatinous  consistency.  The  plug  disappears 
after  the  fifth  month  (Kolliker,  "Entwickelungsges.,"  1879,  7G7). 

Maxillary  Process. — Reference  has  already  been  made  to  that 
thickening  of  the  upper  edge  of  the  mouth,  which  appears  almost  as 
a  continuation  of  the  mandibular  arch,  and  which  is  known  as  the 
maxillary  process,  or  sometimes  as  the  superior  maxillary  process 
(Oberkieferfortsatz).  It  is  termed  a  process,  because  from  its  small 
size  and  position  it  appears  at  first  like  a  bud  from  the  mandibular 
arch.  Later  it  stretches  farther  forward,  and  when  the  mouth  has 
changed  from  its  original  pentagonal  shape  to  a  transverse  slit, 
Fig.  322,  the  maxillary  process  no  longer  appears  specially  con- 
nected with  the  mandibular  arch,  but  is  united  with  the  edge  of  the 
nasal  process  as  above  described,  p.  576.  A  thorough  study  of  the 
primitive  relations  and  growth  of  the  maxillary  process  is  much 
needed.  It  is  possible  that,  as  several  authorities  have  maintained, 
it  is  morphologically  the  upper  part  of  the  mandibular  arch,  which, 
in  consequence  of  the  head-bend,  makes  an  angle  with  the  mandible 
proper.  Although  this  hypothesis  commends  itself  to  the  embryolo- 
gist,  it  needs  a  firmer  basis  than  it  yet  has  to  stand  upon. 

Mandibular  Arch. — The  first  branchial  arch  forms  the  lower 
boundary  of  the  mouth,  and  by  its  long-continued  growth  develops 

into  the  projecting  lower  jaw.  The 
history  of  the  skeleton  and  muscles 
of  the  lower  jaw  are  treated,  p.  444 
and  478,  respectively.  The  chin  is 
at  first  retreating  and  does  not  be- 
come distinctly  prominent  until  the 
fifth  month.  The  growth  of  the 
jaws  increases  the  separation  of  the 
mouth  from  the  heart. 

Lips  and  Gums.  —  Very  soon 
after  the  upper  jaw  has  been  formed 
by  the  union  of  the  maxillary  and 

FIG.  324.— View  of  the  Roof  of  the  Mouth  noool    nrnr>a««^«     if  a  nrtil    tmrfana  rla 

of  a  Human  Embryo,     na,  Nares;  Op,  eye;  nasal  processes,   ITS  Oral  SUriace  C 

L,  portion  of  lip  developed  from  the  nasal  velopS  two  parallel  ridges,  Fig.  324, 

process;  MX.  portion  of  the  upper  lips  devel-  /.       i  .    i     ,-|                                                 ^    n 

oped  from  the  maxillary  process :  Z>,  dental  of  which  the  Outer  and  more  bulky, 

ifterwonmsegums;jPaZ'palate<  X8diams-   £,  MX,  is  the  anlage  of  the  upper 

lip,  and  the  inner  and  smaller,  Z), 

the  anlage  of  the  gums  (gingivce,  dental  ridge) .    At  about  the  same 
time,  or  a  little  later,   similar  ridges  develop  on  the  lower  jaws. 


THE    MOUTH    CAVITY    AND    FACE. 


579 


The  histogenesis  of  the  lips  and  gums  has  not  been  investigated. 
From  the  study  of  sections  of  the  lower  lip  of  a  foetus  of  six  mouths, 
which  I  have  prepared,  I  consider  it  probable  that  the  peculiar  epi- 
thelium of  the  lips  arises,  1,  by  the  disappearance  of  both  the  epitri- 
rhium  and  stratum  lucidum,  and,  2,  the  distention  of  the  remaining 
cells — a  basal  growing  layer  being  retained.  In  a  rabbit  of  thirteen 
days,  the  epitrichium  runs  over  the  region  of  the  future  lip.  In  a 
pig  embryo  of  about  3.5  cm.,  the  epitrichium  is  still  present,  and 
the  cells  below  are  enlarging  and  beginning  to  cornify. 

The  glands  of  the  lips,  according  to  Kolliker,  "  Entwickelungsges. , " 
L876,  ^8,  arise  during  the  fourth  month  as  solid  ingrowths  of  the 
epithelium,  and  later  send  out  each  eight  to  ten  branches,  which, 
while  still  solid,  form  a  pretty  rosette. 

Formation  of  the  Palate. — As  soon  as  the  external  nares  are 
separated  from  the  mouth,  there  is  a  partition  between  the  nasal  pits 


EtK 


FIG.  325.  —Frontal  Section  of  the  Oral  and  Nasal  Chambers  of  a  young  Cow  Embryo,  pat , 
Palate:  Int.  lateral  cartilage;  Eth,  ethmoid  plate:  itb.  inferior  turbmal;  T,  tongue:  d,  dental 
germ;  a,  oral' angle;  Jtf,  Meckel's  cartilage;  Afd,  mandible. 

and  the  mouth.  This  partition,  in  which  the  intermaxillary  bone  is 
differentiated  later,  is  supplemented  by  another  partition,  the  true 
palate,  Fig.  324,  Pal,  which  shuts  off  the  upper  part  of  the  oral  cavity 
from  the  lower,  thus  adding  the  upper  part  to  the  nasal  chambers. 
The  palate  is  a  secondary  structure,  which  divides  the  mouth  into  an 


580 


THE   FCETUS. 


upper  respiratory  passage  and  a  lower  lingual  or  digestive  passage. 
The  palate  arises  as  two  shelf -like  growths  of  the  inner  side  of  each 
maxillary  process,  Fig.  324,  Pal,  and  is  completed  by  the  union  of 
the  two  shelves  in  the  median  line.  As  seen  in  a  side  view  the 
shelves  are  represented  in  Fig.  323,  Gl,  they  arch  so  as  to  descend 


^*-r        ^*jr        \^^ 

FIG  326  —Frontal  Section  of  the  Nasal  and  Oral  Cavities  of  a  Human  Embryo  of  three  Months 

(Minot  Coll.  No.  41X 

a  certain  distance  into  the  pharynx,  but  in  the  pharynx  their  growth 
is  arrested,  though  they  may  be  still  recognized  in  the  adult.  In  the 
region  of  the  tongue,  which  includes  more  than  the  primitive  oral 
cavity,  the  palate  shelves  continue  growing.  At  first  they  project 
obliquely  downward  toward  the  floor  of  the  mouth,  Fig.  325,  pat, 
and  the  tongue,  T,  rises  high  between  them,  and  appears  in  sections 
which,  like  the  one  represented  in  Fig.  325,  pass  through  the  internal 
nares,  to  be  about  to  join  the  internasal  septum.  As  the  lower 
jaw  grows,  the  floor  of  the  mouth  is  lowered  and  the  tongue  is 
thereby  brought  further  away  from  the  internasal  septum.  At  the 
same  time  the  palate  shelves  take  a  more  horizontal  position  and 
pass  toward  one  another  above  the  tongue  and  below  the  nasal  sep- 
tum, and  meet  in  the  middle  line  where  they  unite.  From  their 
original  position,  see  Fig.  325,  pat,  the  shelves  necessarily  meet  in 
front  (toward  the  lips)  first,  and  unite  behind  (toward  the  pharynx) 
later.  In  the  human  embryo  the  union  begins  at  eight  weeks,  and 
at  nine  weeks  is  completed  for  the  region  of  the  future  hard  palate, 
and  by  eleven  weeks  is  usually  completed  for  the  soft  palate  also. 
The  palate  shelves  extend  back  across  the  second  and  third  branchial 
arches ;  by  the  migration  of  the  first  gill  pouch,  or,  in  other  words, 


THE    MOUTH    CAVITY    AND    FACE.  581 

of  the  Eustachian  tube,  the  Eustachian  opening  comes  to  lie  above 
the  palate  (uvula)  while  the  second  cleft  remains  lower  down  and 
lies  below  the  palate,  as  the  anlage  of  the  tonsil,  His,  "Anat. 
menschl.  Embryonen,"  Heft  III.,  82.  The  uvula  appears  during 
the  latter  half  of  the  third  month  as  a  projection  of  the  border  of 
the  soft  palate.  Soon  after  the  two  palatal  shelves  have  united 
with  one  another  the  nasal  septum  unites  with  the  palate  also, 
Fig.  326,  and  thereby  the  permanent  or  adult  relations  of  the  cavi- 
ties are  established. 

Lachrymal  Duct. — The  canal  which  leads  from  the  corner  of 
the  eye  to  the  nose  (Thranennasengang  of  G.  Born)  is  not  found  in 
fishes,  but  only  in  amphibia  and  amniota.  The  site  of  this  duct  is 
very  early  marked  out  by  the  lachrymal  groove,  Fig.  322,  running 
down  from  the  eye  to  the  invagination,  or  to  the  nasal  pit  as  soon  as 
the  latter  appears.  This  groove  is  bordered  above  by  what  is  known 
as  the  lateral  nasal  process  or  prominent  surface  between  the  nasal 
pit  and  the  eye — compare  Fig.  322 — and  is  bordered  below  by  the 
maxillary  process.  This  groove  soon  disappears  and  leaves,  so  far 
as  known,  no  trace.  It  was  supposed  by  Kolliker  ("  Entwickelungs- 
gesch.,"  1879,  469)  to  be  the  anlage  of  the  duct — an  opinion  which 
Bom's  observations  on  amphibians,  76. 1,  and  on  Sauropsida,  78. 1, 
83.3,  followed  by  those  of  Legal,  81.1,  83. 1,  on  mammals,  showed 
to  be  erroneous. 

The  duct  arises  along  the  line  of  the  lachrymal  groove  as  a  thicken- 
ing of  the  under  side  of  the  epidermis,  which  appears  about  the  time 
that  the  cartilage  develops  around  the  nasal  cavities — in  man,  ac- 
cording to  Ewetzky,  88. 1,  at  the  end  of  the  fifth  or  beginning  of  the 
sixth  week.  The  thickening  increases  until  it  forms  a  ridge,  which 
finally  separates  as  a  solid  cord  from  the  epidermis,  except  at  each 
end ;  the  cord  then  acquires  a  lumen,  thereby  becoming  an  epithelial 
canal.  In  man  the  upper  end  of  the  solid  cord  broadens  out  at  the 
inner  canthus  and  then  divides  into  two  forks,  each  of  which  acquires 
a  lumen,  with  the  result  of  producing  a  bifurcation  of  the  duct 
(Ewetzky) .  In  the  pig,  the  bifurcation  is  developed,  but  one  fork 
aborts,  according  to  Legal,  83.1. 

Teeth. — The  development  of  the  teeth  in  man  and  other  mam- 
mals lias  been  much  studied,  and  has  been  repeatedly  described  by 
competent  authorities  in  comprehensive  summaries.  I  have,  there- 
fore, deemed  it  unnecessary  to  go  over  many  of  the  original  articles 
carefully,  and  instead  base  the  following  synopsis  chiefly  upon 
Waldeyer,  72.1,  Kolliker,  79.2,  815,  Tomes  ("Dental  Anatomy"), 
Von  Ebner,  90.1,  and  O.  Hertwig.  The  list  of  authorities  is 
given  in  my  "  Bibliography"  under  "  Teeth,"  but  it  is  far  from  com- 
plete ;  for  further  lists  see  Waldeyer  and  Von  Ebner.  For  a  very 
admirable  critical  synopsis  of  the  various  notions  that  have  been 
advanced  concerning  the  histogenesis  of  the  teeth,  see  Von  Ebner, 
90.1,  240-252.  It  must  be  remembered  that  most  of  the  articles 
upon  the  human  teeth  are  by  more  or  less  incompetent  writers. 

DERMAL  TEETH  OF  SHARKS. — The  teeth  were  primitively  organs 
of  the  skin  and  widely  developed  over  the  surface  of  the  body,  and 
as  stated  before,  p.  401,  they  have  played  an  important  role  in  the 
genesis  of  the  skeleton.  It  is,  therefore,  to  the  fishes  that  we  must 


582 


THE   FOETUS. 


Fio.  327.  —Dental  Papilla  of  a  Dermal  Tooth  of  an 
Acanthias  Embryo  of  10  cm.  En,  Enamel  organ; 
»,  papilla;  Ep,  epidermis;  CM,  dermis.  After  O. 
Hertwig. 


turn  to  ascertain  the  primitive  mode  of  tooth  formation,  choosing 
the  sharks,  since  they  have  been  the  most  thoroughly  studied  in 
this  regard,  thanks  chiefly  to  O.  Hertwig,  74.1,2.  The  teeth  of 
sharks  are  generally  known  as  placoid  scales.  The  tooth  begins  as 

a  mesenchymal  papilla,  Fig. 
327,  composed  of  crowded  cells 
and  projecting  into  the  epider- 
mis. The  layer  of  epidermal 
cells  overlying  the  papilla 
changes  in  character,  its  cells 
gradually  lengthening  into 
very  long  cylinders,  and  be- 
comes the  enamel  organ.  By 
further  development  the  epi- 
dermis thickens,  the  papilla 
projects  into  it,  and  becoming 
narrow  and  longer,  and  taking 
an  oblique  position,  gradually 
assumes  the  shape  of  the  tooth. 
Ossification  now  begins  over 
the  surface  of  the  papilla. ;  there  arises  a  layer  of  epithelioid  osteo- 
blasts,  and  between  these  and  the  enamel  organ  the  development  of 
bone,  or,  as  it  is  called  in  teeth,  of  ivory,  begins ;  the  osteoblasts  per- 
sist, and  the  bony  structure  is  developed  only  between  them  and  the 
epidermis,  forming  a  stratum  which  grows  in  thickness.  At  the 
same  time  the  enamel  organ  begins  to  deposit  the  calcified  layer, 
known  as  enamel,  over  the  papilla.  Later  the  tooth  acquires  a  sup- 
port by  the  direct  ossification  of  the  connective  tissue  at  its  base, 
and  is  then  a  completed  "placoid  scale." 

The  teeth  of  the  mouth  depart  from  this  primitive  mode  of  devel- 
opment, for  they  do  not  arise  on  the  surface,  but  deep  down,  Fig. 
328,  because  the 
dentiferous  epithe- 
lium grows  down 
into  the  dermis, 
forming  an  oblique 
shelf,  which  may  be 
regarded  as  a  spe- 
cial tooth,- forming 
organ.  On  the  un- 
der side  of  the  shelf 
the  teeth  are  devel- 
oped in  the  same 
way  as  over  the 

skin    although  they    en.amel  cells;  P,  dental  papillaj'D.S,*  dental  shelf.    After  O.  Hert- 

are  much  larger. 
The  teeth  are,  however,  in  various  stages  of  development,  and  only 
one  is  fully  exposed ;  when,  as  happens  in  time,  it  is  lost,  the  next 
tooth  behind  replaces  it,  and  since  the  production  of  new  tooth  germs 
goes  on  in  adult  life,  the  replacement  of  teeth  in  the  shark's  jaw 
continues  indefinitely;  hence  sharks  are  termed  polyphyodont. 
Mammals  have  two  sets  of  teeth,  and  hence  are  called  diphyodont. 


D.s 


FIG.  828.— Section  of  the  Lower  Jaw  of  an  Acanthias  Embryo  of 
10  cm.  T,  Tooth;  en,  enamel  cap;  Ep^  epidermis;  Z),  dentine;  En. 
enamel  cells;  P,  dental  papilla;  D.S,  dental  shelf, 
wig. 


THE    MnfTIl    (  AVITV    AND    FACK. 


583 


Lqr •* 


Fio.  329.— Section  of  Part  of  the  Lower  Jaw 
«>t  a  Human  Embryo  of  40  mm.  K/>.l.  Kpi- 
thflimn  of  lip;  o.ep,  oral  epithelium: 


We  leani  from  the  sharks  that  a  tooth  is  a  papilla  which  projects 
into  the  epidermis,  ami,  ossifying  in  a  peculiar  way,  change>  into 
ivory  around  the  soft  ec  re  or  pulp:  to  the  papilla  the  epidermis  adds 
a  layer  of  enamel.  The  tooth  proper  unites  with  a  small  plate  of 
dermal  hone  at  its  hase.  By  a  modification  in  the  jaws,  the  epider- 
mis iir>t  grows  into  the  derinis,  and  then  the  dermal  tooth  papilla  is 
developed.  In  the  higher  vertebrates  the  teeth  of  the  jaws  only  are 
developed,  and  they  arise  in  the  modified  way  we  have  noted  in  the 
selachian  jaw. 

AMNIOTE  TOOTH-GERMS. — The  first  indication  of  the  development 
of  tooth-germ-  in  mammals  is  the  appearance  of  a  thickening  of  the 
epithelium  covering  the  jaw;  the  thickening  forms  a  curving  ridge 
on  the  under  sideof  the  epithelium. 
According  to  ('.  Rose,  91.2,  161, 
the  ridge  appears  in  the  human  em- 
bryo during  the  sixth  week.  The 
ridge  expands,  Fig.  '.*>•>'<.  and  subdi- 
vides into  an  outer  portion,  L.gr, 
the  anlage  of  the  groove  between 
the  lip  and  gum,  and  an  inner  por- 
tion, f/..s//,  the  dental  shelf,  which 
grows  obliquely  inward;  on  the  un- 
der side  (in  the  upper  jaw  on  the 
corresponding  upper  side)  of  the 
shelf  arise  the  dental  papilla',  l'j>. 

mi  ,        .     ,         ,     -I*     I'*     i      /     •    j    \        n-um  o       p;  o.ep,  ora 

J/ic    dciitdl     ahelj      (ZahnleiSie)    anlaseof  lip  groove  ;<*.«*, 

is  homologous  with  the  similar  papilla'  Ah 
structure  in  the  shark.  Its  history  in  the  human  embr}ro  has  been 
investigated  by  C.  Rose,  91.2.  The  papilla  for  the  milk-teeth  are 
formed  on  the  under  side  of  the  shelf,  Fig.  3\!'.»,  and  it  is  thus  possi- 
l»le  for  the  shelf  to  continue  growing  toward  the  lingual  side,  so  that 
a  second  set  of  germs  is  developed  for  the  permanent  teeth.  The  end 
of  the  shelf  toward  the  articulation  of  the  jaws  is  prolonged  without 
retaining  the  direct  connection  with  the  epithelium,  and  from  this 
I  »i  <  >1(  >ngat  i<  m  arise  the  enamel  organs  for  the  three  permanent  molars. 
Wherever  a  tooth-germ  arises,  the  dental  shelf  is  locally  enlarged,  and 
the  local  enlargement  constitutes  an  enamel  organ  which  pn ..;• 
from  the  under  side  of  the  shelf.  The  portions  of  the  shelf  between 
the  enamel  organs  gradually  break  up,  forming  first  an  irregular 
network,  and  later  separate  fragments,  which  may  persist  throughout 
life  and  lead  to  various  pathological  structures;  w^hile  the  permanent 
germs  are  forming  the  shelf  is  solid  between  them,  although  it  has 
a -sumed  the  reticulate  structure  between  the  germs  of  the  milk- 
teeth.  In  consequence  of  the  reticular  formation,  the  fully  developed 
enamel  organs  have  several  bands  or  threads,  by  which  they  are 
connected  with  the  dental  shelf  proper. 

Fig.  330  represents  the  under  side  of  a  model  of  the  epithelium  of 
the  gum  of  the  upper  jaw  of  a  human  embryo  of  40  mm.  reconstructed 
by  C.  Rose  from  the  sections,  Fig.  329.  L.gr  is  the  ridge  corre- 
sponding to  the  groove  between  the  lip  and  gum ;  pal  is  the  surface 
of  the  palate;  a.sh  is  the  dental  shelf,  the  ten  cups  or  depressions 
on  which  correspond  to  the  papilla?  for  the  ten  milk-teeth. 


584 


THE   FCETUS. 


L.gr 


ash 


FIG.  330. —Explanation  in  text. 


After  the  shelf  has  developed  somewhat,  ii?s  line  of  connection  with 
the  epithelium  of  the  gum  becomes  marked  by  a  superficial  groove, 
as  may  be  seen  in  the  human  embryo  of  eight  to  ten  weeks,  Fig. 
324,  D.  This  groove  was  formerly  supposed  to  be  the  first  trace  of 

the  dental  shelf,  but  Rose's 
observations  correct  the  sup- 
position. 

The  second  step  in  mam- 
mals is  the  formation  of  out- 
growths (in  man  ten  in  each 
jaw)  from  the  under  side  of 
the  dental  shelf;  each  out- 
growth is  the  anlage  of  an 
enamel  organ  for  a  milk- 
tooth.  The  derivation  of 
the  enamel  organ  from  the  epidermis  was  discovered  by  Kolliker. 
The  outgrowth  is  covered  toward  the  mesoderm  by  a  layer  of  cy- 
lindrical epithelial  cells,  the  continuation  of  the  basal  layer  of 
the  epidermis,  while  the  core  is  filled  with  polygonal  cells,  which 
resemble  those  of  the  middle  part  of  the  Malpighian  layer  of  the 
skin.  The  outgrowths,  after  penetrating  a  short  distance,  expand 
at  their  lower  ends,  but  remain  each  connected  by  a  narrow  neck 
with  the  overlying  epidermis.  The  expanded  end  is  the  enamel 
germ  proper ;  it  very  soon  assumes  a  triangular  outline  as  seen  in 
sections,  owing  to  the  flattening  of  its  under  side,  and  at  the  same 
time  it  moves  somewhat  toward  the  lips.  Meanwhile  the  shelf  con- 
tinues growing  on  the  lingual  side  of  each  ingrowth,  to  produce  the 
enamel  organs  destined  for  the  second  or  permanent  teeth.  At  this 
stage  we  notice  that  the  mesenchyma  under  the  flattened  end  of  the 
enamel  organ  has  become  more  dense,  to  form  the  anlage  of  the 
dental  papilla,  and  is  beginning  to  develop  fibrillaB  around  both  the 
enamel  germ  and  the  papillary  anlage.  The  fibrillar  envelope  is  the 
future  dental  follicle  (Zahnsack) . 

The  third  step  is  the  final  differentiation  of  the  enamel  organ  and 
the  accompanying  shaping  of  the  papilla.  The  enamel  organ,  Fig. 
331,  continues  growing  and  becomes  concave  on  its  under  side,  so 
that  the  mesoderm  underneath  acquires  the  shape  of  a  papilla.  It 
is  now  that  the  form  of  the  tooth  is  determined  by  the  form  assumed 
by  the  papilla,  which  in  its  turn  is  probably  determined  by  the 
growth  of  the  enamel  organ.  Von  Brunn,  87.1,  has  shown  that 
the  enamel  organ  extends  over  the  papilla  of  various  mammals  not 
only  as  far  as  the  enamel  is  formed,  but  also  as  a  thin  layer  to  the 
base  of  the  papilla,  or  over  the  future  root.  Over  the  root,  after  the 
tooth  is  shaped,  the  enamel  organ  aborts.  The  apex  of  the  root  is 
never  covered.  C.  Rose,  91.2,  has  shown  that  in  man  also  the 
enamel  organ  extends  at  first  over  the  root,  but  subsequently  aborts. 
A  fully  developed  tooth  germ  consists  of,  1,  the  follicle,  2,  the 
enamel  germ  with  its  neck  running  to  the  dental  shelf,  the  edge  of 
which  grows  on,  Fig.  331,  B,  to  form  the  secondary  teeth,  and,  3, 
the  papilla. 

THE  FOLLICLE  is  merely  an  envelope  of  connective  tissue,  Fig. 
331,  in  which  we  can  distinguish,  according  to  Kolliker,  an  outer 


THE    MOUTH    CAVITY    AND    FACE. 


585 


denser  and  inner  looser  layer ;  in  the  latter  the  cells  are  mere  distinct 
and  the  fibrilla3  are  less  numerous  than  in  the  former.     A  rich  net- 
work of  capillar}"  vessels  is  developed  in  the  follicle,  Fig.  :>:;; 
and   appears  in   part  as  a  series  of  villous-like  growths  into  tlir 
enamel  organ.     The  follicle  develops  first  over  the  lower  part  of  the 


: 


•''••'.:'•  •'•'•"  •':  '•'  •*.' '•;• 


•En 


FIG.  331.— Vertical  Section  of  a  Molar  Tooth  Germ  of  a  Human  Embryo  of  160  mm.  Ep<  Epi- 
thelium of  the  dental  furrow ;  /?,  bud  for  secondary  eerm ;  En^  central  cells  of  the  enamel  organ : 
c,  enamel  cells;  p,  mesenchymal  papilla;  v,  follicular  envelope  with  blood-vessels. 

papilla,  then  over  the  enamel  organ,  the  neck  of  which  aborts  and 
the  follicle  closes  over,  completely  separating  the  enamel  organ  from 
its  parent  epidermis. 

THE  ENAMEL  ORGAN  changes  greatly  in  appearance.  The  layer 
of  cylinder  cells  is  well  preserved  only  over  the  concave  side,  Fig. 
331,  c,  where  the  epithelium  is  in  contact  with  the  dental  papilla. 
In  the  neck  the  cells  become  appressed  and  irregular  in  form.  Over 
the  convex  surface  of  the  enamel  organ  the  cells  become  lower  and 


58G 


THE   FCETUS. 


cuboidal,  and  ultimately  atrophy  and  flatten  out,  but,  so  far  as  I 
know,  no  exact  study  has  yet  been  made  of  the  changes  they  pass 
through.  The  convex  surface  becomes  very  irregular  by  upgrowths 
of  cells,  crowded  together ;  it  is  between  these  upgrowths  that  the 
vascular  villi  of  the  follicle  are  formed.  The  layer  of  cylinder  cells 
over  the  papilla  become  much  elongated  and  as  their  nuclei,  after 
the  enamel  has  begun  to  form,  are  nearly  all  placed  at  about  the 
same  level,  they  constitute  one  of  the  most  beautifully  regular  epi- 
thelial layers  known.  These  cells  covering  the  papilla  are  known  as 
the  enamel  cells  (Schmelzzellen,  ameloblasts,  membrana  ad(nn<in- 
tina  of  Raschkow)  because  they  produce  the  enamel,  as  described 
below.  The  enamel  cells  average  about  40/Jt  in  length,  and  at  birth 
about  6-7/;-  in  width ;  their  outer  ends,  i.  e.  away  from  the  papilla, 
are  furnished  with  prickles  or  thread-bridges  by  which  the  cells  are 

connected,  Fig.  332,  with  one  another  and 
the  neighboring  cells  of  the  enamel  organ ; 
the  bodies  of  the  cells  are  finely  granular, 
and  not  infrequently  have  larger  glistening 
granules  at  their  lower  or  papillary  ends; 
their  nuclei  are  elliptical  and  10-12/j-  long; 
before  the  enamel  appears  they  lie  at  various 
levels ;  after  it  appears  they  are  found,  with 
rare  exceptions,  Fig.  332,  fr,  near  the  upper 
ends  of  the  cells,  all  at  one  level.  The  lower 
or  papillary  ends  have  the  processes  of  Tomes, 
so  named  from  their  discoverer ;  these  appear 
when  the  enamel  begins  to  form ;  they  are 
short,  thick,  and  tapering,  one  on  each  cell ; 
they  often  seem  fibrillated,  and  are  always 
separated  from  the  cell  proper  by  a  small 
cuticular  border;  while  in  situ  Tomes'  pro- 
cesses are  fitted  into  sockets  on  the  surface  of 
the  enamel.  The  enamel  cells  have,  probably, 
no  membrane  on  their  sides.  After  the  for- 
mation of  the  enamel  is  completed  the  enamel 
cells  degenerate  and  are  lost,  except,  1,  that 
their  border  persists  as  a  horny  membrane, 
cuticula  eboris,  covering  the  enamel,  and,  2, 
that  a  few  groups  of  cells  may  remain  for  a 
long  time  as  isolated  epithelial  bodies  in  the 
dental  follicle  (Malassez).  The  cells  in  the 
centre  of  the  enamel  organ  undergo  a  very  peculiar  metamorphosis. 
They  remain  united  together  by  a  few  thread-like  processes,  and, 
therefore,  have  a  certain  degree  of  resemblance  to  the  embryonic 
connective  tissue  cells,  but  the  intercellular  spaces  do  not  contain  in 
the  enamel  organ  any  homogeneous  matrix,  but  merely  fluid.  The 
steps  by  which  this  metamorphosis  of  the  central  cells  is  accomplished 
are  still  imperfectly  known.  A  few  layers  of  the  central  cells  of  the 
enamel  organ  retain  more  of  their  primitive  character,  Fig.  332,  c. 
These  cells  constitute  the  intermediate  layer  of  Kolliker ;  they  are 
polygonal,  granular,  and  connected  with  one  another  by  intercellular 
threads  (prickles). 


Incisor  Germ,  a,  Tomes'  pro- 
cesses; 6,  enamel  cells;  c,  mid- 
dle layer  of  prickle  cells;  d, 
central  or  pulp-cells.  After  V. 
von  Ebner.  X  about  550  diams. 


THE    MOUTH    CAVITY    AND    I- A  <   i..  587 

THK  DKNTAL  PAPILLA  consists  at  first,  as  stated  above,  of  crowded 
in  -enchymal  cells.  Blood-vessels  appear  in  it  very  soon  after  the 
enamel  organ  lias  become  concave  on  the  lower  side.  The  papilla 
acquires  very  nearly  its  permanent  shape  before  any  further  differ- 
entiation <«f  its  tissue  begins.  The  shape  of  the  papilla  is  probably 
determined  entirely  by  the  enamel  organ,  by  which  it  is  completely 
einl>ra<-e<l.  see  above.  During  the  fourth  month  the  cells  nearest  the 
surface  enlargt — principally  by  the  growth  of  their  protoplasm.  They 
appear  as  a  continuous  layer  next  the  enamel  organ;  their  function 
is  to  pmi 1 1 ico  the  dentine  between  themselves  and  the  enamel  organ, 
1  icnce  they  are  called  odontoblasts  (membranaeboris,  Kolliker);  they 
are  to  be  regarded  as  modified  osteoblasts.  The  deposit  of  dentine 
lupins  iii  the  milk-teeth  toward  the  end  of  the  fourth  month.  In  a 
•ical  section  of  a  developing  papilla,  one  can  see  several  stages, 
because  the  development  advances  more  rapidly  toward  the  apex  and 
more  slowly  toward  the  base  of  the  papilla.  The  tissues  underneath 
the  odontoblast  layer  constitute  the  so-called  pulp  of  the  tooth.  The 
connective-tissue  cells  of  the  embryonic  pulp  are  small  and  have 
numerous  very  fine  and  branching  processes  which  impart  a  fibril- 
la  ted  appearance  to  the  tissue,  but  so  far  as  known  there  are  no  true 
intercellular  fibrilke  in  the  pulp.  The  cells  are  somewhat  more 
crowded  directly  under  the  odontoblasts  than  in  the  interior  of  the 
papilla. 

K.\  \  M  I.L. — The  deposit  of  enamel  begins  on  the  milk-teeth  toward 
the  end  of  the  fourth  month.  According  to  our  present  knowledge, 
the  formation  of  enamel  must  be  conceived  about  as  follows :  Each 
enamel  cell  forms  an  enamel  prism  by  the  metamorphosis  of  the 
lower  end  of  the  cell  into  a  calcified  column ;  a  cement,  which  is  also 
ified,  holds  the  prisms  together;  the  cement  is  presumably  a 
derivative  of  the  inter-cellular  substance  between  the  enamel  cells. 
Enamel  is,  therefore,  essentially  different  from  bone  and  dentine,  in 
neither  of  which  do  the  cells  calcify,  yet  the  enamel  cells  resemble 
odontoblasts  in  many  respects.  The  first  step  toward  the  production 
of  an  enamel  prism  is  the  change  of  the  protoplasm  at  the  lower  or 
papillary  end  of  the  enamel  cell  into  a  homogeneous  mass,  resembling 
a  cutieular  cell  border;  by  the  union  of  the  borders  of  adjacent  cells, 
a  continuous  membrane  or  cuticula  is  generated.  We  must  assume 
that  this  membrane  grows  upon  its  upper  side  by  apposition  from 
the  enamel  cells,  and  becomes  modified  on  its  lower  or  papillary  side 
at  nearly  the  same  rate.  The  modification  consists  in  the  production 
of  the  fibrous  tuft,  Fig.  332,  a,  described  above,  at  the  end  of 
each  enamel  cell.  The  lower  end  of  this  tuft  (Tomes'  process)  cal- 
cifies and  becomes  the  beginning  of  the  enamel  prism.  The  enamel 
prisms  begin  small  in  diameter  with  considerable  cementing  sub- 
stance between,  but,  as  they  lengthen,  their  diameter  increases  so 
much  that  there  is  little  or  no  space  for  cementing  substance  between 
them.  The  enamel  prisms  lengthen  by  apposition  on  their  ends 
adjoining  the  enamel  cells,  yet  for  a  long  time  the  cells  maintain 
their  size,  perhaps  nourishing  themselves  at  the  expense  of  the  cen- 
tral cells  of  the  enamel  organ,  which  gradually  atrophies  as  the 
enamel  thickens.  From  their  mode  of  growth,  it  follows  that  the 
prisms  stretch  through  the  whole  thickness  of  the  layer  of  enamel. 


588 


THE   FCETUS. 


Since  the  enamel  prisms  widen  out  toward  the  surface  of  the  tooth, 
it  is  probable  that  the  enamel  cells  increase  in  diameter  as  the  enamel 
is  deposited.  The  cells  cease  multiplying  by  the  time  the  enamel 
begins  to  form.  The  enamel  prisms  undergo  further  changes  after 
birth.  They  become  harder  and  thicker  at  the  expense  of  the  ce- 
menting substance  between  them.  At  birth  it  is  still  relatively  easy 
to  break  up  the  enamel  into  its  prisms,  and  to  a  certain  extent  to 
break  the  prisms  so  as  to  obtain  indications  of  fibrillated  structure. 

DENTINE. — The  odontoblasts,  as  stated  above,  are  modified  mesen- 
chymal  cells,  which  form  an  epithelioid  layer  over  the  surface  of  the 
papilla.  The  odontoblasts  are,  at  first,  short  cylinder  cells,  each  with 
an  oval  nucleus  toward  the  end  of  the  cell  farthest  from  the  enamel 
organ.  They  keep  their  mesenchymal  character  in  that  they  are 
connected  by  processes  with  one  another  and  with  the  underlying 
cells  of  the  papilla.  The  first  change  in  the  odoctoblasts  prepara 
tory  to  the  deposit  of  dentine  is  the  appearance  of  the  so-called 
membrana  prcefortnativa,  a  clear  homogeneous  membrane  consist- 
ing apparently  of  anisotropic  intercellular  substance.  The  mem- 
brana  always  lies  next  the  odontoblasts  and  is  best  interpreted  as  the 
layer  of  uncalcified  dentine,  see  C.  Rose,  91.2,  470.  There  now 
arise  the  dental  processes,  which  are  prolonga- 
tions of  the  odontoblasts  toward  the  enamel  organ 
as  far  as  the  membrana  prseformativa.  The  pro- 
cesses vary  much  in  size,  but  are  generally  about 
one-sixth  to  one-fourth  the  diameter  of  the  cells ; 
each  cell  usually  has  one  dentinal  process  only, 
but  sometimes  there  are  two,  and  even  as  many 
as  six  have  been  seen  by  Boll.  Between  the  den- 
tinal processes  a  clear  anisotropic  substance  is 
formed,  which  gradually  increases  in  thickness, 
the  processes  lengthening  correspondingly,  until  a 
considerable  layer,  which  may  be  described  as 
uncalcified  dentine,  intervenes  between  the  odon- 
toblasts and  the  enamel  organ.  Calcification  sets 
in  next  the  enamel  and  progresses  toward  the 
papilla ;  at  the  same  time  the  deposit  of  uncalcified 
dentine  is  continued  by  the  odontoblasts.  The 
calcification  is  incomplete;  the  uncalcified  spots 
are  known  in  the  adult  tooth  as  the  interglobular 
spaces.  The  membrana  pra3formativa  cannot,  as 
suggested  by  Von  Ebner,  90.1,  244,  be  resorbed 
by  the  enamel  organ,  since  it  is  not  in  contact 
with  it,  but  it  is  to  be  observed  in  well-developed 
teeth,  and  is  perhaps  present  throughout  life.  It 
has  given  rise  to  many  misconceptions.  The 
matrix  of  the  dentine  was  supposed  by  Waldeyer 
to  be  produced  by  a  metamorphosis  of  the  proto- 
plasm of  the  odontoblasts,  but  this  point  is  open 
to  discussion.  The  question  is  part  of  the  more 
general  one — What  is  the  origin  of  intercellular 
substance?  Compare  p.  399.  As  the  dentine  increases  in  thickness 
the  odontoblasts  become  longer  and  narrower,  Fig.  333,  B,  and  the 


FIG.  333.  —  Odonto- 
blasts from  Cow  Em- 
bryos. A,  of  30  cm.  ; 
B,  of  24  cm.  After 
Franz  Boll. 


Till.    MOUTH    CAVITY    AND    FACE. 


dentinal  processes  finer,  more  numerous  and  branching,  the  branches 
anastomosing  with  one  another.  The  processes  persist  and  never 
calcify,  the  spaces  they  occupy  being  the  dental  cami/icu/i  of  the 
a- hilt.  The  ends  of  the  odontoblasts  toward  the  dentine  become,  for 
iho  most  part,  as  it  were,  squared  off,  while  the  lower  ends  become 
more  or  less  pointed,  Fig.  333.  The  odontoblasts  lose  much  of  their 
regularity  of  arrangement,  as  the  dentine  nears  completion,  but  they 
are  still  found  in  the  adult.  In  old  age  they  become  comparatively 
inconspicuous  and  assume  a  rounded  or  ovoid  shape  (Tomes'  "  Den- 
tal Anat.,"  L876,  p.  M). 

'I'm:  (  I.MKNT  is  merely  a  layer  of  bone  developed  by  ossification  of 
the  dental  follicle  over  the  root  of  the  tooth.  It  differs  from  ordinary 
bone  by  the  greater  abundance  of  Sharpey's  fibres  in  it.  Its  develop- 
ment begins  on  the  milk-teeth  during  the  fifth  month,  and  takes 
place  after  the  type  of  peri  osteal  ossification. 

\«.K  OF  DEVELOPMENT. — The  following  table  indicates  approx- 
imately the  ages  at  which  the  various  stages  of  development  are 
pa>M«d  by  the  different  teeth.  To  complete  the  table  it  must  be 
added,  i,  that  the  first  permanent  molar  arises  the  fifteenth  or  six- 
t.rnth  week  like  a  milk-tooth  as  a  bud  from  the  epithelium  of  the 
dental  groove;  2,  that  the  second  molar  begins  as  a  bud  from  the 
neck  of  the  first  about  the  third  month  after  birth,  and,  3,  that,  ac- 
cording to  Magitot,  the  germ  of  the  third  molar,  or  wisdom-tooth, 
begins  as  an  enamel  bud  from  the  neck  of  the  second  molar,  about 
the  third  year  (C.  S.  Tomes,  "Dental  Anat.,"  1876,  p.  128.) 


<(&. 

Milk  teeth. 

Permanent  teeth  (except 
molars). 

First  molars. 

7th  

^'\\        .   . 

|i.-;rt,il  -I-<MIV»-  an<l  ridtfe. 
Knami'l  origins  bud 

<itli 

Kiiaint'l  organ  concaves 

10t!i  

F»llieiilar  wall. 

i:.:l  

i  F.nanifl    (ir^an  fully  differen- 

'      tiatt-d 

j  Enamel    bud 

10th  

^  I  ollicle    closes  above    germ. 
Neck   of  enamel  organ  re- 

'            v.,,|-|l,-d 

Enamel  buds  appear 

i     appears. 

17th  

»  Deritine  appears   on   incisors 

Papilla 

isth  

>  It.-ntine  appears  on  first  and 
<     second  molars 

Follicle 

20th 

I><-nt  ine  caps   0  04-0  06  in  high 

Papilla  formed  

Follicle  closes 

•>-,ti, 

0  06-0  07         " 

28th  

0.08-0  09         u 

j  Enamel  organ  fully  differen- 

•V'd 

0  Q9_o  \\         « 

1     tiated;  follicle  well  formed. 


With  

0.11-0.12 

o.l  2-0.  14 

Follicle  closes  above  germ. 

After  birth 

Enamel  and  dentine  appear 

Double  Dentition  of  Mammals. — The  manner  in  which  the 
teeth  are  renewed  in  the  shark's  jaw  has  been  described,  p.  582,  Fig. 
328 ;  the  new  tooth-germs  arise  as  outgrowths  on  the  lingual  side  of 
the  old.  In  mammals  there  is  the  same  relation  between  the  earlier 
milk-teeth  and  the  later  permanent  teeth.  It  is,  therefore,  justifiable 
to  assume  that  the  diphyodont  mammal  preserves  in  a  reduced  degree 
the  piscian  power  of  renewing  the  teeth,  and  that  the  milk-teeth  rep- 
resent the  primary  dentition.  Such,  however,  is  not  the  view  of 
Flower,  67. 1,  who  considers  that  the  present  mammals  are  derived 


590  THE   FCETUS. 

from  monophyodont  ancestors,  and  have  acquired  the  milk-teeth 
secondarily  by  interpolation.  This  conception  has  been  more  recently 
adopted  and  defended  by  Oldfield  Thomas  (Phil.  Trans.,  1887,  451). 
For  criticisms  of  these  authors  gee  Lataste,  89. 1,  who  also  advances 
a  more  complicated  hypothesis.  Flower's  hypothesis  was  based  on 
the  belief  that  marsupials,  which  have  only  one  set  of  teeth,  possess 
the  permanent  set,  but  W.  Kiikenthal,  91.1,  has  found  that  the 
teeth  of  Didelphys  (opossum)  correspond  to  the  milk-teeth,  and  that 
the  germs  of  the  permanent  teeth  are  present  in  the  embryo  and  abort 
without  forming  any  tooth  except  the  third  pra3molar  (so-called  first 
molar)  of  the  upper  jaw,  which  belongs  to  the  second  dentition. 

As  to  the  evolution  of  the  complicated  forms  assumed  by  the  teeth 
of  mammalia,  see  E.  D.  Cope,  74.1,  and  H.  F.  Osborn,  88.2. 

Salivary  Glands. — The  mouth  cavity  of  amniotes  is  furnished 
with  numerous  glands,  which  in  Sauropsida  are  found  in  part  vari- 
ously gathered  into  groups,  in  part  scattered  singly.  In  mammals 
scattered  single  glands  are  found,  but  instead  of  groups  of  glands 
there  are  three  pairs  of  large  glands,  each  with  a  long  single  duct. 
The  three  pairs  are  the  salivary  glands  and  are  known  only  in  mam- 
mals. It  has  been  suggested  that  each  salivary  gland  corresponds 
to  a  group  of  oral  glands  in  reptiles,  but  the  attempts  to  determine 
the  homologies  involved  in  this  assumption  have  failed,  compare 
Reichel,  83.1,  and  Ercole  Giaccomini,  90.1.  On  the  other  hand 
the  development,  I  believe,  indicates  clearly  that  each  salivary  gland 
is  a  single  oral  gland  greatly  enlarged,  for  it  arises  from  a  single 
invagination  and  in  an  early  stage  has  a  marked  resemblance  to  an 
ordinary  branching  gland  of  the  mouth. 

Concerning  the  development  of  the  small  oral  glands  in  man,  a 
few  observations  are  recorded  by  Kolliker  ("  Mikrosk.  Anat./''  II.,  2, 
and  "Entwickelungsges.,"  1879,  828)  who  also  gives  a  few  data  con- 
cerning the  sali varies.  The  development  of  the  latter  glands  is 
known  to  us  chiefly  through  the  researches  of  J.  H.  Chiewitz,  85. 1. 
The  glands  appear  in  the  following  order :  submaxillary,  sublingual, 
parotid.  The  submaxillary  anlage  can  be  seen  in  a  pig  embryo  of 
21  mm.  and  in  a  human  embryo  of  about  six  weeks;  the  parotid 
appears  in  man  by  the  end  of  the  eighth  week.  As  to  the  position 
of  the  anlages :  the  mouth  at  the  time  they  appear  has  a  character- 
istic shape  in  section,  Fig.  325,  being — if  we  imagine  the  tongue 
removed — like  an  inverted  j_,  and  there  is  at  each  side  an  angle, 
a;  it  is  from  the  epithelium  along  this  angle  that  the  solid  outgrowth 
to  form  the  parotis  takes  place.  The  base  of  the  tongue  forms  an 
angle  on  each  side  with  the  floor  of  the  mouth,  Fig.  325;  it  is  from 
this  angle  that  the  solid  outgrowths  of  the  buccal  epithelium  take 
place  to  form  the  sublingual  and  submaxillary  glands,  the  former 
near  the  front,  the  latter  near  the  back  of  the  tongue.  The  anlages 
of  the  parotid  and  submaxillary  are  at  first  at  about  the  same  dis- 
tance back  from  the  frenulum  of  the  tongue,  but  as  development 
proceeds  the  submaxillary  orifice  migrates  forward,  the  parotid  back- 
ward. The  following  measurements  are  from  Chiewitz,  85.1,  422. 

Age  of  embryo  in  weeks 6  8  10        12 

Submaxillary  gland,  distance  from  frenulum.  .0.52  0  32  0.36  0.12  mm. 
Parotid  gland 0.34  1.08  1.10mm. 


THE    Monil    CAVITY    ANI>    FACK. 


59] 


The  outgrowth  of  the  salivary  anlage  is  at  first  a  cylinder, 
which.  h< '\vcv.-r.  s< »<>ii  begins  to  lengthen  and  branch;  the  ends  of 
th<-  branches  enlarge,  and  ultimately  develop  into  the  alveoli.  The 
inland  is  now  further  characterized  by  the  condensation  of  the  con- 
nective tissue  ahout  its  branches  into  a  globular  mass,  which  is 
sharply  defined.  Fig.  :>:'»4,  a,  against  the  neighl>oring  looser 


Ahr. 


*U.—  Section  of  the  Submaxillary  Gland  of  a  Human  Embryo  of  sixty-three  to  sixty -eight 
Days.    Minot  Coll.  No.  188.    Ali\  Alveolus;  a,  connective-tissue  sheath  of  gland;  />,  duct. 

nective  tissue.  The  lumen  of  the  gland  appears  first  in  the  main 
duct,  then  in  its  branches,  and,  last  of  all,  in  the  alveoli;  it  develops, 
not  hy  the  abortion  of  the  cells  in  the  centre,  but  by  the  cells  moving 
asunder  so  as  to  leave  a  central  cavity,  while  they  themselves  assume 
an  epithelial  arrangement.  The  alveoli  are  still  solid  at  the  begin- 
ning of  the  fifth  month,  but  in  an  embryo  of  twenty-two  weeks  were 
found  by  Chiewitz,  /.c.,  427,  to  be  ah1  hollow.  At  this  time  the  epi- 
thelium consists  of  a  single  layer  of  cylinder  cells;  in  the  ducts  the 
nuclei  are  so  placed  that  they  form,  as  in  earlier  stages  also,  Fig. 
:>:>4,  Z),  two  rows;  the  nuclei  of  the  outer  row  are  somewhat  smaller 
and  stain  more  readily  than  those  of  the  inner  row;  in  the  alveoli 
the  cells  are  at  first  all  alike,  but  after  the  alveoli  become  hollow 
some  of  the  cells  become  enlarged  to  form  muciparous  beaker-cells, 


592 


THE   FCETTJS. 


while  others  remain  smaller  and  protoplasmatic ;  these  smaller  cells 
become  partly  covered  in  by  the  neighboring  beaker-cells,  and  thus 
develop  into  the  semilunar  cells  of  the  adult. 

Between  the  anlages  of  the  sublingual  and  submaxillary  glands, 
there  appear  later — twelfth  week  in  man — some  eleven  to  thirteen 
gland  anlages,  which  in  their  mode  of  development  resemble  small 
salivary  glands,  Chiewitz,  85.1,  423.  These  are  termed  by  Chie- 
witz  alveolingual  glands,  and  have  been  often  confounded  with  the 
true  sublingual  gland. 

Tongue. — Although  the  tongue  is  developed  from  the  floor  of  the 
pharynx,  yet  it  becomes  so  entirely  an  appendage  of  the  mouth  that 
it  may  be  appropriately  treated  here.  Our  knowledge  of  the  devel- 
opment of  the  tongue  is  derived  chiefly  from  Dursy,  69. 1,  and  His, 
("Anat.  menschl.  Embryonen,"  III.,  64-81). 

The  first  distinct  trace  of  the  tongue  is  a  small  tubercle  which 
appears  in  the  middle  line  on  the  floor  of  the  pharynx  between  the 

ends  of  the  first  and  second  (i.  e. , 
mandibular  and  hyoid)  arches.  It 
was  supposed  by  Dursy  to  be 
formed  by  the  fusion  of  the  lower 
ends  of  the  mandibular  arches, 
but  His  has  shown  that  it  is  single 
and  median,  and  accordingly  has 
termed  it  tuberculum  impar,  Fig. 
177.  Immediatelj'  behind  the  tu- 
bercle appears  the  evagination  to 
form  the  thyroid  gland,  see  Chap- 
ter XXIX.  Very  soon  after  the 
tubercle  has  appeared  the  lower 
ends  of  the  second  and  third  arches 
fuse — human  embiyos  of  7  mm.— 
a  FHumS-~ES?otai^Ip?f  **£&**£  and  their  fused  ends  constitute  the 
8Ti°nTceeV^  anlages  of  the  back  of  the  tongue. 

glottis;  m.th,  median  thyroid  aniage.    After  The  tubercle  now  rapidly  enlarges, 

Fig.  335,   Tg,  and  becomes  easily 

recognizable  as  the  front  part  of  the  tongue.  The  site  of  the  thyroid 
evagination  remains  as  a  fixed  point,  which  is  often  marked  by  a 
small  depression,  the  foramen  coscum  of  Morgagni ;  the  duct  of  the 
thyroid  sometimes  persists  and  is  then  found  starting  from  the  fora- 
men ccecum.  The  front  and  back  of  the  tongue  are  marked  off,  Fig. 
335,  by  two  oblique  lines,  which  start  from  the  foramen,  and  together 
form  a  widely  open  V.  This  V  can  be  traced — as  pointed  out  by  His, 
I.e.,  79 — in  the  adult  tongue;  the  part  behind  the  V  has  its  surface 
thrown  into  ridges,  and  over  it  there  are  glands,  which  appear  dur- 
ing the  third  month ;  the  part  in  front  has  papillae  developed  under 
its  epithelium,  and  the  papillae  circumvallatae  are  situated  a  little 
(5-8  mm.)  in  front  of  the  V,  but  in  lines  parallel  with  it ;  the  circum- 
vallate  papillae  do  not,  therefore,  represent  the  division  line  between 
the  front  and  back  of  the  tongue.  The  largest  part  of  the  tongue 
is  developed  from  the  tuberculum  impar,  the  less  part  from  the 
region  of  the  second  and  third  branchial  arches — hence  the  tongue  is 
a  derivative  of  the  pharynx  and  not  of  the  oral  cavity. 


Si 


m.iK 


IV 


CHAPTER  XXVII. 
THE     NERVOUS     S  YSTK  \  I 

THE  formation  of  the  vertebrate  cerebrospinal  axis  has  already  been 
treated  at  length,  pp.  173-181.  In  its  first  stage  it  appears  as  the 
medullary  tube  with  ectoderm: il  walls.  The  second  stage  is  the  dif- 
ferentiation of  the  brain  from  the  spinal  cord  hy  the  enlargement  of 
the  anterior  end  of  the  tube.  The  sharp  distinction  which  we  have 
just  drawn  between  the  stages  does  not  maintain  itself  in  the  am- 
ni<  »ta.  In  fact  the  medullary  groove  widens  at  its  cephalic  end  before 
it  closes  to  form  a  tube,  so  that  the  brain  is  indicated  in  the  embryo 
before  the  medullary  tube  is  formed.  Moreover  the  development  of 
the  I »rain  progresses  while  the  groove  is  closing,  so  that  the  brain  is 
already  quite  advanced  before  the  medullary  tube  is  closed  at  its 
caudal  end.  These  irregularities  in  the  development  of  the  central 
nervous  system  render  it  impossible  to  decide  at  present  whether  the 
simple  medullary  tube  without  a  brain  enlargement,  or  a  (perhaps 
solid)  central  nervous  system  with  a  brain  enlargement,  represents 
the  phylogenetically  primitive  condition.  The  difficulty  of  reaching 
a  decision  is  still  further  increased  by  the  fact  that  the  tubular  con- 
dit  ion  of  the  nervous  system  was  probably  acquired  within  the  verte- 

hrate  series,  see  p.  180. 

Definition  of  the  Brain. — The  vertebrate  brain  is  the  anterior 
portion  of  the  medullary  tube,  and  is  characterized  by  two  primary 
features:  1,  the  enlargement  of  the  tube;  2,  its  special  associations 
with  higher  sense  organs  (olfactory,  visual,  and  auditor}*).  The 
brain  is  further  characterized  in  all  true  vertebrates :  1,  by  having 
three  principal  enlargements  separated  from  one  another  by  two  con- 
strictions (H.  Ayers,  90.1,  claims  that  the  three  enlargements 
can  be  traced  in  Amphioxus  also) ;  2,  by  being  bent  at  the  region  of 
the  second  enlargement  (mid-brain)  owing  to  the  development  of  the 
head-bend  of  the  embryo;  3,  by  containing  the  principal  centres  for 
the  co-ordination  of  sensations  and  movements.  All  modifications 
of  the  brain  can  be  traced  back  to  this  primitive  type,  and  it  seems 
probable  that  the  evolution  of  the  brain  has  been  dominated  by  the 
ad  vantages  of  more  perfect  co-ordinating  apparatus,  as  the  special 
senses  on  the  one  hand  and  the  locomotive  organization  on  the  other 
acquired  a  higher  development. 

Cerebral  Vesicles. — The  enlargement  which  produces  the  brain 
extends  about  half  the  length  of  the  embryo,  compare  Figs.  114  and 
155,  and  takes  place  unevenly,  so  that  there  are  produced  three  suc- 
cessive lobes,  which  are  known  as  the  primary  cerebral  vesicles,  Fig. 
11  :•  and  114;  the  second  and  third  vesicles  (mid-brain  and  hind- 
brain)  are  often  imperfectly  divided  from  one  another.  The  three 
vesicles  subsequently  subdivide,  so  as  to  form — to  follow  the  tradi- 
38 


594 


THE   FCETUS. 


tional  description — five  secondary  vesicles.  It  has  long  been  cus- 
tomary to  describe  the  medulla  as  dilating  to  form  the  three  and 
later  five  vesicles,  but  unfortunately  the  descriptions  have  been  s<  > 
much  conventionalized  in  subservience  to  tradition  that  they  are 
misleading  in  several  important  respects.  The  attempt  is  here  made 
to  give  an  untrammelled  objective  account. 

OPTIC  EVAGINATIONS. — The  first  indication  of  brain  formation 
seems  to  me  to  be  the  widening  of  the  extreme  anterior  end  of  the 
medullary  plate  or  groove,  which  can  be  recognized  in  all  vertebrate 
embryos  at  a  very  early  stage.  In  elasmobranchs  it  appears  to  me 
evident  that  the  widening  is  due  to  the  very  process  of  concrescence 
itself,  and  is  initiated  while  the  ectental  lines  are  approaching  one 
another,  and  is  fully  marked  before  the  longitudinal  axis  of  the  embryo 
is  completed  by  concrescence.  Fig.  317  represents  a  dog-fish  embryo ; 
m  is  the  point  at  which  concrescence  has  begun ;  it  will  be  observed 
that  the  embryonic  rim  curves  around  this  point  and  in  consequence 

is  spread  out  laterally;  in  later  stages  the 
lateral  protrusion,  which  we  see  initiated  in 
Fig.  317,  at  m,  becomes  still  more  marked 
and  can  be  followed  until  it  is  evidently  the 
optic  diverticulum.  In  mammals  we  find 
the  medullary  groove  specially  widened  at  its 
anterior  end — noticeably  so  in  the  mole,  Fig. 
99,  op.  A  cross  section  through  the  optic 
vesicle  at  this  stage  offers  a  very  singular 
appearance,  Fig.  100;  the  entoderm,  En,  has 
not  closed  over,  although  the  notochord,  ?/r//. 
is  already  distinguishable  under  the  medul- 
lary groove;  the  ectoderm,  EC,  is  greatly 
thickened  on  the  dorsal  side  to  form  the  very 
wide  medullary  plate,  which  has  a  median 
depression,  Mp,  corresponding  to  the  medul- 
lary groove  proper,  and  two  lateral  depres- 
sions corresponding  each  to  an  optic  vesicle. 
If  we  imagine  the  medullary  plate  to  bend 
upward  and  to  close  over  itself,  then  the  two 
edges  of  the  optic  depressions,  op,  which  are 
outermost  in  Fig.  100,  will  meet  in  the  me- 
dian line,  and  as  soon  as  the  groove,  by  clos- 
ing, becomes  a  tube,  there  will  be  at  this  point 
two  lateral  diverticula,  having  the  same  char- 
acteristically thickened  ectodermal  lining  as 
the  rest  of  the  medullary  tube.  These  diver- 
ticula are  the  so-called  optic  vesicles,  which 
are  ultimately  transformed  into  the  optic 
nerve,  retina,  and  choroid  of  the  eye. 

In  the  chick  the  optic  vesicles  become  clear- 
ly indicated  by  the  twenty-fourth  hour,  when 
there  are  from  five  to  seven  distinct  pairs  of 
primitive  segments,  and  the  head  projects 
slightly  over  the  proamniotic  area.  Before  the  medullary  groove 
has  closed  anywhere  the  optic  diverticula  are  quite  distinct.  In  a 


FIG.  336.— Chick  Embryo  of 
twenty-nine  Hours,  op,  Optic 
vesicle;  pro,  proamnion;  F"2, 
second  cerebral  vesicle  or  mid- 
brain;  F3,  hind-brain;  v.om, 
omphalo  -  mesaraic  vein ;  seg, 
primitive  segment;  Md,  me- 
dullary tube;  pr.g,  primitive 
groove.  After  M.  Duval. 


THK   NMRVOTTfl   BT8TKM, 

chick  of  twenty-nine  hours,  Fig.  330,  the  vesicles,  op,  are  very  large, 
their  growth  being  an  important  factor  in  the  precocious  distention 
of  the  head. 

\VII>I;MN<;  OP  THE  MEDULLARY  TUBE. — While  the  optic  vesi- 
cles are  developing  the  medullary  tube  expands  in  diameter  through- 
out its  cranial  or  anterior  half,  without  there  being  at  first  much 
change-  in  the  structure  of  its  walls  or  much  evidence  of  subdivision, 
but  very  soon  the  expansion  becomes  unequal,  so  that  the  tube  is 
slightly  constricted  in  lined  lately  behind  the  optic  vesicles,  Fig. 
<>l>;  then  follows  a  slight  dilatation,  I'2,  the  mid-brain  (Mittelhirn)* 
which  is  separated  by  a  second  constriction  from  the  long  and  large 
hind-brain,  I  (  Hiiitn •// //•//),  which  is  widest  in  front  and  gradually 
diminishes  in  diameter,  and  merges  without  distinct  boundary  into 
the  posterior  unexpanded  portion  of  the  medullary  tube  or  future 
spinal  cord.  Transverse  sections  show  that  the  widening,  by  which 
the  brain  is  differentiated  from  the  cord,  is  due  chiefly  to  the  en- 
largement of  the  medullary  cavity,  and  that  the  walls  change  but 
little  in  thickness  until  the  three  vesicles  are  differentiated,  when 
the  walls  begin  a  series  of  characteristic  modifications. 

THE  THREE  PRIMARY  VESICLES  (Gehirnbltischen,  vesiculce  cere- 
hi- ales)  were  known  to  Malpighi  and  Haller  according  to  Tiedemann, 
61.1,  u.  Bischoff,  45.1,  170,  appears  to  have  been  the  first  to 
observe  that  they  are  formed  before  the  medullary  groove  is  entirely 
closed  in  the  cephalic  region.  Owing  to  the  fact  that  the  optic  ves- 
icles grow  out  so  early  and  that  the  remainder  of  the  brain  as  a 
whole  widens  out,  we  ought,  perhaps,  to  accept  A.  Goette's  view, 
75. 1,  280,  that  a  double  division  precedes  the  triple.  In  this  case 
w«-  should  have  to  describe  the  mid-brain  and  hind-brain  as  arising 
by  the  subdivision  of  the  second  primary  enlargement. 

1.  The  Fore-Brain. — As  we  have  seen  above,  the  fore-brain  orig- 
inally includes  the  optic  vesicles,  which 
primitively  show  no  trace  of  any  de- 
marcation from  the  central  portion  of 
the  fore-brain.  This  condition,  how- 
ever, does  not  last  long,  for  the  central 
portion  of  the  fore-brain  soon  begins  to 
expand  upward  and  forward,  making 
a  separate  central  enlargement,  which 
may  be  designated  as  the  permanent 
fore-bran/ .  Meanwhile  the  distal  ends 
of  the  optic  diverticula  also  dilate  rap- 
idly, while  the  part  of  each  diverticulum 
nearest  the  fore -brain  proper  grows 
slowly.  It  is  often  erroneously  stated 
that  part  of  the  optic  vesicle  is  con- 
stricted:  in  reality  it  enlarges,  though 
relatively  slowly.  From  these  modifi-  Lepyosteus  .Embryo  of  ei*ht  Days. 

J  .  /o.   Fore-brain :  EC,   ectoderm ;  L,   an- 

cations  there  are  developed  a  wide  me-  la^reof  lens;  op,  optic  vesicle;  En,  en- 

dian    fore-brain    and    two   lateral    Optic    t0derrn'    After  Batfour  and  Parker. 

vesicles  connected  by  tubular  stalks  with  the  ventral  side  of  the  brain 
proper,  Fig.  337.  *  In  short,  the  primitive  vesicle  is  divided  into  three 

*  Compare  also  Figs.  170,  171,  and  179. 


596  THE   FCETUS. 

parts,  one  median  and  two  lateral,  and  it  is  only  the  median  part  that 
enters  into  the  formation  of  the  brain.  The  history  of  the  median 
division  is,  therefore,  treated  in  this  chapter,  while  that  of  the  two 
lateral  divisions  is  dealt  with  in  Chapter  XXVIII.,  on  the  organs  of 
sense.  It  may,  however,  be  stated  now,  in  order  to  facilitate  the 
comprehension  of  the  figures,  that  the  optic  vesicles  expand  dors  1- 
ward,  Fig.  337,  op.  It  should  be  noted  that  the  walls  of  the  fore- 
brain  and  optic  vesicle  are  still  nearly  uniform  in  thickness,  and, 
so  far  as  yet  observed,  in  structure.  The  changes  described  in  this 
paragraph  occur  in  the  chick  at  about  thirty-two  to  forty  hours,  in 
the  rabbit  the  ninth  day,  in  man  about  the  eighteenth  day. 

The  next  series  of  changes  in  the  fore-brain  lead  to  the  differentia- 
tion of  the  cerebral  hemispheres.  By  a  long-continued  tradition  it 
has  become  customary  to  describe  the  process  as  the  subdivision  of 
the  primary  vesicle  into  two  secondary  vesicles,  designated  as  the 
fore-brain  proper  (Vorderhirn,  prosencephalon)  and  'tween-bnu'/i 
(Zivischenhirn,  thalamencephalon) .  Such  a  description,  however, 
seems  to  me  hardly  justified  either  by  embryology  or  comparative 
anatomy,  and  to  be  especially  apt  to  mislead  and  confuse.  In  fact 

every  embryologist  must  admit  that 
it  is  scarcely  correct  to  say  that  the 
fore-brain  divides  into  two  vesicles, 
from  the  anterior  of  which  the  cere- 
bral hemispheres  grow  out.  It  is 
more  in  accord  with  the  actual  facts 
to  describe  the  hemispheres  as  ap- 
pendages of  the  fore-brain,  that  is 
to  say,  of  the  so-called  Zwischenhirn 
or  thalamencephalon.  Accordingly 
I  present  the  history  of  the  origin  of 
the  cerebral  hemispheres  somewhat 
differently  from  usual,  though,  of 
course,  without  changing  the  facts. 
SP-C  I  If  For  convenience  I  defer  mention  of 
the  head-bend  (see  p.  600),  which 
FIG.  asa -Brain  of  Embryo  NO.  22,  p.lor  develops  while  the  hemispheres  are 

(His'  Lg).     H,  Anlage  of  hemispheres;  Mb,    arising.       Soon  after  the    OptlC  V6S1- 

SEJPoi  'MS?  CveeS';un|,ff  '$£i  cles  have  become  stalked  the  extreme 

cord;    Op,  optic  vesicle.      After  W.  His.     X    onforinr  &nr\  rvf    fhn  fivcf  -vroeiVl^    mi 
23  diams     Compare  Fig.  342. 

larges  and  pushes  itself,  so  to  speak, 

forward  and,  owing  to  the  head- bend,  downward.  The  flexure  is 
at  first  slight,  but  increases  as  development  proceeds,  compare  p. 
600.  The  enlarged  end  of  the  medullary  tube  is  in  no  way  divided 
off  from  the  first  cerebral  vesicle  until  the  end  begins  to  dilate  to- 
ward each  side  to  produce  the  hemispheres.  The  manner  in  which 
the  hemispheres  grow  out  can  be  better  understood  from  the  Figs. 
338,  339,  and  340,  than  from  any  mere  description.  At  first,  as 
just  indicated,  they  form  an  undivided  common  anterior  enlargement, 
but  the  lateral  expansion  begins  very  early,  and  with  it  the  anlages  of 
the  two  hemispheres  are  given.  If  the  position  of  the  hemispheres 
is  observed  carefully,  Fig.  338,  jfZ,  it  will  be  seen  at  once  that  it  is 
the  product  of  the  dorsal  side,  and  that  the  ventral  half  of  the  primi- 


THK     NERVOUS     M  -  I  KM. 


597 


01 


hy- 


— Op 


Fio.  839.—  Reconstruction  of  the  Brain  of  His'  Embryo  Ko 
nl&nge,  10.2  mm.).     3/6,  Mid-brain;  /&,  fore-brain;  //, 
hemisphere ;  OI,  olfactory  lobe ;  Op,  optic  nerve ;  hy>  hypophy- 
;  cm,  corpus  mammilare.     After  w.  His. 


ris; 


tive  ion-brain,  as  shown  by  W.  His,  89.4,  does  not  participate  in 
the  outgrowth.  The  consideration  of  this  important  fact  demon- 
strates that  the  hemispheres  cannot  be  strictly  compared  with  one 
of  the  primary  vesicle.;,  each  of  which  includes  a  ventral  as  well  as  a 
dorsal  portion  of  the  medullary  tube.  The  origin  of  the  hemispheres 
from  the  dorsal  side 
h;i-  BO  uTeat  impor- 
tance morphologically 
that  special  emphasis 
mu-t  he  laid  upon  the 
fact.  The  ventral  « 
boundary  of  the  hemi- 
spheres must  he  placed 
near  the  optic  stalks, 
BO  that  the  hemi- 
spheres include  that 
portion  of  the  brain 
wall  which  unites  with 
the  ectoderm  to  form 
the  olfactory  plate,  al- 
ready described,  p.  5 7-V 
The  cerebral  hemispheres  grow  more  rapidly  than  any  other  part  of 
the  brain,  see  Fig.  iJM,  //,  but  their  growth  is  principally  in  their 
distal  par's,  so  that,  like  the  optic  vesicles,  they  become  large  pouches 
connected  by  relatively  small  hollow  stalks  with  the  fore-brain.  The 
stalk  is  short.  The  passage  through  the  stalk  is  called  the  foramen 
Unit  roe,  Fig.  340,  f.m.  As  this  foramen  enlarges  but  little, 
while  the  brain  increases  enormously,  it  appears  in  the  adult  as  a 
small  opening  in  proportion  to  the  size  of  the  whole  brain.  Although 
the  foramen  enlarges  absolutely,  it  is  sometimes  described  errone- 
ously as  becoming  smaller  during  development.  While  the  hemi- 
spheres are  expanding 
the  olfactory  plate, 
Fig.  339,  O/,  acquires 
n  more  marked  differ- 
entiation beneath  them 
;md  shows  traces  of  di- 
vision into  a  dorsal  or 
anterior,  and  ventral  or 
posterior,  lobe.  Even 
at  the  stage  of  Fig. 
339,  it  can  still  be 
recognized  that  the  ol- 
factory region  corre- 
sponds to  what  was, 
before  the  brain  was 
bent,  part  of  the  ex- 
treme anterior  wall  of  the  fore-brain.  But  the  olfactory  region 
is:  already  paired,  and  is  associated  in  its  development  with  the 
hemispheres.  This  leaves  a  part  of  the  wall  of  the  fore-brain  in  the 
median  line,  Fig.  340  (between  the  reference  lines  f.m  and  R.o), 
which  is  known  as  the  lamina  terminalis  and  represents  throughout 


it. 


FIG.  340.— ReconstiiK  t.-.l  Mnlian  View  of  the  Fore-Brain  of 
His'  Embryo  Ko  (\ackenldnge,  10.2  mm.).  H.  Hemisphere; 
/. m,  foramen  of  Munroe;  jR.o,  recessus  opticus;  t.c,  tuber cine- 
reum;  in,  corpus  inuinmilare;  m&,  mid-brain.  After  W.  His. 


598  THE   FOETUS. 

life  the  extreme  anterior  wall  of  the  fore-brain.     As  seen  in  Fig. 

340,  it  extends  from  the  level  of  the  foramen  of  Munroe  to  the  level 
of  the  optic  stalks.     In  the  same  figure  it  can  also  be  seen  that  the 
hemispheres  and  olfactory  lobe  project  further  forward  than  the 
lamina.      The  hemispheres  expand,  not  only  upward  and  forward 
in  regard  to  the  longitudinal  axis  of  the  fore-brain,  but  also  back- 
ward, as  can  be  well  seen  in  Figs.  339  and  341.     The  history  of  the 
hemispheres  is  given  more  fully  and  for  later  stages  below,  p.  690. 

The  primary  differentiations  of  the  floor  or  ventral  wall  of  the 
fore-brain  are  also  clearly  indicated  in  a  human  embryo  of  10-12 
mm.  (Nackenldnge) ,  Figs.  339  and  341.  The  lower  part  of 
the  fore-brain  has  expanded,  forming,  as  it  were,  a  hanging 
pouch,  Fig.  339,  from  which  pass  off  the  optic  stalks,  Op.  Follow- 
ing the  median  wall  of  the  pouch  around  from  the  mid- brain  to  the 
level  of  the  foramen  of  Monroe,  Fig.  340,  f.m,  we  find,  first,  a  pro- 
tuberance, m,  which  extends  nearly  half-way  to  the  optic  stalk,  and 
indicates  the  future  mammillary  bodies ;  second,  a  slight  swelling, 
t.c,  which  marks  the  future  tuber  cinereum;  third,  the  future 
apex  of  the  infundibulum ;  fourth,  the  area  of  the  brain  wall  united 
with  the  hypophysis ;  and  fifth,  the  lamina  terminalis,  just  beyond 
the  recessus  opticus,  R.o. 

2.  The  Mid-Brain. — The  second  cerebral  vesicle  undergoes  less 
modification  than  the  first  and  third.     Its  walls  are  at  first  of  nearly 
uniform  thickness,  see    Duval,  "Atlas,"  Fig.   255.      It  is  oval  or 
round  in  transverse  section.     It  is  situated  at  the  point  where  the 
head-bend  takes  place  (compare  p.   GOO),  and  by  the  head-bend  its 
shape  is  profoundly  altered,  its  dorsal  surface  becoming  more  arched 
and  expanded,  Fig.  338,  Mb,  while  its  ventral  wall  as  seen  in  profile 
becomes  concave ;  further,  the  dorsal  wall  becomes  relatively  much 
thinner  than  the  ventral  wall.     The  cavity  of  the  mid-brain  remains 
very  large,  and  during  the  early  expansion  of  the  brain  the  commu- 
nication between  the  fore-brain  and  mid-brain  enlarges  more  than 
does  the  passage  between  the  mid-  and  hind-brain.    This  is  commonly 
expressed  by  saying  that  the  constriction  between  the  first  and  second 
cerebral  vesicles  is  much  less  marked  than  between  the  second  and 
third. 

In  the  lower  vertebrates  the  fore-brain  and  hind-brain  do  not  ad- 
vance either  in  growth  or  complication  as  in  the  amniota.  In  birds 
and  reptiles  the  mid-brain  develops  to  a  greater  extent  than  in  mam- 
mals, and  in  the  embryo  early  acquires  great  size,  see  Fig.  396,  II. 
In  mammals,  on  the  other  hand,  the  mid-brain  grows  more  slowly. 
Roughly  speaking,  then,  we  may  say  that  the  importance  of  the  mid- 
brain  diminishes  as  we  ascend  the  vertebrate  series,  and  that  it  does 
not  participate  in  the  advance  of  organization  which  characterizes 
the  first  and  third  cerebral  vesicles. 

3.  Hind-Brain. — The  third  cerebral  vesicle  is  especially  charac- 
terized by  the  great  expansion  of  its  very  thin  dorsal  wall,  by  the 
thickening  of  the  dorsal  wall  immediately  behind  the  constriction 
separating  the  second  from  the  third  vesicle,  and  by  the  great  and 
prominent  bend  formed  by  the  ventral  wall  of  the  hind-brain,  Fig. 

341,  Hb.     The  thin  dorsal  wall  corresponds  to  the  epithelial  epen- 
dyma  of  the  adult ;  its  morphological  significance  is  explained  in  the 


THK    NKKV«»rs    SVSTKM.  599 

-ection  on  the  zones  of  His,  p.  nnn.  The  dorsal  thickening  is 
the  anla^e  of  the  cerebellum  and  corresponds  to  a  commissure  found 
in  the  lower  vertebrates.  The  apex  of  the  ventral  flexure  is  the 
anla.uv  of  the  pons  Varolii  of  the 
adult.  The  thickened  floor  of  the  . 
hind-hrain, between  tlie]>onsand 
the  spinal  cord,  .s-/>.r.  gives  rise 
to  the  medulla  oblongata.  We 
thus  have  the  four  chief  struct- 
ures, which  develop  from  the 
hind-hrain,  definitely  mapped 
out  by  the  earliest  changes. 
The  modifications  which  result  H  ,-•  ~V 

in   this  four-fold  differentiation      \  W,  sP-c-i 

all    take    place    simultaneously  R 

and  are  interdependent.     They 

.1                  i.       ,-  .          f                    -,  Fio.  841.— Brain  of  a  Human    Kmbryo  of   flve 

are  the  reSUlt   Ot  two  t;ict0rs:    1,  Wt«.                         <;,l.ry..  S«-li>.      //,  H.-mispl.,-,-,-;  /;. 

tln«  iiiii'.m-d  il»'V«'lnnmpnt  of  rlif-  olfart.n-v    ]..»•••:  »/,.  optic  n.-i-v.-:  Mf,.  nii<l-hrain; 

'  (lll<li  1/fc,  himl-l.ruin;  -V.c,  spinal  cord.    Aft.-r  \V.  His. 

ferent  regions  of  the  medullary 

walls;  2,  the  appearance  of  the  Varolian  bend  (Br&ckenkrUm  ///  '///f/) . 

These  factors  are  considered  later. 

It  is  usually  stated  that  the  hind-brain  subdivides  into  two  vesi- 
< -le>,  for  which  the  names  secondary  hind-hrain  and  after-brain 
(\iirhhint)  have  been  employed;  the  Nachhirn  is  the  part  nearest 
the  spinal  cord.  In  fact,  it  is  convenient  to  designate  the  anterior 
part  of  the  hind-brain,  out  of  which  the  cerebellum  and  pons  Varolii 
arise,  as  the  hind-brain  proper  (nirf<'n,Tj>//<i/<>/,)  and  the  posterior 
part  as  the  Nachhirn  u  /»-//rr/>//»f/o//  <>r  myelencephcdon)  or,  better,  as 
t lie  medulla  oblongata.  ( )n  the  other  hand,  it  is  incorrect  to  speak 
of  the  primitive  hind-brain  as  forming  two  secondary  vesicles.  This 
error  goes  back  to  the  time  of  Von  Baer,  II.,  100,  who  observed  such 
division  in  the  chicken  embryo.  It  has  also  been  described  and  fig- 
ured by  MihalUvics,  77.1,  l>5,  Taf.  IV.,  Fig.  :J:J,  in  a  chick  of  fifty- 
eiLcht  In »urs.  These  authors,  and  most  others  who  have  written  on 
the  subject,  assumed  that  their  observations  were  upon  a  constant 
and  typical  condition.  In  reality  there  is  great  irregularity  in  the 
-r«  >wt  h  « »t  t  he  walls  of  the  hind-brain,  and  sometimes  in  birds  and  per- 
haps in  reptiles  the  third  cerebral  vesicle  is  temporarily  more  dilated 
at  its  anterior  end  than  elsewhere.  The  dilatation  soon  disappears, 
and  no  proof  has  been  brought  yet,  to  my  knowledge,  to  establish  an 
identity  between  it  and  the  region  corresponding  to  the  cerebellum 
and  pons — it  seems  to  take  in  more  than  the  cerebellum,  less  than 
the  pons.  In  chicken  embryos  the  separate  dilatation  is  usually 
wanting,  and  it  has,  so  far  as  I  know,  never  been  observed  in  any 
mammalian  or  ichthyopsidan  embr}To.  It  is  interesting  to  note  that 
Balfour,  "  Comp.  Embryol.,"  II.,  424,  though  he  does  not  expressly 
mention  the  error  of  the  traditional  description,  yet  skilfully  avoids 
adopting  it  in  his  account  of  the  hind-brain. 

The  shape  of  the  hind -brain  requires  more  detailed  description. 
As  seen  in  Fig.  338,  the  hind-brain  at  the  time  of  the  development 
of  the  head-bend  is  more  than  equal  to  all  the  rest  of  the  brain  in 
length.  It  begins  with  the  constriction  or  isthmus  behind  the  mid- 


600 


THE   FCKTUS. 


Mb 


Sp.G 


FIG.  342. —Hind-Brain 

of  a  Human  Embryo 
(No.22,  p.297  His'Lg) 

seen  from  the  dorsal 
side.  m&,  Mid-brain; 
C6,  cerebellum;  IV, 
fourth  ventricle ;  Sp.c, 
spinal  cord.  After  W. 
His.  Compare  Fig. 338. 


FIG.  343.  —  D  o  r  s  a  1 
View  of  the  Hind- 
Brain  of  a  Human  Em- 
bryo of  one  Month 
(His'  Ru).  Mb,  31  id- 
brain;  Obi,  ceivhfl- 
lum;  IV,  fourth  ven- 
tricle; Sp.c,  spinal 
cord.  After  W.  His. 


brain  and  at  first  widens  rapidly,  then  gradually  tapers  to  the  neck- 
bend,  where  it  passes  into  the  spinal  cord.  Viewed  from  the  dorsal 
side,  Fig.  342,  the  anterior  constriction  or  isthmus  is  still  more  no- 
ticeable, and  we  can  also  see  the  kite-shaped  outline 
of  the  thin  roof.  Comparison  of  the  figure  with 
the  following,  Fig.  343,  represent- 
ing a  slightly  older  stage,  affords 
an  idea  of  the  widening  of  the  me- 
dulla, while  comparison  of  Figs. 
338  and  341  will  indicate  its  modi- 
fications as  seen  in  profile.  It  is 
important  to  observe  that  there  is, 
as  yet,  no  cerebellum,  but  only  a 
thickening  of  the  dorsal  wall  close 
to  the  isthmus.  This  thickening 
is  the  anlage  of  the  cerebellum, 
and  is  to  be  homologized  with  the 
commissure  found  in  the  corre- 
sponding position  in  Ichthyopsida. 
Cerebral  Flexures. — The  axis 
of  the  neuron  may  be  described  as 
straight,  for  it  is  actually  very 
nearly  so,  up  to  the  stage  when 
the  optic  vesicles  begin  to  be  con- 
stricted off— see  Figs.  99  and  336. 
While  the  dilatation  to  form  the  second  cerebral  vesicle  or  mid- brain 
is  taking  place,  the  primary  head-bend  of  the  embryo  is  established, 
involving  the  brain.  The  bend  of  the  brain  takes  place  at  the  level 
of  the  mid-brain ;  the  fore-brain  is  bent  over  ventralward  until  it 
forms  a  right  angle  with  the  hind-brain,  Fig.  338,  the  actual  flexure 
being  almost  confined  to  the  mid-brain,  in  which,  as  can  be  seen  in 
the  figure,  the  cerebral  axis  curves  very  much,  while  in  the  hind- 
brain  it  remains  nearly  straight,  and  in  the  fore-brain  is  slightly 
bowed  only.  This  bend  may  be  called  the  mid-brain  or  primary 
flexure.*  During  the  early  stages  of  the  hemispheral  outgrowths 
the  flexure  increases  until  the  axis  of  the  fore-brain  forms  an  acute 
angle  with  that  of  the  hind-brain,  Fig.  320.  Mihalkovics,  77.1, 
39,  proposes  to  distinguish  the  right-angled  stage  as  the  Haken- 
kriimmung,  and  the  later  acute-angled  stage  as  the  Kofpbeuge. 
Such  a  distinction  is  entirely  arbitrary,  and  the  suggestion  has  not 
been  adopted.  The  angle  becomes  ultimately  so  sharp  that  the  floor 
of  the  fore-brain  becomes  nearly  parallel  with  that  of  the  hind-brain. 
The  second  bend  to  appear  is  at  the  junction  of  the  hind-brain 
(medulla  oblongata)  and  spinal  cord,  Fig.  338,  and  is  termed  the 
neck-bend  (Nackenkrummung).  Like  the  primary  bend  it  affects 
the  whole  head ;  the  summit  of  its  angle  appears  in  the  embryo  when 
seen  in  profile,  compare  Figs.  220  and  223,  during  several  early 
stages  as  a  projection  (His'  Nackenhocker) ,  which  is,  however, 
soon  obliterated.  The  neck-bend  develops  later  than  the  head-bend, 
not  appearing  in  mammals  until  the  hemisphere  anlages  have  begun 

*  It  is  called  by  Reichert   Gesichtskopfbeuge ;  by  Dursy,  Kopfbeuge ;  by  Kolliker,  vordere 
Kopfkrummung;  by  His,  Scheitelkriimmung. 


THE   NERVOUS  SYSTEM.  601 

to  grow  out  separately.  It  is  very  slight  in  the  Ichthyopsida ;  in 
the  reptiles  and  birds  it  is  more  developed,  but  it  attains  its  maxi- 
mum only  in  the  mammalia,  and  notably  in  man.  In  human 
embryos  the  neck-bend  increases  from  the  third  to  the  end  of  the 
fifth  week,  when  it  reaches  its  maximum,  the  hind-brain  then  form- 
ing nearly  a  riLcht  angle  with  the  spinal  cord,  Fig.  341.  Later  the 
bend  becomes  less  again,  owing  to  the  gradual  erection  of  the  head 
as  already  described  and  illustrated  in  Chapter  XVIII.  for  the 
human  embryo. 

The  third  cerebral  flexure  is  known  as  the  Varolian  bend  (Kolli- 
ker's  Briickenkrummung)  and  is  essentially  different  from  the  two 
tlexmvs  just  described,  for  it  is  not  a  bend  of  the  whole  medullary 
tube,  as  are  they,  but  a  bend  of  the  ventral  side  of  the  hind-brain, 
Ki.n-.  oil.  the  dorsal  side  remaining  as  seen  in  profile,  nearly  straight, 
i  livady  mentioned,  the  greater  part  of  the  dorsal  wall  of  the  hind- 
hrain  is  a  thin  membrane,  and  this  membrane  takes  no  part  in  the 
formation  of  the  Varolian  bend,  which  depends  on  the  growth  of  the 
thick  walls  of  the  floor  of  the  hind-brain,  and  with  this  growth  the 
bend  increases,  its  formation  being  accompanied  by  the  lateral  ex- 
pansion of  the  hind-brain  at  its  anterior  or  cerebellar  end,  Fig.  34:5. 

The  cause  of  all  the  cerebral  flexures  is,  of  course,  the  unequal 
growth  of  the  various  parts.  Herein  the  growth  of  the  brain  is  cer- 
tainly the  principal  factor  in  determining  the  result.  The  general 
conception  of  the  influence  of  the  unequal  growth  of  the  brain  dates 
back  to  Von  Baer,  and  was  revived  by  Rathke.  W.  His  was  the 
first  to  attempt  an  analysis  of  the  mechanical  conditions,  and  to 
demonstrate  that  the  shaping  of  the  brain  depends  to  a  large  cU'L 
upon  these  conditions,  which  are  many  of  them  relatively  obvious 
and  simple.  His  has  given  in  his  semi-popular  work,  "Unsere 
Korperform,"  74.1,  pp.  93-118,  an  admirable  presentation  of  his  re- 
sults, which  have  not  yet  received  from  embryologists  the  attention 
which  their  exceptional  importance  demands. 

Origin  of  the  Sensory  Ganglia. — To  fully  understand  the 
history  of  the  ganglia  the  reader  should  consult  the  section  on  the 
ganglionic  sense-organs  in  the  following  chapter.  The  origin  of 
the  ganglia  has  been  carefully  traced  in  a  human  embryo  with 
thirteen  segments,  by  M.  von  Lenhossek,  91.1,  three  of  whose  figures 
I  reproduce,  Fig.  344.  As  seen  in  A,  the  ectodermal  cells,  Gl,  which 
immediately  adjoin  the  medullary  plate,  differ  in  size  and  by  their 
rounded  form  from  the  cells  of  the  neighboring  ectoderm  and  of  the 
medulla.  These  cells  constitute  two  bands,  which  unite  in  a  single 
median  band  when  the  medullary  groove  closes.  The  median  band 
has  been  termed  the  Zwischenstrang  in  the  chick  embryo  by  His, 
but  is  more  usually  termed  the  neural  crest  or  ridge  (Neuralleiste), 
as  proposed  by  Balfour.  In  B,  the  cells  are  about  to  unite  in  the 
median  line.  In  C  they  have  united,  and  though  incorporated  in  the 
medulla  and  separated  entirely  from  the  external  ectoderm  are 
readily  distinguished  from  the  cells  of  the  medullary  plate  proper. 
The  cells  are  also  growing  out  on  each  side,  Gr/,  toward  the  myotome. 
The  emigration  continues  until  all  the  cells  are  transferred  from  the 
median  line  to  the  lateral  masses,  #/,  which  are  the  anlages  of  the 

*  This  embryo  is  the  one  designated  as  No.  18,  and  described  p.  295. 


602 


THE   PCETUS. 


Hid 


sensory  ganglia.  As  the  cells  depart  from  the  neural  crest  the 
medullary  plate  proper  closes  over  in  the  median  dorsal  line.  The 
ganglionic  lateral  masses  exhibit  a  segmental  arrangement  very 

early,  so  that  the  cells  ap- 

A       ^m®*  pear  in  clusters,  each  clus- 

ter  on  the  inner  side  ot  a 
myotome.  According  to 
Chiarugi,  90.1,  these 
clusters,  at  least  in  the 
post-auditory  region  of  the 
head,  are  bud -like  growths 
from  the  neural  crest ;  be- 
tween the  clusters  the  crest 
persists  for  a  short  time 
like  a  commissure.  These 
clusters  are  found  in  older 
stages  to  enlarge  rapidly 
and  to  move  farther  down 
toward  the  notochord. 
They  are  the  rudimentary 
ganglia.  The  ganglia  are 
always  strictly  segmental 
in  position,  both  when  first 
formed  and  later.  This  is 
especially  noticeable  when 
they  attain  their  maximum 


FIG.  344.— Sections  through  the  Cervical  Part  of  the  Me- 
dulla of  a  Human  Embryo  with  thirteen  Segments.  A,  In 
front  of^the  segments  ^where  the^  medullary  groove  is  relative  Size,  tor  they  then 

occupy  the  whole  width  of 
a  segment. 


groove  is  just  closing ;  C,  at  the  level  of  the  third  segment. 
EC,  Ectoderm;  Gl,  ganglionic 

After  Lenhossek. 


Ectoderm;  Gl, 
mesoderm. 


widely  open;  B,  a  little  further  back,  where  the  medullary 
"      ;  C,  at  the  level  of  the  third  segment, 
nglionic  anlage;  md,  medulla;  mes, 

W.  His,  90.1,  has  ren- 
dered it  highly  probable  that  the  cells  which  form  the  anlages  of  the 
spinal  ganglia  emigrate  singly  from  the  ectoderm;  these  cells  bear 
an  obvious  resemblance  to  the  germinating  cells,  which  become  the 
neuroblasts  of  the  medullary  tube ;  see  also  the  account  of  the  olfac- 
tory ganglion,  Chapter  XXVIII. 

In  all  vertebrates  the  ganglia  are  developed  essentially  as  in  man, 
but  the  process  varies  considerably  in  detail.  Thus  in  Petromyzon 
according  toKupffer,  90. 1,  486,  Taf.  XXVIII,  Figs.  22,  23,  24,  and 
36,  the  medullary  cord  is  completely  formed,  and  afterward  the  cells 
are  differentiated  to  form  the  dorsal  median  neural  crest  (Q-anglien- 
leiste,  Nervenleiste).  The  account  given  by  Kupffer  differs  from 
that  given  by  Sagemehl,  82. 1,  which  has  been  accepted  by  Shipley, 
88.1,  and  Scott,  87.1.  If  Kupffer  is  right,  then  the  lamprey  "is 
characterized  by  a  very  late  differentiation  of  the  neural  crest.  This 
is  true  also  of  elasmobranchs,  see  Balfour,  "Comp.  Embryol.,"  II. , 
449,  Rabl,  89.2,  223,  Taf.  X.,  Figs.  34  and  35,  also  Kastschenko, 
88.1,  463;  in  this  class  the  medullary  canal  is  completely  formed, 
and  the  neural  crest  appears  afterward,  and  moreover  without  any 
marked  differentiation  of  its  cells  from  those  of  the  medullary  tissue 
proper.  In  the  axolotl,  Lenhossek,  91.1,  19-21,  finds  the  neural 
crest  early  separated  from  the  medullary  canal,  which  closes  dorsally 
by  a  single  row  of  cells,  each  of  which  stretches  completely  across,  see 


THE   NERVOUS    SYSTKM.  G03 

his  Fig.  10.  In  birds,  as  first  observed  by  W.  His,  68. 1,78,  the  neural 
crest  i>  ,-i  -ej.arate  distinct  thickening  of  the  ectoderm,  which  can  be 
Been,  at  least  iii  the  cephalic  region,  while  the  medullary  groove  i> 
still  open.  Fig.  14T,  (rl;  it  is  readily  distinguished  by  the  larger 
si/e  of  its  cells  fnnn  the  tissue  of  the  medullary  plate.  This  band 
was  termed  by  His  the  intermediate  cord  (Zwttchenstrang)  and 
he  was  the  first  not  only  to  demonstrate  the  existence  of  a  neural 
crest,  but  also  its  genetic  relations  to  the  ganglia. 

In  certain  cases  there  appears,  while  the  medullary  groove  is  still 
open,  a  slight  groove  in  the  ectoderm  close  to,  and  parallel  with,  the 
edge  of  the  medullary  plate.  This  groove  has  been  named  by  His, 
68.1,  the  Zn-iscl,t'iirinne.  It  apparently  results,  as  suggested  l»y 
C'hiarugi,  91. 1,  from  the  effort  of  the  ectoderm  to  fit  in  between  the 
edge  of  the  medulla  and  the  myotomes.  The  ectoderm, 'even  when 
there  is  no  groove,  is  thickened  along  this  line,  and  this  thickening 
was  formerly  thought  to  be  connected  with  the  development  of  the 
neural  crest.  This  appears,  as  Beard,  88.3,  100,  has  correctly 
maintained,  not  to  be  the  case.  Beard  has  adopted  with  this  correc- 
tion His'  view  of  the  origin  of  ganglia,  but,  without  giving  his  rea- 
sons for  so  doing,  advances  it  as  a  new  conception. 

In  regard  to  the  early  history  of  the  ganglia  the  following  points 
deserve  special  mention:  1,  the  ridge  appears  first  in  the  region  of 
the  hind-brain,  and  thence  its  development  progresses  forward  and 
tail  ward;  the  same  law  governs  the  appearance  of  the  separate 
gangl  ionic  anlages;  2,  the  ganglia  arise  near  the  dorsal  summit  of 
the  neuron,  as  seen  in  cross  sections,  but  rapidly  migrate  toward  the 
notochord  until  they  reach  their  permanent  level  alongside  the  me- 
dullary tuln?;  3,  as  they  descend  the  ganglion  anlages  lose  all 
connection,  so  far  as  can  be  observed,  with  the  medullary  tube. 
Kolliker,  however,  expressly  states  ("  Grundriss,"  2te  Aufl.,  207)  that 
the  ganglia  always  remain  connected  dorsal ly  with  the  medullary 
tube;  4,  the  continuity  of  the  neural  crest  is  preserved,  it  remain- 
ing as  a  slender  band  connecting  on  each  side  of  the  body  the  dorsal 
parts  of  the  ganglia  with  one  another  longitudinally.  The  connect- 
ing band  maybe  called  the  gau<jl  ionic  commissure.  It  has  beefc 
observed  by  Kolliker  ("  Grundriss,"  2te  Aufl., 268)  in  a  human  embryo 
of  the  fourth  week. 

The  ganglionic  commissure  is  undoubtedly  a  very  important 
morphological  structure,  as  insisted  upon  by  Balfour,  "  Comp.  Em- 
bryol.,"  II.  450-451.  There  are  a  number  of  valuable  observations 
upon  it  scattered  in  various  articles,  but  until  these  shall  have  been 
collated  or  considerably  extended,  it  will  remain  impossible  to  give  a 
satisfactory  account  of  the  commissure,  its  significance  or  its  fate. 
A  special  investigation  of  this  problem  is  much  to  be  desired. 

CEPHALIC  GANGLIA. — As  the  ganglia  of  the  head  differ  somewhat 
in  their  primitive  arrangement  from  those  of  the  rump,  I  add  a  brief 
description  of  them. 

As  long  ago  as  1847  Remak  described  in  chick  embryos  of  sixty 
hours  the  four  ganglia  of  the  head  to  which  the  neural  crest  primar- 
ily gives  rise,  at  least  in  amniota.  W.  His,  68.1,  106,  168,  gave  a 
fuller  description  and  studied  ,also  earlier  stages.  No  study  of  the 
ganglia  corresponding  to  the  present  requirements  and  resources  of 


G04  THE    FOETUS. 

embryology  has  yet  been  attempted.  The  four  ganglia  to  be  seen 
in  the  chick  before  the  head-bend  appears  are  thus  described  by  His, 
88.2,  417:  There  are  two  ganglionic  masses  in  front  of  and  two 
behind  the  auditory  vesicle ;  the  foremost  of  these  is  the  trigeminal 
ganglion,  which  is  very  long,  occupying  nearly  half  the  length  of 
the  head ;  it  begins  in  front  of  the  optic  vesicle,  perhaps  even  at  the 
olfactory  pit,  passes  along  the  dorsal  side  of  the  optic  vesicle,  along- 
side the  mid-brain,  and  ends  a  short  distance  after  the  beginning  of 
the  hind-brain.  Later  this  large  ganglion  separates  into  the  ciliary 
ganglion  and  the  trigeminal  ganglion  proper,  the  former  arising  from 
that  part  of  the  original  anlage  which  is  near  the  optic  vesicle.  A. 
Froriep,  91.2,  has  observed  that  in  torpedo  embryos  of  6  mm.  the 
trigeminal  ganglion  also  sends  a  large  branch,  which  runs  straight 
to  the  dorsal  side  of  the  isthmus  to  the  point  where  the  trochlear 
nerve  arises  later ;  this  branch  may  be  called  the  trochlear  arm ;  in 
embryos  of  9  mm.  the  arm  is  represented  only  by  a  few  groups  of 
cells;  and  in  embryos  of  16  mm.  one  of  these  groups  still  persists  as 
a  small  ganglion  appended  to  the  trochlear  nerve.  In  embryos  of 
20  mm.  even  this  remnant  of  the  trochlear  arm  had  disappeared.  The 
second  ganglion  lies  between  the  trigeminal  and  the  auditory  vesicle, 
and  is  known  from  the  nerves  with  which  it  becomes  connected  as 
the  acustico-facialis.  The  third  and  fourth  ganglia  lie  behind  the 
otocyst,  and  are  concerned  in  the  development  of  the  glosso-pharyn- 
geal  and  vagus  nerves  respectively.  The  second,  third,  and  fourth 
ganglia  are  much  smaller  than  the  trigeminal,  and  in  a  chick  at  sixty 
hours  are  of  about  the  same  size  as  the  otocyst  and  primitive  seg- 
ments at  the  same  stage. 

The  form  of  the  four  cephalic  ganglia  as  seen  in  cross  sections  (of 
the  human  embryo  at  least)  is  very  characteristic,  His,  82.3,  1371. 
The  trigeminal  appears  oval;  the  acustico-facial  subdivided  by 
diverging  bundles  of  fibres ;  the  glosso-pharyngeal  is  almost  circular ; 
the  vagus  is  like  a  long  spindle. 

Neuromares. — The  entire  medullary  tube  undergoes  a  segmen- 
tation by  a  series  of  alternating  slight  enlargements  and  constrictions. 
Each  enlargement  is  supposed  to  give  rise  typically  to  a  pair  of 
ventral  nerve-roots  and  is  joined  by  the  corresponding  dorsal  (or 
ganglionic)  rooi:s.  In  certain  neuromeres  of  the  brain  this  relation 
to  the  nerve-roots  is  modified  and  even  obliterated.  The  neuromeres 
are  most  distinct  in  amniota  at  the  stage  when  the  hemispheres  are 
just  beginning  to  grow  out  from  the  fore-brain,  and,  after  persisting 
for  a  short  time  distinctly  marked,  are  gradually,  but  rapidly,  oblit- 
erated. They  appear  first  in  the  hind-brain  and  cervical  region, 
and  from  thence  they  appear  progressively  toward  the  fore-brain  and 
the  tail.  Their  appearance  seems  to  depend  upon  the  development 
of  the  primitive  segments  of  the  mesothelium  (compare  p.  192). 
When  the  segments  are  fully  formed,  and  before  their  inner  wall  has 
changed  into  mesenchymal  tissue,  they  press  against  the  medullary 
tube,  and  oppose  its  enlargement ;  at  least  one  sees  that  the  tube 
becomes  slightly  constricted  between  each  pair  of  segments  and 
slightly  enlarged  opposite  each  inter segmental  space.  Each  inter- 
segmental  dilation  is  a  neuromere,  and  later  produces  the  nerve  for 
the  segment  (?  behind  it). 


TMK   NERVOUS   SYSTEM. 


605 


Nm. 


A  caution  is  here  necessary.  Each  neuromere  produces  a  pair  of 
nerves,  but  when  the  first  trace  of  roots  appears,  they  are  seen  to 
spring  from  the  constriction  between  the  neuromere,  but  later  from 
the  neuromere.  The  origin  from  the  neuromere  is  therefore  >.-cond- 
ary,  as  pointed  out  by  Julia  B.  Platt,  89.1,  who,  however,  has 
ignored  the  difference  between  the  ganglionic  and  medullary  nerve 
fibres.  I  deem  it  probable  that  the  neuromeres,  as  here  described, 
really  comprise  each  a  half  of  two  adjacent  true 
neuromeres. 

Kach  neuromere  is  separated  from  its  fellows  by 
an  external  dorsal- ventral  constriction  and  opposite 
this  an  internal  sharp  dorsal-ventral  ridge,  Fig. 
•  JJ'i.  aa,  so  that  in  a  longitudinal  horizontal  sec- 
tion. Fig.  :>l~».  each  half  of  a  neuromere  forms  a 
small  arc  of  a  circle.  So  far  as  at  present  known, 
the  constrictions  are  confined  to  the  sides  of  the 
medullary  tube  and  do  not  cross  either  the  dorsal 
or  the  ventral  plate  of  the  neuron.  Fig.  345  shows 
the  arrangement  of  the  cells  in  the  neuromeres  at 
a  very  early  stage.  The  elongated  cells  are  placed 
radially  t«>  the  inner  curved  surface  of  the  neuro- 
mere. The  nuclei  are  generally  nearer  the  outer 
surface,  and  approach  the  inner  surface  only  to- 
ward the  apex  of  the  dividing  ridge.  On  the  line 
between  the  apex  of  the  internal  ridge  and  the  pit 
of  the  external  depression  the  nuclei  are  crowded 
ther,  but  the  cells  of  one  neuromere  do  not 
extend  into  another  neuromere.  Often  a  light 
space  marks  the  boundary  between  the  adjacent 
neural  segments.  FIO. 


As  to  the  number  of  neuromeres  our  knowledge  n£  I 


846.— Longitixli- 
ection 
Hind- 


r.  aniage  of 


h 
s  still  defective.     It  is  not  impossible  that  the  Brain  of  a  vow  Boa 

number  in  the  head,  especially  in  the  fore-brain  sSj£«jf.a  iSf*  N.'iii'.'.- 
and  vagus  region  of  the  hind-brain,  is  less  in  the 
amniota  than  in  primitive  vertebrates,  for  there  is 
evidence  that  the  number  of  mesodermic  segmen  ts  H.eor?h 
has  been  reduced  in  the  head,  and  it  is  probable 
that  the  formation  of  the  neuromeres  is  conditional  upon  the  presence 
of  the  mesodermic  segments.  In  the  spinal  cord  there  is  evidently  a 
neuromere  for  each  pair  of  nerves  ;  for  example,  in  chicken  embrj-os 
of  the  second  day  the  neuromeres  are  readily  seen  to  correspond 
exactly,  as  do  later  the  nerves,  with  the  number  of  segments;  com- 
pare Duval's  Atlas,  Figs.  84,  89,  93,  98,  100,  102.  In  the  hind- 
brain  of  a  lizard  (Anolis)  and  of  the  chick,  McClure,  90.1,  finds 
six  neuromeres  (but  in  amblystoma  five  only);  these  six  he  assigns 
to  the  following  nerves,*  beginning  in  front,  trigeminal,  abducens, 
facial,  auditory,  glosso-pharyngeal,  and  vagus  ;  he  believes  that  the 
abducens  neuromere  is  wanting  in  the  newt.  In  the  mid-brain  we 
find  as  yet  no  evidence  of  neuromeres  among  the  amniota,  but  Kup- 
ffer,  86.1,  states  that  in  teleosts  two  can  be  distinguished,  and  W. 


*It  is  to  IK-  mentioned  that  McClure  overlooks  the  fact  that  the  neuromeres  can   have  no 
genetic  relation  to  the  ganglionic  nerves,    c/.  p.  619. 


COG  THE    FOETUS. 

B.  Scott,  87.1,  PL  IX.,  Fig.  15,  has  given  a  figure  which  suggests 
the  existence  of  two  or  possibly  three  neuromeres  in  the  mid-brain 
of  Petromyzon.  The  fact  that  two  nerves — the  oculo-motor  and  tro- 
chlear — arise  from  the  mid-brain,  renders  it  probable  that  there  are 
corresponding  neural  segments.  In  the  fore-brain  McClure  has 
observed  two  neuromeres  in  Amblystoma,  Anolis,  and  the  chick ;  in 
Anolis  these  are  seen  in  the  region  of  the  Zwischenhirn  (thalamen- 
cephalon)  after  the  optic  vesicles  have  become  stalked  and  the  hemi- 
sphere anlages  have  appeared.  McClure  calls  the  anterior  of  these 
the  olfactory  neuromere,  and  says  it  is  connected  with  the  olfactory 
nerves.  I  question  the  existence  of  such  a  connection,  of  which  no 
evidence  is  given,  because  the  olfactory  nerves  do  not  arise  from  the 
Zwischenhirn.  The  second  neuromere  is  called  the  optic  by  McClure, 
and  is  stated  by  him  to  produce  no  nerve. 

The  total  number  of  neuromeres  in  the  head,  exclusive  of  those 
belonging  to  the  hypoglossus,  is  fixed  at  ten  by  McClure,  90. 1,51. 

Historical  Note. — The  neuromeres  were  observed  by  C.  E.  von 
Baer,  28.2,  64,  74,  in  chicken  embryos  of  the  third  and  fourth  day, 
and  were  figured  in  a  dog  embryo  by  Bischoff,  and  were  noticed  by 
Eemak,  50.1,  §28,  67,  Dursy,  69.1,  A.  Goette,  75.1,  Taf.  VIIL, 
Fig.  151,  Mihalkovics,  77.1,  49,  Beraneck,  84.1,W.  B.  Scott,  87.1, 
273,  Michael  v.  Lenhossek  (in  man),  91.1,  5.  Foster  and  Balfour 
("  Embryology,"  1st  ed.,  137)  were  the  first  to  suggest  their  segmental 
value,  and  this  suggestion  was  adopted  by  Anton  Dohrn,  75.2.  C. 
Kupffer,  86.1,  definitely  asserted  that  they  indicate  a  "primary 
metamerism"  (segmentation)  of  the  medullary  tube.  Orr,  87.1, 
335,  was  the  first  to  clearly  demonstrate  their  relations  to  the  nerves, 
and  these  relations  were  specially  studied  by  McClure,  90. 1.  The 
term  "  neuromere  "  was  introduced  by  Orr. 

The  Zones  of  His. — By  this  name  I  propose  to  designate  the 
four  thickenings  which  run  the  entire  length  of  the  medullary  cord, 
and  the  morphological  significance  of  which  was  first  fully  recog- 
nized and  elucidated  by  W.  His,  88.3,  350.  In  this  connection  we 
have  also  to  consider  the  thin  portions  of  the  medullary  walls  on 
the  dorsal  and  ventral  sides  of  the  neuron.  These  portions  are 
termed  by  His,  86.1,  483,  respectively  " Deckplatte"  and  " Bodrn- 
platte."  L.  Lowe,  80.2,  had  insisted  upon  the  importance  of  the 
thickenings  running  lengthwise  of  the  neuron,  but  failed  to  discover 
their  relations  to  the  nerves.  These  relations  have  been  made  clear 
by  His. 

The  wall  of  the  medullary  tube  is  of  uneven  thickness  even  in  the 
earliest  stages.  As  seen  in  cross  section,  Figs.  161  and  103,  the 
external  outline  is  oval  in  amniota  (more  nearly  round,  however,  in 
ichthyopsida)  while  the  outline  of  the  cavity  of  the  tube  is  compressed 
from  side  to  side.  In  other  words,  the  walls  are  thin  on  the  median 
line  dorsally  and  ventrally,  and  much  thicker  on  each  side.  We  have 
then  from  the  start  two  thickened  bands,  which  can  be  traced  back,  as 
described  in  Chapter  VIII.,  to  the  double  thickening  of  the  medullary 
plate.  In  the  brain  the  thickenings  can  also  be  traced  without  diffi- 
culty, although  in  early  stages  they  are  less  sharply  marked  than  in 
the  spinal  cord,  Fig.  161. 

The  next  stage  is  reached  by  the  subdivision  of  each  lateral  thick- 


TUK 


SYSTEM. 


int«»  a  dorsal  and  a  ventral  thickening.  The  change  is  nn  »t 
readily  studied  in  the  spinal  cord,  lo  which,  therefore,  the  t'<  >ll«»w  in^ 
deM-riptinn  primarily  i-efers.*  The  central  canal  widens  out  in  it> 
dorsal  part.  Fig.  34»'>,  hut  so  that  it  remains  in  its  extreme  upper- 
most part  a  slit,  as  it  does  also  through  most  of  its  vential  part. 
The  widening  of  the  canal  cuts  into  the  lateral  wall  of  the  medulla. 
leaving  a  smaller  upper  thickening,  which  I  propose  to  call  the  dnr- 
W  zone  nf  His.  I),  and  a  larger  ventral  thickening,  which  I  shall 
name  the  rt-nlni!  nmeofHis,  V.  Thedor- 
sal  /.one  forms  in  cross  sections  a  high  round- 
ed prominence  into  the  central  canal,  and 
carries  in  its  outermost  layer  the  longitudi- 
nal bundles  of  nerve  fibres,  which  enter  the 
cord  from  the  ganglia  through  the  dorsal 
roots,  I>./\\  and  constitute  the  anlage  of  the 
posterior  horn;  the  zone  is  connected  by 
means  of  the  thin  deck-plate,  <!./>/,  with  its 
fellow  of  the  opposite  side.  The  ventral 
/.one,  I',  exceeds  the  dorsal  in  both  height 
and  width;  its  boundary  toward  the  central 
canal  is  convex;  externally  it  gives  off  the 
fibres  which  constitute  the  ventral  or  motor 
nerve-  root.  Between  it  and  the  dorsal  col- 
umn there  is,  at  least  in  the  human  embryo, 
a  temporary  external  groove,  but  the  con- 
nection between  the  dorsal  and  ventral  zone 
on  the  same  side  remains  broad.  The  ven- 
tral /.one  is  connected  with  its  fellow  by  the 
thin  Bodenplatte,  l>. 

His  at  first,  86.1,  497,  termed  the  dorsal  and  ventral  zones 
respectively  //////r/r.s-  Mftrkjtrifwiu  and  ro/Wr/v//  Mar&CJfit  wfer  ,  but 
later,  88.3,  350,  named  them  respectively  FIH<i<'li»l«tt<'  and  (irnnd- 
l>}«it<'.  The  external  groove,  which  in  man  separates  the  two  zones, 
has  an  upper  angle  near  the  dorsal  root;  this  angle  corresponds  to 
1  1  is1  Hnml/nrrhe;  and  it  has  also  an  angle  next  the  ventral  column; 
this  1(  >wer  angle  corresponds  to  His'  Cylinderfurche.  As  the  groove 
and  its  angles  are  temporary,  it  seems  to  me  unnecessary  to  give 
tin  -in  special  nam<->. 

We  distinguish,  then,  six  longitudinal  zones  in  the  embryonic 
cord.     These  are: 
Deck-plate. 
Dorsal  zones  of  His. 
Ventral  zones  of  His. 
Bodenplatte. 

The  six  zones  appear  in  each  division  of  the  brain  with  character- 
istic modifications,  which  have  been  thoroughly  studied  by  His, 
88.3,  89.4,  90.2,  and  must  now  be  passed  in  review. 

1.  MEDULLA  OBLONGATA.  —  The  course  of  development  differs 
from  that  of  the  spinal  cord  somewhat,  owing  chiefly  to  the  precocious 
widening  of  the  region  and  the  accompanying  expansion  of  the  deck- 
plate  to  form  a  large  rhomboid  epithelial  membrane,  as  already 

*  Further  details  are  given  in  the  section  on  the  spinal  cord,  p.  658. 


Fio.  .m— Diagrammatic  Sec- 
tion of  the  Embryonic  Spinal 
Cord,  d.pl.  Deck-plate:  D.  dor- 

-al/..n.-;  ni-.l,.  nval  huinllr:  I>  t;. 
dorsal  root;  Ksrh.  Raadachleier; 

I",  v.-ntral  /MIL-:    V.  n.'iir..l.la«.t«.: 
I"./.',     vi-ntral    r.x.t:     I,,     r.i.,1,-1;- 
•  ntral    canal ;    •  /-. 
ependymal  la.vr 


1. 

2,3. 
4,5. 
6. 


608 


THE    FCETUS. 


described  and  figured,  p.  600,  Fig.  343.  Owing  to  the  expansion  of 
the  deck-plate  the  lateral  walls  Ware  outward,  and  consequently  the 
zones  of  His,  which  are  developed  from  those  walls,  are  changed  in 
position.  We  may  distinguish  five  regions  in  the  medulla  oblongata 
(His,  90.2,  5),  as  follows: 

1.  The  transitional  region,  next  the  neck-bend  and  adjoining  the 

spinal  cord. 

2.  The  region  of  the  calamus  scriptorius,  which  is  imperfectly 

separated  from  the  transitional  region  in  the  embryo,  although 
perfectly  distinct  from  it  in  the  adult. 

3.  The  region  of  greatest  width,  which  includes  the  part  nearest 

the  auditory  vesicle  and  about  the  origin  of  the  trigeminal 
nerve. 

4.  The  region  of  the  cerebellum  and  pons  Varolii. 

5.  The  isthmus  or  narrow  connection  with  the  mid-brain. 

The  widening  begins  (in  human  embryos  during  the  third  week) , 
as  indicated  in  Fig.  347,  in  the  headward  part  of  the  medulla,  the 

ventral  part  of  the  central  canal  remaining 
very  narrow ;  the  change  suggests  the  differ- 
entiation of  the  dorsal  and  ventral  zones. 
As  the  widening  continues  (human  embryos 
of  the  fourth  week) ,  the  lumen  becomes  more 
triangular,  and  later  five-sided  in  section, 
Fig.  348.  The  largest  side  is  dorsal  and  is 
constituted  by  the  widened  deck-plate;  the 
other  four  correspond  to  the  zones  of  His ;  the 
dorsal  zones  form  a  decided  angle  with  the 
ventral  ones ;  each  zone  as  seen  in  section  projects  toward  the  inte- 
rior, appearing  concave  on  the  outer,  convex  on  the  inner  side.  The 
assumption  of  the  five-sided  form  is  not  simultaneous  throughout  the 
medulla  oblongata.  The  widening  of  the  medullary  tube  continues, 
and  becomes  so  extreme  in  the  third  region  that  the  zones  of  His  are 
brought  by  the  enormous  expansion  of  the  deck-plate  into  one  plane, 
Fig.  350.  •  While  this  is  be- 
ing accomplished  there  ap- 
pears, along  the  morphologi- 
cally dorsal  edge  of  the  dorsal 
zone  of  His,  a  fold  by  which 
that  edge  is  bent  over  out- 
ward and  downward,  Fig. 
349.  This  everted  edge  has 
been  named  by  His,  88.3, 
356,  the  Eautenlippe;  it  ex- 
tends through  the  regions 
I. -IV.,  and  in  fresh  human 
embryos  of  five  weeks  may  be 
seen  as  a  bright  border  around  the  edge  of  the  rhomboid  sinus  formed 
by  the  deck-plate.  The  Rautenlippe  is  simply  a  fold,  and  is  accord- 
ingly separated  by  an  external  groove  from  the  rest  of  the  dorsal  col- 
umn, while  internally  there  is  another  groove,  Figs.  349  and  350,  C, 
which  is  bounded  on  one  side  by  the  bent-over  edge  of  the  dorsal 
zone,  on  the  other  by  the  lateral  margin  of  the  deck-plate.  The 


FIG.  347.— Section  of  the  Me- 
dulla and  Otocysts,  Gb,  of  a 
human  Embryo  (His1  BB)  of  3.2 
mm.  After  W.  His.  X  70  diams. 


Fia.  348.— Sections  through  the  Regions  3  and  5  of 
the  Hind-brain  of  His'  Embryo  a.  Gb,  Otocyst ;  D, 
deck-plate;  Fl,  dorsal  zone;  Gr,  ventral  zone;  Bd, 
Bodenplatte.  After  W.  His.  X  30  diams. 


THE   NERVOUS   SYSTEM. 


609 


grooves  niv  designated  by  His  respectively  outer  and  inner  li/t-t/r<n>r<- 

(  /.  //>/H'tt  j  'in  •<•/!<')  .    The  junction  of  the  Rautenlippe  and  the  deck-plato 

i>  distinguished  by  His  as  the  Tn>ni<t.     The  Rautenlippe  plays  an 

important  role  in  the  differ- 

entiation of  both  the  medulla 

oblongata  and  of  the  cerebel- 

lum.    \\\  the  end  of  the  fifth 

week   in   the   human  embryo 

the  expansion   is  carried  so 

far  in  the  region  of  greatest 

width  that  the  dorsal  zones 

are  forced  over  so  as  to  be 

in  a  lower  plane  ventral  of 

the  plane  of  the  dorsal  zones. 

I  ,ater  the  process  of  bending 

down  the  dorsal  zones  occura 

also  in  the  region  of  the  cala- 

mus,  though  it  is  not  carried  so  far  as  in  the  region  of  greatest  width. 

In  the  region  of  the  cerebellum,  on  the  contrary,  the  medullary  wall 

constituting  the  dorsal  zone  does  not  bend  over,  but  remains  nearly 

in  a  vertical  plane. 

In  human  embryos  of  the  latter  part  of  the  second  month,  His 
found.  90.2,  20,  the  following  relations:  The  Rautenlippe  begins  as 
a  small  band  in  the  transitional  region  and  runs  forward,  increasing 
in  width  until  it  reaches  the  lower  half  of  the  region  of  the  calamus 
scriptorius,  then  diminishes  in  width  throughout  the  region  of 
greatest  width,  and  finally  attains  its  maximum  size  in  the  cerebellar 
region;  at  its  anterior  extremity  the  lippe  tapers  off  to  end  in  a 
point.  The  external  groove  between  the  Rautenlippe  and  rest  of  the 
dorsal  column  of  His  becomes  obliterated  by  the  walls  of  the  groove 
growing  together.  The  union  of  the  walls  does  not  take  place  simul- 


FIO.  WQ.—  secti-m  tin-ouch  ti»-  K.-^ion  :i  ,,f  ti..-  nimi- 
7  tc°mpare  Fig"  2W'   ** 


Fio.  350.— Four  Sections  of  the  Brain  of  a  Human  Embryo  of  about  five  Weeks.    A,  Mid-brain. 
B,  Isthmus.     C,  fourth  region  of  hind-brain ;  D,  fifth  region  of  hind-brain.    After  W.  His. 

tanously  throughout;    it   occurs  very  early   in  the   region   of  the 
calamus,  much  later  in  the  cerebellar  region,  where  the  groove  be- 
comes  especially  deep.      In  the  region  of  the  medulla  oblongata 
39 


610 


THE   FCETUS. 


proper  the  union  is  in  part  temporary,  while  in  that  of  the  cerebellum 
it  is  permanent. 

The  isthmus,  Fig.  350,  B,  or  part  connecting  with  the  mid-brain, 
is  characterized  by  remaining  smaller  than  the  rest  of  the  third  cere- 
bral vesicle  and  by  the  absence  of  the  expansion  of  its  deck-plate. 
As  seen  in  cross  section,  Fig.  350,  B,  in  a  human  embryo  of  five 
weeks,  it  appears  somewhat  compressed  from  side  to  side,  and  the 
deck-plate  and  Bodenplatte  project  somewhat,  producing  each  a 
slight  external  median  ridge  (His,  88.3,  357). 

The  expanded  deck-plate  in  man,  up  to  the  middle  of  the  second 
month,  arches  over  the  wide  cavity  of  the  medulla  oblongata;  in 
older  human  embryos,  owing  to  the  growth  of  the  cerebellum,  it  be- 
comes bent,  so  as  to  form  a  transverse  fold,  the  plica  chorioidea, 
which  is  situated  close  behind  the  cerebellum  and  projects  inward 
toward  the  floor  of  the  fourth  ventricle.  The  fold  is  anlage  of  the 
choroid  plexus  (His,  90.2,  20). 

2.  MID-BRAIN. — In  the  embryonic  mid-brain,  Fig.  350,  A,  the 
transverse  diameter  exceeds  the  vertical.  The  deck-plate  projects 
as  in  the  isthmus,  but  the  Bodenplatte  is  broadened  and  thickened, 
and  having  become  convex  toward  the  interior,  concave  toward  the 
exterior,  constitutes  an  internal  ridge  and  external  longitudinal 

groove.  The  ventral  zones 
of  His  are  well  defined  and 
are  much  narrower  in  extent 
^  than  the  dorsal  zones,  which 
constitute  the  largest  part  of 
the  wall  of  the  mid-brain, 
and  which  merge  without 
any  distinct  boundary  into 
the  deck-plate  (His,  88.3, 
357).  Later,  that  is  to  say, 
by  the  time  the  oculo-motor- 
ius  has  grown  forth  from  the 
mid-brain,  the  boundaries  of 
the  six  primary  longitudinal 
zones  are  almost  obliterated, 
compare  Fig.  367,  and  still 
later  they  entirely  disap- 
pear.* 

3.   FORE-BRAIN.  —  The 
zones  of  His  are  less  distinct 
the   fore-brain    than 


FIG.  351.— Brain  of  His1  Embryo  Br.  3  (Nackenlange, 
6.9mm.).  Hs,  Hemisphere;  C.s,  corpus  striatum;  Rl, 
olfactory  lobe ;  Rp,  recesses  opticus ;  ^4,  entrance  to  op- 


Ill 


111 


tic  nerve ;  L,  lamina  terminalis ;  TV,  infundibulum ;  Hp,  the    hind-brain,  bllt  may  be 

hypophysis;    Ri,  recessus  infundibuli;    P.s,   pars  sub-  .  j 

thalamica;    T/i,  thalamus;    Z,  anlage  of  pineal  gland;  traced      in     >TOUng     em  Dry  OS 

v.  F,  anterior,  h,v,  posterior  region  of  the  future  cor-  without  diffimiltir 

Mh,  mid-brain;  H6,  tegmentum;  Wlt  llt>  ' 


pora  quadrigemina ; 

Is,  isthmus;  Po,  pons  Varolii;  C6,  cerebellum;  G,  ven 


tral  zone  of  His;  F,  dorsal  zone  of  His.     After  W.  His. 


m    Sections    at    right   angles 

to  the  axis  of  the  fore-brain. 
The  ventral  zone  tends  to  arch  inward,  while  the  larger  dorsal  zone 
tends  to  arch  outward.  His,  89.4,  679-685,  has  endeavored  to  trace 
out  the  exact  course  of  the  zones  in  the  fore-brain,  a  most  difficult 


*  Since  writing  this  1  have  been  led  to  think  that  the  division  of  dorsal  and  ventral  zones  can 
be  traced  in  the  aqueductus  in  the  adult;  further  observations  are  needed. 


THE   NERVOUS   SYSTEM.  611 

ta>k,  owing  to  the  flexures  and  to  the  outgrowth  of  the  optic  vesicles 
and  of  the  hemispheres.  He  concludes  that  the  two  ventral  zones 
extend  primitively  to  the  optic  chiasma  and  include  at  least  a  part 
(the  retinal)  or  the  whole  of  the  optic  evaginations.  As  shown  in  the 
diagram,  Fig.  351,  this  makes  the  regio  sub-thalamica,  P.s,  the 
maminilary  process,  Ma,  tuber  cinereum,  Tc,  and  recessus  infundi- 
buli,  AV,  derivatives  of  the  ventral  zone  (Grundplatte) ;  while  on 
tin-  other  hand,  the  optic  thalami,  Th,  hemispheres,  Hs,  corpus  stria- 
t  um,  Cs,  and  olfactory  lobe,  Rl,  are  derivatives  of  the  dorsal  column 
(Flugelplatte). 

The  Bodenplatte  loses  its  individuality  in  the  fore-brain,  but  the 
deck-plate  becomes  much  specialized,  as  described  in  connection  with 
the  history  of  the  fore-brain,  given  later. 

The  division  between  the  ventral  and  dorsal  zones  is  readily  traced 
in  the  wall  of  the  third  ventricle  of  the  adult;  it  is  the  sulcus  Munroi 
<  »t  Rei chert,  and  extends  from  the  lower  edge  of  the  foramen  of 
Muiiroe  to  the  aqueductus  Sylvii;  this  groove  is  figured  in  Ober- 
steiner's  "  Handbook"  and  elsewhere,  but  is  often  omitted  in  anatom- 
ical figures  in  which  it  should  be  represented.  As  a  morphological 
division  it  is,  of  course,  of  fundamental  importance. 

Origin  of  Nerve-Cells. — The  first  step  in  the  histological  differ- 
entiation of  the  medullary  walls  is  the  separation  of  the  cells  into 
two  classes:  1,  the  spongioblasts,  or  young  neuroglia  cells;  2,  the 
<l<'nninating  cells,  which  are  the  parents  of  the  young  nerve  cells 
or  neuroblasts.  This  section  deals  with  the  germinating  cells  and 
their  transformation  into  neuroblasts.  The  history  of  the  spongio- 
blasts  is  sketched  in  the  two  following  sections. 

The  medullary  tube  is  at  first  composed  of  a  single  layer  of  simple 
epithelial  cells  of  a  nearly  uniform  character — a  fact  which  was  dis- 
covered by  Victor  Hensen,  76.1,  383;  the  discovery  has  since  been 
verified  t<  >r  all  classes  of  vertebrates.  There  soon  appear  special  cells 
of  a  rounded  form  in  the  medullary  epithelium  on  the  side  of  the 
epithelium  toward  the  central  cavity.  These  cells  divide  actively 
and  have  been  named  the  germinating  cells.  The  germinating  cells 
(Eeimzeltendes  Markes,  His,  89.1,  314)  are  the  only  ones  which 
undergo  division,  and  as  their  nuclei  divide  indirectly,  we  can  read- 
ily determine  the  distribution  of  these  cells  by  that  of  the  karyo- 
kinetie  figures  in  the  embryonic  neuron.  Altmann,  in  1881,  first 
\"  »inted  out  that  the  figures  of  nuclear  division  in  parts  of  the  central 
nervous  system  of  the  embryo  are  found  next  the  central  canal,  and 
that,  therefore,  the  pericentral  stratum  is  the  growing  layer.  These 
observations  have  since  been  confirmed  and  extended  by  Uskoff, 
82.1,  Rauber,  86.1,  Merk,  85.1,  and  W.  Vignal,  84.1,  208-210, 
who  appears  to  have  been  unacquainted  with  the  earlier  German 
observations.  In  his  last  paper,  87.1,  Merk  points  out  that  there 
is  much  greater  variety  in  the  distribution  of  karyokinetic  figures 
in  the  medullary  canal  than  appeared  from  previous  researches,  and 
that  each  region  has  its  characteristics.  Thus  in  the  retina  the 
growing  layer  is  external  *  or  next  the  mesoderm ;  in  the  corpus 
striatum  and  thalamus  opticus  the  proliferation  takes  place  through 
the  whole  thickness  of  the  wall,  etc.  Special  stress  is  laid  by  Merk 

*I  feel  much  doubt  as  to  Merk 's  accuracy  in  regard  to  this  point. 


612  THE    FCETUS. 

upon  the  difference  between  cell  multiplication,  which  does  not 
necessarily  mean  synchronous  increase  of  substance,  and  cell  growth, 
which  does  mean  increase  of  substance.  The  growth  of  the  nervous 
system  depends  chiefly  on  the  enlargement  of  the  cells,  as  Boll,  73. 1, 
and  Eichhorst,  75.1,  maintained  long  ago,  and  it  is  incorrect  to 
follow  the  custom  of  using  the  terms  proliferation  and  growth  as 
synonymous. 

The  typical  germinating  cells  (His,  89.1,  315)  are  round  or 
slightly  oval,  and  measure  from  10  to  14  /j.  in  diameter.  The  nuclei 
measure  from  4  to  8,u;  in  the  resting  stage  they  are  oval,  with  a  dis- 
tinct outline,  and  scattered  chromatine  granules;  but  most  of  the 
nuclei  in  young  embryos  are  in  some  stage  of  indirect  division  and 
therefore  have  no  distinct  outline,  while  their  chromatine  granules 
are  large,  conspicuous,  and  variously  grouped  according  to  the  stage 
of  karyokinesis,  Fig.  352.  The  protoplasm  forms  a  clear,  broad  cell- 
body,  and  with  higher  powers  can  be  seen  to  form  a  granular  endo- 
plasm  and  a  non-granular  ectoplasm.  The  cells  lie  between  the 
processes  of  the  neuroglia  cells,  and  lie  typically  as  in  Fig.  352,  in 
the  rounded  spaces  between  those  processes,  close  to  the  thin  mem- 
brana  limitans  interna,  which  is  described  in  the  next  section  on 
the  neuroglia.  The  number  of  the  germinating  cells  is  very  large 
in  the  human  embryo  at  four  weeks,  so  that  in  places  they  seem  to 
form  an  almost  continuous  layer.  Later  they  gradually  diminish  in 
number,  and  the  spaces  occupied  by  them  persist  empty  for  a  time. 
As  to  the  disappearance  of  the  cells  our  information  is  incomplete, 
but  it  is  probable  that  they  are  all  changed  into  neuroblasts.  That 
most  of  them  do  so  change  has  been  proved  by  His,  89. 1,  318;  see 
the  sections  on  the  origin  of  the  neuroglia,  below,  and  of  the  nerve 
fibres,  p.  616. 

Origin  of  the  Neuroglia. — The  following  account  refers  es- 
pecially to  the  human  embryo,  and  is  based  on  His'  observations. 
The  cells  of  the  medullary  tube  have  at  first  a  distinctly  epithelial 
character,  and  in  very  thin  sections  (^-J-^-^^mm.)  of  well-pre- 
served specimens  each  cell  can  be  seen  to  extend  radially  through 
the  entire  thickness  of  the  wall.  So  long  as  the  epithelial  character 
is  preserved,  there  is  an  outer  and  an  inner  zone  without  nuclei 
with  a  middle  layer  containing  all  the  nuclei,  which  increase  in 
number  as  the  development  progresses.  There  next  appear  cells  in 
the  inner  non-nucleate  layer ;  these  are  the  germinating-cells ;  they 
differ  from  the  other  cells  of  the  cord,  and  according  to  His,  89. 1, 
321,  give  rise  to  the  young  nerve-cells.  All  the  remaining  cells,  the 
nuclei  of  which  remain  in  the  middle  zone,  give  rise  to  the  neuroglia, 
and  are  accordingly  named  spongioblasts  by  His.  The  change  of 
the  epithelial  cells  into  spongioblasts  can  be  particularly  well  studied 
in  elasmobranch  embryos  (e.  g.,  Pristiurus  of  44  mm.).  The  elon- 
gated cells  acquire  a  vacuolated  appearance;  the  cell  boundaries 
become  indistinct ;  the  substance  of  the  cell-body  takes  on  more  and 
more  of  a  trabecular  character,  and  there  results  a  network  of  meta- 
morphosed cell  material  instead  of  a  layer  of  discrete  epithelial  cells 
(His,  89. 1 ,  350) .  While  the  spongioblast  network  (myelo-spongium, 
neurospongium,  neuroglia)  is  developing,  the  protoplasm  alters  into  a 
substance  which  is  more  homogeneous,  more  highly  refractile,  and 


THE   NERVOUS   SYSTEM. 


U13 


more  readily  stained  than  protoplasm.  In  other  vertebrates  the  con- 
version of  the  epithelial  cell  into  a  spongioblast  takes  place  in  a  similar 
manner,  as  has  been  <le*nonstrated  by  His'  observations  on  mammals, 
hirds,  amphibians,  and  tishes.  Each  spongioblast  has  (His,  89. 1, 
:;•.'!)  two  main  processes,  an  outer  and  an  inner,  and  several  smaller 
lateral  processes.  The  inner  processes  run  to  the  inner  boundary, 
where  their  ends  unite  to  form  the  iitcnihraiid  liniituiix  infcrnti:  the 
character  of  these  processes  calls  for  further  study,  because,  though 
they  usually  run  without  dividing,  yet  in  certain  cases  they  have 
been  found  giving  off  branches ;  the  ends  of  the  fibres  break  up  into 
fine  branches  which  unite  to  make  a  close  network,  and  this  network 
is  the  membrana  limitans.  The  outer  processes  always  branch,  their 
branches  being  most  developed  in  the  outer  non-nucleated  layer,  Fig. 


FIG.  :r>-,'.  N.  nrotflia  of  the  Dorsal  Zone  of  the  Spinal  Cord  of  a  Human  Embryo  of  about 
three  and  one-half  Weeks.  N,  Young  neuroblast;  l.in,  limitans  interna;  Rsch,  Randschleier. 
Alt.-,-  \V.  His.  X  SBOdiums. 

352,  Rsch  (His'  Randschleier) ;  the  branches  form  a  network,  which 
appears  most  distinctly  in  the  outer  layer.  The  nuclei  are  oval,  and, 
as  just  remarked,  lie  at  various  levels;  around  each  nucleus  is  an 
accumulation  of  protoplasm,  which  may  for  convenience  be  dis- 
tinguished as  the  cell-body;  the  cell-bodies  also  give  off  processes, 
which  anastomose  with  one  another.  The  cells  become  still  more 
elongated  as  the  embryo  advances,  and  tend  to  gather  more  or  less 
in  little  groups,  as  may  be  seen  in  human  embryos  of  the  sixth  and 
seventh  week. 

It  is  to  be  noted  that  neither  the  ventral  plate  (His'  Bodenplatte) 
nor  the  dorsal  plate  (His'  Deckplatte)  undergo  the  same  histological 
differentiation  as  the  lateral  zones  of  His.  Neither  plate  develops 
any  young  nerve- cells  (neuroblasts) ;  the  ventral  plate  changes  entirely 
into  neuroglia,  into  which  the  nerve  fibres  penetrate  secondarily  to 
make  the  anterior  commissure.  The  dorsal  plate  retains  its  primitive, 
simple,  epithelial  character  wherever  there  is  an  ependyma,  but  else- 
where its  cells  also  become  spongioblasts. 

The  history  of  the  neuroglia  shows  that  it  is  in  no  wise  related  to 


614 


THE    FCETUS. 


the  mesenchyma  or  true  connective  tissue.  This  relationship  was 
for  a  long  time  and  generally  assumed.  Golgi  was  the  first  to  dis- 
cover the  ectodermal  character  of  the  neuroglia  ("  Studi  s.  f.  anat.  d. 
organi  centr.  syst.  Nerv. , "  p.  178) .  Without  reference  to  Golgi's  dis- 
covery, Gierke,  85.1,  498,  upon  a  somewhat  imperfect  basis  of 
observation  positively  asserted  the  exclusively  ectodermal  origin, 
and  the  question  was  definitely  settled  by  W.  His.  Since  then  the 
neuroglia  in  the  embryo  has  been  asserted  by  Lachi,  90.1,  to  be 
partly  at  least,  immigrant  connective  tissue,  but  that  Lachi 's  view 
is  erroneous  has  been  more  than  sufficiently  demonstrated  by  Cajal, 
90.1,  and  M.  voii  Lenhossek,  91.2. 

Specialization  of  the  Neuroglia. — We  know,  chiefly  through 
Gierke's  researches,  85.1,  that  the  neuroglia  assumes  various  char- 
acteristic modifications  in  the  different  regions  of  the  adult  central 
nervous  system.  Gierke,  I.e.,  496-505,  gives  some  observations  on  the 
differentiation  of  the  neuroglia  in  the  embryo,  but  I  have  been  able 
to  find  little  in  these  pages  sufficiently  definite  for  use.  Gierke  held 
that  the  matrix  of  the  neuroglia  was  a  modification  of  the  peripheral 
parts  of  the  embryonic  cells,  an  opinion  which  I  deem  erroneous. 

All  the  spongioblasts  in  the  embryo  stretch  through  the  entire 
thickness  of  the  medullary  wall  and  have  a  correspondingly  elongated 

form.  When  treated  by  Golgi's 
chromic-osmium  method  a  portion  of 
the  spongioblasts  are  found  in  two  to 
four  days  to  be  colored,  and  may  bo 
easily  followed,  as  in  the  same  length 
of  time  the  nerve-cells  are  not  colored, 
though  the  blood-vessels  are.  The 
method  has  been  applied  by  Golgi 
himself,  90.3,  and  by  Ramon  y  Ca- 
jal, 90.1,  to  the  chick  embryo,  by 
M.  von  Lenhossek,  91.2,  to  the  hu- 
man embryo,  and  by  Nansen*  to 
Myxine.  From  these  investigations 
we  have  learned  that  the  spongio- 
blasts become  very  much  elongated 
and  remain  very  slender ;  where  the 
nucleus  is  situated,  the  cell  is  thick- 
ened. At  first  the  nuclei  are  confined 
to  the  gray  matter,  but  as  develop- 
ment progresses  the  nuclei  appear  to 
migrate,  so  that  gradually  their  num- 
ber through  the  gray  matter  dimin- 
ishes, while  they  accumulate  in  a 
closer  layer,  Fig.  354,  around  the 
central  canal  and  in  the  outer  neuro- 
glia layer  (Randschleier) .  Many  of 
the  cells  now  appear  to  lose  their  central  ends,  Fig.  353,  so  that  only 
the  prolongation  of  the  nucleated  body  toward  the  outer  surface  of  the 
medulla  is  preserved.  Later  the  distal  prolongation  is  also  lost,  and 
the  secondary  branches,  which  have  been  meanwhile  developed,  con- 

*  Nansen's  paper  was  published  by  the  Museum  at  Bergen  in  1886.     I  have  not  seen  it. 


FIG.  353.— Cross  Section  of  the  Spinal 
Cord  of  a  Human  Embryo  of  14  mm.  treated 
by  Golgi's  Chromium-Osmic  method,  to 
show  the  Neuroglia  Cells.  The  gray  mat- 
ter is  indicated  by  an  outline.  After  von 
Lenhossek. 


THK  NERVOUS  SYSTKM. 


615 


vert  the  elongated  <•« -11  into  the  so-called  ])eiters'  or  spider  cell.  Fig.  364. 
Lenhossek,  91.2,  has  traced  the  changes  both  in  the  gray  and  white 
mat ter,  and  found  typical  modifications  in  each  part  of  each  layer. 
\V.  Vignal,  88. 1,  JJ-Ju,  though  he  failed  to  recognize  the  neuroglia 
until  advanced  stag 
reports  some  observa- 
tions on  the  later  differ- 
entiation of  the  neuro- 
glia in  the  eerehrillll  Of 
the  human  ftetns;  at 
seven  months  the  cell- 
have  transparent  bodif- 
with  numerous  granules 
which  can  be  seen  when 
the  cells  are  examined 
in  water,  but  not  when 
they  are  mounted  in 
glycerin  ;  they  have  mi-  • 
merons  processes  and  a 
round  or  oval  nucleo- 
lated  nucleus.  At  eight  ' 
months  the  cerebral 
neuroglia  cells  vary  in 
>i/e,  but  some  are  much 
enlarged  and  their  pro- 
068868  show  trao-sof  the 
change  into  a  homoge- 
neous refringent  sub- 
stanc.  .  Bee  Vignal,  I.e., 
PI.  XL,  Fig  2,  a. 

While  some  of  the 
spongioblasts  have  their 
main  nucleated  bodies 
retained  in  the  gray  or  white  matter,  others  have  their  bodies  placed 
close  around  the  central  canal,  Fig.  354,  where  they  form  the  so- 
called  epithelium  of  the  central  canal,  or  ejH'mlf/iiia,  as  it  maybe 
better  called.  These  ependymal  cells  stretch  out  through  the  entire 
diameter  of  the  medullary  wall,  there  being  a  fine  radial  process, 
Fig.  :!.VJ,  which  passes  outward  through  the  gray  and  white  matter, 
rafl  tii-t  recorded  by  Golgi,  and  later  by  Gierke,  85.1,  499,  and 
has  since  been  more  fully  described  by  Cajal  and  Lenhossek.  The 
cilia  011  the  inner  ends  of  the  ependyma  cells  appear  in  the  human 
embryo  about  the  end  of  the  fifth  week  (His,  89.1,  330).  Eich- 
horst,  75.1,  records  that  in  a  three  months'  embryo  the  cilia  are 
present  on  some  of  the  cells  around  the  canal  but  not  on  others. 
Lowe,  83.1,  observed  that  the  ependyma  cells  resemble  spongio- 
blasts, but  failed  to  recognize  their  identity. 

One  sees  readily  in  embryos  of  mammals,  when  about  10  mm. 
long,  a  broad  layer  of  nuclei  close  to  the  cavity  of  the  medullary 
tube ;  later,  where  the  canal  obliterates  no  trace  of  this  layer  is  pre- 
served, but  where  the  lumen  of  the  canal  is  permanent  there  persists 
a  narrow  layer  of  crowded  ependymal  nuclei.  This  is  because  many 


Fio.  854.—  Part  of  a  Tran>\  •  -i>f  section  of  the  Spinal  Cord 
of  a  Human  Embryo  of  28  cm.  The  specimen  was  treated 
by  Golgi's  method  (chromium-osmic),  and  shows  the  differ- 
entiation of  the  ependyma  and  the  Deiters*  cells. 


616  THE   FCETUS. 

of  the  cells  have  changed  into  true  neuroglia  cells,  and  the  broad  layer 
has  been  in  part  annexed  to  the  gray  matter  or  neuroblast  layer. 

Layers  of  the  Medullary  Wall. — By  the  time  the  neuroblasts 
are  differentiated  we  can  distinguish  three  primary  layers  which 
persist  throughout  life  with  sundry  secondary  modifications.  There 
is  an  outer  layer  of  neuroglia  network,  Fig.  352,  which  is  the 
anlage  (or  homologous  with  the  anlage)  of  the  white  matter  of  the 
spinal  cord ;  it  has  been  named  the  Randschleier  by  His,  and  this 
name  I  have  adopted,  although  it  might  be  better  named  the  outer 
neuroglia  layer.  There  is  a  middle  layer,  in  which  all  the  neuroblasts 
are  situated,  and  which  is  the  anlage  of  the  gray  matter  of  the  cen- 
tral nervous  system  throughout ;  it  has  been  named  the  mantle  layer. 
Finally  there  is  an  innermost  layer,  in  which  at  first  germinating 
cells  are  situated,  but  which,  after  their  emigration,  consists  merely 
of  spongioblasts ;  this  is  the  Innenschicht  of  His,  and  may  be  defined 
as  the  ependymal  layer;  during  development  it  is  reduced  by  the 
encroachment  of  the  mantle  or  neuroblastic  layer. 

We  distinguish  then : 

1.  Randschleier,  or  outer  neuroglia  layer  (white  matter). 

2.  Mantle  layer ,  or  middle  layer  in  which  all  the  neuroblasts  are 

situated  (gray  matter) . 

3.  Inner  layer ,  or  inner  neuroglia  layer  (ependyma). 

Origin  of  Nerve  Fibres. — We  know  through  the  researches  of 
Wm.  His,  86.2,  88. 1,  88.3,  that  there  are  two  sets  of  nerve-fibres 
developed  in  the  vertebrate  embryo — one  set  from  the  medulla,  and 
another  from  the  ganglia.  Each  medullary  fibre  arises  as  an  out- 
growth from  one  pole  of  a  nerve-cell,  situated  in  the  wall  of  the 
medullary  tube ;  each  ganglionic  fibre  arises,  on  the  contrary,  by  the 
outgrowth  of  two  opposed  poles  of  a  nerve- cell.  The  cell  of  the 
medullary  fibre  is  terminal,  while  that  of  the  ganglionic  fibre  is 
interpolated  in  the  course  of  the  fibre.  There  is,  in  fact,  a  profound 
morphological  difference  between  the  two  classes  of  nerve-fibres,  and 
it  is  necessary  to  consider  their  development  separately. 

MEDULLARY  FIBRES. — In  the  section  upon  the  neuroglia,  it  was 
pointed  out  that,  when  the  medullary  tube  closes,  the  cells  which 
form  its  walls  are  all  similar  to  one  another.  About  the  end  of  the 
third  week  in  human  embryos  the  cells  lose  their  uniform  character 
and  become  differentiated  into  the  neuroglia  cells,  which  form  a 
network,  and  the  nerve-cells,  which  lie  scattered  about  and  produce 
nerve-fibres,  while  the  neuroglia  is  developing.  While  these  changes 
are  going  on,  the  medullary  tube  grows  rapidly  and  in  the  nucleated 
layer  of  its  walls  two  primary  layers  become  distinguishable :  these 
are  the  so-called  inner  layer  and  the  outer  or  mantle  layer,  Fig.  377. 
In  the  latter  are  situated  all  the  cells  which  give  rise  to  nerve-fibres, 
but  later,  that  is,  after  the  blood-vessels  have  penetrated  the  medulla, 
nerve-cells  encroach  more  and  more  upon  the  inner  layer  also  (His, 
86.2,  509).  It  is  to  be  noted  that  the  more  superficial  position  of 
the  nerve-cells,  which  is  permanently  maintained  in  the  cerebellum 
and  in  the  cerebral  hemispheres,  is  originally  characteristic  of  the 
entire  neuron.  The  deep  position  which  the  cells  have  in  certain 
parts  in  the  adult — as,  for  example,  in  the  spinal  cord — is  produced 
secondarily  by  the  growth  of  nerve-fibres  in  the  Randschleier.  This 


THE    NERVOUS   SYSTEM. 


617 


change  is  very  early  indicated  in  the  spinal  cord  by  the  growth  of 
the  Randschleier.  In  the  inner  layer,  the  cells  and  their  oval  nuclei 
arc  crowded,  and  it  is  here  only  that  all  division  goes  on;  the 
peculiar  posit  ion  of  the  karyokinetic  figures  has  been  described,  p.  611. 

The  nerve-cells,  according  to  W.  His,  89.1,  31S-:>\|t;,  arc  all  de- 
scended from  the  germinating  cells  described  above,  p.  612,  and 
migrate  t'rum  the  inner  layer  into  the  mantle  layer.  That  the  nerve- 
cells  arise  near  the  central  canal  and  migrate  into  the  mantle  layer, 
was  discovered  in  1884  by  Herms,  84.1,  in  his  studies  upon  the 
t'acialis  neuroblasts  of  the  lamprey. 

The  metamorphosis  begins  by  the  protoplasm  of  the  germinal  cell 
accumulating  on  the  side  of  the  nucleus  away  from  the  cavity  of  the 
medulla  and  there  elongating  itself  into  a  point,  which  in  its  turn 
soon  elongates  into  the  beginning  of  the  nerve-fibre ;  the  fibre,  there- 
fore, always  points  away  from  the  cavity,  Fig.  355,  N.  The  elonga- 
tion of  the  fibre  continues  apparently  at  the 
expense  of  the  protoplasm  already  accu- 
mulated in  the  cell;  the  fibre  accordingly 
grows  very  rapidly  at  first,  and  soon  passes 
beyond  the  medulla,  but  later  the  elonga- 
tion is  much  slower,  for  it  then  depends 
upon  the  actual  growth  of  the  fibre  itself. 
When  the  fibre  begins  to  develop,  the  cells 
begin  to  migrate  toward  the  outer  part  of 
the  medulla  to  form  the  mantle  layer  and 
are  found  a  short  distance  from  the  mem- 
brana  limitans  interna.  The  reason  that 
the  young  nerve-cells  migrate  only  to  a 
certain  point  is  found  apparently  in  the 
structure  of  the  outer  non-nucleated  zone, 
as  pointed  out  by  His,  89.1,  336.  The 
neuroglia  network,  as  can  be  seen  in  Fig.  352,  is  so  dense  in  this 
zone  that  it  blocks  the  way  for  all,  or  nearly  all,  the  neuroblasts.  In 
later  stages  the  meshes  become  larger  again,  and  the  blood-vessels 
are  able  to  penetrate  it  to  enter  the  neuron.  The  nuclei  of  the  mi- 
grating cells  are  oval  and,  for  the  most  part,  have  a  single  nucleolus ; 
the  protoplasm  is  principally  accumulated  in  a  pyramidal  mass  at 
the  distal  end  of  the  nucleus,  the  apex  of  the  pyramid  being  prolonged 
as  the  axis-cylinder  process;  the  protoplasm  forms  only  an  exceed- 
ingly thin  layer  around  the  sides  and  proximal  end  of  the  nucleus. 
At  this  stage  of  the  cells,  the  protoplasm  stains  deeply,  and  in 
stained  sections  the  distal  ends  of  the  nuclei  are  often  obscured  or 
even  hidden ;  when  this  is  not  the  case,  the  distal  ends  of  the  nuclei 
are  pointed — a  peculiarity  which  becomes  more  marked  in  a  slightly 
more  advanced  stage.  The  cells  continue  their  migration  and  de- 
velopment until  they  reach  the  mantle  layer  as  fully  differentiated 
young  nerve-cells,  which  are  characterized  by  having  an  oval  non- 
nucleolated  nucleus,  with  only  a  very  thin  envelope  of  protoplasm, 
the  rest  of  the  protoplasm  having  been  converted  into  the  nerve-fibre. 

According  to  W.  His,  88.1,  370,  89.1,  316,  the  mantle  layer 
consists  in  the  human  embryo  at  four  weeks  almost  entirely  of  young 
nerve-cells,  and  contains  only  very  few  neuroglia  cells.  The  nerve- 


Fio.  856.— From  a  Section  of  the 
MedullaOblonffataof  His'  Embryo 
Br3.  ngl,  Neuroj?lia  cells;  (/,  ger- 
minating cell :  A',  neuroblast.  After 
W.  His.  x8HOdiams. 


618 


THE    FCETUS. 


cells  he  names  neuroblasts;  they  present  the  following  character- 
istics: they  have  an  oval  nucleus  (9-1 1^  long  and  -1.5-5.5//  wide)  at 
the  distal  end  of  which  is  a  small  cone  of  protoplasm,  which  is  con- 
tinued as  the  nerve-fibre ;  the  nucleus  contains  considerable  chroma- 
tin  in  the  form  of  scattered  granules  connected  by  delicate  threads; 
the  envelope  of  protoplasm  is  exceedingly  thin,  so  that  when  the 
nuclei  are  cut  transversely  or  obliquely  they  seem  almost  without 
protoplasm,  and  represent  the  so-called  naked  nuclei  of  the  mantle 
layer ;  the  nerve-fibre  is  of  nearly  uniform  diameter,  and  presents, 
as  does  also  the  protoplasmatic  cone  from  which  it  springs,  alongi- 
tudinally  fibrillated  appearance.  The  neuroblasts  often  lie  in  groups ; 
in  such  cases  the  fibres  from  one  group  unite  in  a  long  cone  or  bun- 
dle, and  continue  their  growth  in  association,  Fig.  350.  The  young 
cells  have  no  other  outgrowths,  the  branching  processes,  which  are  so 
characteristic  of  the  adult  nerve-cell,  not  developing  until  much  later. 
The  paths  taken  by  the  rapidly  lengthening  medullary  nerve-fibres 
have  next  to  be  considered.  The  fibres  may  be  divided  into  two 
classes,  according  as  they  make  an  immediate  exit  from  the  neuron  or 
first  grow  within  it.  The  latter  class  include,  first,  fibres  which  cross 

through  the  Bodenplatte  to  the 
opposite  side;  they  constitute 
the  formatio  arcuata,  Fig.  377; 
second,  fibres  which  take  a  lon- 
gitudinal course  in  the  Rand- 
schleier ;  it  may  be  noted  that, 
according  to  M.  von  Leiihos- 
sek,  91.3,  123,  the  nerve-cells, 
which  give  off  fibres  to  run  lon- 
gitudinally, can  be  first  seen  in 
the  chick  the  sixth  day,  which 
is  later  than  the  other  cells; 
their  fibres  are,  moreover,  char- 
acterized by  their  branching 
(Collateralen  of  Kolliker) ; 
third,  fibres  which  join  the 
ganglionic  or  dorsal  root.  M. 
von  Lenhossek,  91.3,  has  ob- 
medullary  fibres  which  joined  the 


FIG.  356.— Group  of  Motor  Neuroblasts  and  Nerve- 
Fibres  from  a  Transverse  Section  of  the  Spinal 
Cord  of  a  Cat  Embryo  of  6  mm.  (N  L).  After  W. 
His. 


chick 


served   in   a  four-days' 
ganglionic  root. 

The  neuroblasts  of  His  have  now  been  found,  described,  and  fig- 
ured in  every  class  of  vertebrates  except  the  dipnoans.  They  have 
everywhere  the  same  essential  character,  though  presenting  minor 
variations.  They  are  unusually  small  in  Petromyzon ;  in  amphibians 
unusually  large ;  in  the  frog  they  are  pigmented ;  in  the  trout  they 
are  particularly  numerous  and  distinct ;  the  trout  is  further  remark- 
able for  having  a  few  unusually  large  neuroblasts  on  the  dorsal  side 
of  the  embryonic  spinal  cord.  For  further  details  see  His,  89.1, 
331-356. 

Elasmobranchs  offer  the  peculiarity  that  the  motor  nerve-roots 
become  invaded  by  mesenchymal  cells  very  soon  after  the  fibres  grow 
out  of  the  medulla,  hence  the  roots  contain  nuclei  at  a  very  early 
stage.  The  nuclei  were  first  observed  by  Balfour,  78.3,  76,  who 


THK    M.KVOUS   SYSTEM. 


619 


drew  the  erroneous  conclusion  that  the  roots  arose  as  cords  of  cells, 
and  that  tin-  nerve-libres  were  devel.  >ped  later.  This  error  has  been 
kept  up  by  Van  \Vijhe  and  J.  Beard,  88.3,  1  '.»•>,  hut  discussion  of 
it  is  pas-ed,  compare  His,  89.1  :U4,  Kastschenko,  88.1,  -l  ''..">,  and 
Dohrn,  91.8,  who  have  proven  that  in  the  cartilaginous  fishes  the 
motor  nerve-fibres  gprow  o^  from  the  medulla  as  in  all  other  verte- 
brates. ])olmi  has  further  maintained,  88.1,  that  in  elasmobranchs 
medullary  cells  migrate  from  the  medulla  with  tin-  nerve-fibres. 
\\i>,  89.1,  questioned  the  correctness  of  this  opinion,  but  Dohrn, 
91.1,  lias  renewed  his  assertion  and  offers  additional  evidence.  He 
asc«  Ttained  nothing  as  to  the  ultimate  fate  of  the  emigrant  cells. 

GAN<;LI<»NIC  FIBRES.  —  The  exact  history  of  the  earliest  changes 
in  the  cells  of  the  ganglia,  p.  601,  has  still  to  be  worked  out.  His, 
however,  has  shown  that  in 
the  human  cml>ryo  they  all 
become  bipolar,  that  is  to 
.  much  elongated;  one 
end  pointing  toward  the 
dorsal  side  of  the  ganglion 
and  lengthening  out  as  a 
nerve  -fibre,  which  pene- 
trates the  myelon,  the  other 
end  pointing  toward  the 
ventral  side  of  the  ganglion 
and  lengthening  out  as  a 
peripheral  nerve-fibre.  Fig. 
:'.M  represents  a  group  of 
bipolar  cells  from  a  spinal 
ganglion  of  a  young  human 
embryo.  The  cells  are  gath- 
ered jn  groups  and  the  fibres 
from  one  group  unite  in 
primary  bundles.  When  the 
cell  is  turned  so  as  to  be 
viewed  in  profile,  it  is  seen 
that  the  oval  nucleus  occu- 
pies an  eccentric  position 
and  is  surrounded  by  a  mass 
of  protoplasm,  which  gives 
off  the  nerve-fibre  in  two 
opposed  directions,  so  that 
one  might  almost  say  that 

there  is  a  nerve-fibre  with  a       FIG.  357.—  Bipolar  Cells  from 

cell   appended  to  its  side.   g£Z°  (Hi8'  embr5">  NX  A 
In  an  embryo  of  six  weeks, 

the  cells  are  still  of  this  type,  and  resemble  the  bipolar  cells  described 
by  Freud  in  the  ganglia  of  Petromyzon.  In  an  embryo  of  seven  weeks 
the  mesenchymal  cells  had  begun  to  grow  into  the  ganglion  between 
the  ectodermal  cells,  which  thereafter  begin  to  change  into  pear- 
shaped  appendages  of  the  fibres,  with  the  result  of  developing  the 
T  -joints  of  Ranvier.  On  the  development  of  the  cells  proper  see 
p.  626. 


Spinal  Ganglion  of  an 

-H'8 


620 


THE    FCETUS. 


Gl 


The  dorsal  processes  of  the  cells  enter  the  myelon  as  sensory  roots. 
The  number  of  entering  fibres  is  at  first  small,  but  gradually  in- 
creases. Within  the  myelon  the  fibres  at  first  all  take  a  longitudinal 
course  in  the  outer  layer  (Randschleier  of  His),  some  of  the  fibres 
passing  headward,  others  tailward,  but  later  fibres  course  within  the 
myelon  directly  toward  the  nerve-cells  of  the  mantle  layer.  Accord- 
ing to  Ramon  y  Cajal,  90.1,  02,  the  ganglionic  fibres  penetrate  the 
medulla  and  there  fork ;  each  branch  curves  around  and  becomes  a 
longitudinal  fibre,  but  the  two  branches  run  in  opposite  directions 
as  fibres  in  the  Randschleier ;  these  fibres  give  off  fine  branches  nearly 
at  right  angles,  which  penetrate  the  gray  matter  and  there  ramify, 
but  without  forming  a  true  network,  compare  Fig.  358 ;  the  branches 
running  to  the  gray  matter  our  author  names  "  collaterals ;"  their 
ramifications,  at  least  at  first,  are  confined  to  the  gray  matter  of  the 
dorsal  zone  of  His.  Ramon  y  Cajal's  important  discovery  has  been 
confirmed  by  Kolliker,  who  has  also  made  important  additions  to 

our  knowledge  of  the  dis- 
tribution of  the  sensory 
fibres  within  the  cord. 

The  distal  or  ventral 
processes  extend  in  one 
group  from  each  ganglion 
as  the  sensory  root. 

The  formation  of  the 
nerve  roots  may  be  su- 
perbly demonstrated,  as 
discovered  by  Ramon  y 
Cajal,  90. 1,  by  the  appli- 
cation of  Golgi's  bichro- 
mate -  silver  method  to 
embryos  (chicks  of,  four 
to  sixteen  days,  and 
mammals  of  correspond- 
ing stages),  see  Fig.  358. 
Such  preparations  de- 
monstrate further  the 
early  development  of  the 
dendrites  of  the  medullary  nerve-cells,  and  the  abundant  intra-mye- 
lic  ramifications  of  the  "  collaterals"  of  the  ganglionic  fibres. 

Historical  Note. — The  first  suggestion  that  all  sensory  nerve- 
fibres  arise  from  the  ganglia  and,  grow  centrifugally  and  centripetally, 
was  made  by  W.  His  (His5  Archiv,  1881,  p.  477)  who  brought 
positive  proof  of  the  correctness  of  his  view  in  1886,  86.2,  490,  and 
later  showed  that  it  was  true  not  only  of  the  spinal,  but  also  of  the 
cephalic,  ganglia,  88.1,  374,  88.3,  368. 

MEDULLARY  SHEATHS.— All  the  nerve-fibres  are  at  first  simple 
processes  of  the  nerve  or  ganglion  cells,  and  they  persist  in  that  con- 
dition for  a  long  time,  but  finally  there  is  developed  around  those 
fibres,  which  are  destined  to  form  medullated  fibres  in  the  adult,  a 
covering  of  mesenchymal  cells.  No  trace  of  this  covering  can  be 
seen  in  mammals  until  after  the  nerves  have  grown  out  and  ramified 
through  the  entire  embryo.  We  have  at  this  stage,  as  well  as  later, 


FIG.  358.— Transverse  Section  of  the  Dorsal  Cord  and  Gan- 
glion of  a  Chick  of  nine  Days.  F>,  Anterior;  Z>r,  posterior 
root;  Gl,  ganglion  of  dorsal  root;  coZ,  collateral,  with  its 
branches;  JV,  medullary  neuroblasts  with  dendrites  and 
axis-cylinder.  After  Cajal. 


TIIK    NKKV01    -    M  -TIM. 


621 


C      C 


Jj 


no 


to  distinguish  between  the  bundles  of  fibres  or  nerves  proper,  and 
the  fibres  running  singly  or  terminal  branches  of  the  ner\ 

When  a  nerve  consists  of  several  or  more  fibres,  the  mesenchyma 
forms  an  envelope  around  it  (Vignal,  83.1,  518),  which  in  certain 
66  at  least,  and  perhaps  always,  is  very  distinct  and  sharply  de- 
fined (Kolliker,  "Gewebelehre,"  6te  Aufl.,  p.  152,  Fig.  11$).  There 
next  follows  the  penetration  of  the  nerve  by  the  mesenchymal  cells, 
which  make-  their  way  in  between  the  fibres.  In  the  case  of  very 
small  nerves  and  of  single  fibres,  the  cells  of  the  connective  tissue 
have  direct  access  to  the  single  fibres. 

Whether  the  cells  reach  the  fibres  directly  or  not,  has  no  influence 
on  their  further  differentiation.  They  lay  themselves  against  the 
nerve-fibre,  from  place  to  place,  and  grow 
around  it  so  intimately  that  it  becomes  diffi- 
cult to  distinguish  the  boundary  between  the 
original  fibre  and  its  accessory  envelope,  and 
one  is  inclined,  at  first  sight,  to  conclude  that 
the  fibre  has  merely  become  thicker  and  nu- 
cleated.* In  reality,  the  mesenchymal  cells 
dose  around  the  fibre,  which  they  cover  like  a 
chain  of  elongated  beads.  Each  cell  is  the 
anlage  of  a  medullary  segment ;  the  junction 
of  two  adjacent  cells  is  the  anlage  of  a  node 
of  Ranvier ;  the  nucleus  becomes  the  internodal 
nucleus  of  Schwann's  sheath.  Each  cell  is  at 
first  short  and  protoplasmatic.  The  cells  mul- 
tiply; Kolliker,  85.2,  has  observed  them  di- 
viding in  amphibians;  W.  Vignal  maintains 
that  new  cells  are  interpolated  in  mammalian 
embryos  between  those  already  enveloping  a 
fibre.  It  seems  possible  that  the  cells  may  in- 
crease in  number  by  both  means.  They  also 
grow  quite  rapidly  in  both  length  and  diameter. 

The  differentiation  of  the  cells  into  the  three 
sheaths  of  an  adult  fibre  depends  upon  their 
forming  each  a  membrane  and  an  internal  de- 
posit of  myeline.  The  nucleus  takes  and  keeps 
its  position  near  the  centre  of  the  cell  and  re- 
tains a  small  quantity  of  granular  protoplasm 
permanently  alxmt  itself.  In  regard  to  the 
formation  of  the  membrane,  I  know  of  no  sat- 
•tory  observations,  but  I  think  it  probable 
that  a  membrane  is  formed  over  the  entire 
surface  of  the  cell,  and  that  it  is  this  mem- 
brane  on  the  outside  of  the  cell  which  is  known 

as  the  sheath  of  Schwann,  and  on  the  inside  lary  sheath,  no,  Node  of  Ran - 
of  the  cell  next  the  axis-cylinder  may  be  %£j*yfi^4tfS& 
termed  the  periaxial  sheath.  This  supposi- 
tion needs  to  be  verified  by  observation.  The  medulla  or  myeline 
appears  quite  late — in  the  cow  toward  the  fourth  month,  in  the 
sheep  at  seventy  days,  according  to  W.  Vignal's  observations, 

*See Kolliker,  Zeit.  wiss.  Zool. ,  XLIII. ,  Taf7T~ 


no 


FIG.  359.  —  Isolated  Nerve 
Fibres  from  the  Sciatic  Nerve 
of  a  Sheep  Embryo  of  150mm. 


622  THE   FCETUS. 

83.1,  523,  on  the  fcciatic  nerve.  The  myeline  begins  to  appear  at 
the  same  time  in  many,  but  not  in  all  the  fibres  of  a  nerve,  and  it 
develops  later  in  the  peripheral  than  in  the  proximal  portion  of  a 
nerve,  and  can  be  earliest  observed  in  the  spinal  cord.  It  appears 
at  first  as  a  very  thin  layer  in  the  mesenchymal  cell  and  next  to  the 
axis-cylinder;  it  is  usually  deposited  simultaneously  throughout  the 
entire  length  of  the  cell,  but  sometimes  the  deposit  begins  at  the 
centre  of  the  cell ;  the  myeline  layer  is  usually  continuous  from  the 
start,  but  sometimes  it  constitutes  a  series  of  separate  masses,  which 
grow  and  unite  into  a  continuous  layer ;  at  this  stage  one  observes 
that  the  axis-cylinder  is  pressed  aside  by  the  nucleus  of  the  myeline 
cell.  The  deposit  of  myeline  gradually  increases,  and  forms  a  more 
regular  layer;  at  the  same  time  the  boundary  (Ranvier's  node)  be- 
tween adjacent  cells  becomes  more  distinct  and  the  cells  (internodal 
segments)  elongate. 

Historical  Note. — Our  knowledge  of  the  history  of  the  peripheral 
nerve-fibres  is  largely  based  on  the  study  of  the  tail  of  tadpoles,  see 
Kouget,  75.1,  W.  Vignal,  83.1,  83.2,  and  Kolliker  both  for  obser- 
vations and  references  to  the  literature.  The  development  of  the 
fibres  in  mammals  has  been  studied  by  Vignal,  /.c.,  and  by  Axel  Key 
and  Retzius  (Arch,  mikrosk  Anat.,  IX.,  308). 

Origin  and  Growth,  of  Nerves. — There  are  two  sets  of  nerves, 
corresponding  to  the  two  classes  of  nerve-fibres.  Every  nerve  con- 
sists of  a  bundle  of  nerve-fibres.  Each  ganglion  and  each  lateral 
half  of  a  neuromere  sends  out  a  bundle  of  nerve-fibres,  or  a  nerve,  as 
we  may  better  say.  There  are,  therefore,  typically  for  every  segment 
four  primary  nerves,  two  on  each  side,  a  dorsal  ganglionic  and  a 
ventral  medullary  nerve ;  usually  the  two  nerves  on  the  same  side  of 
a  segment  unite  at  a  short  distance  from  the  myelon  into  a  single 
trunk ;  in  this  case  the  ganglionic  nerve  becomes  the  dorsal  or  pos- 
terior root  of  anatomy,  and  the  medullary  nerve  the  ventral  or  ante- 
rior root  of  the  nerve  trunk.  Nearly  all  the  spinal  and  several  of 
the  cranial  nerves  conform  to  this  type.  In  certain  cranial  nerves, 
however,  we  have  only  ganglionic,  in  others  only  medullary  fibres. 
The  development  of  the  various  nerves  is  considered  later ;  that  of 
the  nerve-fibres  is  described  in  the  .preceding  section;  we  shall, 
therefore,  treat  here  only  the  general  principles  of  embryonic  nerve 
growth. 

As  to  the  mechanical  means  by  which  the  fibres  are  first  gathered 
into  bundles,  we  have  little  positive  information.  In  the  case  of  the 
medullary  fibres  the  paths  are  probably  prescribed,  as  suggested  by 
His,  by  the  structure  of  the  previously  developed  neuroglia.  In  the 
case  of  the  ganglionic  fibres  they  seem  to  be  brought  together  by  the 
pointed  shape  assumed  by  the  ganglion  as  a  whole. 

The  nerve-fibres,  as  they  grow  peripherally,  are  gathered  into 
short  stems  (nerve-trunks) .  Each  stem,  whether  motor  or  sensory, 
consists  (His,  88. 1,  375)  of  a  number  of  fine  fibres  without  nuclei; 
within  the  stem  the  fibres  run  all  in  the  same  general  direction,  but 
some  of  them  take  partly  crooked  courses.  Paterson,  91.1,  168, 
has  observed  that  the  nerve-fibres  increase  in  thickness  in  the  spinal 
nerves  of  mammals,  while  they  are  growing  to  their  destinations; 
the  fibres  in  these  nerves  take  characteristic  wavy  courses.  Meso- 


THK    NKRVors    SYSTEM. 


633 


Mastic  colls  penetrate  the  stem,  which  then  becomes  nucleated ;  in 
the  human  embryo  th«-  number  of  mesoblastic  nuclei  in  the  nerves 
remains  small  for  a  long  period,  during  which  the  nerves  appear 
light  and  conspicuous  in  stained  sections,  owing  to  their  poverty  in 
cells.  The  ends  of  the  nerves  are  at  first  broad  and  blunt,  and  it  is 
only  by  repeated  branching  that  the  nerves  acquire  finer  endings. 
The  ends  are  at  first  so  blunt  that  the  nerves  appear  as  if  chopped 
off,  Fi->.  86  and  :)«;n,  a  pe- 
culiarity which  formerly 
misled  many  observers  to 
conclude  that  they  had  not 
found  the  end  of  the  nerve 
at  all.  All  t he  nerves  take 
a  Mraight  course  at  first 
and  always  tend  to  grow  in 
a  Mraight  line  represent- 
ing the  prolongation  of 
the  direction  of  the  nerve- 
tibres.  This  law,  which 
iras  discovered  by  His,  ap- 
plic*;  to  all  nerves,  even  to 
those  which  take  a  compli- 
cated course  in  the  adult. 
This  is  well  illustrated  by 
the  early  stages  of  the 
nerves  to  the  eyes,  or  of 
the  vagus,  or  of  the  cervical 
nerves,  etc.  The  straight  course  of  a  nerve  is  modified  in  two  ways : 
by  encountering  an  obstacle,  or  by  a  change  in  the  relative  positions 
of  parts  with  which  the  nerve  has  become  connected.  When  a  nerve 
encounters  an  obstacle  it  is  either  deflected  from  its  course  or  forced 
to  divide.  The  most  important  obstacles  are  cartilages,  blood-vessels, 
and  cavities  lined  by  epithelium,  and  it  is,  therefore,  necessary  that 
these  tissues  be  differentiated  at  the  proper  points  in  the  embryo,  be- 
fore the  nerve  arrives,  or  else  the  necessary  mechanical  conditions  for 
effecting  the  normal  distribution  of  the  nerve  are  not  established. 
K"i  example,  the  third  branch  of  the  trigeminus  when  it  strikes 
Meckel's  cartilage  divides  into  the  ramus  lingualis  and  the  ramus 
mandibularis,  and  the  hypoglossal  nerve  when  it  strikes  the  wall  of 
the  jugular  vein  divides  into  its  descending  and  lingual  branches 
(His,  88. 1,  376).  After  a  nerve  is  deflected  it  grows  forward  in  the 
direction  of  the  fibres  at  the  growing  blunt  end  of  the  nerve.  Simi- 
larly when  a  nerve  is  divided  each  branch  tends  to  grow  straight 
forward  in  the  direction  of  the  fibres  at  the  end  of  the  branch.  After 
a  nerve  has  entered  a  given  part  of  an  embryo  it  retains  a  permanent 
connection  with  that  part,  and  it  is  largely  owing  to  the  secondary 
migration  of  organs  that  the  distribution  of  the  nerves  becomes  so 
complicated  in  the  adult.  It  is  evident  that  the  migration  of  the 
organs  must  take  place  after  the  nerves  have  reached  them.  Perhaps 
the  most  striking  illustration  of  the  translation  of  an  organ  with  its 
nerve  is  afforded  by  the  descent  of  the  testis — compare  also  the  re- 
current laryngeal,  Fig.  3GO,  R. 


K   IS 


Fio.  860.— Part  of  the  Nerves  of  a  Human  Embryo  of 
13.8  mm.  V-XII,  cephalic  nerves  according  to  the  usual 
enumeration;  1-8,  cervical  nerves ;  R,  recurrent  laryngeal. 
Affr  W.  His. 


(524 


THE   FCETUS. 


As  the  nerves  all  grow  forth  in  planes  at  nearly  right  angles  to  the 
axis  of  the  neuron,  it  follows  that  the  direction  taken  by  each  nerve 
depends  largely  upon  the  cerebral  flexures  and  the  curvature  of  the 
spinal  cord.  This  is  admirably  illustrated  in  the  human  embr}'o, 
Fig.  360.  The  figure  also  shows  that  certain  of  the  nerves,  as  is 
more  fully  explained  in  the  section  on  the  spinal  nerves,  are  brought 
into  contact  with  one  another  and  unite,  forming  the  plexuses. 

What  has  been  said  suffices  to  indicate  some  of  the  simple  and 
almost  self-evident  mechanical  conditions  of  nerve  development. 

Hensen  has  suggested,  76.1,  that  the  nerve  fibres  have  from  the 
start  their  permanent  connections,  and  that  as  the  cells  divide  and 
move  apart,  the  nerve-fibres  divide  and  lengthen  out,  and  he  has 
referred  to  the  filaments  seen  in  the  mesoderm  of  young  embryos  as 
being  such  nerve-fibres.  This  suggestion  cannot  be  adopted,  since 
the  outgrowth  of  the  nerve-fibres  has  been  observed ;  moreover  Alt- 
mann,  85.1,  has  pointed  out  that  the  fibres  seen  in  the  embryonic 
mesoderm  are  really  processes  of  the  mesodermic  cells,  and,  as  shown 
in  the  excellent  Fig.  2  of  his  plate,  are  quite  distinct  both  from  the 
ectoderm  and  entoderm;  Kolliker  also,  85.2,  remarks  that  in  the 
tail  of  the  tadpole  the  number  of  nerve-fibres,  and  of  the  branches 
and  anastomoses  thereof,  increases  with  the  age  of  the  animal,  they 
being  at  first  very  few  in  number,  so  few  that  it  is  evident  that  the 
innervation  of  most  parts  must  be  developed  later,  there  not  being  at 
first  branches  enough  to  supply  all  the  terminal  organs,  which  are 
ultimately  furnished  with  nerves. 

Union  of  Nerves  and  Muscles. — Trinchese,  86.1,  gives  a 
few  details  as  to  the  changes  in  the  muscle-fibres  which  precede  and 
coincide  with  the  union  of  the  nerve-fibre  with  the  muscle-fibre,  but 
as  he  gives  no  figures,  I  am  unable  to  follow  his  description. 

Further  Development  of  Nerve-Cells. — The  early  history 
of  the  nerve-cells  has  already  been  given,  and  the  final  differentia- 
tion of  the  nerve-fibres  traced.  We 
have  now  to  consider  the  histogenetic 
changes  in  the  main  cell  bodies,  and 
their  nuclei,  first  in  the  medullary, 
second  in  the  ganglionic  nerve-cells. 
1.  MEDULLARY  NERVE-CELLS. — 
We  possess  little  satisfactory  infor- 
mation concerning  the  phases  of  the 
young  nerve-cells.  The  protoplasm 
of  the  neuroblast  of  His  is  apparently 
utilized  to  make  the  nerve-fibre,  so 
that  very  little  is  left  around  the  nu- 
cleus ;  hence,  in  sections  and  in  cells 
isolated  by  maceration  the  nucleus 
appears  almost  naked.  The  nerve- 
cell  nucleus  early  becomes  recogniz- 
able by  its  distinct  nucleolus,  Fig. 

vessels,    x  about  1,000  diams.  361,  uu.     The  next  change  consists 

in  a  growth  of  the  nucleus  and  of  the 

court  of  protoplasm  around  it.     The  outline  of  the  cell  now  becomes 
irregular  and  the  production  of  the  protoplasmatic  process  begins, 


FIG.  861.— Cells  and  Nuclei  from  the  Cer- 
vical Region  of  the  Spinal  Cord  of  a  Human 
Embryo  (Minot  Coll.  66)  of  160  Days.  A, 
Young  ganglion  cell;  B.  older  ganglion 
cell;  ngl,  neuroglia  nuclei;  nit,  ganglion 


THE   NERVOUS   SYSTKM. 

Fig.  :'>'>!,  A .  The  first  of  these  processes  (dendrites)  probably  ai 
during  the  second  month,  not,  as  formerly  supposed,  during  the  fourth 
month.  W.  His,  90.2,  50,  observed  that  the  neuroblasts  had  one  or 
two  short,  blunt  processes  running  off  from  the  pole  opposite  the  nerve- 
fibres;  in  the  medulla  oblongataof  the  human  embryo  these  processes 
were  probably  the  beginning  dendritic  branches.  In  later  periods 
(r.f/.,  sixth  month)  1  lind  various  stages  at  once.  In  more  advanced 
ganglion  cells  the  nucleus  is  very  much  enlarged,  Fig.  301,  B,  as  is 
also  its  nueleolus,  and  the  nucleoplasma  is  vacimlated.  The  proto- 
plasm has  grown  very  much,  and  I  find  it,  at  least  in  the  motor  cells 
<>f  the  spinal  cord  of  the  human  foetus,  divided  into  an  inner  finely 
gr;  i  n  i  ilar  layer,  and  an  outer  layer  with  coarser  granules,  which  I  have 
not  ol  >s<  TY»  id  after  birth.  The  further  development  consists,  so  far  as 
known,  simply  in  growth  of  all  the  parts.  As  to  the  progress  of  the 
dendrites,  or  protoplasmatic  processes,  the  observations  are  unsatis- 
factory, owing  chiefly  to  the  failure  of  investigators  to  recognize  the 
difference  between  the  neuroglia  and  nerve-cells.*  In  the  chick  the 
dendrites  arise  very  early,  as  shown  by  Cajal  and  Lenhossek,  91.3, 
1  is,  beginning,  namely,  during  the  third  and  fourth  days  of  incuba- 
tion ;  t  he  first  motor-cells  of  the  spinal  cord  have  branching  dendrites 
t  he  1  i  f t  h  day.  The  branches  of  the  nerve-cells  become  very  numerous 
and  extend  into  the  Randschleier  of  the  embryo,  and  their  interlacing 
causes  a  large  part  of  the  network  appearance  which  is  so  character- 
i>t  it;  of  the  embryonic  cord.  There  is  no  evidence  sufficient,  I  think, 
to  prove  that  the  processes  of  neighboring  nerve-cells  unite;  compare 
W.  Vignal,  88.1,  22G,  and  Kolliker,  "Verh.  Anat.  Ges.,"  V.,  7, 
His,  90.1,  M.  von  Lenhossek,  91.3. 

The  cord  and  each  part  of  the  brain  has,  as  is  well  known,  in  the 
adult  its  special  and  characteristically  shaped  nerve-cells.  Concern- 
ing the  evolution  in  the  foetus  of  these  modifications,  we  know  very 
little.  An  isolated  motor-cell  from  the  cord  of  a  sheep  embryo  of 
10  cm.  is  figured  and  described  by  W.  Vignal,  84.1,  231-233.  In 
older  stages  the  cells  become  larger,  their  processes  larger  and  more 
branched,  and  fibrillated — Vignal,  /.c.,  3G9-375,  describes  the  forms 
in  human  embryos  of  six,  seven,  eight,  and  nine  months.  In  the 
(•('rchclhun,  Vignal,  88.1,  329,  observed  the  first  trace  of  the  en- 
largement of  the  cells  of  Purkin je  in  a  foetus  of  five  months ;  a  month 
later  the  cells  are  larger  and  conspicuous,  and  they  offer  the  peculi- 
arity that  their  protoplasm  is  gathered  almost  wholly  on  the  side  of 
the  nucleus  toward  the  surface  of  the  brain.  At  six  months  Vignal 
could  distinguish  also  the  bodies  of  the  small  nerve-cells  of  the  gran- 
ular layer.  In  the  cerebral  hemispheres  the  enlargement  of  the 
nuclei  and  protoplasm  of  the  large  pyramidal  cells  (Meynert's  third 
layer)  begins,  according  to  W.  Vignal,  88. 1,  250,  at  five  and  a-half 
months  in  the  human  embryo ;  the  protoplasm  presents  an  irregular 
outline;  the  nuclei  stain  more  deeply  than  the  neighboring  ones. 
During  the  sixth  month  the  cells  elongate  toward  the  exterior  and  so 
assume  their  characteristic  u  pyramidal "  form ;  their  protoplasm  is 
finely  granular  without  very  distinct  outlines,  and  their  processes  or 
dendrites  are  neither  long  nor  much  branched.  At  birth  the  cells 

*  This  is  notably  the  case  with  Vignal,  who  failed  to  recognize  the  neuroglia  cells  before  the 
fourth  month,  88.1,  319. 

40 


626 


THE    FCETUS. 


are  found  in  various  stages,  both  in  the  second  and  third  layer  of 
Meynert,  but  the  most  advanced  of  the  large  cells  differ  but  little 
except  in  size  (see  Vignal,  Z.c.,  PI.  IX.,  Fig.  2,  a)  from  those  at 
seven  months.  The  enlargement  of  the  nerve-cells  of  the  second 
layer  occurs  during  the  eighth  month.  Magini,  88.1,  affirms  that 
the  cells  do  not  have  the  pyramidal  shape  in  the  foetal  hemispheres, 
but  resemble  rather  the  cerebellar  Purkinje's  cells,  and  states  that 
when  the  cells  are  colored  with  Golgi's  osmio-bichromate  silver 
mixture,  their  processes  appear  varicose,  having  scattered  nodular 
thickenings. 

As  .regards  the  time  of  development  of  the  nerve-cells,  Below, 
88. 1,  reports  that  the  cells  appear  first  in  the  spinal  cord  and  then 
in  the  brain  in  the  following  order :  In  the  medulla  oblongata,  cere- 
bellum, mid-brain,  cerebrum.  He  further  states  that  in  animals 
born  helpless  (man,  dog,  cat,  rat,  mouse,  rabbit)  the  cells  are  much 
less  developed  in  the  brain  than  in  those  animals  which  are  immedi- 
ately active  (horse,  cow,  pig,  sheep,  Guinea-pig).  Vignal  states, 
88. 1,  that  the  Purkinje's  cells  (nerve-cells  of  the  cerebellum)  acquire 
their  cell-bodies  in  man  about  the  sixth  month,  while  the  pyramidal 
cells  of  the  cerebral  cortex  do  not  become  equally  distinct  until  the 
eighth  month. 

2.  GANGLIONIC  NERVE-CELLS. — These  are  all  spindle-shaped  bi- 
polar cells  in  early  stages,  as  above  described;  the  cell-body  and 

nucleus  draw  early  to  one  side  so  as  to 
appear  as  a  lateral  appendage  to  the  nerve- 
fibres,  Fig.  362.  There  can  be  little  doubt 
that  the  cell-body  draws  more  and  more 
to  one  side,  and  becomes  pear-shaped ; 
and  that  then  the  pointed  end  elongates 
until  it  becomes  a  nerve-fibre,  which  joins 
at  an  angle  the  earlier  fibre  developed 
from  the  two  poles  of  the  cell.  That  the 
cells  thus  develop  appears  probable  from 
the  scanty  observations  we  possess,  and 
also  because  the  development  would  agree 
with  the  series  of  forms  which  have  been 
traced  by  G.  Retzius,  80.1,  through  the 
vertebrate  series.  For  example,  in  the 
lowest  true  vertebrate  (Petromyzon)  Freud,  78.1,  finds  that  the 
bi-polar  form  of  the  cells  is  permanent  in  the  adult.  The  unipolar 
form  is  found  in  all  amphibians  and  amniota.  In  a  human  em- 
bryo of  the  tenth  week,  I  find  the  cells  in  various  stages  of  pro- 
gress, Fig.  362;  the  nuclei  are  round,  as  seen  in  horizontal  sections 
of  the  ganglia,  granular  with  distinct  intra-nuclear  network ;  they 
vary  in  size ;  the  smaller  have  so  little  protoplasm  about  them  that 
they  appear  almost  naked ;  the  amount  of  protoplasm  increases  with 
the  size  of  the  nucleus ;  the  protoplasm  lies  on  one  side  of  the  nucleus, 
and  assumes  a  triangular  or  quadrilateral  outline  in  the  sections : 
between  the  cells  lie  the  triangular  sections  of  the  nerve-fibres,  the 
fibrillse  of  which  appear.as  dots. 

In  the  sympathetic  ganglia,  the  nerve-cells,  the  origin  of  which  is 
discussed  p.  630,  retain  the  bi-polar  form,  but  the  two  poles  are 


FIG.  362. -Spinal  Ganglion  Cells 
from  a  Longitudinal  Horizontal 
Section  of  a  Human  Embryo  of  the 
tenth  Week. 


THE  NERVOl  ^   SYSTEM. 

brought  near  together,  and  one  pole  gives  rise  to  the  spiral,  the  other 
to  tin-  straight  nbre  of  Beale  and  Arnold.  Concerning  the  develop- 
ment of  these  cells,  I  know  of  no  detailed  observations. 

Spinal  Nerves. — It  is  singular  that,  although  the  early  history 
of  the  spinal  nerves  up  to  the  period  of  the  union  of  the  nerve-roots  has 
heen  the  object  of  much  investigation,  yet  their  later  history  has 
I  ie«  MI  very  little  studied.  Almost  the  only  observations  of  importance 
are  those  of  W.  His,  88.3,  380-385.  More  has  been  done  to  eluci- 
date the  history  of  the  hypoglossus  and  spinal  accessory  nerves, 
which,  though  morphologically  derived  from  the  spinal  cord,  have 
heen  annexed  by  the  head,  and  may  be  conveniently  regarded  as 
cephalic  nerves. 

The  results  obtained  by  His,  /.  c. ,  are  as  follows :  The  nerves  toward 
tin-  head  develop  more  rapidly  than  those  toward  the  tail.  The  nerve 
trunk  formed  by  the  union  of  the  two  roots,  p.  622,  grows  at  first  in 
a  plane  approximately  at  right  angles  to  the  axis  of  the  spinal  cord, 
but  owini;-  to  changes  in  the  curvature  of  the  cord  the  cervical  and 
lumbar  nerve>  very  early  appear  oblique,  Fig.  3G3.  The  obliquity 
increases  especially  in  the  neck,  where  the  neck-bend  is  gradually 
1>— ened  as  the  head  of  the  embryo  rises  (compare  Chap.  XVIII.). 
After  the  trunk  has  grown  a  short  distance  the  fibres  at  the  distal 
end  are  seen  to  tend  to  spread  apart,  and  this  spreading  seems  to 
initiate  the  branching  of  the  nerve  without  any  special  obstacle 
causing  it  to  divide  in  the  way  described  on  p.  623,  for  by  their 
spreading  the  ends  of  adjacent  nerves  are  brought  into  contact  in 
the  cervical  and  lumbar  regions,  and  by  uniting  begin  the  formation 
< >t  the  brachial  and  lumbar  plexus.  A  portion  of  the  fibres  from  one 
nerve  join  those  of  another,  and  the  united  portions  constitute  a  new 
nerve  trunk.  I,i  an  embryo  of  ?  mm.  (His,  /.c.,  Tab.  II.,  Fig.  4) 
th"  anlages  of  the  cervical  and  brachial  plexus  are  present,  that  of 
the  lumbar  flexure  about  to  develop.  In  an  embryo  of  10  mm.,  Fig. 
:;».:;,  one  can  recognize,  1,  the  A',  invijtitfilix  minor  arising  from  the 
first  and  second  nerve;  2,  3,  the  A".  <iuricnl(iri'x  IHHI/HHX  and  N. 
rr/v/V<///.v  siijtrrfirialis  coming  from  the  second  and  tl  ves; 

•!.  A'//,  xiiitnirlnriciiltin-s.  and  :.,  the  A',  phrcnims.  The  phrenic 
nerve,  /\  descends  steeply  past  the  brachial  plexus  and  the  wall  of 
the  ^horax  where  it  is  lodged  in  a  small  ridge  immediately  behind 
the  vena  cava  superior.  The  brachial  plexus  is  formed  by  the  fifth 
to  eighth  cervical  and  rirst  dorsal  nerves.  In  Fig.  363,  the  position 
of  the  arm  anlage  is  indicatd  by  a  dotted  line;  it  will  be  seen  that  it 
i<  Mich  that  one  branch  from  the  fifth  nerve  does  not  enter  the  arm; 
the  fibres  which  enter  the  arm  become  grouped  in  three  main  stems, 
1 M it  the  steps  by  which  they  become  so  grouped  have  not  been  clearly 
worked  out.  The  second  and  third  dorsal  nerves  have  each  an 
interc.  >sto-humeral  branch  running  toward  the  brachial  plexus.  The 
remaining  dorsal  nerves  at  this  stage  require  no  special  description. 
Turning  to  the  sacral  nerves  we  find  the  first  gives  off  two  inde- 
pendent branches,  the  ileo-liypogastric,  ih,  and  ileo-ingualis,  ii,  and 
a  third  branch,  which  unites  with  fibres  from  the  second  nerve  to 
form  the  yenito-cruralis,  gc.  The  second  to  fifth  sacral  nerves 
together  with  the  first  to  third  coccygeal  nerves  unite  to  form  four 
nerve  trunks,  which  enter  the  leg,  and  one  which  does  not.  The 


628 


THE   FCETUS. 


attachment  of  the  leg  is  indicated  by  a  dotted  line;  the  four  ner\ «  s 
of  the  extremity  are  the  cutaneus  externus,  c.e;  the  cruralis,  c.r; 


FIG.  363.— Peripheral  Nervous  System  of  a  Human  Embryo  of  about  10  mm.  (His'  Ito),  recon- 
structed from  the  sections  IH-XII,  cephalic  nerves.  F,  Froriep's  ganglion ;  1-8,  cervical  gan- 
glia; 1-12,  dorsal  ganglia;  1-5,  lumbar  ganglia ;  1-5,  sacral  ganglia;  Ot,  otocyst;  Ven,  ventricle; 
Au,  auricle;  Li,  liver;  P,  phrenic  nerve;  th,  anlage  of  thyroid;  In,  intestine.  (For  the  remain- 
ing letters  see  text;  the  positions  of  the  limbs  are  indicated  by  dotted  lines.)  After  W.  His. 

obturatorius,  o;  and  the  ischiadicus,  i.s.     The  nerve  stem  below 
the  leg  is  the  pudendus  communis,  pic. 

We  know  very  little  concerning  the  development  of  branches  of 
the  spinal  nerves,  other  than  those  resulting  from  the  contact  of 
nerves  with  one  another,  and  which  are  concerned  in  the  production 
of  the  plexus.  W"e  know  from  comparative  anatomy  that  a  spinal 
nerve  has  typically  a  dorsal  branch,  which  carries,  1,  motor  fibres 
to  the  myotome  (or  its  product  the  muscles)  and,  2,  sensory  fibres  to 
the  skin  of  the  back,  and  a  ventral  branch,  which  itself  divides  into 
two  branches,  one  running  to  the  somatopleuric  wall  of  the  splanch- 
nocoele  and  the  other  running  to  the  splanchnopleure  or  viscera. 
This  type,  as  we  know  from  Paterson's  observations,  87.1,  91.1, 
reappears  in  the  development  of  mammals.  The  trunk  formed  by 
the  union  of  the  sensory  and  motor  roots  grows  only  a  very  short 


THE   NERVOUS    SYSTEM. 


629 


D. 


S.D 


distance  before  it  undergoes  its  first  or  primary  division,  one  branch 
running  to  the  primitive  segment,  the  other  continuing  obliquely 
downward  and  outward.  The  cause  of  this  division  I  do  not  know, 
hut  I  think  it  possible  that  it  may  be  due  to  the  nerve  encountering 
the  edge  of  the  muscle 
plate.  We  now  have 
the  dorsal  and  ventral 
branches  ;  the  latter 
grows  on  until,  as  shown 
by  Paterson's  observa- 
tions, 9 1 . 1,,  it  encounters 
tin'  nirsothelium  of  the 
dors  almost  angle  of 
splanchnoccele,  where- 
upon the  branch  is  forced 
to  divide  (rat  embryo 
eight  to  nine  days)  into  a 
somatic  and  a  splanchnic 
branch,  Fig.  364,  N.som, 
and  X.x/rf.  In  this  case 
the  mecbanical  cause  of 
the  division  seems  unmis- 
takable.  The  splanchnic 
branch,  at  least  in  the 
case  of  the  dorsal  and 
lumbar  nrrvrs  of  mam- 
malia, is  Mill  further  de- 
flected to  a  horizontal 
course  by  the  cardinal 
vein,  and  is  thus  directed 
toward  the  aorta  and  en- 
abled to  join  Paterson's 
sympathetic  cord.  The 
somatic  branch  grows 
into  the  somatopleure,  but 

VeFV  SOOn  divides — cause    seventeen  to  eighteen  Days,  through  the  Lumbar  Region. 

'    Md,  Medulla  spinalis;  D.r,  dorsal  root;  Gl,  ganglion;  S.Z>, 
Unknown intO      tWO    superior  division  of  nerve;  EC,  ectoderm;  Spl,  splanchnic 

branches.  The  further 
history  of  the  somatic 
nerve  branches  has  still 
to  be  ascertained .  About 
the  time  these  changes  are  going  on,  there  is  developed  an  increased 
separation  of  the  roots  of  the  primary  dorsal  and  ventral  rami,  so 
that  each  has  its  discrete  bundle  of  ganglionic  and  medullary  nerve- 
fibres,  Fig.  304. 

CERVICAL  NERVES.—  W.  His,  88.3,  360,  points  out  that,  while 
the  medullary  netiroblasts  send  their  fibres  all  into  the  ventral  roots 
throughout  the  greater  part  of  the  spinal  cord,  yet  in  the  upper  cervi- 
cal region  the  neuroblasts  in  the  zone  of  the  future  lateral  horn  send 
their  fibres  out  in  nerve  bundles  near  the  entrance  of  the  ganglionic 
fibres.  We  have  in  this  peculiarity  a  transition  to  the  cerebral  type, 
in  which  the  dorsal  root  is  formed  partly  by  medullary  fibres. 


w 


Ao 


v.card 
Fio.  864. — Transverse  Section  of  a  Mouse  Embryo  of  about 


branch ;  Som,  somatic  branch  of  nerve ;  Pan,  pancreas ;  Spl, 
spleen;  Ki,  kidney;  v.card,  cardinal  vein;  m*f,  mesentery; 
Ao,  aorta;  Sy,  sympathetic  anlage;  Vtb,  vertebral  anlage; 
ncfc,  notochord;  Sp.A,  spinal  artery;  C.c.  central  canal. 
After  A.  M.  Paterson.  (The  figure  is  compiled  from  several 
successive  sections.) 


630  THE   FCETUS. 

In  birds  and  reptiles  the  first  and  second  cervical  ganglia  are  pres- 
ent only  during  a  very  short  early  embryonic  period  (Chiarugi,  89.2, 
334)  and  then  disappear  entirely,  as  was  discovered  by  Froriep,  82. 1, 
83. 1.  Froriep  also  observed  that  in  mammals  the  ganglia  continue 
their  development,  being  present  in  the  adult. 

Sympathetic  System. — Two  views  have  been  advanced  in  re- 
gard to  the  origin  of  the  sympathetic  system.  The  older  view,  that 
of  Remak,  was  that  it  arose  in  situ  from  the  mesoblast ;  the  later 
view,  that  of  Balfour,  was  that  it  arose  as  a  series  of  buds  from  the 
spinal  nerves,  the  buds  afterward  becoming  connected  to  form  two 
main  chains  of  sympathetic  ganglia.  Remak's  view  has  been  re- 
established by  A.  M.  Paterson,  upon  whose  memoir,  91.1,  I  base 
the  following  account.  It  is  possible  that  His'  suggestion,  90. 1,  is 
correct,  and  that  the  cells  of  the  sympathetic  are  not  mesenchymal, 
but  cells  which  have  emigrated  singly  from  the  ganglia.  Good 
summaries  of  the  literature  on  the  subject  are  given  by  Onodi,  86.1, 
and  Paterson. 

The  first  trace  of  the  sympathetic  may  be  seen  in  a  mouse  embryo 
of  eight  days  (rat  of  7  mm.)  at  a  stage  when  the  spinal  nerve  has 
nearly  reached  the  mesothelium  of  the  splanchnoccefe,  and  the 
Wolffian  tubules  have  just  appeared.  In  the  interval  between  the 
aorta  and  the  cardinal  vein  the  uniformity  of  the  mesenchyma  is 
now  broken  by  a  group  of  cells,  which  differ  strikingly  from  their 
neighbors ;  the  cells  stain  deeply ;  their  nuclei  are  large  and  often 
possess  a  considerable  number  of  nucleoli.  This  mass  of  .specialized 
cells  is  bilaterally  symmetrical  and  extends  from  the  fevel  of  the 
cephalic  border  of  the  fore-limb  to  the  level  of  the  stomach.  It  con- 
stitutes a  cord  on  each  side,  and  is  the  anlage  of  the  sympathetic 
system.  The  cord  is  comparatively  large  anteriorly,  and  gradually 
tapers  off  and  becomes  indistinct  posteriorly.  It  has  no  connection 
with  the  spinal  nerves  or  ganglia.  Longitudinal  sections  show  that 
the  cells  are  fusiform  and  elongated  lengthwise  of  the  cord,  and  that 
the  cord  offers  no  trace  of  segmentation. 

The  next  step  in  the  development  is  the  union  of  the  spinal  nerves 
with  the  sympathetic  cord ;  the  union  takes  place  only  in  the  dorsal 
and  lumbar  region,  not  in  the  neck  or  in  any  segment  of  the  body 
posterior  to  the  bifurcation  of  the  aorta.  It  is  the  splanchnic  branch 
only  which  joins  the  sympathetic  cord,  Fig.  364,  Spl.  In  rat  em- 
bryos of  8.5  mm.  (eight  to  nine  days)  the  cord  is  slightly  larger  than 
before,  but  is  still  in  close  proximity  to  the  aorta  and  presents  no  sign 
of  constriction  or  segmentation ;  the  ventral  branch  of  the  nerve  has 
just  reached  the  angle  of  the  splanchnoccele  and  is  dividing.  In  mice 
embryos  of  nine  days  the  branch  has  grown  about  half-way  to  the  cord ; 
in  those  of  ten  days  it  has  almost  reached — in  those  of  eleven  days  it 
has  actually  joined — the  cord.  The  cord  itself  now  has  ventral  branches 
and  its  cells  mingle  with  the  nerve-fibres,  and  later  the  cells  migrate 
along  the  nerves.  In  the  anterior  thoracic  region  the  whole  of  the 
splanchnic  branch  joins  the  cord,  but  in  the  lower  thoracic  and  in 
the  abdominal  regions  some  of  the  fibres  pass  beyond.  In  the  neck 
above  the  point  of  origin  of  the  vertebral  artery  the  splanchnic 
branches,  as  already  stated,  have  no  connection  with  the  sympathetic 
cord.  After  union  with  the  nerve,  the  cord  loses  its  boundaries,  and 


I  UK    NKKVOUS    SYSTEM. 


631 


Spl 


it-  cells  acquire,  Kig.  &65,  N//,  greater  >i/.e  and  branching  processes. 
Though  the  splanchnic  nerve  branch  elo ngates  considerably  it  con- 
tinues to  end  in  the  cord.  It  was  the  observation  of  this  condition 
coupled  with  the  assumed 
necessity  of  tracing  all  sup- 
posed nerve-cells  to  an  ecto- 
dennal  origin,  which  led 
I  Jail  our  to  hifl  theory  of  the 
origin  of  the  sympathetic 
1.  The  splanchnic  nerve- 
t  i  b  res  d  i  s  t  r i  bute  themselves 
through  the  cord  and 
branches,  also  penetrating 
the  cervical  portion  of  the 
c.  «rd  which  does  not  receive 
any  of  the  cervical  nerves. 

"In  transverse  sections," 
says  Paterson,  /.c.,  p.  171, 
••  of  a  human  embryo  about 
the  end  of  the  first  month, 
hardened  in  spirit  and 

Stained    with    aniline    blue-       Fio.  385.  -Transverse  Section  of  the  Sympathetic  Cord 
vi      i      ,1  .i     4.-  i    from  the  Ix>wer  Dorsal  Region  of  a  Rat  Embryo  of  about 

black,  the  sympathetic  cord  ti.irt,-,,  0*7*  -v/.  spiai» -imu-  nerve  brand?;  sy.*ym- 
has  very  much  the  character  Kg**".00**'  Jo'  aorta;  me*'  me8en<*J™a.  After 
just  described.  The  cord  it- 
self is  large  and  uniform  in  width,  widening  out  anteriorly  to  form 
the  inferior  cervical  ganglion ;  beyond  this  it  narrows,  encloses  the 
subclavian  artery,  and  forms  a  fibrous  cord;  this  again  becomes 
cellular,  and  widens  out  into  the  "superior"  cervical  ganglion.  No 
splanchnic  branches  join  the  cord  in  front  of  the  level  of  the  inferior 
cervical  ganglion.  In  the  thorax  (Plate  28,  Fig.  1C)  the  splanchnic 
branches  are  seen  (spl)  arising  from  both  roots  of  the  spinal  nerve 
(/,  D),  and,  as  in  the  figure,  terminating  wholly  in  the  sympathetic 
cords  (sy).  Sometimes  a  small  portion  of  a  splanchnic  branch  can  be 
traced  round  the  ventral  side  of  the  cord,  accompanied  by  a  cellular 
branch  from  it.  In  the  hinder  thoracic  region,  a  small  part  only  of 
the  splanchnic  branch  joins  the  cord,  the  greater  part,  along  with  cel- 
lular outgrowths  from  the  sympathetic,  passing  onward  to  form  the 
solar  plexus  and  semilunar  ganglia,  which  are  seen  in  process  of  for- 
mation on  the  ventral  aspect  of  the  aorta.  A  similar  fibro-cellular 
bundle  passes  to  join  the  supra-renal  body.  In  the  lumbar  region 
the  splanchnic  branch  can  be  seen  for  a  considerable  distance  almost 
entirely  unconnected  with  the  sympathetic  cord,  and  separated  by  an 
interval  from  it.  The  cord  gradually  narrows  as  it  is  followed  back- 
ward, and  becoming  attenuated  disappears  at  the  point  of  bifurcation 
of  the  aorta/' 

The  third  step  is  the  gangliation  of  the  cord,  that  is  to  say,  the 
formation  of  the  series  of  enlargements,  which  constitute  the  adult 
ganglia,  the  thinner  portions  of  the  cord  persisting  as  the  inter- 
ganglionic  commissures.  The  commissures  come  gradually  to  con- 
sist chiefly  of  nerve-fibres.  The  ganglionic  thickenings  first  appear 
(human  embryo  of  18-19  mm.,  mouse  embryo  nineteen  days)  where 


632 


THE   FCETUS. 


S.G 


CP 

-  C.I 

2 

3 


the  nerves  join  the  mesenchymal  sympathetic,  and  presumably  result 
from  the  growth  locally  of  both  the  nerve-fibres  and  the  sympathetic 
cells.  As  the  parts  gradually  attain  their  adult  form,  the  regularity 
of  the  alternate  swelling  and  constriction  does  not  persist,  but  as  the 
ganglia  become  defined  in  form  their  position  tends 
to  become  irregular ;  while  one  may  lie  in  the  in- 
terval between  two  vertebra,  the  next  may  be  seen 
opposite  the  vertebra  itself.  The  parts  derived  from 
the  sympathetic  cord  in  the  neck  above  the  inferior 
cervical  ganglion  may  be  regarded  as  belonging  to 
the  peripheral  or  collateral  distribution  of  the  sym- 
pathetic nerve,  because  they  have  no  direct  connec- 
tion with  the  cervical  nerves.  A  fibro-cellular 
bundle  springs  from  the  cord  and  accompanies  the 
vertebral  artery;  beyond  this  the  original  cord, 
which  is  at  first  terminated  at  the  level  of  the 
mouth,  becomes  constricted  by  the  formation  of  a 
fibro-cellular  commissure  separating  off  the  superior 
cervical  ganglion.  This  ganglion  ends  headward 
in  a  fibrous  bundle,  which  accompanies  and  is  lost 
upon  the  internal  carotid  artery  beneath  the  auditory 
capsule.  The  middle  cervical  ganglion,  when  pres- 
ent, is  to  be  regarded  as  formed  of  a  group  of  cells, 
which  have  been  included  in  the  commissure,  Fig. 
366,  M G.  The  connections  of  the  sympathetic  cord 
with  the  cranial  nerves  have  yet  to  be  investigated. 
As  regards  the  caudal  termination,  the  sympathetic 
cord  is  at  first  ill-defined  behind  the  region  of  the 
kidneys ;  it  gradually  extends  further  back,  along- 
side the  aorta  and  middle  sacral  artery,  where  the 
two  cords  become  closely  approximated.  They  be- 
come gradually  more  and  more  attenuated,  and 
finally  disappear.  Near  their  termination  they  are 
joined  together  on  the  dorsal  aspect  of  the  middle 
sacral  artery  by  cellular  commissures,  from  which 
the  connecting  loop  and  ganglion  impar  are  devel- 
oped. No  fusion  of  the  two  cords  can  be  seen  until 
they  have  reached  their  permanent  posterior  limit. 
The  sympathetic  cord  behind  the  lumbar  region 
may  be  regarded  as  belonging  to  the  peripheral 
distribution  of  the  cord  for  the  same  reasons  as  the 

i'iur.uuo. — Sympatnet-  ,  ,. 

ic  Ganglia  of  One  Side    Cervical  portion. 

The  peripheral  branches  from  the  sympathetic 
cord,  including  the  collateral  ganglia,  as  well  as  the 
medullary  portions  of  the  supra-renal  bodies,  the 
superior  cervical  ganglia,  etc.,  are  formed  by  out- 
/  «""Tnfe6rTor  growtns  from  the  cord,  which  are  at  first  cellular. 

ganglion-  d.spi,  great  These  give  rise  to  ganglia,  nerves,  and  plexuses, 

teraAC^f.icpaterson Af*  and  are  accompanied  by  the  parts  of  the  splanchnic 
branches  of  the  spinal  nerves,  which  do  not  join 

the  ganglia.     In  this  category  are  placed  doubtfully  the  gray  rami 

communicantes. 


numbers  refer  to  the 
nerves  connected  with 
the  ganglionic  chain. 
C.P,  Carotid  plexus; 
S.G,  superior  gangli- 
M.G,  middle  gan- 


THE    NKKVot'S    SYS  I  I.  M. 

General  Morphology  of  the  Cephalic  Nerves.*— It  is  n.,\v 
rally  believed  by  einhrvoln^ists  that  the  nerves  wliich  spring 
from  the  brain  form  a  part  of  the  same  morphological  series  a^  the 
spinal  nerves,  t'nlike  the  spinal  nerves  they  vary  greatly  among 
theniM-lves  both  in  their  development  and  in  their  permanent  char- 
acter, and  at  least  one  of  them,  the  optic  nerve,  appears  to  have  a 
ditVerent  morphological  value  from  a  true  nerve.  It  is,  therefore, 
impos>ible  to  give,  as  was  attempted  for  the  spinal  nerves,  a  compre- 
hensive history  of  the  nerves  of  the  head,  but  instead  wo  must  study 
each  nerve  separately. 

The  following  table  gives  a  list  of  the  cerebral  nerves  and  shows 
with  which  division  of  the  brain  each  is  connected: 

TABLE  OF  THE  CRANIAL  NERVES. 

Vesirl.-.  Nerve. 

First I.  Olfactory. 

II.  Optic. 

Second III.  Oculo-motor. 

IV.  Trochlearis. 

Third V.  Trigeminus. 

VI.  AbduceDs. 

VII.  Facial. 

VIII.  Auditory. 

IX.  Glosso-pharyngeal. 

X.  Vagus. 

Spinal  cord XL  Spinal  accessory. 

XII.  Hypoglossus. 

I  give  below  the  separate  history  of  each  nerve,  and  in  the  follow- 
ing paragraphs  of  this  section  I  have  discussed  certain  general  ques- 
tions of  the  morphology  of  the  cerebral  nerves. 

The  first  point  to  be  emphasized  in  regard  to  the  cephalic  nerves 
is  that,  as  discovered  by  W.  His,  88.3,  there  are  three  sets  of  roots, 
one  ganglionic,  the  other  two  medullary.  The  ganglionic  roots  are 
part  of  the  same  series  as  the  sensory  roots  of  the  spinal  cord.  The 
two  sets  of  medullary  roots  are  parts  of  the  same  series  as  the  single 
set  of  spinal  motor  roots.  It  is,  therefore,  a  peculiarity  of  the  brain, 
that  its  medullary  fibres  have  their  points  of  exit  along  two  longitu- 
dinal lines  on  each  side.  Both  lines  are  situated  in  the  ventral 
zone  of  His :  one  is  toward  the  Bodenplatte  and  may  be  regarded  as 
the  prolongation  of  the  line  of  the  ventral  roots  of  the  spinal  cord ; 
the  other  is  close  to  the  edge  of  the  dorsal  zone  of  His,  and, 
therefore,  immediately  below  the  ganglionic  root.  It  appears  a  jus- 
tifiable hypothesis  to  assume  that  every  segment  in  the  head  had 
originally  its  segmental  nerve,  and  that  every  nerve  had  three  roots, 
one  sensory  and  two  motor,  i.  e.,  one  lateral  and  one  ventral  motor 
root.  The  lateral  root  is  the  distinguishing  characteristic  of  a  typi- 
cal cephalic  nerve,  f  but  its  existence  has  been  long  overlooked  because 

*  For  an  admirable  resnm£  of  the  progress  up  to  1888  of  our  knowledge  of  the  development 
of  cephalic  nerves  see  W.  His.  ss  •_>.  379-409. 

t  I  cannot  but  think  that  the  spinal  nerves  also  will  be  found  to  have  lateral  roots. 


THE   FOETUS. 

it  is  so  closely  joined  to  the  ganglionic  or  dorsal  root  that  it  has  been 
generally  mistaken  for  a  part  of  a  dorsal  root.  It  is  this  mistake 
which  has  been  the  principal  obstacle  in  the  way  of  investigations 
upon  the  morphology  of  the  cephalic  nerves,  and  the  correction  of 
this  mistake  by  His  is,  to  my  mind,  the  most  important  contribution 
to  the  morphology  of  the  brain  which  has  been  made  for  a  long  time 
past.  The  relation  of  the  three  roots  is  well  illustrated  in  Fig.  370. 

As  stated  in  Chapter  IX.  there  are  probably  seventeen  or  eighteen 
segments  in  the  vertebrate  head,  and  perhaps  seventeen  or  eighteen 
neuromeres  in  the  brain  (see  above).  As  yet,  however,  only  twelve 
nerves  have  been  observed  in  any  adult  vertebrate.  Of  these  nerves 
some  are  purely  ganglionic,  others  are  purely  medullary,  and  still 
others  are  mixed,  and  one  of  them  (hypoglossus)  arises  by  the  fusion 
of  parts  of  four  nerves;  of  the  medullary  nerves,  some  represent 
lateral  roots,  like  the  accessorius,  others  ventral  roots  like  the  abdu- 
cens.  If,  therefore,  the  cephalic  nerves  were  derived  from  seventeen 
or  eighteen  segmental  nerves,  they  must  have  undergone  very  ex- 
tensive modifications.  Morphologists  are  endeavoring  to  trace  out 
these  modifications,  and  to  establish  thereby  the  hypothesis  that  the 
cranial  nerves  represent  a  series  of  segmental  nerves.  That  these 
endeavors  will  be  successful  can  hardly  be  doubted  by  competent 
embryologists. 

The  second  point  to  be  emphasized  is  that  the  gill-clefts  are  not 
segmentally  arranged,  and  that  all  attempts  to  ascertain  the  seg- 
mental value  of  cranial  nerves  by  determining  their  relations  to  the 
gill-clefts  are  based  upon  an  erroneous  assumption.  As  explained  in 
Chapter  IX.,  each  of  the  three  anterior  gill-clefts,  counting  the 
mouth  as  one,  corresponds  to  several  segments.  It  is  possible  that 
the  posterior  clefts  are  segmentally  arranged,  but  these  clefts  are 
without  branchial  nerves  of  their  own,  being  innervated  from  the 
vagus.  As  regards  the  nerves  connected  with  the  clefts,  to  wit, 
the  trigeminal,  facial,  glosso-pharyngeal,  and  vagus,  we  can  conceive 
them  as  representing  each  several  segmental  nerves,  either  by  being 
the  product  of  the  fusion  of  several  primitive  nerves,  or  by  being 
one  each  of  a  group  of  nerves,  the  rest  of  which  are  aborted.  The 
branchial  nerves  are  recurred  to  in  a  paragraph  below. 

A  third  important  point  is  the  subdivision  of  each  primary  cephalic 
ganglion  into  an  upper  (lateral  or  main)  ganglion,  and  a  lower  (or 
epibranchial)  ganglion.  The  development  of  the  lamprey,  as  worked 
out  by  C.  Kupffer,  suggests  that  every  cephalic  ganglion  had  primi- 
tively two  direct  connections  with  the  epidermis  to  make  the  lateral 
and  epibranchial  organs,  and  the  development  in  the  amniota  sug- 
gests that  two  ganglia  are  differentiated  from  the  primitive  one,  and 
that  in  some  cases  a  cephalic  ganglion  represents  the  primitive,  in 
others  one  of  the  secondary,  ganglia.  Thus  we  may  hypothetically 
regard  the  ciliary  and  trigeminal  ganglia  as  primary ;  the  acoustic 
as  a  secondary  lateral  line  ganglion ;  the  facial  as  a  secondary  epi- 
branchial ganglion  ;  while  in  the  case  of  the  glosso-pharyngeal  and 
vagus  nerves,  both  secondary  ganglia  are  preserved,  Ehrenritter's 
and  the  jugular  ganglia  being  assigned  to  the  lateral,  the  petrosum 
and  nodosum  to  the  epibranchial  series.  I  can,  of  course,  only 
suggest  this  hypothesis  as  an  obvious  corollary  of  Kupffer's  discov- 


Tin-;   mBYOUfi   >v.  STEM, 

cry.  and  though  its  justification  must  be  left  to  the  future,  y«-t  it 
is  to  me  now  very  plausible. 

The  nerves  of  the  head  have  very  different  values,  and  are  by  no 
means  morphologically  equivalent  one  to  another.  It  seems  cer- 
tain, however,  that  not  one  can  be  homologizcd  with  a  single  com- 
plete Moment  al  nerve,  that  is  to  say,  a  nerve  in  which,  aside  from 
it-  c. 'inn ii>su res,  there  are  to  be  found  all  the  nerve-fibres,  both  gan- 
glionic  and  medullary,  of  one  segment  united  in  one  main  trunk. 
( )n  the  contrary,  no  cephalic  nerve  is  the  equivalent  of  more  than  a 
part  of  a  complete  segmental  nerve.  Even  those  cerebral  nerve- 
which  are  derived  from  the  fusion  of  several  nerves  do  not  include 
the  whole  of  each  nerve  component. 

\Ve  may  conveniently  distinguish  between  those  nerves  of  the 
head  which  are  derived  from  part  of  a  single  segmental  nerve,  and 
tho>e  derived  from  the  fusion  of  parts  of  several  segmental  nerves. 
Unfortunately  this  distinction  rests  at  present  chiefly  on  hypotheti- 
cal identifications.  We  have  to  class  provisionally,  as  single  nerves, 
olfactory,  oculo-motor,  trochlear,  and  abducens — and  perhaps  acous- 
tic.  as  compound  nerves,  trigeminal,  facial,  glosso-pharyngeal  (?), 
ifl  i  /),  accessorius,  and  hypoglossal. 

Concerning  the  roots,  a  few  general  remarks  may  be  made.  We 
have  already  insisted  upon  the  triple  division  into  dorsal  sensory 
i,  lateral  motor  roots,  and  ventral  motor  roots.  The  dorsal  and 
lateral  roots  are  situated  so  closely  together,  the  former  at  the  ven- 
tral edge  of  the  dorsal  zone  of  His,  the  latter  at  the  dorsal  edge  of 
the  vmtral  zone,  that  they  appear  as  one  root  so  long  as  the  origin 
<  >i  the  fibres  is  not  considered.  We  have,  in  fact,  several  nerves, 
which  arise  apparently  from  one  root,  but  which  in  reality  arise 
from  two  roots  closely  united;  such  are  the  trigeminal,  facial,  glosso- 
]  ih a  ry  1 1  i?eal,  and  vagus  nerves.  If  the  lateral  root  aborts,  the  sensory 
root  may  remain ;  such  nerves  are  the  olfactory  and  acoustic.  In 
the  reverse  case  the  lateral  root  persists,  as  occurs  with  the  oculo- 
motor ( ?) ,  trochlear,  and  spinal  accessory  nerves.  The  ventral  motor 
roots,  like  those  of  the  spinal  cord,  to  which  they  are  partially  equiv- 
alent, have  an  independent  exit :  they  persist  only  in  the  abducens 
and  hypoglossus. 

A  constant  feature  of  the  persistent  ganglia  is  probably  that  the 
t^m glionic  fibres  as  soon  as  they  enter  the  medulla  form  a  longitu- 
dinal bundle,  which  grows  tailward  close  to  the  outer  surface  and 
in  the  lower  part  of  the  dorsal  zone  of  His.  This  bundle  is 
homologous  with  the  similar  bundle  in  the  spinal  cord.  The  bundle 
is  known  as  the  ascending  tract  in  the  anatomy  of  the  brain  and 
behind  the  vagus  as  the  tractus  solitarius.  It  has  been  shown  to 
receive  fibres  in  the  embryo  from  the  trigeminal,  facial,  glosso-pha- 
ryngeal,  and  vagus  ganglia. 

I  will  now  give  a  synopsis  of  the  interpretations  of  the  twelve 
cerebral  nerves,  which  appear  to  me  indicated  by  our  present  knowl- 
edge of  the  development  of  the  nerves,  as  reviewed  in  the  following 
twelve  sections,  and  by  our  knowledge  of  the  position  of  the  cephalic 
segments  as  described  in  Chapter  IX. 

I  append  a  table,  modified  from  Zimmermann,  91.1,  109,  which 
indicates  the  relations  of  the  nerves  to  the  neuromeres  so  far  as  at 


636 


THE    FCETUS. 


present  rendered  probable.     The  assignments  made  in  the  table  are 
in  my  judgment  all  more  or  less  problematical. 


1. 


2. 
3. 


4. 

5. 
6. 


Olfactory.  Probably  ganglionic,  though  the  development  of 
its  ganglion  differs  from  that  of  the  other  ganglia;  belongs  to 
the  first  (and  second?)  segment. 

Optic.     Probably  not  a  true  nerve. 

Oculomotor.  Lateral  root  with  sensory  ganglion,  which  aborts 
very  early ;  belongs  to  the  first  or  second  segment  of  the  mid- 
brain. 

Trochlear.  Lateral  root  with  sensory  ganglion,  which  aborts 
very  early;  belongs* to  third  segment  of  mid-brain. 

Trigeminus.     Sensory  and  lateral  roots  of  several  segments. 

Abducens.     Ventral  root,  perhaps  of  a  single  segment,  and  of 

the  same  segment  to  which  the  facial  nerve  belongs. 
7,  8.  Facialis-acusticus.  Sensory  and  lateral  roots  of  several 
nerves.  The  acustic  may  include  two  distinct  ganglia  and 
would  then  represent  two  sensory  roots.  The  facial  intervenes 
between  the  two  parts  of  the  acustic,  and  may  prove  to  be  the 
sensory  and  lateral  roots  of  one  segment. 

Olosso-pharyngeus.  Sensory  and  lateral  roots  of  one,  possibly 
two  segments. 

Vagus.  Sensory  and  lateral  roots  of  a  single  segment,  but 
secondarily  connected  by  means  of  a  persistent  epibranchial 
commissure  with  the  innervation  of  several  gill-clefts  of  the 
hypoglossal  region. 

Accessorius.  Lateral  roots  of  four  hypoglossal  nerves,  of 
which  the  ganglia  are  temporarily  developed,  with  accessions 
of  fibres  from  cervical  nerves. 

12.  Hypoglossus.  Ventral  roots  of  four  occipital  nerves  of  which 
the  ganglia  are  temporarily  present  and  of  which  the  lateral 
roots  form  the  accessorius. 


9. 


10. 


11 


Neuromere. 

Dorsal  root. 

Lateral  root. 

Ventral  root. 

FORE-BRAIN. 

1 

Olfactory. 

2 

MID-BRAIN. 

3 

Motor-oculi. 

4 

(?Motor-oculi.) 

5 

Trochlear. 

HIND-BRAIN. 

6) 

j 

I 

Trigeminus. 

>•  Trigeminus. 

9 

?  Acusticus. 

10 

Facial  is. 

Facialis. 

Abducens. 

11 

Acusticus. 

12 

Glosso-pharyngeus. 

ftlosso-pharyngeus. 

13 

Vagus. 

Vagus. 

14 

Accessorius. 

Hypoglossus. 

15 

Accessorius. 

Hypoglossus. 

16 

17 

Accessorius. 
Accessorius. 

Hypoglossus. 
Hypoglossus. 

BRANCHIAL  NERVES. — The  relations  of  the  nerves  to  the  segments 
(myotomes  and  neuromeres)  are  primitive,  the  relations  to  the 
branchial  arches  and  gill-clefts  are  secondary.  Indeed  we  must 
assume  that  the  vertebrates  had  segmented  ancestors,  who  acquired 


THE   NERVOUS    SYSTEM. 

gill-clefts,  segments  being  phyli >genet  it-ally  much  older  than  gill- 
clefts.  The  ancestral  nerves  were  adapted  to  the  gill-clefts,  and  we 
may  some-  day  know  the  history  of  that  adaptation  and  the  modifica- 
tions consequent  upon  it.  At  present  we  can  on'y  say  that,  contrary 
to  the  assumption  which  has  prevailed  for  twenty  years,  the  ^ill- 
clefts  are  not  segmental  and  therefore  the  branchial  nerves  are  not 
in  segmental  order. 

The  unquestionable  branchial  nerves  are  the  facial,  glosso-pharyn- 
geus,  and  vagus.  To  the  same  series  we  must  probably  a»ign  the 
trig* -mi nus  after  subtraction  of  its  ophthalmic  branch,  for  it  enters 
into  the  same  relations  to  the  mouth  as  the  other  nerves  mentioned 
to  the  gill-cleft>:  as  we  have  seen,  the  mouth  is  probably  a  modified 
pair  of  gill-clefts.  Counting  the  mouth  as  a  gill-cleft,  we  may  say 
that  each  of  the  four  nerves  arises  by  the  union  of  a  lateral  root  with 
mglion  to  form  a  common  nerve-trunk,  which  springs  from  or 
parses  by  the  epibranchial  organ  of  the  ganglion  and  descends  behind 
the  deft  with  which  the  nerve  is  associated,  in  the  visceral  arch 
between  that  cleft  and  the  next  following.  Later  there  arises  a 
branch  which  passes  in  front  of  the  cleft;  the  main  stem  is  then 
known  as  the  post-trematic  branch,  the  secondary  branch  as  the 
pra •-treniatic  branch.  In  the  lamprey  the  whole  series  of  epi- 
branchial organs  are  connected  by  a  continuous  longitudinal  commis- 
sure. In  mammalia  all  trace  of  the  commissure  is  lost  except  behind 
the  vagus,  which  thus  is  permanently  associated  with  the  fourth  and 
tifth  clefts  of  amniota,  to  which  it  does  not  morphologically  belong. 
Gegenbaur's  hypothesis  that  the  vagus  represents  several  branchial 
nerves  is  not  tenable,  for  reasons  explained  below.  I  regard  it  as 
probable  that  the  hypoglossus,  with  which  I  include  the  accessorius, 
will  l)e  ultimately  recognized  as  including  the  branchial  nerves  of 
the  fourth  and  fifth  clefts,  if  indeed  these  clefts  ever  possessed  true 
branchial  nerves. 

I.  Olfactory  Nerve.— Van  Wijhe,  82.1,  18,  has  sought  to 
prove  that  the  olfactory  nerve  is  not  really  the  first  but  the  second  of 
the  cerebral  nerves,  and  that  it  arises  further  back  morphologically 
than  the  optic  nerve.  The  development  of  the  fore-brain,  as  worked 
out  by  His  in  the  human  embryo,  p.  595,  renders  it  very  difficult 
to  accept  this  notion,  and  the  arguments  presented  by  Chiarugi, 
91.1,  seem  to  me  conclusive  that  the  olfactory  nerve  is  really  in 
front  of  the  optic. 

\\\*.  89.4,  717-723,  finds  in  the  human  embryo  that  the  nerve 
develops  as  follows:  The  first  step  is  the  separation  of  the  olfactory 
plate,  p.  575,  from  the  wall  of  the  brain  by  an  ingrowth  of  mesen- 
chyma.  This  separation  has  been  observed  by  Kolliker,  90.5,  in 
chicken  embryos  of  the  fourth  day  and  in  a  cow  embryo  of  10  mm. 
The  second  step  is  the  production  of  the  olfactory  ganglion;  the 
ectodermal  cells  of  the  olfactory  plate  multiply,  the  karyokinetic 
h'gures  being  found  next  the  outer  or  free  surface  of  the  layer;  the 
cells  thus  produced  assume  the  appearance  of  medullary  neuroblasts, 
and  at  four  weeks  are  found  migrating  toward  the  mesenchymal 
surface,  so  that  the  base  of  the  layer  of  the  olfactory  ectoderm  be- 
comes crowded  with  nuclei ;  the  protoplasm  of  these  neuroblasts  is 
collected  on  one  side  of  the  nucleus  in  a  pointed  mass ;  the  cells  now 


638  THE   FOETUS. 

grow  forth  from  the  ectoderm  and  constitute  the  anlage  of  the 
ganglion  between  the  ectoderm  and  the  brain.  The  third  step  con- 
sists in  the  assumption  of  the  bi-polar  form  *  by  the  cells  of  the 
ganglion,  and  the  elongation  of  the  poles  on  the  one  side  as  centrip- 
etal nerve-fibres  which  join  the  brain,  on  the  other  as  centrifugal 
fibres  which  join  the  olfactory  epithelium  (embryos  of  five  weeks). 
It  thus  appears  that  the  development  of  the  nerve  is  accomplished 
during  the  fifth  week  in  the  human  embryo.  Kolliker  has  observed 
that  in  the  rabbit  of  thirteen  days  the  ganglion  has  reached  the 
olfactory  lobe,  but  its  centripetal  fibres  have  not  penetrated  the  wall 
of  the  lobe ;  he  also  observed  in  the  same  rabbit  that  the  nuclei  of 
the  ganglion  were  dividing  karyokinetically,  and  he  considers  it 
probable  that  these  divisions  result  in  forming  chains  of  cells,  each 
chain  developing  into  one  nerve-fibre,  and  he  thinks  that  in  the 
adult  the  fibres  are  multinucleate.  Chiarugi  states,  91.1,  that  the 
olfactory  nerve  is  present  in  the  guinea-pig  embryo  of  4.7  mm., 
before  there  is  any  olfactory  lobe,  and  that  it  extends  from  the  brain 
wall  to  the  olfactory  plate.  Miss  Platt,  91.1,  260,  affirms  that  the 
olfactory  ganglion  is  derived  from  the  neural  crest,  but  has  published 
no  proof  of  this  affirmation. 

Concerning  the  morphological  interpretation  of  the  olfactory  nerve 
no  satisfactory  conclusions  are  yet  possible.  Marshall,  78. 1,  82. 1, 
advanced  the  theory  that  it  is  a  true  segmental  nerve,  or  at  least 
the  dorsal  root  of  one,  but  its  development  differs  so  much  from  that 
of  the  ordinary  ganglionic  nerve  that  I  hesitate  to  accept  this  theory. 
Marshall  has  sought  to  strengthen  his  theory  by  homologizing  the 
nasal  pits  with  a  pair  of  gill-clefts,  but  the  observations  he  has 
reported,  79. 1,  do  not  seem  to  me  to  justify  the  homology,  and  he 
has  failed  to  attribute  weight  to  the  fact  that  gill-clefts  are  primarily 
evaginations  of  the  entoderm,  while  the  nasal  pits  are  invaginations 
of  the  ectoderm  and  have  no  connection  with  the  pharynx  in  any 
vertebrate.  J.  Beard,  85.1,  modified  Marshall's  theory,  and  ho- 
mologizes  the  olfactory  plate  and  its  ganglion  with  an  epibranchial  or 
lateral  sense  organ.  We  know  (Chap.  XXVIII.)  that  the  ganglionic 
sense  organs  arise  by  a  union  of  the  ganglion  with  the  ectoderm,  but 
the  olfactory  sense  organs  arise  by  a  differentiation  of  both  the  sen- 
sory ectoderm  and  the  ganglion  from  a  common  ectodermal  plate. 
Nevertheless,  it  remains  a  tempting  hypothesis,  which  places  the 
nose  in  the  series  of  segmental  sense  organs,  but  at  present  it  is  still 
merely  an  hypothesis  with  no  secure  basis.  If  it  is  verified  hereafter, 
we  may  recognize  in  the  olfactory  nerve  a  true  ganglionic  nerve  or 
dorsal  root,  or  perhaps  the  representative  of  a  series  of  roots,  since  it 
is  possible  that  a  number  of  segments  have  disappeared  from  the 
pra3-oral  region,  and  each  segment  may  be  supposed  to  have  had  its 
nerve. 

That  the  olfactory  nerve  corresponds  to  a  spinal  dorsal  root  is 
rendered  probable  by,  1,  the  formation  of  its  fibres  from  bi-polar  cells; 
2,  the  ingrowth  of  the  fibres  from  the  ganglion  into  the  wall  of  the 
neuron. 

II.  The  Optic  Nerve. — The  development  of  the  optic  nerve  is 
treated  together  with  that  of  the  eye,  Chapter  XXVIII.  Concerning 

*  Chiarugi,  91.1,  suggests  that  some  of  the  cells  may  be  more  than  bipolar. 


THE   NERVOUS   SYSTEM, 


839 


FIG.  8C7.— Transverse  Se<'ti..n  through  tli»« 
Posterior  Part  of  the  Mill-brain  i.t'a  Human 
Embryo  of  five  weeks  (His4  rmbry. 


the  morphological  value  of  the  optic  nerve  nothing  is  known,  nor 
can  we  hope  t«>  form  any  satisfactory  hypothesis  as  to  its  value  until 
the  development  of  the  optic  nerve-fibres  is  thoroughly  nnderst* ><,<!. 
At  present  we  are  unable  to  say  whether  it  is  to  he  regarded  as  a 
nioilitication  of  a  true  nerve  or  of  a  cerebral  commissure. 

111.  The  Oculo-motor  Nerve. — The oOTfo-motor nerve, accord- 
ing to  \Y.  His,  88.3,  :>r>rs  aiises  from  neuroblasts  of  the  ventral 
column  of  His  in  the  mid-brain,  Fiu.  867;  transverse  sections 
of  the  brain  of  this  embryo  are  represented  in  Fi^  '»70. 

W.  His,  88.3,  ]  .  has  figured  the  nucleus  of  the  third  nerve 

as  a  broad  t^roup  of  pear-shaped  neu- 
roblasts, which  Ljive  off  the  centrifu- 
gal fibres  of  the  nerve;  some  of  the 
oculo-motor  neuroblasts  point  cent  ral- 
\vard  (His,  I.e.,  P.Martin,  90.1), 
and  Martin  states  that  he  has  ob- 
ed  bi-polar  forms  in  the  cat;  as 
to  the  further  history  of  these  two 
peculiar  kinds  of  cells  we  have  no  in- 
formation. As  shown  in  Fig.  368, 
the  nerve  grows  in  a  perfectly  straight 
line  to  t he  caudal  edge  of  the  eyeball, 
where  it  joins  the  anlage  of  the  eye- 
muscles.  Here  the  nerve  must 
branch,  since  it  is  distributed  in  the 
adult  to  live  muscles,  viz.:  the  levator  palpabrse,  rectus  superior, 
rectus  interims,  rectus  inferior,  and  obliquus  inferior.  No  observa- 
tions on  the  development  of  these  branches  in  the  mammalian  em- 
bryo are  known  to  me. 

The  development  of  the  motor  oculi  in  elasmobranchs  has  been 
much  studied,  with  conflicting  results.  In  Scylliinn  and  Pristiuris 
it  appears,  according  to  Van  Wijhe,  82.1,  22,  while  the  third  i^ill- 
cleft  is  developing,  which  is  about  the  stage  when  the  anterior  roots 
of  the  spinal  nerves  develop  according  to  Balfour.  In  Balfour's  stage 
L,  the  nerve  after  crossing  the  opthalmicus  prof undus runs  to  the  p<»- 
teri..r  edi;e  of  the  "first  myotome"  of  Van  Wijhe;  compare  A.  M. 
Marshall,  81.2.  The  path  of  the  nerve  passes  the  ciliary  ganglion 
(ganglion  mesocephalicum  of  Beard  and  Dohrn) ,  but  has  no  connec- 
tion with  that  ganglion  (Dohrn,  91.1,  G),  as  has  been  erroneously 
assumed  by  some  writers.  Miss  Platt,  on  the  contrary,  says,  91.2, 
99,  that  the  nerve  begins  as  a  single  cell  thrown  off  from  the  ciliary 
ganglion.  This  view  rests  probably  on  erroneous  interpretation  of 
observations,  for  it  cannot  be  admitted  that  a  motor  nerve  is  formed 
by  ganglionic  fibres.  Dohrn,  I.e.,  affirms  positively  that  medulla ry 
cells  leave  the  wall  of  the  brain  and  enter  the  nerve,  and  he  traces 
to  these  cells  the  development  of  those  which  constitute  the  ganglion 
of  the  nerve ;  but  his  observations  are  very  far  from  convincing  to 
me,  and  I  still  regard  it  as  possible  that  the  cells  observed  in  the 
nerves  are  mesenchymal,  and  if  this  is  the  case  then  it  is  also  possi- 
ble that  the  ganglion  of  the  nerve  is  of  mesenchymal  origin  and 
homologous  with  a  sympathetic  ganglion. 

The  ganglion  of  the  oculo-motor  nerve  in  selachians  was  discovered 


640 


THE    FCETUS. 


by  G.  Schwalbe  (Jenaische  Zeitschr.,  1879),  and  was  identified  by 
him  with  the  ciliary  ganglion  of  human  anatomy.  Van  Wijhe 
found  the  oculo-motor  ganglion  in  his  embryos  in  Balfour's  stage  O, 
and  pointed  out  that  it  was  distinct  from  the  true  ciliary  ganglion, 
which  belongs  to  the  ophthalmicus  profundus  nerve.  C.  K.  Hoff- 
mann, 85. 1,  302,  recognized  the  two  ganglia  in  reptiles,  but  applied 
the  term  ciliary  to  the  ganglion  of  the  oculo-motor,  and  the  term 
ophthalmic  to  that  of  the  ophthalmicus  profundus.  J.  Beard,  87.2, 
put  an  end  to  confusing  the  two  ganglia,  but  unfortunately  proposed 
to  restrict  the  term  ciliary  to  the  oculo-motor  ganglion,  andnto  intro- 
duce the  name  of  mesocephalic  for  the  ophthalmic  or  true  ciliary 
ganglion.  Beard's  nomenclature  is  erroneous,  for,  as  shown  by  His, 
88.2,  421,  the  ciliary  ganglion  of  the  embryo  is  identical  with  the 
ciliary  ganglion  of  the  adult,  and  the  oculo-motor  ganglion  is  always 
morphologically  distinct  from  the  ciliary.  Beard's  proposal  added 
to  the  existing  confusion  by  misapplying  the  term  ciliary.  Antonelli, 
so  far  as  one  can  judge  from  the  abstract  of  his  researches,  90. 1,  has 
again  confounded  the  oculo-motor  and  ciliary  ganglion.  The  true 
oculo-motor  ganglion  has  yet  to  be  discovered  in  mammalia.  For 
notices  of  the  conflicting  descriptions  of  the  structure  of  the  adult 
oculo-motor  ganglion,  see  A.  Dohrn,  91.1,  16-28. 

If  the  known  oculo-motor  ganglion  is  sympathetic,  then  it  is  possible 
that  the  thalamic  nerve  discovered  by  Miss  Platt  and  described  in 
the  following  section,  is  really  the  true  ganglion  of  the  third  nerve. 

Ill.a.  The  Thalamic  Nerve.— Julia  B.  Platt,  91.2,  97,  dis- 
covered a  rudimentary  ganglion  in  Acanthias  embryos  appended  to 
the  dorsal  part  of  the  mid-brain  close  to  the  fore-brain.  In  a  subse- 
quent paper,  91.1,  she  has  added  further  details.  The  ganglion  is 
developed  from  the  neural  crest  and  retains  a  connection  with  the 
ciliary  ganglion  along  what  must  be  regarded  as  the  epibranchial  line 
commissure.  The  commissure  is  stated  to  give  rise  to  the  ramus 
ophthalmicus  profundus  of  the  adult.  The  ganglion  proper  has  a 
transitory  existence.  It  seems  to  me  probable  that  the  ganglion 
may  prove  to  be,  as  suggested  in  the  last  sec- 
tion, the  true  primitive  ganglion  of  the  oculo- 
motor. 

IV.  The  Nervus  Trochlearis,  or  Pathe- 
ticus. — The  origin  of  this  nerve  in  the  embryo 
long  eluded  investigation;  thus  Marshall  and 
Spencer,  81.1,  and  Van  Wijhe,  82. 1,  25,  failed 
to  ascertain  its  early  history.  His,  in  1888, 
88.3,  365,  reported  that  in  a  human  embryo  of 
the  fifth  week  the  fourth  nerve  can  be  traced, 
Fig.  368,  from  its  point  of  exit  from  near  the 
median  dorsal  line  of  the  isthmus  (compare 
Fig.  363,  IV)  as  a  bundle  of  fibres  running 
down  through  the  mantle  layer  of  the  medullary 
wall  to  a  group  of  neuroblasts,  from  which  the 
nerve  arises,  and  which  are  situated  in  the  part 
of  the  medullary  tube  corresponding  to  the  ventral  zone  of  His.  It 
must  be  assumed  that  the  neuroblasts  send  out  the  fibres  in  a  differ- 
ent direction  from  what  we  find  in  the  case  of  all  other  medullary 


•K 


FIG.  368.— Section  of  the 
Brain  of  a  five  Weeks  Em- 
bryo (His'  Ko).  IF.  Fourth 
nerve;  N,  neuroblasts  of 
the  nerve  in  the  ventral 
zone  of  His.  After  W.  His. 


THE    NKKVoTS    S^  >TK.M.  641 

nerve-roots,  but  Martin's  observations,  noted  below,  indicate  that 
the  peculiar  course  <»t  the  fibres  results  from  migration  of  the  neuro- 
blasts.  It  may  In*  added  that  the  position  of  the  nucleus  of  the  nerve 
in  the  adult  agrees  with  that  of  the  neuroblasts.  as  observed  in  the 
embryo  by  His.  P.  Martin,  90.1,  reports  that  in  the  cat  the  fibres 
do  not  cross  in  the  earliest  stage,  but  make  their  exit  on  the  same 
side  on  which  their  neuroblasts  are  situated,  and  that  the  neuroblasts 
them>elvi'<  lie  at  first  higher  up,  and  later  migrate  to  the  ventral 
position,  in  which  they  were  seen  by  His,  as  just  stated.  Froriep, 
90.1,  .~»T,  has  observed  in  young  torpedo  embryos  that  the  nerve  of 
ei  tiier  side  receives  fibres  from  both  sides,  and  both  he  and  Dohrn, 
91.1,  have  oh- -rv.-d  in  elasmobranch  embryos  that  the  nerve  forms 
a  plexus  of  its  own  fibres  on  its  way  from  the  brain  to  the  muscles  it 
innervates. 

Dohrn,  91.1,  t»-ll,  has  observed  cells  in  the  course  of  the  nerve, 
especially  at  certain  points  where  they  are  accumulated  so  as  to  pro- 
duce a  thickening  of  the  nerve.  Dohrn  designates  these  cells  as  nerve 
cells  derived  from  the  medullary  canal,  but  neither  his  description 
nor  figures  justify  this  conclusion.  It  is  more  probable  that  these 
cells  are  surviving  remnants  of  the  trochlear  ganglion  or  possibly 
meivly  immigrated  mesenchymal  cells. 

The  ganglion  of  the  trochlearis  was  discovered  independently  by 
A.  Froriep,  91.2,  and  Julia  B.  Platt,  91.2,  in  elasmobranchs.  It 
i  -  a  part  of  the  neural  crest,  and  is  continuous  for  a  time  with  the 
anlage  of  the  trigeminal  ganglion;  the  connected  band  of  cells 
breaks  down  irregularly,  but  its  scattered  remnants  persist  for  a 
time  along  the  original  line.  At  this  stage  the  motor- fibres  grow 
out  from  the  medulla  near  the  dorsal  summit  of  the  ganglion,  and 
the  permanent  trochlearis  is  developed.  Miss  Platt  *  speaks  of  the 
ganglion  as  the  "primitive  trochlearis, " and  she  interprets,  p.  97, 
the  ramus  ophthalmicus  superficialis  trigemini  as  a  survival  of  the 
original  connection  between  the  trigeminal  and  trochlear  ganglia. 
As  the  connection  here  mentioned  is  on  the  level  of  the  dorsal  line  of 
the  neuron,  it  may  be  regarded  as  a  part  of  a  lateral  line  commissure. 
The  discovery  of  the  ganglion  of  the  fourth  nerve  further  demon- 
strates that  the  motor  fibres  represent  a  lateral  root.  In  torpedo 
embryos  of  W  mm.  Froriep,  /.r.,  56,  has  found  a  small  group  of  gan- 
glion cells,  which  soon  disappear,  but  at  this  stage  are  appended  to 
the  caudal  side  of  the  nerve  a  short  distance  below  the  ventral  limit 
of  the  mid-brain.  These  cells  are  probably  a  remnant  of  the  original 
ganglion.  Miss  Platt  thinks  that  the  trochlear  ganglion  also  con- 
tributes to  the  ciliary  ganglion,  but  her  proof  of  this  appears  unsat- 
isfactory to  me. 

V.  The  Trigeminal  Nerve. — This  is  one  of  the  most  compli- 
cated nerves  of  the  head.  It  is  developed  from  both  the  ganglia 
and  the  medullary  tube,  and  has  permanently  both  sensory  and  motor 
roots.  Its  ganglionic  portion  is  double,  comprising  the  ciliary  or 
ophthalmic  ganglion  and  the  Gasserian,  and  it  will  be  advantageous 
to  consider  these  two  parts  as  morphologically  distinct.  The  motor 
root  forms  a  single  bundle ;  the  nerve  enters  into  special  relations 

*  Miss  riatfs  il.'srriptinii  H  si>nn-\\  hat  obscured  by  her  overlooking  the  fundamental  differ- 

.•iic.-  i.i-twi-.-it  nifdullary  and 
41 


642  THE   FOETUS. 

with  the  epidermis,  and  finally  it  develops  a  typical  system  of 
branches.  Each  of  these  fundamental  characteristics  forms  the 
subject  of  a  separate  paragraph  following. 

GANGLION  CILIARE  AND  NERVUS  OPHTHALMIC  us  PROFUNDUS.— 
This  is  the  ganglion  which  has  been  long  and  generally  known  as 
the  ciliary,  and  becomes  the  ciliary  of  the  adult ;  for  mention  of  other 
names  applied  to  it  see  p.  640.  The  centrifugal  nerve  arising  from 
the  ganglion  is  known  as  the  ramus  ophthalmicus  profundus,  the  cen- 
tripetal nerve  as  the  radix  longa,  which  joins  the  trigeminal  ganglion 
before  the  radix  enters  the  brain.  How  the  ciliary  ganglion  becomes 
separated  from  the  trigeminal  is  unknown,  so  far  as  amniota  are 
concerned,  but  in  elasmobranchs  Van  Wijhe  thinks,  82.1,  20,  that 
a  considerable  middle  portion  of  the  originally  continuous  ganglionic 
mass  disappears.  In  the  human  embryo  at  one  month  the  ciliary 
ganglion  is  connected  with  the  trigeminal  by  a  bundle  of  fibres 
without  cells,  His,  88.3,  372.  Beard,  85.1,  30,  was  the  first  to 
observe  that  the  ganglion  unites  with  an  epidermal  thickening  of 
the  lateral  line.  He  says :  "  Cells  are  then  proliferated  off  from  the 
skin  to  form  the  ganglion,  and  the  outer  portion  of  the  thickening 
begins  to  form  the  primitive  branchial  *  sense  organ.  From  the 
thickening  cells  are  given  off  for  some  time  until  a  large  ganglionic 
mass  is  formed,  which  still  for  some  time  remains  fused  with  the 
skin."  C.  Kupffer,  91.1,  has  found  in  Petromyzon  embryos  a 
large  ganglion,  Fig.  407,  which  lies  in  front  of  the  trigeminal  gan- 
glion ;  this  ganglion  is  probably  the  ciliary  and  it  has  connection  in 
the  larva  (Ammoccetes)  of  4  mm.  with  an  epibranchial  organ;  this 
suggests  that  there  may  be  an  epibranchial  organ  of  the  ciliary 
ganglion  in  the  higher  vertebrate  embryos. 

It  is  probable  that  the  cells  of  the  ciliary  ganglion  become  bi-polar 
and  produce  ganglionic  fibres,  but,  so  far  as  I  am  aware,  no  observa- 
tions on  the  origin  of  the  nerve  have  been  published.  If  the  nerve 
arises  as  suggested,  then  the  centrifugal  fibres  must  constitute  the 
ophthalmic  nerve,  the  centripetal  the  radix  longa,  or  as  it  is  called  in 
human  anatomy  the  ophthalmic  branch  of  the  trigeminal,  compare 
Fig.  363.  In  this  figure  the  ciliary  ganglion  overlies  the  eye  and  is 
united  with  the  trigeminal  ganglion,  G.  Gr,  and  sends  its  nerve  for- 
ward toward  the  fore-brain.  Why  the  fibres  pass  to  the  brain  by 
way  of  the  trigeminal,  instead  of  making  an  independent  entrance, 
is  unknown.  A.  M.  Marshall  found  the  nerve  to  run  forward  from 
the  ganglion  in  elasmobranch  embryos  in  Balfour's  stage  K,  past  the 
upper  border  of  Van  Wijhe's  first  segment  and  the  inner  side  of  the 
eye,  to  end  at  a  point  just  dorsal  of  the  nasal  pit.  Some  further 
details  are  given  by  Van  Wijhe,  82.1,  20-22. 

2.  GANGLION  G  ASSERT,  OR  TRIGEMINAL  PROPER. — After  the  sep- 
aration of  the  ciliary  ganglion  the  Gasserian  (His,  88.3,  372)  has 
in  side  view,  Fig.  363,  G.  6r,  a  somewhat  triangular  form  in  the 
human  embryo ;  its  apex  points  dorsalward  and  sends  the  centripetal 
nerve-fibres  into  the  brain.  The  peripheral  nerves  it  gives  off  are 
accompanied  by  some  of  the  ganglion  cells,  which  are  thought  by 
His  to  be  destined  to  form  the  anlages  of  the  ganglion  rhinicntn 
arid  g.  oticum.  The  fibres  which  enter  the  brain  do  so  near  the 

*  In  consequence  of  later  researches  we  should  substitute  "lateral  "  for  "branchial." 


THK    NERVOUS    M  STEM. 


alible  formed  by  the  junction  of  the  dorsal  and  ventral  /ones  of 
His,  and  then-  take  a  longitudinal  course  as  a  Imndleof  lihres  lioinol- 
ogous  with  the  longitudinal  bundle  formed  by  tlie  spinal  ner 
Tliis  bundle  is  the  tract  HX  t  ri(/cm  ii/'ixur  aseendini;  tri.uvminal  root  of 
authnrs;  it  lies  close  to  the  surface  of  the  hrain  and  is  oval  in  section, 
being  flattened  laterally,  His.  I.e.,  Fitj.  ^T.  The  lumdle  j^rows  slowly 
do\vn  toward  the  spinal  cord.  In  the  adult  it  is  said  to  extend  into 
the  cervical  cord.  • 

Kupffer,  91.1,  41,  has  observed  that  in  Pt'tromy/oii  larvae  of  4 
mm.  the  trigeminal  ganglion  overlies  the  mouth  cavity,  IM-.  -1'C;  it 
has  a  strong  root  and  the  root  contains  fibrillae,  and  its  main  peripheral 
st« -in  branches  near  the  ganglion  to  form  the  maxillary  and  mandib- 
ular  branches,  both  of  which  are  compact  cords  of  fibres  with  nuclei 
among  them  and  partially  covered  by  a  cellular  sheath.  The  main 
trunk  is  also  connected  with  the  thickening  of  the  epidermis,  which 
constitutes  the  third  of  the  four  epi branchial  organs  overlying  the 
mouth  at  thN  stage.  Froriep,  85.1,  43,  searched  carefully  but  un- 
sncivs>fully  for  an  epibranchial  organ  connected  with  the  Gasserian 
-a n -lion  in  mammalian  embryos. 

;  MOTOR  ROOT  OR  PORTIO  MINOR. — The  motor  root  of  the  tri- 
in  us  is  developed  from  neuroblasts  of  the  ventral  zone  of  His 
in  the  hind-brain  at  the 
1  of  the  Varolian 
bend,  Fig.  363.  These 
neuroblasts  are  gathered 
together,  forming  the  tri- 
-•  'iiiinal  nucleus,  which 
early  becomes  recogniz- 
able. The  nucleus  lies, 
Fi:;.  301),  near  the  junc- 
tion of  the  ventral  and 
d<  irsal  columns  and  there- 
fore close  to  the  ascending 
sensory  root,  or,  of  the 
trigeminus.  The  fibres 
from  the  neuroblasts  are 
gathered  into  a  single 
stem  and  make  their  exit, 
as  shown  in  Fig.  309,  near 
the  dorso-lateral  edge  of 
the  ventral  zone  (His, 
88.3,  365). 

4.  PERIPHERAL 
BRANCHES. — The  trige- 
minus is  so  named  be- 
cause in  man  it  was 
observed  to  have  three 
branches.  One  branch,  as  we  have  seen,  runs  to  the  ciliary  ganglion, 
and  must  be  considered  as  belonging  morphologically  rather  to  that 
ganglion,  than  to  the  Casserian.  The  other  two  branches  run  respec- 
tively to  the  maxillary  and  mandibular  regions.  In  the  lamprey, 
Kupffer,  91.1,  41,  the  two  branches  arise  from  a  common  stem,  but 


FIG.  369.— Section  of  the  Brain  of  a  five  Weeks  Embryo 
(His'  Ko).  Ep,  Ependyma;  D,  dorsal  zone;  Ve,  ventral 
zone ;  ov,  oval  oundle  or  ascending  tract ;  V,  fifth  or  trige- 
minus nerve;  G.G,  ganglion  Gasseri.  After  W.  His. 


('•I  !  THE   FCETl'S. 

in  the  human  embrj'o  they  arise  separately  from  the  ganglion. 
'Whether  the  maxillary  and  mandibular  nerves  are  to  be  regarded  as 
branches  of  one  nerve  or  not,  must  be  decided  by  further  investiga- 
tions. It  is  possible  that  they  are  distinct  and  their  union  secondary, 
but  the  usual  view  is  that  they  are  primitive  branches.  This  view 
has  found  favor  chiefly  from  theoretical  considerations :  if  the  mouth 
be  interpreted  as  representing  a  pair  of  gill-clefts,  then  the  trigeminus 
may  be  interpreted  as  the  nerve  of  that  cleft,  and  its  two  branches, 
one  in  front  of,  the  other  behind,  the  mouth,  may  be  compared  with 
the  branches  of  the  branchial  nerves. 

No  satisfactory  observations  on  the  growth  of  the  branches  are 
known  to  me.  The  subject  would  well  repay  a  careful  investigation. 

VI.  Abducens  Nerve. — This  nerve  is  formed  exclusively  of 
medullary  nerve-fibres.  The  neuroblasts  which  produce  these  fibres 
have  been  found  by  His,  88.3,  365,  in  a  human  embryo  of  five 
weeks,  to  be  situated  in  the  ventral  zone  of  His  toward  the  median 
ventral  line,  Fig.  370,  and  the  fibres  pass  out  directly  from  the  wall 
of  the  brain,  hence  the  exit  of  the  root  lies  in  a  line  with  that  of  the 
hypoglossal  nerve  and  much  nearer  the  ventral  line  than  the  exits  of 
the  main  branchial  nerves  (trigeminus,  facialis,  glosso-pharyngeus, 
and  vagus) — compare  Fig.  363.  Fig.  370  also  shows  the  peculiar 
manner  in  which  the  abducens  is  embraced  by  the  inner  root  of  the 
facial.  The  fibres  do  not  pass  out  in  one  bundle,  but  as  first  observed 
by  A.  M.  Marshall,  78.1,  in  several  (four  to  seven)  small  bundles. 

The  facts  that  the  abducens  has  no  ganglion  and  arises  from  the 
ventral  side  of  the  brain,  were  discovered  by  A.  M.  Marshall,  78. 1, 
and  verified  by  Van  Wijhe,  82.1,  28.  Both  authors  interpreted  it 
as  a  ventral  root,  homologous  with  a  spinal  ventral  root,  and  corre- 
lated with  a  dorsal  root  represented  by  the  facialis.  His,  88.3, 
has  shown  that  the  relations  are  more  complicated,  and  has  rendered 
Marshall's  simple  hypothesis  untenable. 

As  regards  the  growth  of  the  nerve,  little  is  known.  In  torpedo 
embryos  of  16  mm.  (Froriep  91.2,  Fig.  1)  it  runs  straight  forward  to 
the  caudal  end  of  Van  Wijhe's  third  segment,  which  is  the  anlage 
of  the  external  rectus  muscle  of  the  eye.  A.  Dohrn,  91.1,  11-16, 
states  that  in  elasmobranchs  the  nerve  appears  in  Balfour's  stage  L; 
at  first  only  two,  later  more  fibres  could  be  observed.  The  nerve  at 
the  time  it  reaches  the  rectus  anlage  is  very  thin,  later  it  is  much 
thicker.  Dohrn  also  asserts  that  medullary  cells  continue  to  enter 
the  nerve  and  migrate  along  it  during  a  prolonged  period. 

VII.-VIII.  The  facial  and  acoustic  nerves  are  developed  in  all 
vertebrates  in  such  intimate  connection  with  one  another,  that  they 
are  necessarily  treated  together.  We  shall  take  up:  1,  the  develop- 
ment of  the  ganglion ;  2,  the  motor  roots :  3,  separation  of  the  acoustic 
ganglion :  4,  the  peripheral  branches. 

1.  GANGLION  ACOUSTIC-FACIALE. — As  already  stated,  p.  604,  this 
is  the  smaller  and  posterior  of  the  two  primary  ganglionic  masses, 
which  may  be  seen  in  front  of  the  otocyst  in  a  chick  of  thirty  to  forty 
hours  and  in  corresponding  stages  of  other  amniote  embryos.  His, 
88.3,  372,  gives  the  following  description  of  the  ganglion  in  a  five 
weeks'  human  embryo :  It  lies  close  in  front  of  the  auditory  vesicle, 
Fig.  363,  Gv;  it  is  somewhat  triangular  in  form,  with  its  apex  to- 


THK   NKKVnls    M  STEM. 


645 


ward  the  dorsal  side;  in  sections  its  elements  present  a  characteristic 
fun-like  grouping,  which  recurs  in  no  other  ganglion,  and  which  is 
due  lo  the  Iwisted  course  of  the  fibres  of  the  vcstibular  and  cochlcar 
branches  of  the  acoustic  nerve.  The  ganglion  includes  three  maaeee  of 
neurohlasN  :  the  innermost  or  medial  ma—  .  VITI.,isthean- 

•f  the  ganglion  <-<>cli- 
/<  /••  .  and  sends  its  centri- 
petal fibres  as  shown  in 
the  illustration  dorso-lat- 
erally:  the  outermost  or 
lateral  ma->  i-  the  an 
/'.  \"  1  I  1,  of  the  tfiunj/iim 
rrst ilml<ir< .  and  its  tibres 
enter  the  brain  with  a 
dors*  .-medial  inclination; 
the  middle  mas-  is  theail- 

*  .f  the  f acialjs  or  gran- 

////o;/     1/rnirn/i  ;    it     lies 

•omewnat  lower  down 
than  the  other  two,  and 
its  centripetal  fibres  form 
a  strikingly  compact  cord 
within  the  substance  of 
the  brain.*  Paul  Martin, 
90.3,  v!.!'.',  has  observed 
in  cat  embyrosof  0.8-0. li 
mm.  that  certain  fibres  of 
the  facialis  bend  over  so 

•i     t<»fnrm  n  Irmtyifnrliml       ]  tertion  of  the  BnUn  o«  •  Hunmii  Kmhtyo  of  Ihre 

as  to  lorm  a  longiimnnai  w.-rksdus-  K....  i-;,,.  K|,,.n.iviiia:  i>.  .i..i-xai  tone;  r..  vm- 
cord  which  later  joins  the  tral  zone:  ot'<  oval  '»«»'«"'•  ;"'  as,-,.i..iin>r  tra.-t:  vi.  sixth 

,       '     J    ,  nerve;  N'lII,  fi^lith  ncrvf  (intra-rraiiial  uaiiK'lioii);   r.N'III. 

I-    restibalar  branch;     VII,  wventli  n.-r\.-;     •  \  ill.  rochlear 


fore  not  formed  merely  by  glosso-pharyngeal  fibres.  The  whole  of 
the  triple  ganglion  becomes  later  included  in  the  cartilaginous  mass 
of  tiie  o.s  jtt'fmsum,  but  a  few  cells  are  retained  on  the  cerebral  side 
and  form  a  ganglion,  which  is  known  by  various  names,  and  which 
His  proposes  to  call  the  intracranwl.  According  to  C.  Kupffer, 
91.1,  the  acustico-facialis  ganglion  of  the  lamprey  unites  in  the 
embryo  with  four  spots  of  the  epidermis,  two  along  the  lateral  and 
two  along  the  epibranchial  line.  Of  the  former  one  is  a  union  with 
the  epithelial  wall  of  the  auditory  invagination,  the  other  lies  fur- 
ther head  ward,  being  situated  between  the  otocyst  and  the  trigeminal 
ganglion ;  where  the  anterior  union  takes  place  the  epidermal  cells 
contribute  to  the  development  of  the  facial  ganglion.  The  two  lower 
unions  take  place  by  means  of  ventral  prolongations  of  the  ganglion, 
which  unite  with  epidermal  thickenings  above  the  first  and  second 
gill-clefts  respectively.  Kupffer's  statements  suggest  that  the  gan- 
glion is  really  double,  otherwise  it  is  difficult  to  understand  why  it 
should  have  two  lateral  line  organs  and  two  epibranchial  organs. 
Van  Wijhe  observed  in  elasmobranchs,  82.3,  20,  that  the  facial  gan- 

*  His,  I.e.,  f»x>t-nc»t»'.  points  out  that  the  figure  of  this  ganglion   in  his  "Anat. menschl.  Em- 
bryonen,"  Heft  1,  p.  44,  is  not  correct. 


THE    FCETUS. 

gliou  unites  along  what  \ve  now  regard  as  the  lateral  line  and  again 
above  the  first  gill-cleft  with  the  epidermis ;  the  latter  connection  can 
be  seen  in  Balfour's  etage  K.  Beard,  85. 1,  also  observed  the  epi- 
branchial  connection.  In  amniota  the  lateral  line  connection  has 
not  yet  been  described,  but  Kupffer,  91.1,  52,  states  that  it  has 
been  found  in  birds.  The  epibranchial  connection  of  the  facial  gan- 

flion  has  been  very  carefully  studied  in  mammals  by  A.  Froricp, 
5.1 ;  it  is  present  in  cow  embryos  of  G-12  mm.,  and  is  most  dis- 
tinct in  those  of  from  7-9  mm.,  that  is  to  say,  with  three  gill-clefts, 
well  developed  externally ;  the  lower  end  of  the  ganglion  is  somewhat 
pointed  and  joins  a  small  thickened  area  of  the  epidermis  exactly  at 
the  dorsal  margin  of  the  first  or  hyomandibular  cleft  (Froriep,  I.e., 
Taf.  I.,  Fig.  I.);  there  is  no  distinct  boundary  between  epidermis 
and  the  ganglion,  and  it  is  possible  that  the  former  contributes  cells 
to  the  latter ;  the  thickened  area  is  slightly  invaginated  below  the 
level  of  the  surrounding  epidermis;  a  little  later  the  ganglion  is 
found  to  have  made  a  clean-cut  separation  from  the  skin. 

The  fate  of  the  facial  ganglion  proper  has  yet  to  be  traced.  The 
embryonic  facial  nerve  has  in  its  ganglion,  of  course,  ganglionic 
neuroblasts,  and  must  be  regarded  as  originally  a  mixed  nerve. 

2.  MOTOR  ROOTS. — Our  knowledge  of  these  is  derived  almost  ex- 
clusively from  the  observations  of  His,  88.3,  362,  for  His  is  almost 
the  only  embryologist  who  has  studied  the  histological  development 
of  nerves,  and  it  is  only  by  such  study  that  the  history  of  the  motor 
roots  can  be  followed.     In  a  human  embryo  of  five  weeks,  the  facial 
nerve-fibres  leave  the  brain  as  a  compact  bundle,  a  little  distance 
headward  of  the  auditory  vesicle  and  at  a  point  just  ventral  of  the 
root  of  the  acusticus;  this  bundle  may  be  followed,  Fig..  370,  for 
some  distance  within  the  brain,  ascending  at  first,  then  arching  over 
and  descending  near  the  border  between  the  mantle  layer  and  the 
inner  layer  toward  the  median  ventral  line,  where  its  fibres  spread 
out  and  apparently  take  a  longitudinal  course ;  the  facialis  neuro- 
blasts are  situated  in  the  lateral  part  of  the  ventral  zone  of    His 
and  lie  in  the  region  of  the  otocyst ;  the  course  of  the  fibres  from  the 
neuroblasts  to  the  actual  root  has  not  been  fully  traced,  but  His  thinks 
they  join  the  formatio  arcuata,  then  enter  the  longitudinal  bundle 
near  the  median  line  and  form  there  the  arching  bundle  of  fibres  just 
described.     The  circuitous  course  of  the  motor  fibres  is  very  early 
developed,  but  no  reason  for  that  course  is  yet  known. 

3.  HISTORY    OF    THE    ACOUSTIC    GANGLION    AND    ITS   NERVE 
BRANCHES. — The  following  account  is  based  on  a  paper  by  Wilh. 
His,  jun.,  89. 1,  in  which  the  development  in  the  human  embryo  is 
described,  and  the  previous  researches  of  others  are  reviewed.     As 
stated  in  the  previous  paragraph,  traces  of  the  triple  division  of  the 
ganglion  are  evident  toward  the  end  of  the  fourth  week.     By  the 
middle  of  the  fourth  week  the  auditory  vesicle,  Fig.  371,  shows  the 
anlages  of  the  cochlea  and  the  semicircular  canals,  and  the  ganglion 
shows  clearly  its  triple  division ;  the  facial  nerve  has  its  characteristic 
bend,  for  it  descends  from  the  brain  very  steeply,  passes  through  the 
horizontal  ganglion  geniculi,  G.g,  and  then  descends  again.  The  acus- 
tic  ganglion  lies  closer  to  the  brain-wall  than  the  facial  and  is  divided 
by  the  latter  into  the  upper  and  outer  ganglion  vestibuli,  Gv,  and  the 


THE    NERVOUS    sN  -I  !.M. 


647 


lower  and  inner  ganglion  cochlea?,  Gco.  The  facial  ganglion  descends 
t«>  a  ]<>\vr  level  than  the  ac<  uist ic,  and  then '\vith  the  two  have  linally 
separated.  A  few  days  pater  the  division  of  the  acoustic  uan-linu 
into  an  upper  and  lower  part  becomes  still  more  marked,  bec;m>c  the 
root  of  the  facialis  take>  ;i  more  nearly  horizontal  course  to  ihe  facial 
^•an-'lion  and  then  descends.  Both  parts  of  the  acoustic  i^an^lion  li -• 
in  front  of  the  otocyst  and  come  in  contact  only  with  its  front  wall, 
and  it  is  «.nly  on  this  wall  that  the  macula4  acusticae  are  developed. 
At  live  weeks  t he  semicircular  canals  having  formed  and  the  twi>t- 
inic  of  the  cochlea  having  beu-un.  the  fibres  of  the  acoustic  ganglion 
are  found  united  with  the  auditory  vesicle.  The  fibres  from  the 
cochlear  pin^Hon  form  a  stem,  the  ncrrns  OOcAfcorttf,  and  two 
Mnaller  branches,  corresponding  to  the  middle  branch  of  otologists, 
which  run  resperti vely  to  the  anlage  of  the  macula  saccnli  and  the 
anlagoof  the  macula  <im/tnllce  posteriarit*.  The  fibres  from  theves- 

tibular  ganglion  form  a  single  stem 
\^R.v  running  to  a  spot  which  includes 
Br  I  1  the  anlages  of  three  maculae, 

namely,  of  the  vestibule  and  of  the 


vra 


Cch 


J 


*27 


1  NVrves  of  a  Human 

Kmliryo  <>f  four  ami  a  half  Wr»-ks  ("Nacken- 
'  in.;!  nun.  i  K  •••onstrurtinii.  Hi.  Wall 

<>f  'Tain;  A.xr,  antfrinr:  P.«c,  postrrior: 
mat  s<Miiii-ii-c-iilar  canal  anlage; 

VIII.  auditory  H.TVI-;  VII,  fm-ial  nervo;  Or, 
'ii  v.'stihiili;  a.!/,  K:iiiKrli""  ^'••niculi; 

ll)-li..H    ,-o,-|i|,-;l  \.-Stl- 

Joili;  I't.  iitrii-ulii^:  .s<*r,  saccubus;  C'c/t,  coch- 
Lftar  w.  UK.  junior. 


IILriOC 


Fio.  372.— Acoustic  Ganglia  of  a  Human  Em- 
bryo of  two  Months.    Tfie  flmra repnMata a 

nio<ifl  of  the  ganglia  of  the  l«-ft  si-l.-  s«-»-n  from 
l>.-liin«l.  Mr,  Surfm-f  of  hrain  ;  r.  ccH-hlcar  root; 
il.ular  root  :  /.  facial  root:  m.*<»'.  branch 
to  macula  sac.-uli:  <;<•<>,  ^an^lion  cochleae; 
r.nt.  ln-anch  to  P-C«^<US  utriculi;  Gg,  Ran- 
Kli«'n  genicoli;  VII.  facial  IHM-VI-;  <>  > ;/,  branch 
to  i-xti-rior  ampulla:  n.,mt.  I. ranch  to  anterior 
ampulla.  After  W.  II is.  junior. 


anterior  and  external  ampulla ;  in  more  advanced  stages  the  maculae 
separate  and  each  receives  a  separate  branch  of  the  vestibular  nerve; 
this  is  an  excellent  illustration  of  the  dependence  of  nerve  branches 
upon  secondary  changes  in  their  peripheral  connections.  While  the 
nerve  branches  are  developing  the  ganglia  elongate  ventralward,  and 
at  t  lie  same  time  changes  occur  in  the  distribution  of  the  ganglion 
cells.  In  the  cochlear  ganglion  most  of  the  cells  remain  near  the 
<•«  -I  -hlea,  where  they  are  ultimately  converted  into  the  spiral  ganglion; 
others  a s» vi id  with  the  fibres  to  the  brain,  at  the  edge  of  which  they 
accumulate,  being  stopped  by  the  dense  neuroglia  (Randschleier) , 
and  give  rise  to  His'  inter-cranial  ganglion,  mentioned  above;  still 
others  remain  strung  out  along  the  line  where  the  cochlear  ganglion 
is  in  contact  with  the  vestibular;  this  line  of  ganglion  cells  is  called 


648  THE   FCETUS. 

the  Zwischen-fjanglion  by  W.  His,  jun. ;  the  fibres  from  these 
cells  constitute  the  branches  running  to  the  sacculus  and  the  poste- 
rior ampulla.  In  the  vestibular  ganglion  the  cells  are  more  evenly 
scattered  and  persist  in  the  adult  distributed  along  the  nerve.  The 
further  development  consists  in  little  more  than  a  series  of  adapta- 
tions to  the  advancing  differentiation  of  the  membranous  labyrinth. 
Fig.  372  represents  the  parts  just  described,  but  in  a  somewhat  more 
advanced  stage.  As  to  the  course  of  the  fibres  within  the  brain,  we 
possess  no  satisfactory  information;  see  W.  His.  jun.,  /.c.,  p.  17-11). 

4.  PERIPHERAL  BRANCHES  OF  THE  FACIALIS. — In  mammalian 
embryos,  soon  after  the  facial  ganglion  has  united  with  the  epidermis 
to  form  the  epibranchial  organ  over  the  hyo-mandibular  cleft,  the 
nerve  proper  grows  down  into  the  hyoid  arch,  and  thus  develops  the 
homologue  of  Van  Wijhe's  post-trematic  branch.  Somewhat  later. 
Froriep,  85.1,  44,  another  branch  is  formed  from  the  oral  side  of 
the  ganglion,  and  this  branch,  which  is  probably  homologous  with 
the  rami  prse-trematicus  and  pharnygeus  of  selachians,  extends  into 
the  mandibular  arch.  Froriep  has  observed,  87.1,  in  torpedo  em- 
bryos in  Balfour's  stage  L,  a  branch  of  the  post-trematic  facial  run- 
ning forward  below  the  gill-cleft  into  the  mandibular  region  to  there 
innervate  a  mucous  canal ;  this  branch  Froriep  considers  the  homo- 
logue of  the  chorda  tympani  of  amniota ;  the  union  of  the  chorda 
with  the  trigeminus  is  secondary.  The  branches  in  elasmobranch 
embryos  have  been  carefully  described  by  Van  Wijhe,  82. 1,  2o-'.)(.», 
who  refers  also  to  the  earlier  observations  of  Balfour  and  of  Mar- 
shall and  Spencer. 

Rabl,  87.1,  ascribes  the  peculiar  distribution  of  the  facialis  in 
the  adult  mammal  to  the  fact  that  it  innervates  the  myothelium  of 
the  hyoid  arch ;  this  myothelium  develops  into  the  embryonic  pla- 
tysma,  and  the  platysma  spreads  out  and  is  ultimately  differentiated 
into  the  superficial  facial  muscles.  The  nerve  follows  the  muscle, 
and  as  the  latter  subdivides  the  former  branches  correspondingly. 

IX.  The  glosso-pharyngeal  nerve  has  been  taken  by  many 
embryologists  as  the  most  typical  nerve  of  the  head,  because  it  has 
two  distinct  roots  and  its  relations  to  the  second  gill-cleft  are  very 
clear,  and  it  has  been  assumed  that  the  cranial  nerves  typically  all 
have  two  roots  and  are  similarly  related  to  gill-clefts ;  compare  His, 
88.2,  423.  It  is  to  be  remembered  that  the  assumption  that  the 
glosso-pharyngeus  is  par  excellence  the  typical  cerebral  nerve  .is 
the  outcome  of  the  necessities  of  a  certain  school  of  speculative  mor- 
phologists.  The  assumption  is  by  hypothesis,  and  is  by  no  means 
sufficiently  upheld  by  observation.  We  will  consider:  1,  the  gan- 
glion and  its  sense  organs;  2,  the  motor  roots;  3,  the  peripheral 
branches. 

1.  THE  GANGLION  AND  ITS  SENSE  ORGANS.— The  ganglion  is 
the  third  of  the  four  primary  ganglionic  masses  of  the  head,  and  is 
situated  immediately  behind  the  otocyst.  It  forms  at  first  a  contin- 
uous anlage  with  the  vagus  ganglion.  In  a  chick  of  thirty  to  forty 
hours,  seen  from  above,  it  appears  as  a  rounded  mass  about  equal  to 
the  auditory  vesicle  in  size  (His,  88. 1,  417) .  It  has  been  commonly 
stated  since  Marshall's  paper,  78. 1,  in  1878,  that  there  is  first  formed 
a  common  ganglionic  mass  behind  the  ear,  and  that  this  mass  divides 


TIIK  NEKvors  M  STKM.  649 


into  two  ganglia,  the  giono-pharyngeft]  and  vagus. 
however.  90.  1,  :;:'»»:,  helieves  that  the  ganglion  of  the«  eighth  nerve 
;iri>«-s  in  Saun  -psida  as  an  independent  outgrowth  of  the  ganglion  i<- 
conl  (neural  cn-M  ).  and  a  pjM'ars  before  the  vagus.  He  tind>.  p. 
that  in  tin-  rahhit  tin*  two  ganglia  are  distinct  though  they  appear  ;;t 
nearly  tin-  same  time  (emhryos  of  4.5  mm.  ).  In  the  human  emhry.  > 
tin-  cells  In-come  bi-polar  and  produce  nerve-tilnv*  during  tin-  lit'th 
\\.-.-k.  The  primitive  mass,  according  to  His,  88.3,  '•>',  I,  early 
divides  into  an  upper  or  dorsal  smaller  spindle-shaped  part,  /•////••  //- 
/•///rr'.s  <i<in<il  inn.  and  a  lower  or  ventral  larger  oval  part,  the  V<m- 

glion  petroswn  proper,  Kig.  </>.     The  former  lies  close  behind 

the  auditory  vesicle  and  later  is  covered  by  the  cochlea  ;  the  latter 
moves  away  t  V«  >m  the  otocyst  to  take  a  place  on  a  level  with  the 
pharynx.  The  centripetal  tibres  form  a  single  hnndle,  which  enters 
the  hrain  near  the  lower  edge  of  the  dorsal  zones  of  His,  and  there 
1-iking  a  longitudinal  <  'nurse  descends  toward  the  spinal  cord;* 
within  the  medullary  wall  the  lihres  eon-til  ute  the  ascending  gl<>>-« 
phary  lineal  tract.  In  an  emhrvo  of  r.  '.'  mm.  NL  (His'  Br1)  His, 
/./•..  found  the  tract  not  t«.  have  reached  the  vagus  region,  but  later 
it  is  longer  and  the  tihres  mingling  with  those  of  the  vagus  form  a 
very  characteristic  coj-.l.  t  he  //  -in-fiix  xn/ifuriux.  which  can  IM>  fol- 
lowed into  the  spinal  cord.  It  is  probable  that  both  the  triuvminal 
and  facial  ganglia  send  fibres  to  this  tract  u-. 

The  nerve  was  erroneously  supposed  by  Balfour  ("  Works,"  I.,  1  .'.'.  L 
Marshall  and  Van  Wijhe,  82.1,  !»,  to  arise  exclusively  from  the 
ganglion,  as  owing  to  their  neglect  to  consider  the  origin  of  the 
nerve-  lihres  they  failed  to  see  the  true  motor  roots.  Proceeding  upon 
this  false  assumption  they  have  endeavored  to  interpret  the  nerve  a> 
the  morphological  equivalent  of  a  dorsal  spinal  root.  His'  observa- 
tions oblige  us  to  discard  this  interpretation. 

C.  Kupffer,  91.1,  -II,  found  in  the  lamprey  that  the  glosso-pha- 
ryngeal  ganglion  is  differentiated  later  than  the  other  cephalic  ^;\u- 
glia,  and  is  at  first  intimately  associated  with  the  anlages  of  the 
auditory  vesicle  and  facial  ganglion.  Like  the  other  ganglia  it  is 
soldered  in  the  emhrvo  to  the  epidermis  of  the  lateral  line,  and  after 
widening  out  at  its  ventral  end  it  unites  (ammoccetes  of  4mm.) 
broadly  with  the  epidermis  a  second  time  to  form  the  epibranchial 
organ  above  and  in  front  of  the  third  gill-cleft,  Fig.  407. 

In  Petromyzon,  as  just  stated,  the  ganglion  has  the  lateral  and 
epibranchial  organs,  and  it  is  probable  that  both  exist  in  other  ver- 
tebrates; but  as  yet  only  the  mammalian  epibranchial  organs  have 
been  accurately  studied  by  Froriep,  85.1,  although  the  lateral  line 
oman  was  seen  by  Van  Wijhe,  82.1,  29,  in  shark  embryos  in  P>al- 
four's  stage  K.  Froriep,  Z.c.,  p.  12,  observed  in  cow  embryos  of  8.5 
mm.  that  at  the  dorsal  border  of  the  second  gill-cleft  there  is  a 
slightly  depressed  area  of  thickened  epidermis,  which  is  united  with 
the  lower  part  of  the  ganglion;  in  embryos  of  10  mm.  the  organ 
has  disappeared,  but  its  final  history  is  somewhat  uncertain  (p.  46). 

•.'.  MOTOR  ROOT.  —  The  origin  of  the  motor-roots  in  the  embryo 
has,  so  far  as  I  am  aware,  been  studied  only  by  W.  His,  88.3,  361. 

*  The  arrangement  is  figured  by  W.  His,  88.3,  Fig.  22;  it  is  similar  to  that  of  the  vagus.    See 

Fig. 


050  THE   FCETUS. 

The  neuroblasts  are  gathered  in  the  upper  part  of  the  ventral  zone 
of  His,  as  a  group  which  is  quite  clearly  separated  from  the  neuro- 
blasts of  the  facial  and  vagus  nerves.  The  fibres  from  these  neuro- 
blasts are  gathered  into  a  single  bundle,  which  leaves  the  medullary 
wall  near  the  dorsal  end  of  the  ganglion,  so  that  it  seems  to  form, 
if  we  disregard  the  origin  of  the  fibres,  a  part  of  the  true  dorsal  or 
ganglionic  root ;  compare  p.  048. 

3.  PERIPHERAL  BRANCHES. — The  glosso-pharyngeus  enters  into 
close  relations  with  the  second  gill-cleft.  As  long  known  through 
comparative  anatomy,  the  nerve  typically  .forms  two  branches  when 
it  reaches  the  gill-cleft,  and  the  general  history  of  these  brand irs 
has  been  followed  in  elasmobranch  embryos  by  Balfour,  Van  Wijhe, 
and  Beard.  One  branch  runs  in  front  of  the  gill-cleft — in  other 
words,  in  the  posterior  part  of  the  hyoid  arch ;  this  branch  is  the  prce- 
trematic  of  Van  Wijhe,  82. 1  (prse-branchial  of  Beard).  The  other 
branch  runs  behind  the  gill-cleft — in  other  words,  in  the  anterior 
part  of  the  first  branchial  arch  ;  this  branch  is  the  post-trematic  of 
Van  Wijhe  (post-branchial  of  J.  Beard).  These  branches  are  both 
developed  after  the  epi branchial  organ,  and  in  fishes  are  nearly  equal 
in  size. 

In  mammals,  according  to  Froriep,  85.1,  13,  20,  44,  the  post- 
trematic  becomes  the  main  stem,  which  is  found  in  cow  embryos  of 
8.8  mm.  running  through  the  first  branchial  arch  and  curving  for- 
ward below  the  gill-cleft,  while  the  prse-trematic  branch  is  a  very 
small  bundle  of  fibres  at  this  stage,  and  apparently  persists  as  the 
nervus  tympanicus  of  the  adult.  The  post-trematic  is  the  retain* 
lingualis  of  the  adult,  the  ramus  pharyngeus  being  added  in  later 
developmental  stages.  It  may  be  noted  that  the  so-called  pharyn- 
geus of  elasmobranch s  belongs  to  the  prse-trematic. 

In  the  human  embryo  the  nerve  grows  straight  down  from  the 
medulla  at  first  (His'  Br3),  but  in  an  embryo  of  four  and  one-half 
weeks  (His'  Ko)  it  is  already  bent  at  its  end  owing  to  the  dislocation 
of  the  parts  of  the  pharynx,  His,  88. 3.  ,379.  Noteworthy  is  the 
early  union  of  the  ganglion  petrosum  with  the  ganglion  nodosum 
by  an  oblique  anastamosing  branch,  Fig.  303,  the  development  of 
which  has  not  yet  been  followed. 

X.  The  Vagus  Nerve.— A  few  words  on  the  general  morphol- 
ogy of  this  nerve  may  be  prefixed  to  the  history  of  its  development. 
Gegenbaur,  71.1,  72.1,  directed  especial  attention  to  the  fact  that, 
unlike  any  other  nerve  of  the  head,  the  vagus  supplies  several  gill- 
clefts;  all  the  clefts,  whatever  their  number,  behind  the  glosso- 
pharyngeal  cleft  being  innervated  by  the  tenth  nerve,  which  in 
fishes  shows  its  relations  clearly,  since  it  sends  off  a  prse-trematic  and 
post-trematic  branch  for  each  gill-cleft  of  the  vagus  series.  The 
number  of  the  branches  in  any  form,  of  course,  depends  upon  the 
number  of  clefts  preserved  in  that  form.  As  Gegenbaur  had  formed 
the  theory  that  the  cephalic  nerves  correspond  with  the  gill-clefts, 
there  being  a  nerve  for  each  cleft,  he  necessarily  concluded  that  the 
vagus  was  the  morphological  equivalent  of  several  branchial  nerves. 
This  conception  of  the  vagus  has  been  generall}r  adopted,  and  has 
been  so  generally  taught,  that  many  of  the  younger  morphologists 
seem  to  have  forgotten  that  it  has  remained  a  bold  hypothesis,  and 


TI1K    NKK\  <»l   -    M  ^TEM.  651 


that   there  is   no  evidenee  whatever  of  an   actual  fusion  of  Beveral 
nerve-   into  one  vagus  nerve  to  be  obtained  from  vertebrate  embry- 
ology.     Ni  •v«Tth«-l«-».  (  ie-vnbaur's  theory  has  dominated  all  in 
tigations  of  tin-  last  twenty  years. 

\Ye  now  know  —  compare  p.  200  —  that  the  gill-pouches  only  imitate 
tin-  >eginental  arrangement,  and  are  in  reality  much  less  nmnen.us 
than  tin-  true  segments  of  the  hranchial  region.  and  that  the  D6ITQa 
do  not  correspond  to  the  number  of  segments.  In  view  of  the  -Teat 
irregularities  of  the  nerves  as  compared  with  the  myotomes  of  the 
head,  we  are  no  longer  justified  in  interpret  in-  the  vagus  so  as  to 
make  n  conform  to  a  theoretical  order,  which  is  definitely  ascertained 
not  to  agree  with  the  real  order  —  in  other  words,  it  is  not  necessary 
to  suppose  that  each  gill-cleft  had  a  separate  nerve  and  just  one 
nerve.  Further  we  must  conceive  that  there  was  primitively  a  chain 
of  epi  branchial  organs,  which  were  connected  longitudinally  with 
one  another,  and  transversely  with  several  hypoglossal  nerves,  but 
we  have  at  present  no  reason  for  assuming  that  the  series  of  cephalic 
nerves  extended  as  far  as  the  epihran*  hial  organs.  On  the  contrary 
the  series  of  epi  branchial  organs  (like  those  of  the  lateral  line)  may 
have  extended  tailward,  by  the  growth  of  a  branch  consisting  of 
nerve  fibres  derived  from  probably  several  hypoglossal  nerves;  both 
the  lateral  and  epihranchial  branches  while  they  grow  are  united 
with  the  epidermis. 

I  conceive  the  primitive  condition  to  have  been  one  in  which  there 
were,  presumably,  four  cephalic  nerves  behind  the  vagus,  and  that 
these  nerves  had  each  it>  epibranchial  organ;  the  four  nerves  are 
now  represented  by  the  hypoglossus  and  accessorius.  The  epi- 
hranchial organs  were  connected  with  one  another  by  a  longitudinal 
commissure,  which  persisted  while  the  four  hypoglossal  ganglia  dis- 
appeared. and  thus  the  epi  branchial  organs  and  the  nerve  branches 
running  from  them  to  the  gill-clefts  became,  apparently,  branches  of 
the  vagus.  While  one  thus  recognizes  the  relation  of  the  vagus  to 
several  gill  -clefts,  that  relation  is  not  primary,  but  secondary  and 
acquired,  and  does  not  in  my  judgment  lend  support  to  Gegenbaur's 
hy  p«  >t  hesis.  Another  consequence  of  the  abortion  of  the  hypoglos-a  I 
ganglia  has  been  to  leave  their  lateral  medullary  roots  to  be  modified 
into  a  separate  nerve-stem,  the  accessorius,  and  to  join  the  ganglion 
of  the  vagus. 

The  considerations  advanced  above  lead  me  to  the  conviction  that 
Gegenbaur's  conception  of  the  vagus  as  morphologically  equivalent 
to  several  nerves  can  no  longer  be  maintained,  and  instead  we  must 
return  to  the  older  view  and  again  look  upon  the  relation  of  the 
vagus  to  the  posterior  gill  -clefts  as  the  result  of  the  distribution  of  a 
branch,  which  may  be  named  the  nervus  epibranchialis,  and 
which,  so  far  as  its  connections  with  the  epidermis  are  concerned, 
may  be  compared  with  the  lateral  nerve.  That  Gegenhaur's  theory 
is  untenable  is  shown  by  the  development  of  the  hypoglossal  nerve. 
which  includes  the  nerves  of  the  segments  immediately  behind  the 
vagus  nerve  and  above  the  posterior  branchial  clefts,  so  that,  as  a 
matter  of  fact,  the  segmental  nerves  of  the  posterior  branchial  region 
are  incorporated  not  in  the  tenth,  but  in  the  twelfth  nerve. 

1.  U  A.M.  LION   AND  GANGLiONic  ORGANS.  —  The  ganglionic  crest 


i;,V>  THE    FCETUS. 

behind  the  otocyst  develops  its  two  large  ganglia  somewhat  later 
than  does  the  mass  in  front  of  the  otocyst ;  thus  in  a  torpedo  embryo 
of  6  mm.  Froriep,  98.2,  GO,  found  the  two  anterior  ganglia  divided, 
but  the  two  posterior  were  undivided.  The  ganglion  is  in  amniota 
at  first  a  rounded  mass,  which  may  be  seen  in  a  chick  of  thirty  to 
forty  hours  lying  immediately  behind  the  glosso-pharyngeal  ganglion, 
which  it  about  equals  in  size  (W.  His,  88.2,  417,  Fig.  2).  The 
exact  history  of  the  ganglion  has  never  been  followed.  Chiarugi, 
90. 1,  observed  that  the  ganglion  arises  in  reptiles  as  a  conical  bud, 
which  grows  down  from  the  neural  crest;  later  (Lacerta  embryos  of 
5.5  mm.)  it  arises  by  three  bundles  of  fibres,  of  which  the  first  and 
last  represent  the  persistent  neural  crest  and  unite  the  ganglion  re- 
spectively to  the  glosso-pharyngeus  and  first  cervical  ganglia,  while 
the  middle  bundle  is  the  root  proper,  connecting  the  vagus  ganglion 
with  the  neuron.  This  stage  has  been  described  by  Beraneck  and 
was  the  earliest  seen  by  him.  In  mammals  (Chiarugi,  90.1,  42-46) 
the  ganglion  also  arises  independently,  and  as  it  grows  ventralward, 
passes  outside  of  the  jugular  vein  and  aorta,  unlike  the  glosso- 
pharyngeal  ganglion,  which  passes  inside  these  vessels.  In  rabbit 
embryos  of  G.5  mm.  Chiarugi  found  the  ganglion  attached  by  slender 
commissures  to  the  glosso-pharyngeal  ganglion  in  front  and  the  first 
cervical  behind.  The  medullary  root  next  lengthens  considerably. 
and  in  embryos  of  11  mm.  the  ganglion  is  subdivided  into  the  dorsal 
ganglion  jucjulare  and  the  ventral  ganglion  nodosum.  In  a  cow 
embryo  of  8-0  mm.  the  ganglion  is  much  larger  than  the  glosso- 
pharyngeal  or  facial,  and  extends  over  the  third,  fourth,  and  fifth 
clefts  (Froriep,  85.1).  It  elongates  in  an  oblique  dorso- ventral 
direction.  In  a  human  embryo  of  four  and  one-half  weeks  (W.  His, 
88.3,  375)  it  is  very  long  and  divided,  as  just  mentioned  for  rabbits, 
into  an  upper  ganglion  jugulare  and  a  lower  ganglion  nodosum, 
Fig.  363,  Gj  and  Gn,  connected  by  a  narrower  fibrous  tract,  along 
which  are  scattered  a  few  ganglion  cells.  The  jugular  ganglion  is 
spindle-shaped,  and  has  on  its  inner  side  a  bundle  of  fibres  which 
enters  the  lower  edge  of  the  dorsal  zone  of  His,  Fig.  379,  then' 
takes  a  longitudinal  course  toward  the  spinal  cord  as  a  well-marked 
ascending  vagus  tract;  the  tract  is  at  first  very  short;  it  is  soon 
joined  by  the  fibres  of  the  trigeminal  tract,  and  the  two  sets  of  fibres 
uniting  constitute  the  so-called  tractus  solitarius,  as  mentioned 
above  in  describing  the  trigeminal  ganglion,  p.  642.  The  tractus 
solitarius,  as  shown  in  Fig.  379,  has  at  first  a  superficial  position, 
but  later  it  loses  this  place  in  appearance,  being  covered  over  by 
the  Randlippe  of  His,  compare  p.  666  and  Fig.  381. 

The  vagus  ganglion  probably  has  both  lateral  and  epibranchial 
organs  in  the  embryos  of  all  vertebrates.  In  elasmobranchs  both 
were  seen  by  Van  Wijhe,  82.1,  and  have  been  more  accurately  de- 
scribed by  Froriep,  91.2.  In  Petromyzon  they  have  been  described 
by  Kupffer,  91.1.  In  teleosts  the  lateral  line  organs  are  greatly 
developed,  and  there  are  a  good  many  observations  on  the  organs 
themselves,  but  I  recall  none  on  the  dependence  of  the  organs  upon 
the  ganglion  proper.  Kupffer,  Z.c.,  states  that  in  birds  both  the  lat- 
eral and  epibranchial  fusion  of  the  ganglion  with  the  epidermis  can 
be  seen.  The  epibranchial  organ  in  mammalian  embryos  has  been 


THE   NKKVolS    M  STEM. 


carefully  studied  by  Froriep,  85.1.  Fig.  373  represents  a  trans- 
verse  section  of  a  torpedo  emhryo  of  I  :>  nun.  in  which  the  vagus 
ganglion  shows  its  two  connections  with  the  epidermis,  first  at  the 
Lateral  line,  secondly  over  the  fourth  gill-cleft,  where  the  thickening 
nt  ectoderm  is  very  considerable.  In  this 
emhryo  (Froriep,  91.2,  ill)  the  vagus  gan- 
glion is  connected  with  the  epibranehial 
MS  over  the  second  to  sixth  cleft,  and 
with  six  smaller  lateral  organs,  which  all 
lie  in  the  region  of  the  fourth  and  fifth 
clefts,  and  in  the  head  ward  prolongation 
of  the  lateral  line  proper.  The  section 
figured  passes  through  the  fourth  epi- 
hranchi.il  orpin.  In  a  lamprey  larva  of  4 
mm.  the  vagus  ganglion,  Fig.  4<>7,  as  seen 
from  the  >[(!<•.  i>  triangular,  the  apex 
pointing  tailward  and  being  prolonged  as 
the  lateral  line;  the  upper  angle  forms  the 
d<>rs,d  root;  the  lower  angle  is  prolonged 
and  joins  the  epibranchial  organ  in  front 
of  and  above  the  fourth  gill-cleft;  this  or- 
gan is  the  seventh  at  this  stage,  and  is 
connected  with  the  epibranchial  organ  in 
front  and  the  chain  of  five  organs  over  the  SSlk 
fourth  to  dghth  Deleft.  As  regards 

mammals,  I1  ronep,     85.1,  States   that   the    <•/»-,    epibranchial   or^an:    . 

vagus   ganglion   is   found   in  cow's  em-  Aft 

hryos  of  8.7  mm.  to  be  the  largest  of  the  cephalic  ganglia,  and  to 
overlie  the  third  cleft  and  the  region  of  the  still  undeveloped  fou  rt  1 1 
and  fifth  clefts;  above  the  third  cleft  and  from  these  down  beyond 
the  level  of  the  fifth  aortic  arch,  it  is  found  united  with  the  epider- 
mis over  an  area  about  0.75  mm.  long  and  from  0.10  to  0.23  mm. 
wide.  In  an  embryo  of  12  mm.  the  epidermis  of  the  area  of  epi- 
hranchial contact  has  become  invaginated  and  lies  at  the  bottom  of 
a  narrow  fissure,  but  is  much  reduced  in  size.  The  fissure  and  con- 
tact can  be  still  found  in  embryos  of  15  mm. 

.'.  MOTOR  ROOTS. — The  neuroblasts,  which  form  the  motor  roots 
of  the  vagus,  are  situated  according  to  His,  88.3,  360-362,  in  the 
ventral  zone  of  His,  but  toward  its  dorsal  side,  and  the  fibres 
make  their  exit  from  near  the  dorsal  limit  of  the  ventral  zone  and 
close  to  the  entrance  of  the  ganglionic  root.  The  vagus  neuroblasts 
are  situated  along  the  same  line  as  those  of  the  spinal  accessory  nerve 
and  are  not  marked  off  from  them  in  any  way ;  compare  Fig. 

3.  PERIPHERAL  BRANCHES. — The  vagus  ganglion  in  young  elas- 
mobranchs  sends  off  four  branches  to  the  gill-clefts;  each  branch 
runs  behind  the  gill-cleft  to  which  it  belongs,  and  is  associated  with 
the  corresponding  epibranchial  organ ;  the  first  to  third  branches  are 
nearly  alike  in  size,  but  are  smaller  than  the  large  fourth  branch, 
which  is  further  distinguished  by  continuing  on  beyond  the  pharyn- 
geal  region  and  by  becoming  the  raniHx  intent  i  itulisot  the  adult  (Van 
Wijhe,  82. 1,  32,  Froriep,  91.2,  r,i).  Later  this  fourth  branch  is  also 
connected  with  the  epibranchial  organ  of  the  sixth  cleft.  The  four 


654  THE   FOETUS. 

branches  behind  the  gill-clefts  are  to  be  regarded  as  post-trematic 
nerves  and  the  fourth  is  presumably  equivalent  to  two  post-tivmatic 
nerves.  The  pra3-trematic  branches  arise  later  as  outgrowths  of  the 
ganglion  from  the  region  of  the  epibranchial  organs,  and  also  the  ]»1  ia- 
ryngeal  branches  arise  similarly.  Van  Wijhe,  7.C.,  p.  31,  found  that  in 
Balfour's  stage  K  the  ganglion  has  one  dorsal  branch,  and  gives  off 
the  so-called  lateral  nerve ;  the  dorsal  branch  Van  Wijhe  identifies  as 
the  ramus  supra-temporalis ;  it  is  connected  with  the  epidermis  of 
the  lateral  line.  It  is  probable  that  both  the  dorsal  and  the  lateral 
nerves  are  derivatives  of  the  connection  of  the  ganglion  with  the 
lateral  line.  As  Van  Wijhe  neglected  the  difference  between  the 
ganglion  and  the  nerve  his  investigations  must  be  extended  before 
we  can  decide  whether  the  four  branches  to  the  gill-cleft  arise  from 
the  ganglion  proper  or  from  a  nerve-trunk  which  was  mistaken  for 
a  prolongation  of  the  ganglion.  The  question  raised  is  important, 
since  upon  the  answer  must  depend,  to  a  large  extent,  our  notion  of  the 
origin  of  the  nerve,  whether  it  represents  one  nerve  much  branched 
or  several  nerves  which  have  been  fused.  Balfour's  account  of  the 
development  of  the  vagus  in  sharks  differs  somewhat  from  Van 
Wijhe's— see  Balfour's  "Comp.  Embryology,"  II.,  457. 

In  mammals  the  early  condition  of  the  vagus  branches  has  been 
partially  described  by  A.  Froriep,  85. 1.  In  cow  embryos  of  8.7-8.8 
mm. the  vagus  surpasses  all  other  nerves  in  size;  in  those  of  12  mm. 
the  ganglion  jugulare  is  well  differentiated  from  the  ganglion  IK ><!<>- 
sum,  and  from  the  former  the  main  trunk  extends  for  about  0.4  nun. 
as  the  anlage  of  the  ramus  intestinalis ;  the  trunk  at  this  stage  con- 
sists entirely  of  nerve-fibres  and  contains  no  cells;  the  fibres  pass 
through  the  medial  half  of  the  ganglion.  As  to  the  branches  to  the 
gill-arches  and  the  lateral  line  no  published  observations  are  known 
to  me. 

Lateral  Nerve. — This  branch  of  the  vagus  is  one  of  the  best 
known  nerves  of  the  Ichthyopsida,  and  is  connected  with  the  sensory 
organs  of  the  lateral  line.  The  homologues  in  amniota  of  the  lateral 
nerve  have  never  been  satisfactorily  determined.  The  nerve  itself  is 
perhaps  a  partial  survival  of  a  connection  of  the  epidermis  with  the 
ganglia,  which  originally  extended  along  the  head  as  well  as  along 
the  body,  and  which  was  associated  with  the  series  of  lateral  sense 
organs;  compare  C.  Julin,  87.3.  In  amphibia  (A.  Goette,  75.1, 
672)  and  in  elasmobranchs  (C.  Semper,  76.3,  398,  Van  Wijhe,  82. 1, 
33)  the  growing  end  of  the  lateral  nerve  has  been  seen  to  merge  in 
the  epidermis,  and  these  observers  suggest  that  the  nerve  may  grow 
at  the  expense  of  the  epidermis ;  but  this  notion  is  scarcely  compatible 
with  our  present  knowledge  of  the  genesis  of  nerve-fibres. 

XI.  The  spinal  accessory  nerve  (accessorius  Willissi)  is 
characteristic  of  the  amniota  and  is  not  found  in  the  anamniota. 
It  must,  therefore,  be  regarded  as  a  nerve  which  has  been  evolved 
within  the  vertebrate  series,  and  its  development  indicates  that  it 
arose  by  a  collective  modification  of  the  motor  fibres  of  the  dorsal 
roots  of  the  hypoglossus.  It  comprises  no  ganglionic  fibres.  Chia- 
rugi,  90.1,  found  in  reptiles,  birds,  and  mammals  that  the  neural 
crest  persists,  as  it  does  in  elasmobranchs  according  to  Van  Wijhe, 
82.1,  32,  between  the  vagus  and  first  cervical  ganglion,  and  con- 


Tin:    NERVOUS    8^  STEM. 


655 


tinues  as  a  cellular  cord.  In  >th  while  tin-  hypoglos>al  ganglia  gr«>\v 
out  from  it  ami  after  tin  — •  ganglia  abort.  He  regards  it  as  the 
anlage  of  tin*  accesM»rius.  mid  thi>  is  probably  correct,  hut  Dot  in  the 
861186  that  its  c.-lls  produce  the  ner\<.  for  the  nerve  contain^  u.. 
ganglionic  tihivs,  hut  in  tin-  n-iise  that  it  prescribes  the  path  tor  the 
niot.»r-iibres  and  conducts  them  to  the  vagus  ganglion.  1  venture 
tlie  hyp«>the>i>,  that  if  the  hypoglossal  ganglia  were  preserved  tJie 
iihres  of  the  accessori  us  would  not  run  tothe  vagUO,  hut  chieHy  if 
not  wholly  to  the  twelfth  nerve.  11  is,  88.3,  :>i;n-:;«;-.>,  found  the 
neurol»la-t>  wliich  give  rise  to  the  accessori  us  fibres  to  be  distributed, 
Be  -liown  in  Fig.  374,  along  the  dorsal  part  of  the  ventral  zone, 
throughout  the  vagus  and  hypogl..^al  regions,  i.e.,  roii-hlv  th-- 
lower  third  of  the  medulla  oblongata; 
the  tibn-..  unlike  iln.se  <>f  the  hypogl 
pDS,  make  their  exit  near  the  d«»i>al 
XOIH  :  the  tihres  leave  the  medullary 
wall  ;u4  a  series  of  little  bundles,  which 
unite  into  a  nerve  which  runs  forward 
nearly  parallel  with  the  medulla,  being 
prohahly  guided  by  the  ganglionic  cord, 
and  joins  first  the  vagus  g-mglioii,  then 
the  main  vagus-trunk,  Fig.  6G3,  XI. 
The  longitudinal  trunk  of  the  accesso- 
i  in-  is  n 'uarded  by  Chiarugi,  90.1,  31  •. 
as  a  modification  of  the  original  neural 
•  raiisformed  in  the  occipital  region 
into  a  c«  >m  m  i ssural  cord.  Some  further 
details  are  given  by  Froriep,  85.1,  as 
to  this  nerve  iii  ruminant  embryos.  As 
'ds  the  branches  of  the  nerve,  His, 

Rft   R     ",S<)    finds;  tint  in  th«»  hiiman  pm- 
10. t3,    -.Ml,  mi(b 

1  »i  \  < )  of  four  and  one-half  weeks  the  adult 
relations  are  already  e>tahlished,  Fig.  :jr,:5,  in  that  the  fibres  all  join 
the  vagus  and  run  for  the  greater  part  with  its  descending  stem,  but 
a  part  of  them  pass  off  as  the  independent  ramus  externus  N.  ac- 

/•//;  compare  also  Kroriep,  85.1,  lo-14. 

XII.  The  hypoglossal  nerve  of  mammals  has  been  shown  by 
Froriep,  85, 1,  to  be  the  result  of  the  fusion  of  several  nerves,  prob- 
ably four,  closely  similar  to  the  true  spinal  nerves  in  character. 
Froriep's  results  have  had  confirmation  by  P.  Martin's  observations, 
90.3  011  the  cat,  Chiarugi's,  89.2,  on  several  mammals,  and  Van 
Bemmelen's,  89.1,  on  reptiles.  As  the  homologies  of  the  hypo- 

flossus  among  Ichthyopsida  are  not  clearly  understood,  I  shall  con- 
ne  myself  to  the  development  of  the  nerve  in  the  higher  forms.     We 
shall  consider  in  order,  1,  the  ganglia;  2,  the  motor  roots;  3,  the 
branches. 

The  development  of  the  hypoglossus  suggests  that  it  arose  by 
modification  and  fusion  of  at  least  four  segmental  nerves  situated 
between  the  vagus  and  the  first  cervical  nerve.  The  modifications 
consist  in  the  disappearance  of  the  ganglia  and  the  conversion  of  the 
motor-fibres  of  the  dorsal  roots  into  the  accessorius  nerve,  and  in  the 
disappearance  of  at  least  the  anterior  of  the  ventral  roots.  The  nerve 


Fia.  374.  — Section  of  tin-  Medulla  Ob- 
longataof  aliv.'-w.H-ks'  Human  Embryo 
'lii>'  Km.  Ay>.  Bpeodyma;  A  dorsal 
zone  of  His:  I  .  ventral  zone;  XI,  acces- 
sor! us  ;  XII,  hypoglossus.  After  W.  His. 


650  THE   FCETUS. 

retains  its  primitive  relations,  since  the  lingual  muscles  it  innervates 
are  developed  from  the  occipital  myotomes. 

1 .  THE  GANGLIA. — There  are  found  in  the  occipital  region  of  young 
mammalian  embryos  three  ganglia,  which  abort  before  they  are 
fully  dfferentiated.  These  ganglia  have  a  marked  resemblance  to 
the  true  spinal  ganglia.  They  are  connected  with  a  part  of  the 
neuron  which  belongs  presumably  not  to  the  medulla  oblongata,  but 
to  the  spinal  cord.  If  this  is  really  the  case  the  ganglia  are  true 
spinal  ganglia,  not  cephalic.  Chiarugi,  89.2,  found  that  the  ganglia 
are  preceded  by  a  continuous  stretch  of  the  neural  crest,  which  ap- 
pears as  if  a  commissural  link  between  the  vagus  and  first  cervical 
ganglia,  e.g.,  in  Lacerta  embryos  of  2.7  mm.  From  this  pseudo- 
commissure  there  grow  out  in  Lacerta  at  first  two  ganglia,  which 
overlie  and  extend  in  front  of  respectively  the  third  and  fourth 
occipital  myotomes,  and  there  is  perhaps  a  third  ganglion,  that  is 
to  say,  one  for  the  second  occipital  myotome;  the  three  ganglia  have 
only  a  fugitive  existence,  and  are  no  longer  present  in  embryos  of 
5.5  mm.  It  may  be  well  to  recall  that  the  first  cervical  ganglion 
also  aborts  in  Sauropsida  during  early  embryonic  life,  compare  p. 
G30.  Chiarugi,  /.c.,  339,  found  the  three  rudimentary  occipital  gan- 
glia in  the  chick  embryo  of  the  third  day,  corresponding  to  the 
second,  third,  and  fourth  occipital  myotomes.  In  the  rabbit  only 
two  ganglia  are  known  in  the  occipital  region ;  these  have  been  ob- 
served by  Chiarugi,  /.c.,  430,  in  embryos  of  6.5  mm.  associated  with 
the  third  and  fourth  occipital  myotomes;  the  posterior  of  the  gan- 
glia is  the  larger.  In  cow  embryos  of  8.7  mm. ,  Froriep,  82. 1 ,  85.1, 
16,  found  one  occipital  ganglion  in  association  with  the  last  occipital 
myotome,  there  being  three  myotomes.  We  may  assume  that  there 
are  earlier  two  ganglia  and  four  segments  in  the  cow  embryo  as  in 
the  rabbit,  and  that  by  the  stage  studied  by  Froriep  the  foremost 
segment  and  foremost  ganglion  have  disappeared.  In  cow  embryos 
of  12  mm.,  Froriep,  85. 1,  24,  found  the  ganglion  of  the  fourth  seg- 
ment still  present  and  its  ventral  end  united  with  the  hypoglossal  motor 
roots  of  the  same  segment,  but  in  embryos  of  15  mm.  the  ganglion 
shows  indications  of  abortion,  1.  c.,  p.  33.  In  the  human  embryo, 
Fig.  360,  the  ganglion  of  the  fourth  occipital  segment  has  been  ob- 
served by  His  ("Anat.  menschl.  Embryonen,"  Heft  III.,  89,  also 
88. 1,  401)  in  embryos  of  13-14  mm. ;  later  it  is  found  to  have  dis- 
appeared. His  proposes  to  name  the  ganglion  after  its  discoverer, 
Froriep' s  ganglion.  Kazzander,  9 1 . 1 ,  has  directed  attention  t<> 
various  cases  in  which  a  hypoglossal  (Froriep's?)  ganglion  has 
been  observed  in  man  and  other  mammals  in  the  adult  stage, 
and  reports  a  new  case  of  its  presence  in  a  human  adult. 

The  facts  presented  in  the  preceding  paragraph  render  it  probable 
that  in  all  amniota  there  are  at  least  three  *  ganglia  present  during 
very  early  stages  in  the  occipital  region ;  that  these  ganglia  belong 
to  the  second,  third,  and  fourth  segments  of  the  region,  and  to  the 
hypoglossal  nerve,  and  that  they  successively  disappear,  the  last 
persisting  for  some  time  longer  in  mammalian  than  in  sauropsidan 
embryos.  I  think  that  we  may  expect  to  obtain  evidence  that  there 
is  still  another  hypoglossal  ganglion,  namely,  for  the  first  segment. 

*P.  Martin,  90.3,  230,  affirms  that  he  finds  in  the  cat  five  rudimentary  hypoglossal  ganglia. 


TNI:   M-:I:\  <  >i  -  En  3TKM, 

Although   the  occipital   ganglia   entirely   di>app« .;,r.    tin- 
e..rd,  from  which  they  arise,  persists  and  srrvo  as  the  anlage  of  the 
-orius  as  stilted  in  the  preceding  section. 

N'>  u-;inurlionic  sense  organs  connected  with  the  hypoglossus  have 
y«-t  heen  recognized,  but  it  is  to  me  probable  that  the  part  of  the 
lateral  line  near  the  vagus  represents  hypoglossal  lateral  organs. 
Suitable  investigations  on  Ichthyopsida  might  result  in  confirming 
this  Mi.u-v-tion. 

llixtnrirul  \<>f<'. — The  last  hypoglossal  ganglion  was  discovered 
by  Froriej)  in  1882,  in  ruminant  embryos,  and  its  history  has  since 
I.e.  n  further  studied  by  him.  His,  in  1888,  recognized  its  presence 
in  the  human  embryo.  Chiarugi,  89.2,  90.1,  has  studied  the 
ganglia  in  reptiles,  birds,  and  mammals,  and  our  present  knowledge 
resta  to  a  large  extent  solely  upon  his  observations.  P.  Martin, 
90.3,  lias  observed  the  ganglia  in  cat  embryos. 

2.  MOTOR  ROOTS. — The  neuroblasts  which  give  rise  to  the  hypo- 
.n'lossus  lie,  in  the  human  embryo,  in  the  ventral  part  of  the  ventral 
zone  of  His.    Ki-.  :;;  I.  find  their  fibres  make   their  exit  from  the 
m.-dulla  not  far  from  the  Bod<'ni>l<itte  (His,  88.3,  :;»;i).     The  fibres 
are  gathered  into  bundles.      According  to  His,  these  bundles  are 
•  pii  to  numerous  and  are  found  even  below  the  vagus  ganglion.     I 

-ider  it  probable  that  His  is  mistaken  in  regard  to  this,  and  that 
the  fibres  leave  the  medulla  in  man  only  in  the  region  U'liind  the 
vagofl  in  other  words,  in  the  region  of  the  four  occipital  segments, 
and  in  four  segmen tally  arranged  bundles.  That  there  are  three, 
and  probably  four,  segmen tal  motor  roots  in  cow  embryos  has  been 
shown  by  Froriep,  85.1,  HI,  but  P.  Martin  records,  90.3,  230,  that 
in  cat  embryos,  representing  younger  stages  than  Froriep  studied, 
tliere  are  five  distinct  roots  (?  of  which  one  cervical).  Chiarugi, 
89.2,  90.1,  has  observed  five  segmen  tally  arranged  roots  in  Lacerta, 
the  first  root  lying  in  front  of,  the  remaining  four  corresponding  to, 
the  four  occipital  segments;  four  roots  in  Tropidonotus ;  three  roots 
in  chicks  toward  the  end  of  the  third  day,  the  first  occipital  segment 
having  no  root;  and  finally  two  roots  in  rabbit  embryos.  Van  Bern- 
melon,  89.1,  :>4.'J,  describes  in  Lacerta  five  well -developed  hypoglossal 
ventral  roots,  and  has  noticed  fibres  further  forward  toward  the 
vagus,  which  suggest  to  him  the  possibility  of  yet  more  roots;  he 
further  records  that  motor  fibres  are  added  from  the  first  and  a  little 
later  also  from  the  second  cervical  nerve. 

3.  BRANCHES. — It  will  be  remembered  that  the  posterior  branchial 
arches  are  invaginated,  the  invagination  constituting  the  sinus  cer- 
r inil is.      The  hypoglossal  nerve  in  a  human  embryo  of  the  fifth 
week,  Fig.  363,  was  observed  by  W.  His,  88.3,  380,  to  pass  around 
this  sinus,  going  behind  and  below  it  and  there  curving  forward  into 
the  tongue;   as  shown  in  the  figure,  the  nerve  crosses  the  vagus 
below  the  ganglion  nodosum,  and  after  crossing  gives  off  a  branch, 
nun  us  descendens,  which  runs  along  the  lateral  side  of  the  jugular 
vein  parallel  to  the  vagus  trunk.     The  mechanical  cause  of  the  for- 
mation of  this  branch,  I  do  not  know.     Chiarugi,  90.1,  432,  has 
observed  that  the  distribution  of  the  nerve  is  essentially  the  same  in 
rabbit  embryos  as  in  human. 

In  Lacerta,  Van  Bemmelen,  89.1,  finds  that  the  course  of  the 
42 


658  THE   FCETUS. 

nerve,  as  it  curves  around  to  enter  the  tongue,  is  closely  parallel  to  the 
united  prolongation  of  the  five  myotomes  (four  occipital  and  one  cer- 
vical) which  grow  like  a  single  cord  (Froriep's  Schulterzungenstrang) 
into  the  tongue  to  produce  the  lingual  muscles.  Chiarugi,  90. 1 ,  32 1 , 
states  that  in  lizard  embryos  the  nerve-trunk  runs  outside  the  jugular 
vein,  from  which  it  is  separated  by  the  intervention  of  the  vagus  and 
of  the  carotid  artery,  and  accompanies  a  branch  of  the  jugular,  which 
runs  to  the  mandible  and  is  probably  the  sub-maxillary  vein. 

Spinal  Cord. — The  differentiation  of  the  cord  and  brain  is  effected 
by  the  development  of  the  cerebral  vesicles.  The  histogenesis  of  the 

cord  has  been  described  in  the  sections  on 
the  neuroglia  and  the  nerve-fibres.  The 
following  paragraphs  refer  chiefly  to  the 
cord  without  regard  to  the  peculiarities 
offered  by  the  lower  end  of  cord,  Fig.  :>I  ~>. 
in  which  we  find  the  typical  developmental 
features  very  imperfectly  followed.  This 
is  due,  presumably,  to  the  partially  abor- 
tive history  of  the  caudal  end  of  the  neu- 
ron in  mammalia.  The  following  descrip- 
.  375. -Lower  end  of  the  spinal  tions  are  based  in  large  part  on  His' 

Cord  of  a  Human  Embryo  of  three  •        Q  r>    a 

Months.     Epy,  Ependymal   layer;     memoir,    OD.^. 

1-  GENERAL  GROWTH. -The  following 
account  is  based  upon  that  of  Kolliker 
("  Grundriss, "  2te  Aufl.,  260).  The  medullary  groove  is  found  com- 
pletely closed  in  the  region  of  the  spinal  cord  in  a  chick  embryo 
with  thirteen  primitive  segments,  and  in  slightly  more  advanced 
human  embryos.  But  the  posterior  end  remains  for  a  while  as  a 
solid  mass,  which  terminates  by  fusion  with  the  ectoderm.  When 
the  primitive  segments  are  all  formed,  the  end  of  the  cord  sepa- 
rates from  the  ectoderm.  At  this  stage  the  cord  extends  as  far 
as  the  segments.  In  human  embryos  the  cord  equals  the  vertebral 
column  in  length  up  to  the  end  of  the  third  month.  After  the  fourth 
month  the  vertebral  column  outgrows  the  spinal  cord,  which,  although 
it  absolutely  lengthens,  becomes  relatively  shorter,  so  that  the  dis- 
tance from  its  end  to  the  end  of  the  spinal  canal  increases.  This 
apparent  ascent  of  the  cord  (ascensus  medullce  spinalis)  might  be 
more  properly  described  as  a  descent  of  the  vertebrae.  A  secondary 
result  of  the  changed  position  is  that  the  nerves  running  out  from 
the  lower  end  of  the  cord,  since  their  exits  between  the  vertebras  are 
below  the  end  of  the  cord,  are  forced  to  take  a  more  and  more  longi- 
tudinal course  within  the  spinal  canal.  There  results  a  series  of  nerve- 
roots,  which  after  the  fourth  month  elongate  as  the  vertebra  descend, 
and  thus  gradually  produce  the  so-called  cauda  equina.  The  filum 
terminalis  is  developed,  according  to  Kolliker,  from  the  pia  mater,  and 
is  therefore,  properly  speaking,  not  a  nervous  structure.  The  upper 
part  of  the  filum,  however,  even  in  the  adult  contains  a  prolongation 
of  the  spinal  cord  with  its  central  canal;  compare  Tourneux  et 
Hermann,  87.3. 

The  cervical  and  lumbar  enlargement  of  the  spinal  cord  are  indi- 
cated in  the  human  embryo  at  two  months  and  are  well  developed  at 
three  months. 


THE  NERVOUS  SYsTEM.  G59 

2.  CENTRAL  CANAL. — The  central  canal  has  at  first  the  form  in 
sections  which  is  shown  in  Kigs.  1  "»'•',  1  •'>«',  and  161,  being  flattened 
from  side  and  elongated  dorsal-ventrally,  but  is  often  more  or  less 
invgular  in  shape.  I  have  observed  that  in  birds  and  mammals 
there  is  a  tendency  for  the  walls  of  the  canal  to  come  temporarily 
into  close  contact  along  two  longitudinal  lines,  so  that  the  canal 
appears  at  the  first  glance  to  be  divided  into  three  channels.  This 
condition  may  be  well  seen  in  the  rabbit,  and  it  is  probably  of  wide, 
possibly  of  constant,  occurrence.  As  to  its  significance,  I  have  no 
clew.  The  contact  is  soon  lost,  and  the  canal  becomes  freely  open 
n  throughout  its  extent. 

There  now  occurs  a  change  of  shape,  cf.  p.  607,  by  which  the  canal 
cuts  into  the  thick  medullary  wall  on  each  side,  dividing  it  into  the 
upper  and  small  dorfcal  zone  of  His,  and  the  lower  and  larger  ventral 
/one  of  His,  see  W.  His,  86.2,  p.  197,  Kig.  G.  This  change  occurs  in 
the  human  embryo  toward  the  end  of  the  fourth  week  and  attains  its 
maximum  about  the  beginning  of  the  sixth  week.  It  is  precisely 
during  this  period  that  the  medullary  nerves  grow  out  from,  and  the 
gangl ionic  nerves  grow  into,  the  spinal  cord;  the  former  arising 
from  the  ventral  zone,  the  latter  entering  the  dorsal  zone.  The 
dorsal  plate  is  curved  inward,  making  a  median  ridge  internally 
and  a  median  groove  externally;  on  either  side  of  the  latter  there  is 
a  projecting  fold,  where  the  deck-plate  curves  over  (the  fold  is  the 
anlage  of  Goll's  cord. 

About  the  eighth  week  the  canal  begins  to  contract  (compare  His, 
86.2,  Figs.  (5-9)  between  the  dorsal  zones  until  the  walls  first 
meet  and  then  unite.  Thus  in  a  fcetus  of  the  tenth  week,  Fig.  376, 
the  union  has  already  taken  place  except  at  the  very  dorsalmost  part 
of  the  canal,  where  a  small  remnant  of  the  original  cavity  persists; 
whether  this  is  always  the  case,  I  do  not  know.  In  older  stages  all 
traces  of  the  canal  (both  its  cavity  and  its  epithelium)  have  disap- 
peared, not  only  between  the  dorsal  zones  of  His,  but  also  between 
the  upper  part  of  the  ventral  zones.  In  Fig.  376,  the  boundary 
between  the  dorsal  and  ventral  zones  is  marked  by  the  insertion 
of  the  dorsal  nerve-root.  The  lower  part  of  the  central  canal  remains 
open,  and  presents  in  section  certain  definite  curves  of  outline,  which 
deserve  closer  study.  The  open  part  of  the  canal  is  elongated  dorsal- 
ventrally,  but  toward  the  close  of  fostal  life  it  becomes  more  rounded 
in  form,  and  in  the  adult  is  elongated  transversely.  In  the  caudal 
end  of  the  human  cord  the  cavity  is  large,  Fig.  375,  and  does  not 
go  through  the  same  changes  of  shape  as  in  the  rest  of  the  cord. 

It  seems  to  me  that  the  dorsal  part  of  the  central  canal  is  obliter- 
ated by  the  union  of  its  walls  and  the  subsequent  atrophy  of  its 
so-called  epithelium,  although  the  exact  steps  of  the  atrophy  are 
unknown.  In  the  adult  the  line  of  the  central  canal  on  the  dorsal 
side  is  represented  by  the  posterior  fissure,  which  is  merely  a  thin 
partition  of  vascularized  tissue  and  not  a  true  fissure.  It  seems 
probable  that  the  tissue  as  claimed  by  Barnes,  84. 1,  is  really  derived 
from  the  cells  lining  the  central  canal,  and  is,  therefore,  to  be  classed 
morphologically  with  neuroglia.  Waldeyer,  76. 1,  and  others  speak 
of  a  contraction  of  the  canal,  and  of  its  being  pushed  in  by  the  ingrowth 
of  the  posterior  columns.  This  view  is  incorrect,  for,  as  shown  in 


660 


THE   FCETUS. 


Fig.  376,  the  central  canal  exists  between  the  posterior  columns, 
even  after  the  columns  of  Goll  and  Burdach  can  be  recognized,  in 
what  are  essentially  their  permanent  positions.  A.  Robinson,  91.1, 
90,  calls  attention  to  the  fact  that  the  cord  widens  at  an  early  stage 
in  rodents,  so  that  in  section  it  appears  nearly  round  instead  of  oval ; 
this  change  causes  a  slight  diminution  in  the  dorsal- ventral  diameter 


FIG.  876.  —Section  of  the  Spinal  Cord  of  a  Human  Embryo  of  sixty-three  to  sixty -eight  Days. 
Minot  Coll.,  No.  138.  (Dorsal  region),  a,  GolPs  cord;  b.  Burdach ^s  cord  (Keilstrang) ;  D.i\ 
dorsal  root:  C.c,  central  canal;  A.c,  anterior  cornu;  F,  anterior  fissure. 

of  the  central  canal.  I  cannot  regard  this  diminution  as  a  step 
toward  the  obliteration  of  the  canal. 

The  anterior  fissure  begins  to  develop  during  the  early  part  of 
the  eighth  week,  and  arises  by  the  growth  of  the  cord,  which  takes 
place,  as  indicated  in  Fig.  377,  so  as  to  produce  two  bulging  ridges 
on  the  ventral  side  of  the  cord.  The  median  space  between  the 
ridges  is  the  future  anterior  fissure ;  it  is  occupied  by  fibrous  con- 
nective tissue  enveloping  the  cord;  it  is,  therefore,  a  true  fissure, 
for  across  it  there  is  no  connection  between  the  nervous  tissue  of  the 
two  sides.  Indeed,  part  of  the  original  surface  of  the  cord  bounds 
the  fissure  on  either  side,  and  therefore  we  may  correctly  describe 
the  tissue  in  the  fissure  as  part  of  the  envelope  (pia  mater)  of  the 
cord.  As  the  embryo  advances  the  ridges  grow  and  the  fissure 
deepens ;  the  growth  of  the  ridges  is  largely  due  to  the  expansion  of 
the  gray  matter  to  form  the  anterior  horns. 

There  is  no  atrophy  of  the  ventral  portion  of  the  canal  as  Lowe, 
80. 1,  114,  asserted,  but  the  central  canal  of  the  adult  represents  the 
ventral  portion  of  the  primitive  canal. 


THK    NKKYorS    SVSTKM. 


661 


or.b 


<;I;I»\VTH  OF  THE  MANTLE  LAYER. — The  mantle  layer  in  man 
(His)  and  rodents  (A.  Robinson,  91.1)  first  appears  in  the  region 
of  the  ventral  zones  of  His,  forming  in  sections  a  triangular  mass 
on  each  side  between  the  inner  layer  and  the  Randschleier ;  it  grad- 
nally  thickens,  and  at  the  same  time  its  development  progresses 
d<  >rsal  ward  and  encroaches  also  upon  the  inner  layer.  There  is  thus  a 
40  (in  rats  when  the  cartilaginous  bodies  of  the  vertebrae  arise) 
in  which  the  inner  layer  is  very  much  reduced  in  the  ventral  col- 
umns, and  gradually  increases  in  thickness  dorsalward,  becoming 
in  the  dorsal  zone  thicker  than  the  mantle  layer,  which,  however, 
soon  grows  at  the  expense  of  the  inner  layer,  which  is  ultimately 
reduced  to  the  lining  or  so-called  epithelium  of  the  central  canal. 
The  mantle  layer  is  easily  recognized  by  the  large  size  of  its  elon- 
gated nuclei,  and  by  the  fact  that  some  of  the  nuclei  are  elongated 
dorsal-ventrally  and  others  radially ;  in  the  inner  layer  the  nuclei 
are  smaller  and  all  point  radially. 

4.  DORSAL  ZONE  OF  His. — The  origin  of  this  division  of  the 
cord  has  already  been  described,  p.  607.  In  a  human  embryo  of 
1  .'..")  mm.,  W.  His,  86.2,  497,  found  the  dorsal  zone  to  begin  with  a 
broad  arch  from  the  deck-plate,  to  form  a  marked  projection  into  the 
o-iitral  canal,  and  to  have  upon  its  outer  sur- 
face a  rounded  projection,  ov.b,  which  His 
calls  the  oval  bundle  (ovales  Biindel) ;  the 
projection  is  a  product  of  the  Randschleier 
and  contains  the  ganglionic  fibres,  which 
have  entered  the  medullary  wall  and  run 
lengthwise  within  it ;  the  oval  bundle  at  this 
stage  extends  about  half  the  distance  from 
the  sensory  root,  which  enters  the  ventral 
border  of  the  bundle,  to  the  median  dorsal 
line;  the  oval  bundle  is  the  anlage  of  the 
greater  part  of  the  posterior  column  of  the 
adult.  The  oval  bundle  now  steadily  en- 
larges and  creeps  dorsalward  until  it  reaches 
thr  arch  formed  by  the  passage  of  the  dorsal 
zone  into  the  deck-plate,  d.pl.  The  arch  gives 
rise  to  Goll's  cords.  The  two  cords  of  Goll 
become  closely  united  with  one  another  by 
the  obliteration  of  the  central  canal  between 
them,  Fig.  378.  The  oval  bundle  meanwhile 
creeps  still  further  and  makes  its  way  between 
the  cords  of  Goll  and  the  gray  matter  until  it 
meets  its  fellow  from  the  opposite  side  below 
the  cords  of  Goll;  thus  arise  the  cords  of 
llnrdach  (Burdachsche  Keilstra nge) ,  Fig.  378,  b.  At  this  stage — 
<-mbryo  of  the  tenth  week — the  dorsal  zone  of  His  is  no  longer  dis- 
tinctly marked  off  from  the  ventral  zone  except  by  the  position  of 
the  sensory  root.  The  inner  and  mantle  layers  have  become  the  gray 
matter  and  they  are  completely  covered  by  the  expansion  of  the  oval 
bundle,  that  is  to  say,  by  a  layer  produced  from  the  primitive  Rand- 
s'-bleier  of  His,  p.  616,  and  containing  chiefly  longitudinal  ganglionic 
fibres.  The  layer  developed  from  the  oval  bundle  may  be  subdivided 


Hscb 


Fio.  877. -Transverse Section 
of  the  Spinal  Cord  from  the 
Upper  Dorsal  Re trion  of  a  Hu- 
man Embryo  of  six  Weeks 
(His1  Zw).  d.pl.  Deck-plate: 
ov.6,  oval  bundle  of  dorsal 
zone;  D.K,  dorsal  root;  Rsch, 
Randschleier  of  ventral  zone; 
b, floor-plate;  r./?,  ventral  root 
After  W.  His.  X  44  diams. 


G62  THE   FCETUS. 

into  two  parts :  the  medial  Burdach's  cords  and  the  lateral  portion  of 
the  posterior  columns.  Outside  of  and  above  Burdach's  cords  are 
GolPs  cords,  which  are  developed  from  the  arch  by  which  the  deck- 
plate  originally  passed  into  the  dorsal  zone  of  His  in  the  embryo. 
The  fibres  in  Goll's  cords  are  developed  rather  late. 

The  neuroblasts  of  the  mantle  layer  of  the  dorsal  zone  send  their 
nerve-fibres  ventralward ;  the  fibres  constitute  the  formatio  arcuata. 
As  indicated  in  Fig.  378,  only  part  of  the  posterior  horn  of  the  adult 
probably  is  developed  from  the  dorsal  zone.  The  inner  and  man- 
tle layer  give  rise  to  the  gray  matter,  which  increases  rapidly  after 
the  middle  of  the  second  month,  owing  partly  to  the  multiplication 
of  its  cells,  partly  to  the  penetration  of  blood-vessels,  and  the  accom- 
panying loosening  of  the  tissue ;  this  loosening  (Auflockerung)  pro- 
gresses from  the  head  backward.  At  three  months  the  posterior 
horn  is  still  broad  and  short  in  section,  but  it  gradually  becomes  long 
and  narrow. 

Substantia  Gelatinosa  Eolandi. — This  tissue  is  probably  the 
neuroglia  plus  numerous  nerve-cells  of  the  tip  of  the  anteriorlhorn, 
developed  in  the  mantle  layer.  As  the  cells  of  the  embryonic  mantle 
layer  are  apparently  all  neuroblasts,  His,  86.2,  508,  assumed  that 
there  are  cells,  which  migrate  into  the  layer  to  form  the  gelatinous 
substance.  The  origin  of  these  cells  he  did  not  observe.  Gierke, 
86.1,  144,  pointed  out  that  most  of  the  elements  are  very  small 
nerve-cells.  H.  K.  Corning,  88.1,  found  that  in  the  dorsal  part  of 
the  inner  layer  of  the  dorsal  zone  of  His,  the  development  is 
greatly  retarded,  and  he  interprets  the  substance  of  Roland  as  a  tis- 
sue persisting  in  a  somewhat  embryonic  condition,  and  not  having 
the  same  differentiation  of  its  cellular  elements  that  we  find  in  other 
parts  of  the  cord. 

5.  VENTRAL  ZONES  OP  His. — The  ventral  zones  are  larger 
and  more  complex  than  the  dorsal  zones.  At  six  weeks,  Fig.  377, 
they  comprise  at  least  three-fourths  of  the  cord;  each  zone  con- 
sists of  an  upper  connecting  piece,  His'  Schaltstiick,  and  a  wider 
lower  segment;  the  width  of  the  latter  is  due  to  the  great  thickening 
of  the  inner  and  mantle  layers ;  the  Randschleier  or  anlage  of  the 
white  substance  extends  completely  over  the  outer  surface  of  the 
zone  as  a  layer  or  envelope,  which  varies  but  little  in  thickness. 
Owing  to  the  projection  of  the  oval  bundle  and  of  the  lower  segment, 
the  Schaltstiick  is  marked  externally  as  a  wide  groove ;  His  desig- 
nates the  angle  of  this  groove  next  the  oval  bundle  as  the  Randfurche, 
the  angle  next  the  lower  segment  as  the  Cylinder furche  (His,  86.2, 
498).  As  development  progresses,  the  Schaltstiick  relatively  di- 
minishes, while  the  lower  segment  increases,  so  that  the  groove 
just  described  is  gradually  obliterated ;  nevertheless  it  can  long  be 
recognized.  The  gray  matter  of  the  Schaltstiick  is  to  be  considered 
as  the  anlage  of  the  cervix  cornu.  For  a  considerable  period  the 
Randschleier  or  anlage  of  the  white  substance  of  the  connecting  piece 
remains  thin,  compare  Fig.  377,  but  toward  the  end  of  the  second 
month  it  begins  to  thicken  until  the  groove  is  obliterated,  but  the 
thickened  portion  still  retains,  according  to  His,  a  certain  individu- 
ality, and  may  be  identified  as  the  anlage  of  the  lateral  pyramidal 
cord  (Hinterseitenstrang) . 


THI:  NKIJVOI  -  M  STEM. 


The  lower  sennit-lit  of  the  ventral  /one  is  the  anlage  of  the  anteri- 
or horn,  the  anterior  column,  and  a  large  part  of  the  lateral  column. 
It  is  characterized  hy  its  early  and  rapid  growth,  at  first  chiefly  of 
the  --ray  matter,  later  of  the  white  matter  ( Kandschleier)  also,  com- 
pare Fi.n-s.  :;;;  and  376  The  «-\it  of  the  ventral  root  divides  the 
white  sul»tance  into  the  anla^e  of  the  lateral  column  and  the  anlage 
of  the  anterior  column  or  cord 

The  growth  of  the  i;ray  matter  depends  chietly  on  the  multiplica- 
tion ot  the  germinating  cells  and  the -ro\\  tli  of  the  neuroblasts  in 
the  mantle  layer.  As  the  neuroblasts  are  most  numerous  in  the 
v. -ntral  part,  there  results  the  precocious  enlargement  of  the  lower 
segment  as  compared  with  that  of  the  rest  of  the  cord.  The  neuro- 
blasts of  the  lower  segment  send  out  their  nerve-fibres  mostly  in 
Mnall  In i m lies.  The  nerve-fibres  of  the  formatio  arcuata  coming 
fn»m  the  neuroblasts  of  the  dorsal  zone  also  enter  the  lower  seg- 
ment, and  as  some  of  these  fibres  are  developed  later  their  paths 
croefl  those  of  earlier  fibres,  owing  to  the  changed  relative  positions. 
Not  all  the  neurohlasts  send  their  fibres  directly  into  the  ventral 
roots;  on  the  contrary,  some  of  them  are  found  placed  longitudinally 
in  the  lower  segment.  Thus  the  gray  matter  of  the  anterior  horn 
Becomes  very  complicated  at  an  early  stage.  The  growth  of  the 
nerve-cells  of  the  ventral  column  has  already  been  described,  p.  »'»•'  I. 

5.  GRAY  AND  WHIII:  MATTER  OF  THE  FCETUS. — Concerning 
the  development  of  the  cord  during  the  foatal  period  (middle  of  the 
third  month  until  birth)  we  know  very  little. 

As  regards  the  outline  of  the  gray  matter  we  find  that  the  anterior 
and  posterior  horns  at  three  months  are  of  about  the  same  size  and 
shape,  and  have  a  very  1 » road  a 

connection    with    one    an-  ^^  /          b 

other,    compare    W.     His,  j^*^ 

86.2,  :>n.v  At  five  months 
the  cord  has  grown  very 
much,  Fig.  378,  and  the  cen- 
tral  canal  having  remained 
stationary  is  relatively 
much  smaller.  In  sections 
from  an  embryo  of  this  age, 
I  observe  a  peripheral  den- 
ser layer  sum  »unding  a  cen- 
tral looser  area,  which  is 
divided,  Fig.  378,  into  two 
parts  hy  the  anterior  fissure 
and  Burdach's  cords ;  if  this 
lighter  area  corresponds  to 
the  uray  matter,  then  at 
this  stage  the  anterior  and 
posterior  horns  are  fused, 
and  the  horns  are  not  finally 
shaped  out  until  later. 

As     regards    the    white 
matter:   some  scattered  observations  are  recorded  by   Kolliker  in 
the  second   edition   of  his   "  Entwickelungsgeschichte, "  and  there 


FIG.  378.— Lower  Orviral  Cord  of  a  Human  Embryo 
of  about  five  Months,  a,  Goll's  cord ;  6,  Burdach's  cord : 
P. c.  posterior  horn;  A.c,  anterior  horn;  C.c,  central 
canal;  F,  anterior  fissure. 


6G4  THE   FCETUS. 

are  a  good  many  observations  by  various  authors  on  the  appear- 
ance of  the  medullary  sheaths  of  the  nerve-fibres,  which  are  at 
first  naked.  Flechsig,  76.1,  drew  attention  to  the  fact  that  the 
sheaths  appeared  at  different  periods  for  different  tracts,  and  he 
sought  by  extended  observations  to  trace  the  course  of  the  fibres 
within  the  cord  of  the  embryo  by  following  the  course  of  the 
tracts  with  sheaths  as  distinguished  from  those  without.  Flech- 
sig's  observations  have  been  extended  by  Bechterew,  M.  von  Len- 
hossek, 89. 1,  and  several  others.  References  to  the  various  author- 
ities are  given  by  von  Lenhossek.  A  proper  collation  of  the  results 
obtained  has  yet  to  be  made.  Lenhossek  finds  that  the  medullary 
sheaths  appear  on  the  fibres  of  the  posterior  roots  and  on  the  fibres  of 
Burdach's  column  about  the  same  time,  but  that  the  fibres  of  GolTs 
column  are  not  medullated  until  a  few  days  later.  He  has  discov- 
ered, further,  that  at  a  certain  period  the  fibres  of  the  lower  part  of 
Goll's  column  are  medullated,  while  in  the  lower  cervical  region  only 
those  fibres  which  form  the  ventral  part  of  the  column  have  received 
white  sheaths,  and  that  in  the  upper  cervical  region  none  of  the  fibres 
of  this  column  are  medullated.  He  concludes,  therefore,  that  the 
medullation  of  Goll's  column  is  centripetal  in  direction,  and  that  the 
fibres  which  form  it  have  a  long  course,  but  he  thinks  that  there  is 
no  anatomical  proof  that  any  of  the  fibres  of  the  posterior  roots  pass 
directly  into  the  posterio-internal  columns.  It  is  now  generally 
allowed  that  the  deposition  of  white  matter  upon  the  axis  cylinders 
takes  place  first  in  the  neighborhood  of  the  cells  from  which  they 
spring,  and  proceeds  thence  toward  the  termination  of  the  axial 
process.  This  being  the  case,  it  follows  that  as  the  columns  of 
Burdach  consist  principally  of  posterior  root  fibres  which  have  just 
entered  the  cord,  they  will  become  medullated  very  shortly  after  the 
fibres  of  the  posterior  roots  themselves,  while  the  column  of  Goll, 
which  is  formed  of  fibres  of  the  posterior  roots  which  have  entered 
the  cord  at  a  considerably  lower  level,  will  become  medullated  at  a 
later  period. 

Cajal,  90.1,  discovered  that  the  fibres  of  the  white  substance  of 
the  spinal  cord  give  off  fine  branches  nearly  at  right  angles,  which 
penetrate  the  gray  matter ;  these  branches  he  names  the  collaterals, 
2.C.,  p.  88,  and  they  have  since  been  found  in  the  adult  by  Kolliker, 
90.2.  They  appear  very  early  in  the  embryo,  and  after  the  medul- 
lary sheaths  appear  they  are  seen  to  go  off  from  the  main  fibre  at 
the  nodes  of  Ranvier. 

6.  BLOOD-VESSELS. — The  first  appearance  of  the  blood-vessels  in 
the  cord  has  been  studied  by  W.  His,  65.1,  15,  86.2,  493.  The 
spinal  cord  lies  in  a  canal,  the  walls  of  which  are  formed  by  embry- 
onic connective  tissue  and  repesent  the  anlage  of  the  pia  mater. 
During  the  embryonic  period  of  the  human  embryo  the  cord  is  in 
contact  with  the  wall  of  the  spinal  canal  only  along  the  median  dor- 
sal line.  The  walls  of  the  canal  contain  capillaries  which  are 
developed  during  the  third  week  in  the  region  of  the  head  from  the 
aorta,  in  the  rump  from  the  inter  vertebral  arteries. 

These  capillaries  form  anastomoses  which  produce  four  longitu- 
dinal vessels,  two  near  the  ventral  median  line,  one  close  below  each 
sensory  root.  From  these  four  vessels  vascular  buds  penetrate  the 


THE  tmvoufl  SYSTEM. 

spinal  cord,  the  brand u-s  from  the  ventral  arteries  ] (receding  the 
others  in  their  development:  <•/.  His,  86.2,  Taf.  1.,  Fig.  2.  The  two 
ventral  arteries  become  included  in  the  anterior  fissure ;  during  the 
sixtli  or  seventh  week  they  unite  into  a  single  median  vessel  at  the 
hot  torn  of  the  fissure.  /'.  e.,  near  the  central  canal;  this  vessel  is  the 
tirfrriii  N '///•/,  and  is  the  principal  source  of  supply  for  the  gray 
matter.  From  the  two  vessels  next  the  sensory  roots  branches  enter 
to  the  region  of  the  future  posterior  horn. 

The  vascular  buds  consist  of  elongated  vasoformative  cells,  which 
force  their  way  tin « >ugh  the  neuroglia  network;  by  the  time  the  buds 
have  become  vessels,  there  are  considerable  perivascular  spaces,  as 
it'  the  neuroglia  had  contracted  away  from  the  blood-vessel. 

A  tier  the  vessels  have  penetrated  it,  the  cord  develops  more  rapidly, 
as  if  Letter  nourished  (His,  86.2,  4!>6). 

Medulla  Oblongata. — The  term  is  now  restricted  to  the  por- 
tion of  the  brain  extending  from  the  spinal  cord  to  the  Varolian 
bend.  <  Mir  knowledge  of  its  development  is  derived  mainly  from  the 
superb  researches  of  His,  whose  predecessors  had  given  us  little 
more  than  generalized  descriptions  of  the  external  form.  This  sec- 
tion is,  therefore,  based  on  His'  paper,  90.2,  which,  however,  deals 
with  the  development  of  the  region  of  the  calamus  scriptorius  only, 
to  \\hich  region  accordingly  the  following  account  mainly  refers. 
The  presence  of  the  zones  of  His  and  the  appearance  of  the  Rau- 
tenlippe  have  already  been  described,  p.  608.  The  division  of  the 
medullary  walls  into  four  zones  (p.  806)  dominates  the  structure 
of  the  medulla  oblongata  throughout  life,  and  the  division  of  the 
ventral  and  dorsal  zones  of  His  can  be  traced  in  the  floor  of  the 
fourth  ventricle  of  the  adult.  The  secondary  complications  of  the 
medulla  are  largely  owing  to  the  modifications  due  to  the  transfor- 
mation of  the  Rautenlippe,  and  in  lesser  degree  to  the  fact  that  the 
anterior  fissure  of  the  spinal  cord  is  replaced  by  a  thickening  of  the 
Bodenplatte,  which  allows  the  nerve-fibres  to  cross  from  one  side  to 
the  other  directly.  The  following  more  detailed  history  may  be 
more  easily  understood  if  these  general  characteristics  of  the  me- 
dulla are  born  in  mind,  than  would  be  otherwise  possible. 

As  His  points  out,  90.2,  6G,  the  adult  medulla  contains  in  every 
transx •« -i •-••  -Action  parts  which  have  been  present  from  the  start  and 
others  which  have  been  added  later;  the  former  as  a  rule  lie  nearer 
the  ventricle ;  the  added  parts  lie  nearer  the  outside,  but  a  portion  of 
them  mingle  with  the  older  parts,  it  being  especially  the  fibres  which 
traverse  the  medulla  as  they  develop  in  manifold  directions.  Never- 
theless, in  a  general  way,  we  may  affirm  that  the  further  from  the 
ventricle  in  the  adult,  the  later  was  the  development  in  the  embryo. 
The  first  cells  to  be  differentiated  are  the  spongioblasts,  which  con- 
stitute the  ependyma  in  the  adult.  Next  arise  the  neuroblasts  which 
migrate  into  the  mantle  layer;  the  earliest  nerve-fibres  alone  give 
rise  directly  to  nerve-roots;  the  later  ones  take  their  paths  within 
the  medulla.  Third  arise  the  fibres  of  the  formatio  arcuata,  which 
lies  in  the  outer  part  of  the  mantle  layer  and  sends  its  fibres  from 
side  to  side,  and  the  homologue  (tractus  solitarius)  of  the  oval  bundle 
of  spinal  cord  sensory  fibres.  Fourth,  the  parts  already  formed  are 
covered  in  by  the  Rautenlippe  and  the  stream  of  neuroblasts  which 


666 


THE  F<r/rrs. 


it  sends  toward  the  median  line.  Outside  of  all  these  finally  ensues 
a  development  of  transverse  and  longitudinal  fibres,  the  latter  in- 
cluding the  funiculusrestiformis  and  thetractus  intermedius  of  His. 
ZONES  OF  His  IN  THE  ADULT. — As  will  be  shown  below,  the 
tractus  solitarius  is  a  bundle  of  fibres  running  longitudinally  and 
homologous  with  the  "  oval  bundle"  of  sensory  fibres  in  the  spinal  c<  >rd, 
and  it  indicates  permanently  the  lower  boundary  of  the  dorsal 
zone  of  His.  In  the  embryo  the  two  columns  primitively  meet  at 
a  decided  angle,  and  this  angle  is  marked  by  a  groove  in  the  wall  of 
the  central  canal,  or,  as  we  should  say  in  referring  to  the  adult,  in 
the  floor  of  the  fourth  ventricle.  There  is  always  a  median  groove, 
which  extends  from  the  opening  of  the  central  canal  to  the  aque- 
ductus  SylviaB,  and  marks  the  limit  between  the  two  ventral  zones  <  >f 
His,  although  they  partially  concresce  during  embryonic  life;  on 
each  side  of  the  groove  is  the  ventral  zone,  the  surface  of  which 
projects  slightly  and  is  known  in  descriptive  anatomy  as  the  emi- 
nentia  teres.  The  groove  between  the  dorsal  and  ventral  zone  is 
very  shallow  and  partially  obliterated  in  the  adult;  it  persists,  how- 
ever, in  three  depressions,  namely,  the  fovea  posterior  of  the  ala 
cinerea,  the  fovea  anterior,  and  the  sharp  depression  between  the 
eminentia  teres  and  the  peduncles  of  the  cerebellum;  opposite 
Schwalbe's  tuberculum  acusticum  the  groove  is  almost  obliterated. 
By  this  division  we  see  that  the  al»  cinereaa  and  corpora  restiformia 
are  parts  of  the  dorsal  zone  of  His. 

DORSAL  ZONE  OF  His. — This  part  of  the  medulla  (Flugeleiste 
of  W.  His)  undergoes  fundamental  modifications  owing  to  the  devel- 
opment of  the  Rautenlippe,  p.  608.  It  also  changes  its  position 
with  relation  to  the  ventral  zone  in  consequence  of  the  long  con- 
tinued expansion  of  the  deck-plate,  or,  in  other  words,  in  consequence 
of  the  so-called  opening  of  the  medulla.  When  first  fully  differenti- 
ated the  ventral  zones,  as  seen  in  cross  sections,  Figs.  o-i-S  and 
349,  ascend  obliquely  from  the  median  line,  but  the  dorsal  zones 

appear  nearly  parallel  with 
the  median  plane.  In  the 
next  stage,  Fig.  370,  the 
ventral  zones  diverge  so 
much  from  one  another  that 
they  both  lie  nearly  in  the 
same  horizontal  plane;  at 
the  same  time  (beginning  of 
the  fifth  week)  the  dorsal 
zone  bends  over  throughout 
its  length  to  form  the  Rau- 
tenlippe, RL;  the  lower  limit 

FIG.  379. —Transverse  Section  of  the  Medulla  Oblon-  f  +v     ^r-oal  ryrvnck  ie  -mar-Varl 

gataofHis'  Embryo  Ru  (Nackenlange,  9.1  mm).     After  ot  Hie  dorsal  ZOne  IS  marKeQ 

W.  His.      #L,  Rautenlippe;    Ts,  tractus  solitarius;  X,  by  the  position  of  the  tractus 

vagus  nerve ;  XII,  hypoglossal  nerve,     x  40  diams.  J.       F            „,           TTT--I- 

solitarius,    Ts.      Within    a 

few  days  the  Rautenlippe  unites  with  the  main  fold  of  the  zone 
and  continues  to  grow  toward  the  median  ventral  line  passing  out- 
side of  the  tractus  solitarius,  which  thus  becomes  buried,  and, 
instead  of  lying  superficially,  is  thereafter  deep  below  the  outer 
surface.  The  modified  dorsal  zone  formed  by  the  union  of  the 


i  m:  NEKVors  ^  STKM.  667 


tw.>  f..lds  is  termed  by  His,  90.2,  83,  riii'li'liriilxt.  With 
beu-inniiiLC  «>f  the  sixth  week  the  Fliip-hvulst  bends  over  outward  so 
that  its  inner  surt'are  faces  dorsalward  and  its  outer  surface  ventral  - 
ward.  The  dorsal  and  ventral  -  are  now  nearly  in  the  same 

plane,  and  the  groove  on  the  inner  surface  between  the  zone 
nearly  obliterated,  Fig.  380.     There  next  arises  the  secondary  Rau- 


^^\ 

it1 

r* 


Fio.  880.-Tran8verae  Section  of  the  Medulla  Oblon*ata  of  Hi»'  Embryo  Mr.      7*.  Tnu-ms 
Bolitarius;  /?/,  secondary  Rautenlippe;  /-'.r,  funieulus  ivstifonnis;  a.Tr,  aacvi:  min.il 

tract.    After  W.  His.     X  10  diams. 

tenlippe  of  His,  Fig.  380,  RL,  which  is  apparently  not  a  nervous 
stru. -tur.  -.  but  merely  a  transition  from  the  dorsal  zone  to  the 
ej.endyina  or  expanded  deck-plate;  it  must  not  be  confounded  with 
tin  true  or  primary  Rautenlippe.  If  the  size  of  the  parts  developed 
i  the  dorsal  zone  be  compared  with  that  of  the  ventral  eoluinn 
in  Fin.  -580 — the  tractus  solitarius,  T,  marks  the  boundary — it  will 
be  evident  that  scarcely  a  fifth  of  the  adult  medulla  is  developed 
from  the  dorsal  zone. 

The  dorsal  zone  becomes  the  corpus  restiforme  of  the  adult, 
including  the  tracts  of  longitudinal  fibres  associated  with  it;  these 
are  the  f mi-tux  .W/7ff /-/MS,  Fig.  380,  T7,  the  funicnlus  resti/onm*. 
/•'./-,  and  probably  the  ascend imj  //-/f/r/// ///*//  tract,  a.Tr.  This  la-t 
probably  only,  l>ecause  at  the  time  and  place  it  appears  the  exact 
boundary  between  the  two  zones  cannot  be  determined.  Further 
toward  the  spinal  cord  the  restiform  body  merges  into  the  clava, 
which  passes  into  the  fasciculus  gracilis,  which  in  its  turn  is  pro- 
1.  >ni;ed  into  the  columns  of  Goll  in  the  spinal  cord.  During  the  fifth 
month  the  clava  occupies  nearly  a  transverse  direction  (Kolliker, 
"  Kntwickelungsges.,"  2te  Aufl.,  549).  The  detailed  history  of  the 
restiform  body  has  still  to  be  traced.  The  tractus  solitarius  arises 
very  early,  ( miug  to  the  penetration  of  fibres  from  the  cerebral  ganglia 
into  the  medulla ;  these  fibres,  like  those  of  the  spinal  nerves,  take 
a  longitudinal  course  and  appear  in  sections  as  a  compact  bundle  sit- 
uated in  the  Randschleier  of  the  dorsal  zone  of  His,  as  has  been 
already  described  in  detail  in  the  account  of  the  cephalic  nerves. 
A-  Mated  above,  the  Rautenlippe  during  the  fifth  week  buries  the 
solitary  tract.  Its  development  shows  that  it  is  homologous  with 
the  columns  of  Burdach  of  the  spinal  cord,  although  in  the  medulla 
it  loses  its  original  superficial  position,  which  it  retains  in  the  cord. 
Alter  the  Rautenlippe  has  united  with  the  inner  fold  of  the  dorsal 
zone  a  layer  of  neuroglia  is  developed  over  the  new  external  sur- 


668  THE   FCETUS. 

face  of  the  zone;  this  layer  is  continuous  with  the  Randschleier 
of  the  ventral  zone,  compare  Fig.  300;  in  it  appear  two  bundles 
of  longitudinal  fibres,  Fr  and  a.  Tr,  also  transverse  or  so-called 
arcuate  fibres.  The  most  lateral  of  these  bundles,  Fr,  is  the  funiculus 
restiformis ;  it  is  scarcely  noticeable  until  the  secondary  Rautenlippe 
is  formed ;  the  fibres  are  coarse ;  the  ventro-medial  portion  is  pene- 
trated by  arcuate  fibres ;  the  fibres  of  the  bundle  first  appear  near 
the  cord,  later  higher  up;  most  of  its  fibres  are  arcuate  ones,  which 
bend  and  take  a  longitudinal  course;  these  arcuate  fibres  of  the  funi- 
culus probably  arise  from  the  cells  of  the  olivary  body  of  the  ventral 
zone  of  the  opposite  side  (His,  90.2,  57).  The  ventro-medial 
bundle,  Fig.  380,  a.  Tr,  in  the  outer  neuroglia  layer,  is  the  tract  UN 
intermedius  of  His,  a  term  which  he  employs  because  the  bundle 
includes  not  only  ascending  sensory  fibres,  but  probably  also  fibres 
running  from  the  cerebellum  to  the  spinal  cord ;  in  descriptive  anat- 
omy the  bundle  is  usually  known  as  the  ascending  trigeminal  tract 
or  root ;  the  bundle  is  oval  in  section  and  consists  of  coarse  longitu- 
dinal fibres,  and  is  crossed  by  arcuate  or  transverse  fibres ;  its  devel- 
opment begins  anteriorly  and  progresses  tailward  (His,  90.2,  56). 

The  neuroblasts  of  the  dorsal  zone  have  a  remarkable  history, 
according  to  W.  His,  88.3,  90.2,  35-44.  They  arise  early  and 
rapidly  become  abundant  (see  p.  611),  and  their  production  continues 
until  the  end  of  the  second  month,  when  it  ceases  altogether,  His, 
90.2,  47.  The  neuroblasts  develop  during  the  fourth  week,  that  is 
to  say,  before  the  formation  of  the  Rautenlippe  begins,  and  produce  the 
arcuate  fibres  and  the  primitive  cerebral  motor  roots,  as  above  de- 
scribed for  the  single  nerves.  These  neuroblasts,  therefore,  resemble 
in  their  development  those  of  the  spinal  cord.  The  neuroblasts 
which  arise  later  have  in  large  part  a  different  history,  accomplish- 
ing a  peculiar  migration,  which  has  no  parallel  in  the  spinal  cord. 
In  the  medulla,  as  in  the  cord,  the  production  of  neuroblasts  begins 
on  the  ventral  side  and  progresses  for  a  week  or  more  dorsalward, 
and  consequently,  when  the  germinating  cells  or  parent  cells  of  the 
neuroblasts  have  disappeared  in  the  ventral  zone,  they  are  still 
present  in  the  dorsal  zone  and  continue  to  change  into  young 
nerve-cells  while  the  Rautenlippe  is  bending  over.  The  concrescence 
of  the  Lippe  with  the  main  fold  of  the  dorsal  zone  opens  the  way 
for  the  neuroblasts  of  the  Rautenlippe  to  migrate  from  their  site  of 
origin  past  the  outside  of  the  tractus  solitarius  toward  the  ventral 
zone  of  His,  which  they  enter,  and  accumulating  in  its  lower 
part,  Fig.  381,  there  contribute,  together  with  other  neuroblasts 
which  come  from  the  dorsal  zone  by  migrating  in  paths  inside 
the  tractus  solitarius,  to  the  development  of  the  olivary  bodies.  The 
cause  of  the  migration  of  the  neuroblasts  is  entirely  unknown,  but 
their  wandering  from  the  Rautenlippe  is  one  of  the  most  distinctive 
characteristics  of  the  medulla  oblongata. 

VENTRAL  ZONE  OF  His. — This  zone  is  at  first  about  the  same 
as  the  dorsal  in  size,  Fig.  379,  but  it  rapidly  outgrows  the  dor- 
sal zone  and  constitutes  more  than  three-fourths  of  the  adult  me- 
dulla. Its  development  has  an  obvious  resemblance  to  that  of  the 
ventral  zone  in  the  spinal  cord,  for  there  is  a  similar  rapid  expan- 
sion and  consequent  bulging  inward  and  outward,  and  the  expansion 


TIIK  NKBVOUH   SYSTEM. 


is  due  chiefly  to  the  mantle  lay«-r.  the  Raiidschleier  remaining  thin. 
Tin-re  are  three  chief  factors  which  cause  the  development  to  differ 
t'mm  that  in  the  spinal  cord.  These  are,  1,  the  bending  of  the 
zones  nut  ward  and  downward  until  they  come  to  lie  in  nearly  t  he 
same  huri/mital  plane,  compare  Fig.  879  with  Kip.  .  the  ab- 

wnceof  tin-  anterior  ti>Mire,  which  is  obliterated  by  the  growth  <>f 
the  l»i  xlen  plat  te  t«»  constitute  the  raphe;  3,  the  peculiar  arrangement 
which  is  gradually  assumed  by  the  gray  matter,  developed  out  of 
the  mantle  layer;  4,  the  nerve-fibres  in  the  Randschleier  also  take 
different  courses  from  that  which  they  take  in  the  white  matter  of 
the  >pinal  cnnl.  These  four  sets  of  features  are  considered  in  the 
four  following  paragraphs. 

1.  Tin-  l><  -mli  at/  of  the  ventral  zones,  like  that  of  the  dorsal,  is 
part  of  the  process  of  the  so-called  opening  of  the  medulla  correlated 
with  the  expansion  of  the  deck-plate.  The  general  character  of  the 
mnvfiiient  has  been  already  described,  p.  609.  We  have  merely  to 
add  that,  while  it  is  going  on,  the  inner  surface  of  the  zone,  which 
t  it  utes  the  larger  part  of  the  floor  of  the  fourth  ventricle,  becomes 
pmtiiberant  and  bulges  inward,  forming  a  wide,  rounded,  longitu- 
dinal ridge,  Fig.  381  ;  the  two  ridges  are  separated  from  one  another 


xn 

Fio.  381.— Section  through  the  Medulla  Oblonjfata  of  His'  Enihryo  CR     T.&,  Tractus  solitarius; 
X.  \-ju'ii>  n.-rv,.;  Rsch,  Randschleier;  XII,  hypoglossal  nerve;  R,  raphe.     After  W.  His. 

by  a  narrow,  deep  median  fissure  or  groove,  which  in  later  stages 
opens  somewhat,  so  as  to  appear  V-shaped  in  cross  section,  and  per- 
sists throughout  life  in  that  form.  As  the  groove  deepens  but  little, 
if  at  all,  after  the  second  month,  while  the  medulla  continues  to 
enlarge,  it  follows  that  the  groove  becomes  not  absolutely,  as  some- 
t lines  stated,  but  relatively  smaller.  His  speaks  of  its  opposite  walls 
uniting  and  the  groove  thus  diminishing,  but  he  gives  no  direct 
evidence  of  such  concrescence,  and  his  figures  show  no  diminution  of 
size  in  the  groove  during  later  stages.  In  Fig.  381,  another  effect  of 
the  interior  bulging  is  shown,  namely,  that  that  part  of  the  surface 
of  the  ^ventral  zone  is  brought  into  nearly  the  same  plane  as  the 
inner  surface  of  the  dorsal  zone,  and  as  the  groove  between  the 
two  zones  is  nearly  obliterated,  the  floor  of  the  medullary  cavity 
(fourth  ventricle)  is  rendered  comparatively  even. 

2.  The  raphe  arises  by  a  thickening  of  the  Bodenplatte  and  is 
primitively  a  partition  of  neuroglia,  which  is  subsequently  penetrated 
by  fibres  crossing  from  side  to  side.  In  the  spinal  cord  the  Boden- 
platte remains  thin  though  it  gives  rise  to  neuroglia,  and  by  the  pas- 
sage through  it  of  nerve-fibres  forms  the  anterior  white  commissure. 
We  must,  therefore,  homologize  the  raphe  with  this  commissure. 


670  THE   FOETUS. 

As  the  ventral  zones  thicken  during  the  second  month  and  pro- 
ject more  and  more  ventral  ward,  the  growth  of  the  Bodenplatte  in 
the  medulla  oblongata  obliterates  the  fissure  almost,  but  not  quite, 
completely,  which  would  otherwise  be  formed  between  them,  as  in 
the  cord.  The  growth  of  the  Platte  depends  on  the  elongation  of  its 
cells  (spongioblasts,  for  it  contains  few  or  no  neuroblasts) ,  which  is 
accompanied  by  a  movement  outward  of  some  of  its  nucleated  cell- 
fibres,  which  are  at  first  all  situated  close  to  the  central  canal.  By 
the  end  of  the  first  month  fibres  cross  the  septum,  and  thereafter  the 
number  of  fibres  crossing  it  steadily  increases.  It  allows  no  neuro- 
blasts to  pass  (His,  90.2,  27,  55). 

3.  The  gray  matter  or  mantle  layer  increases  very  rapidly  and  is 
the  principal  factor  in  the  enlargement  of  the  ventral  zone.  Its 
development  involves,  as  elsewhere  in  the  neuron,  the  gradual  reduc- 
tion of  the  inner  layer  until  only  the  ependyma  remains.  The  gray 
matter  is,  of  course,  homologous  with  the  anterior  horn  of  the  spinal 
cord;  but  whereas  in  the  spinal  cord  the  nerve-cells  and  nerve- 
fibres  are  irregularly  arranged,  in  the  medulla  they  produce  a  highly 
characteristic  pattern  by  their  distribution.  The  greater  part  of  the 
gray  matter  in  the  ventral  zone  of  the  medulla  is  converted  into 
the  formatio  reticularis,  His,  90.2,  51.  The  formatio  reticularis 
has  from  the  very  start  a  more  or  less  distinctly  four-sided  outline, 
as  seen  in  cross  sections ;  it  is  marked  out  by  the  bundles  of  nerve- 
fibres  crossing  one  another  at  right  angles.  One  side  faces  the 
fourth  ventricle,  Fig.  381;  one  faces  theraphe;  the  third  faces  the 
outer  wall  of  the  medulla,  and  the  fourth,  which  is  irregular  and 
somewhat  undefined,  faces  the  dorsal  zone.  The  reticulate  ap- 
pearance of  this  area  is  due  to  the  crossing  of  the  fibres  at  right 
angles  to  one  another.  The  fibres  are  first  radial,  second  arcuate 
or  transverse  running  toward  or  from  the  raphe,  and  third  longi- 
tudinal; the  last  set  of  fibres  are  developed  later  than  the  first  two. 
The  fibres  are  united  in  bundles,  which  grow  in  size  by  the  ad- 
dition of  fibres  which  join  them  as  development  progresses;  the 
fibres  are  accompanied  by  a  limited  number  of  neuroblasts  migrat- 
ing along  the  bundles.  The  formatio  reticularis  is  clearly  mapped 
out  by  the  end  of  the  fourth  week,  and  its  development  commences  as 
soon  as  the  nerve-fibres  begin  to  grow  out  from  the  neuroblasts,  for 
the  fibres  at  once  follow  their  definite  courses,  one  set  taking  radial 
paths,  another  set  taking  transverse  courses.  A  similar  arrangement 
is  found  in  the  mantle  layer  of  the  spinal  cord,  but  is  obscured  by 
the  further  development,  instead  of  being  preserved  and  emphasized 
as  in  the  medulla. 

In  embryos  of  six  weeks  and  older  the  formatio  reticular js  is  en- 
tirely surrounded  by  a  crowd  of  neuroblasts.  Of  these  the  accumu- 
lation on  the  inner  side,  or  toward  the  fourth  ventricle,  is  the  oldest 
and  consists  of  neuroblasts  developed  in  loco;  it  is  very  distinct  at 
the  beginning  of  the  fifth  week.  The  neuroblasts  on  the  lateral  side 
are,  of  course,  those  which  belong  to  the  dorsal  zones  of  His. 
Those  on  the  medial  and  outer  side,  on  the  contrary,  are  immigrant 
cells,  which  have  travelled  to  their  location  after  the  union  of  the 
Rautenlippe  with  the  main  wall.  The  stream  of  cells  passes,  as  we 
have  seen,  on  both  sides  of  the  tractus  solitarius,  Fig.  381 ;  that 


THI-:  NKKVOUS  M  M  KM.  •  ;;  i 

the  tractus  comes  from  the  Rautenlippe,  and  is  at  first  (fifth 
we«-k)  >mall,  Itut  later  increases  very  much.  The  stream  j»r 
around  the  edge,  or,  better  expressed,  over  the  surface  of  the  formatio 
until  its  outer  and  medial  surfaces  are  covered  by  scattered  neuro- 
Masts,  forming  a  continuous  sheet  ((in'iit  :/>l<ttte  of  His,  90.2,  42), 
which  the  subsequent  development  transforms  into  the  cellular  layer 
of  the  it/inu-fi  l» »lt/,  compare  Fig.  3S1.  The  olivary  cell  band  has 
at  tir>t  no  very  definite  boundary  ;  t lie  cells  are  here  and  there  more 
crowded  than  elsewhere  (His,  90.2,  5x});  the  fibres  which  spring 
from  them  gather  into  bundles  and  run  to  ward  the  raphe ;  by  the 
end  of  the  third  month  the  olivary  band  has  become  folded  and  ap- 
pears to  contain  all  the  cells  it  is  to  re< -ei  ve.  The  band  gives  rise 
ultimately  to  hoth  the  upper  and  lower  olivary  bodies — in  the  region 
of  the  hypoglossus  to  the  accessory  olivary  body  (  Nenbtnoliv*  )  and 
in  the  region  of  the  pons  to  the  z«<  /,-/,/,  /  /;/ •//,•/., -nl-f rn  of  His.  The 
layer  of  neuroblasts  between  the  formatio  reticularis  and  the  epen- 
dyma  is  the  anlage  of  the  sub-ependymal  motor  nuclei,  His,  I.e.,  p. 
50.  It  may  prove  an  assistance  in  following  the  description  of  the 
medullary  structure  to  point  out  that  in  a  rough  way  there  are  four 
layers  distinguishable:  1,  externally  is  the  layer  of  white  matter 
developed  from  the  Randschleier,  and  which  may  be  followed  into 
the  dorsal  zone,  see  Fi^.  :M  :  .'.  internally  is  the  sub-ependymal 
layer  of  neuroblasts,  which  is  continued  laterally  into  the  gray  mat- 
ter of  the  dorsal  zone  (corpus  restiforme) ;  3,  inside  the  external 
tiln-oiis  layer  is  the  sheet  of  olivary  neuroblasts,  which  merge  into 
the  lateral  gray  matter  of  the  dorsal  zone;  4,  the  layer  of  the  for- 
matio reticularis  between  the  sub-ependymal  layer  and  the  olivary 
body;  this  layer  may  be  considered  as  continued  laterally  by  the 
tractus  solitarius,  but  topographically  only,  for  the  formatio  reticu 
laris  arises  from  the  gray  matter,  the  tractus,  as  we  have  seen,  from 
the  primitive  Randschleier,  so  that  one  cannot  be  the  morphological 
continuation  of  the  other. 

4.  The  Randschleier  includes  the  homologues  of  the  anterior  and 
lateral  cords  of  white  substance  in  the  spinal  cord,  being  divided  by 
the  exit  of  the  ventral  roots  (hypoglossus  and  abducens)  into  two 
regions,  Fig.  381,  one  medial  region  adjoining  the  raphe,  the  other 
ventral,  situated  at  the  exposed  outer  ventral  surface;  the  former 
corresponds  to  the  anterior,  the  latter  to  the  lateral  columns,  and  the 
latter  spreads,  as  we  have  seen,  over  the  dorsal  zone  after  the 
concrescence  of  the  Rautenlippe.  The  two  regions  meet,  of  course, 
along  the  line  of  the  ventral  roots,  forming  a  rounded  angle  with 
one  another  (His,  90.2,  54).  The  medial  region,  as  soon  as  the 
nerve-fibres  begin  to  develop,  acquires  both  longitudinal  fibres  and 
transverse  fibres,  the  latter  running  to  the  raphe  or  thickened  Boden- 
platte ;  as  development  progresses  the  number  of  fibres  increases  and 
they  group  themselves  into  bundles :  the  primitive  longitudinal  fibres, 
like  those  of  the  spinal  cord,  are  derived  from  the  gray  matter  of  the 
opposite  side ;  this  primitive  longitudinal  bundle  persists  through- 
out life;  it  is  the  hinterer  Langsbundel  of  Flechsig.  During  the 
second  month  the  medial  region  grows  rapidly,  expanding  at  the  same 
rate  as  the  raphe,  but  the  primitive  longitudinal  bundle  is  kept  con- 
fined near  the  ventricle  so  that  below  it  is  a  layer  of  netiroglia 


672  THE    FCETUS. 

between  the  f ormatio  reticularis  and  the  raphe ;  this  layer  is  crossed 
by  bundles  of  arcuate  fibres,  which  enter  through  the  raphe  from 
the  opposite  side  and  most  of  which  join  the  formatio  reticularis ; 
during  the  fourth  month  longitudinal  fibres  constituting  the  so-called 
pyramids  are  developed  in  this  ventral  part  of  the  medial  region. 
The  very  late  development  of  the  pyramids  was  discovered  by  Flech- 
sig,  76.1,  132,  142. 

The  ventral  region  of  the  Randschleier,  extending  from  the  exit 
of  the  ventral  roots,  Fig.  381,  XII,  to  the  dorsal  zone,  is,  of 
course,  homologous  with  the  region  of  the  lateral  columns  of  the 
spinal  cord;  it  is  identical  with  the  medial  part  of  the  iceisse 
Randzone  of  Flechsig,  76.1.  When  the  olivary  band  of  neuro- 
blasts  becomes  folded,  some  of  the  folds  cut  so  deep  into  the  white 
layer  that  it  is  almost  obliterated  at  those  points.  About  the  middle 
of  the  second  month  fibres  from  the  raphe  enter  the  layer  and  ulti- 
mately pass  on  to  form  the  f uniculus  restiformis ;  the  number  of 
these  fibres,  though  small  at  first,  is  large  by  the  end  of  the  second 
month.  There  appear  during  the  second  month  fine  longitudinal 
fibres  in  the  layer. 

Pons  Varolii. — The  pons  is  developed  out  of  the  floor  of  the  third 
primitive  vesicle  of  the  brain,  in  front  of  the  Varolian  bend.  Con- 
cerning its  history  we  possess  no  detailed  information.  Kolliker 
(Grundriss,  2te  Aufl.,  250)  states  that  the  characteristic  transverse 
fibres  appear  during  the  third  month  as  a  narrow,  thin  band,  and 
that  the  pons  grows  as  the  lobes  of  the  cerebellum  become  larger  and 
more  distinct.  He  notes  further  as  characteristic  of  the  foetal  brain 
that  the  corpus  restiforme  seems  to  merge  in  part  with  the  lateral 
part  of  the  pons,  and  apparently  some  of  the  fibres  of  the  corpus  bend 
toward  the  median  ventral  line  and  enter  the  pons.  The  growth  of 
the  pons  is  rapid.  In  embryos  of  the  fourth  month  and  older,  the 
pons  can  be  at  once  recognized  as  a  commissure  between  the  two 
sides  of  the  cerebellum. 

As  to  the  fate  of  neuroblasts  present  in  the  pons,  and  as  to  the 
origin  of  the  nerve-fibres  of  the  pons,  nothing  is,  as  yet,  known.  It 
will  probably  be -found  that  the  development  of  the  pons  is  similar 
to  that  of  the  ventral  zones  of  His  in  the  medulla  oblongata. 

Cerebellum. — The  morphologically  primitive  relations  and  posi- 
tion of  the  cerebellum  are  well  shown  in  the  frog's  brain,  Fig.  393.  It 
is  a  thickening  of  the  brain  walls  extending  across  the  median  dorsal 
line ;  its  formation,  therefore,  involves  the  thickening  of  the  deck- 
plate  ;  the  cerebellum  is  situated  between  the  medulla  oblongata  and 
the  isthmus  or  constricted  portion  of  the  medullary  tube  connecting 
the  hind-  and  mid-brains.  His,  90.2,  24,  states  incidentally  that 
it  is  developed  in  man  at  least  from  the  dorsal  zone  of  His  (Flti- 
gelplatte) ;  unfortunately  his  investigations  on  the  cerebellum  are 
still  unpublished. 

The  following  account  of  the  development  of  the  external  form  of 
the  cerebellum  is  based  on  Mihalkovics,  77.1,  53-57.  In  its  first 
stage  the  cerebellum  is  merely  a  lamella  across  the  dorsal  side  of  the 
hind-brain;  its  posterior  limit  is  marked  by  the  point  where  the 
expansion  of  the  deck-plate  begins ;  toward  the  mid-brain  the  lamella 
merges  into  the  isthmus  without  any  demarcation  (human  embryo 


THI:  NKKV<  >rs  M  STKM. 


678 


cbl 


of  tho  fourth  week),  compare  Kig.  842.  At  this  stage  the  cerebellum 
rises  as  a  transverse  plate  inclined  at  a  wide  angle  to  the  axis  of  the 
medulla  nblnngata.  and  hears  an  ob- 
vious reeemblanoe  to  the  amphibian 
(vivU'llinn,  compare  Figs.  382  and 
While  the  Varolian  bend 
(Bruckenbeuge)  i-  developing  the 
lamella  thickens  and  widens;  its  pos- 
terior larder  passes  gradually  int.- 
the  thin  and  expanded  deck-plate  of 
the  medulla:  the  transit  ional  part  i- 
the  secondary  Rautenlippe,  and  dors 
not  participate  in  the  formation  of 
the  cerebellum,  but  is  the  anlage  of 
the  re/nut  inol  n//n  rr  i»>.sf  ten  ni  (liin- 
f<  /-r.s-  Murlcxi'yh  p.  677.  The  further 
development  of  the  lamella  in  the 

•  •hick   has  been  investigated  by  La- 
housse,  88.1;  it  continues  growing 
in  all  dimensions;  by  the  third  day 
it  begins  to  arch  forward  and  upward 

until  it  encloses  a  space,  which  is  a  £SS/""SH?i  V''  rt"«rofmedullaob: 

.       *  A    •    i          lonjjaia.     AIMT  JJIIKKISSI*. 

•  iivcrticuluni  of  the  fourth  ventricle, 

Ki-.  383;  the  convolutions  are  distinctly  marked  on  the  ninth  day 
and  are  merely  superficial  transverse  ridges,  not  folds  of  the  wall. 


>.-<  tion  of  the  Brain  of 

uCl.i.-k  Kinl.rv..  ,,t  HlMiiit  fimr  Hays.  ,-l,l. 
C.-rflM-lliiiu ;  //./•.  iniil-hrain :  i>m.  pineal 
anlajre;  /T>,  fore-brain;  EC.  ectoderm  IV. 


V  if 


$$ 


FIG.  .m—  Longitudinal  Median  Section  of  the  Cerebellum  of  a  Chick  of  about  twelve  Days. 
/i.  Nodulus;  r.i.  vrrmis  inferior;  Pfc,  layer  of  Purkinje's  cells:  wes,  mesoderm :  t\«,  vermis 
superior;  /t«,  lingula;  pi,  plexus;  m.o,  membrana  obturator.  After  E.  Lahousse. 

It  is  probable  that  in  the  mammalian  embryo  a  similar  bending 
of  the  lamella  takes  place,  but  that  the  diverticulum  is  obliterated 
43 


674 


THE    FCETUS. 


by  the  growth  of  the  cerebellar  walls,  but  observations  are  want- 
ing to  verify  this  supposition.  There  is  never  present  any  large 
open  diverticulum  in  the  mammalian  embryo  (Kolliker,  "  Ent- 
wickelungsges. "  2te  Aufl.,  537).  The  lamellar  anlage  of  the  mam- 
malian cerebellum  grows  rapidly  into  a  rounded  protuberance,  the 
transverse  diameter  of  which  exceeds  the  longitudinal.  As  seen 
from  above,  the  cerebellum  now  appears  somewhat  pointed  later- 
ally. The  lateral  ends  of  the  lamella  expand  and  form  the  anlage  of 
the  cerebellar  hemispheres,  leaving  the  median  portion  as  the  anlage 
of  the  vermis.  There  now  soon  appear  (beginning  of  the  fourth 
month  in  man,  cow  embryo  of  80  mm.)  a  series  of  four  transverse 
grooves,  by  which  the  surface  of  the  vermis  is  divided  into  five  pri- 
mary ridges  (gyri),  which  persist  as  five  primary  lobes  throughout 
life ;  two  of  the  transverse  lobes  belong  to  the  upper  surface ;  three  to 

the  lower  surface ;  they 
are  respectively  the 
quadrate  or  antero- 
superior,  the  postero- 
superior,  the  postero- 
inferior,  pyramidal, 
and  the  uvula.  During 
the  fourth  month  the 
hemispheres  grow  rap- 
idly, so  that  at  five 
months  they  equal  and 
thereafter  surpass  the 
central  vermis  more 
and  more  in  size.  The 
primary  transverse 
lobes  spread  onto  the 
hemispheres  during 
the  fourth  month,  and 


K 


FIG.  384.  —Section  through  the  Cerebellum  and  Medulla  Ob- 
longata  of  a  Human  Embryo  of  one  hundred  and  sixty  Days. 
MinotColl.  No.  66.     V,  Vermis;  H,  hemispheres;  Fl,  fl< 
Md,  medulla  oblongata;  C,  Central  canal.     X  4  diams. 


MinotColl.  No.  66.     V,  Vermis;  H,  hemispheres;  Fl,  flocculus;    they  persist  there  as  ill 

~"  the  vermis  throughout 

life.  In  descriptive  anatomy  an  astounding  variety  of  names  are 
applied  to  the  various  parts  of  each  lobe ;  it  would  be  an  essential 
gain  if  at  least  three-fourths  of  these  names  could  be  discarded. 
Each  of  the  five  primary  lobes  becomes  subdivided  by  additional 
grooves,  most  of  which  are  approximately  parallel  to  the  primary 
grooves ;  the  subdivision  continues  until  the  full  number  of  folia  are 
produced,  which  is  probably  accomplished  before  birth.  The  fifth  or 
most  posterior  lobe  forms  an  independent  expansion  on  each  side, 
beginning  in  the  fourth  month  to  form  the  flocculus,  Fig.  384,  FL 
A  number  of  additional  details  as  to  the  human  cerebellum  at  various 
stages  are  given  by  Kolliker  ("Entwickelungsges.,"  2te  Aufl.,  542- 
548). 

The  histogenesis  of  the  cerebellum  has  been  studied  in  the  chick 
by  Lahousse,  88.1,  and  in  man  by  Bellonci  et  Stefani,  89.1,  and 
Vignal,  88.1.  In  the  chick  at  six  days  (Lahousse,  Fig.  28)  both 
the  mantle  and  inner  layers  are  crowded  with  nuclei  and  form  about 
three-fourths  of  the  wall  in  section,  the  remaining  fourth  being  con- 
stituted by  the  Randschleier  in  which  there  are  a  few  nuclei ;  between 


TMK    NEBVOUS    >N  STEM, 


G75 


tin'  spon.nioblasts  are  seen  the  dividing  germinating  cells  (p.  613) 
<•!•»»•  to  the  central  canal.  The  Kaudschleier  of  the  cerebellum  is 
the  (/nine  niulccnln  re  I  )ecl:l<t  melle  of  Lowe,  80.2,  or  envellope 
nndeenl'iire  <jrise  of  Lahousse,  I.C.,  p.  ''•:».  At  the  sixth  day  there 
I.,  -ins  to  appear  in  the  Randschleier  a  stream  of  cells,  which  proba- 
l.ly  come  from  the  Rautenlippe  of  the  cerebellum,  but  as  the  Lippe 
\\as  not  known  to  Lahousse  he  gives  no  information  on  this  point.* 
These  cells  are  elongated  parallel  to  the  surface  of  the  cerebellum, 
close  to  which  they  appear.  Their  immigration  results  finally  in 
the  conversion  of  the  Randschleier  into  the  outer  layer  of  the  adult. 
1  Miring  the  eighth  day  the  nerve-fibres  appear,  the  differentiation  of 
the  mantle  and  inner  layers  is  easily  recognized,  and  the  Handschleier 
now  comprises  three  layers,  a  thin  outer  or  superficial  neuroglia 
layer,  a  middle  gray  richly  nucleated  layer,  in  which  the  immigrant 
cells  are  >itiiated,  and  a  third  layer  next  the  mantle  layer,  having 
tiered  nuclei.  The  ninth  day  we  can  make  out  the  following 
layers  beginning  within:  1,  the  ependyma:  2,  the  inner  layer;  3, 
the  mantle  or  gray  molecular  layer,  some  of  the  cells  along  the  outer 
edge  of  which  are  changing  into  Purkinje's  cells,  making  another 
layer,  j;  5,  the  neuroglia  layer  or  inner  part  of  the  Randschleier; 
and,  ».,  t  he  outermost  layer  containing 
immigrant  cells  (Lowe's  Zellstrief), 
cells  which  are  probably  neuroblasts. 
The  >i\  layers  just  enumerated  can 
be  recognized  in  the  mammalian  em- 
bryo, and  have  been  described  by  W. 
Vignal,  88.1,  :;•.»;  334,  PL  XII., 
who,  however,  failed  entirely  to  re- 
<  ••  ionize  the  early  differentiation  of  the 
neuroblasts  and  neuroglia.  There  is 
a  t  hin  outermost  layer  without  nuclei, 
next  follows  a  broader  layer  crowded 
with  nuclei ;  these  belong  to  the  cells 
which  have  migrated  into  the  em- 
bryonic Randschleier,  and  they  form 
a  well-marked  layer  throughout  foe- 
tal life;  this  layer,  so  far  as  I  know, 
was  first  observed  by  Obersteiner 
(Sitzber.  Wiener  Akad.Wiss.,  1870), 
in  the  cerebellum  of  new-born  chil- 
dren; and  it  may  be  conveniently 
designated  as  the  outer  nuclear  lay- 
er: it  disappears  as  a  distinct  layer 
during  childhood.  Bellonci  et  Ste- 
fani,  89.1,  23,  state  that  two  zones 
may  be  distinguished  in  Obersteiner's 
layer,  an  outer  zone  with  numerous 
karyokinetic  figures  and  crowded 
round  nucleolated  nuclei,  and  an  inner  zone  with  the  nuclei  elongated 
and  less  crowded.  In  pigeons  of  twelve  days'  incubation,  some  of 

*  Lowe  cainr  very  near  discovering  the  Rautenlippe,  for  he  observed  that  the  ependyma  was 
reflected  on  to  the  outside  of  the  cerebellum. 


Fio.  386.— Section  of  the  Cerebellum  of  a 
Human  Embryo  of  one  hundred  and  sixty 
Days.  Minot  Coll. ,  No.  66.  Under  the  sur- 
face is  seen  the  crowded  outer  nuclear  lay- 
er, and  deeper  down  the  outer  portion  of  the 
mantle  layer,  with  crowded  nuclei,  some 
of  which  are  elongated. 


676  THE   FCETUS. 

the  cells  of  the  outer  nuclei  have  developed  dendritic  processes,  which 
extend  even  into  the  inner  nuclear  layer.  Immediately  below  the 
outer  nuclear  layer  is  one  with  few  nuclei,  and  then  we  come  to  the 
broad  band  of  crowded  nuclei  belonging  to  the  mantle  layer  proper ; 
everything  outside  the  mantle  layer  is  derived  from  the  primitive 
Randschleier.  The  cells  of  Purkinje  are  recognizable  in  the  human 
foetus  of  five  months  and  their  external  branches  at  six  months ;  at 
seven  months  their  inner  ends  are  rounded,  but  at  birth  pointed  and 
apparently  prolonged  as  a  process  running  into  the  mantle  layer 
(axis-cylinder  process?). 

The  Fourth  Ventricle  and  its  Roof.— The  fourth  ventricle 
has  long  been  known  to  embryologists  as  the  expanded  central  canal 
of  the  hind-brain,  and  as  enclosed  completely  by  the  medullary  wall. 
The  expansion  of  the  deck-plate  and  consequent  thinness  of  the  dor- 
sal wall  of  the  ventricle  was  known  to  Von  Baer,  but  he  supposed 
that  this  wall  was  lost  in  the  adult,  28.2,  74,  37. 1,  108.  Remak, 
50. 1,  33,  maintained  this  opinion  for  the  chick ;  Rathke,  39. 1,  37,38, 
20.1,  Th.  IV,  14,  for  reptiles  and  anamniota.  We  owe  to  Kolliker 
("  Entwickelungsges. , "  Iste  Aufl. ,  243)  the  discovery  of  its  persistence ; 
to  Hensen  (Arch.  f.  Mikrosk.  Anat.,  II.  424)  the  demonstration  that 
it  forms  the  epithelial  covering  of  the  choroid  plexus.*  Several 
writers  have  thought  that  the  membrane  was  broken  through  at 
certain  points,  but  it  probably  is  really  continuous  throughout 
life.  The  fourth  ventricle  is  to  be  regarded,  then,  as  an  expansi<m 
of  the  central  canal  permanently  bounded  by  the  original  medullary 
walls. 

The  fourth  ventricle  has,  as  seen  from  above,  a  rhomboid  shape, 
Figs.  342,  343 ;  it  tapers  down  anteriorly  to  the  central  canal  (aque- 
ductus  Sylvia)  of  the  mid- brain,  posteriorly  to  the  central  canal  of 
the  spinal  cord.  It  is  widest  at  the  level  of  the  Varolian  bend  and 
in  the  adult  the  lateral  angles  of  the  embryo  persist  as  the  recessus 
later  ales.  The  so-called  floor  of  the  ventricle  is  constituted  by  the 
inner  surface  of  the  dorsal  and  ventral  zones  of  His,  already  des- 
cribed. 

The  roof  of  the  ventricle  behind  the  cerebellum  is  derived  from 
the  deck-plate,  compare  p.  608;  it  becomes  subdivided  into  three 
parts,  the  dorsal  ependyma,  the  secondary  Rautenlippe,  and  the  epi- 
thelial covering  of  the  choroid  plexus  of  the  fourth  ventricle.  The 
deck-plate  is  a  layer  of  epithelium  and  preserves  its  simple  epithelial 
character  through  most  of  its  extent  and  throughout  life.  In  the 
human  embryo  at  four  weeks  (His,  90.2,  29)  it  is  a  single  layer  of 
cells,  8/4  high  by  10/A  wide,  but  toward  the  edges  of  the  plate  the 
cells  become  a  little  higher  and  narrower ;  the  number  of  cells  in- 
creases (whether  by  their  own  division  or  not,  is  uncertain)  so  that 
the  cells  become  higher  (11-13(«)  during  the  second  month,  although 
the  area  of  the  membrane  greatly  enlarges. 

Where  the  deck-plate  joins  the  lateral  wall  of  the  medulla  it 
becomes  thickened,  forming  the  secondary  Rautenlippe,  p.  667. 
When  the  main  deck-plate  and  the  choroid  plexus  are  removed  from 
an  embryo  of  two  months  or  older,  the  Rautenlippe  appears  as  a 

*  These  references  are  taken  without  verification  from  Mihalkovics,  77.1,  60.  The  reference  to 
Hensen  has  been  verified,  being  to  an  incidental  observation  in  a  paper  on  the  eyes  of  snails. 


THE   NERVOUS    SYSTK.M.  677 

i 

narrow  whitish  band  along  the  edges  of  the/o/v//  rAomlotdoft*,  or, 
in  other  words,  of  the  medulla  oMonpita  and  cerebellum.  The  band 
per>i>t>  throughout  life  and  is  known  in  descriptive  anatomy  l»v 
three  different  names;  the  part  attached  to  the  cerebellum  is  termed 
the  rt'hnn  incdiiUiirt'  i>uxtiruin  (hintm's  Murlcxri/i-l)  ;  the  part  along 
the  ed.Lce  of  the  medulla  ohlungata  is  termed  the  Tcenia  fossce  rh<nn- 
boidaiis  «>r  liijnln  ( l!i<>im-licii) ;  the  part  at  the  posterior  apex  of  the 
rhomboid  opening  is  t<  rmed  tin-  uln\r  (/.'/V^/i. 

Tlie  choroid  j)le.\iis  is  an  ingrowth  of  the  deck-plate  accompanied 
by  v  a -<•  i  ilar  mesenchyma  and  projecting  into  the  cavity  of  the  fourth 
ventricle.  In  the  amphibia,  Fig.  393,  the  half  of  the  deck-plate 
nearest  the  cerebellum  forms  a  series  of  irregular  rounded  projec- 
tions into  the  cavity  of  the  fourth  ventricle,  and  each  of  these  projec- 
ti«  -us  contains  mesench}-ma  (i.  e.  connective  tissue  and  blood-vessels). 
In  mammals  we  find  the  same  choroid  area,  but  it  is  pushed  in,  as 
a  whole,  into  the  cavity.  In  the  placentalia  at  least,  the  invagina- 
tion  of  the  whole  area  precedes  in  the  embryo  th,  formation  of  the 
irregularities  of  the  surface.  The  invagination,  cf.  Fig.  38G,  may  be 
seen  in  the  human  embryo  of  five  or  six  weeks  as  a  transverse  fold  <  -f 
the  deck-plate  extending  quite  deep  down,  and  resulting,  apparently, 
from  the  excessive  development  of  the  Varolian  bend.  The  fold  is  the 
anlage  of  the  choroid  plexus.  By  its  further  development  the  anlage 
assumes  a  more  and  more  complex  and  irregular  form,  but  it  remains 
always  a  fold  of  mesenchyma  richly  vascular  and  covered  by  the 
epithelial  deck-plate.  In  the  human  embryo  at  four  months  (Kolli- 
ker,  "  Kntwick('limv>L;e^."  ,'te  Antl.,  A-jn)  the  position  of  the  fold  can 
be  seen,  when  the  medulla  oblongata  is  viewed  from  above,  as  a 
narrow  transverse  line,  along  which  the  mesenchj'ma  (connective 
tissue  of  the  pia  mater)  enters  the  fold,  and  which  is  situated  close 
behind  the  cerebellum ;  in  front  of  and  behind  this  line  the  deck- 
plato  forms  a  transverse  rid^e  (f////v/.s-  r//o/vm/^/.s-  (inferior  and  pos- 
terior) ;  the  two  ridges  might,  at  first  sight,  be  mistaken  for  portions 
of  the  cerebellum. 

The  Mid -brain. — Concerning  the  second  cerebral  vesicle  our 
information  is  very  imperfect,  and  amounts  to  little  more  than  a 
knowledge  of  its  general  form  at  successive  stages;  it  is  derived 
chiefly  from  MihalkovioB,  77.1,  63-68,  and  Kolliker,  "Entwicke- 
lungsges."  2te  Aufl.,  535.  The  mid-brain  is  remarkable  for  its  pre- 
cocious expansion,  Fig.  341,  and  for  the  fact  that  in  young  embryos 
it  occupies — owing  to  the  cephalic  flexures — the  highest  part  or 
summit  of  the  head,  Fig.  338.  In  both  the  figures  just  referred  to, 
the  mid-brain  appears  as  a  vesicle  with  a  large  cavity  and  thin  walls 
constricted  in  front  as  it  joins  the  fore-brain — behind,  as  it  joins 
tlie  hind-brain.  We  have  no  knowledge  of  the  separate  histories  of 
the  six  longitudinal  zones  (deck-plate,  the  four  zones  of  His,  and  the 
Bodenplatte).  The  floor  of  the  mid-brain  very  early  begins  to  thicken, 
and  the  thickening  includes  the  Bodenplatte,  for  it  extends  across  the 
median  line.  On  the  dorsal  side  the  median  line  has,  in  young 
human  embryos  at  least,  an  external  ridge  with  a  corresponding  in- 
ternal groove,  both  resulting  from  a  median  fold  of  the  medullary  wall. 
The  whole  dorsal  side  of  the  mid-brain  expands  considerably  (human 
embryos  of  four  weeks) ;  especially  is  this  the  case  in  Sauropsida, 


678 


THE    FCETIS. 


as  may  be  well  seen  in  a  chick  embryo  of  the  fourth  day,  Fig.  :>s:>. 
The  mid-brain  now  grows  steadily,  though  much  less  than  the  fore- 
and  hind-brains,  so  that  the  cerebrum  and  cerebellum  outstrip  it. 
Its  growth  is  principally  a  thickening  of  its  walls  and  an  increase  of 
its  length,  but  with  little  enlargement  of  its  cavity ;  hence  the  cavity 
becomes  relatively  smaller,  though  it  persists  throughout  life  as  the 
part  of  the  central  canal  known  as  the  aqueductus  Sylvife,  and 
intervening  between  fore-brain  (third  ventricle)  and  hind-brain 
(fourth  ventricle) . 

The  ventral  part  (?  ventral  zones  of  His)  of  the  mid-brain  de- 
v61ops  into  the  peduncles  of  the  cerebrum;  the  projecting  of  the 
peduncles  as  rounded  longitudinal  ridges  on  either  side  of  the  median 


11, 


FIG.  386.— Median  Section  of  the  Head  of  a  Sheep 
Embryo  of  36  mm.  s,  Septum  marium;  /,  falx 
cerebri;  /.m,  foramen  of  Munro;  Mo,  thalannis: 
cp,  commissura  posterior;  ms,  teementum;  mh, 
midbrain;  f,  tentorium;  cl,  cerebellum;  pi,  chorid 
plexus.  After  Kolliker.  x  3  diams. 


FIG.  387.—  Brain,  Human  Fnetus.  live 
Months,  rt..  Corpus  striatum;  <>.  thala- 
mus  opticus;  la,  lobus  lunatus  anterior; 
Ip,  lobus  lunatus  posterior:  NX.  s.'milu- 
uaris  superior;  si,  semilunaris  inferior: 
/?,  pyramis.  After  Kolliker.  Natural 
size. 


ventral  line  becomes  noticeable  during  the  third  month ;  they  remain 
small  until  the  fifth  month,  when  the  fibres  from  the  pyramids  of 
the  medulla  oblongata  begin  to  penetrate  them,  and  thereupon  they 
enlarge  and  at  the  same  time  the  longitudinal  concavity  of  the  ven- 
tral side  is  obliterated.  It  is  probable  that  the  Bodenplatte  thickens, 
somewhat  as  in  the  medulla,  and  persists  as  a  median  raphe. 

The  corpora  quadrigemina  arise  from  the  dorsal  side  of  the  mid- 
brain,  and  will,  perhaps,  be  found  to  represent  the  dorsal  zones  of 
His.  The  dorsal  wall  of  the  mid-brain  is  at  first  evenly  arched 
and  smooth ;  at  five  weeks  there  is  a  median  ridge,  as  already  noted ; 
during  the  third  month  the  ridge  is  replaced  by  a  groove ;  during 
the  fifth  month  there  appear  two  oblique  grooves  which  run  inward 
and  backward,  one  on  each  side,  Fig.  387,  and  complete  the  subdi- 
vision of  the  surface  into  the  four  corpora  quadrigemina.  Concern- 
ing the  development  of  the  posterior  commissure,  which  is  a  bundle 
of  fibres  crossing  the  dorsal  wall  of  the  brain  just  in  front  of  the  cor- 
pora, see  p.  686. 


THE    NERVOUS    SYSTEM 


(  hving  to  the  fact  that  the  mid-brain  grows  inucli  less  than  the  fore- 
and  hind-brains,  it  is  gradually  covered  over,  principally  by  the  ex- 
pansion of  1 1 ie  hemispheres.  At  the  beginning  "f  the  third  montli 
the  hemispheres  have  expanded  to  the  edge  of  the  mid-brain  ;  at  three 
months  they  half  cover  it  ;  at  four  months  they  cover  all  but  a  small 
piece;  at  live  months  the  whole  of  the  mid-brain. 

Median  Portion  of  the  Fore-Brain. — The  manner  in  which 
the  primitive  fore-brain  is  divided  into  two  lateral  parts  or  hemi- 
spheres and  one  median  part  (  Tlinlninrnrr/ihn/ini,  Zii'isclicnh  i  n<\ , 
after  the  outgrowth  of  the  optic  vesicles,  has  been  described.  The 
c a  \  i  t  y  ( en  la  rgi  M  1  cen  t  ral  canal)  of  the  median  part  is  the  third  ventricle 
of  descriptive  anatomy  :  therefore,  the  median  part  is  sometimes  called 
the  rc'/ion  <>f  f/ic  f/iir<l  mil  rirlc. 
Kor  convenience  the  hemispheres  are 
treated  in  a  separate  section.  It  has 
already  been  pointed  out  that  it  is 
misleading  to  describe  the  primitive 
irsi  vesicle)  as  dividing 
into  two  secondary  vesicles.  Todivide 
the  median  ]>ortion  of  the  fore-brain 
intotwo  parts,  as  is  traditionally  done, 
is  arbitrary.  We  shall,  therefore,  in 
this  section  consider  not  only  the 
thalamencephalon  as  usually  defined, 
but  also  the  lamina  terminalis  and  the 
commissures, 

1.  (ii  \i  I;\L  SHAPE.— By  the  fifth 
week  the  median  portion  of  the  fore- 
brain  has  Mourned  nearly  its  definite 
form.  Across  the  anterior  median  line 
extends  that  portion  of  the  medullary 
wall  connecting  the  two  hemispheres 
known  as  the /<n//n/r/  tmin'mi/ix.  Fig. 
340,  between  /.  m  and  R.  o.  Above  and 
around  the  dorsal  side  of  the  foramen 
of  Monro  the  medullary  wall  is  con- 
t in ued  in  the  median  line,  Fig.  340, 
but  is  modified  first  to  form  the  corpus 
callosum,  second  the  choroid  plexus, 

*  1        '         1  J  1  1       *         J  "  J-'HJ.     »J»^'. M.    CM  \t  W*.    J_F»  *****   W»     M.M.H—       *-.»»»---  ^  f 

tWO  Structures   Ot  Which  the  history  IS    CR,  (Nackenlan^e,  18.6  mm.)   Plf.  Plex- 

presented  below.  The  corpus  callosum  ^^c^^^^Sffa^SK 
is  a  thickening  produced  by  fibres,  BSrtBllBi^'m?*1**1*^5  1A  mld" 
forming  a  transverse  commissure  be- 
tween the  two  hemispheres.  The  choroid  plexus  is  a  fold  of  the  me- 
dullary wall  which  projects  into  the  cavity  of  the  brain,  Fig.  388,  Plx. 
The  cerebral  hemispheres  are  outgrowths  from  the  anterior  part  of  the 
forebrain,  Fig.  330 ;  the  passage  from  the  cavity  of  the  hemispheres  to 
the  median  cavity  is  the  foramen  of  Monro,  Fig.  300,  ?» ;  the  part  of 
the  fore-brain  between  the  foramen  of  Monro  and  the  mid-brain  corre- 
sponds to  the  thalamencephalon,  or  Zwischenhirn  as  ordinarily  defined. 
The  thalamencephalon  as  viewed  in  dorsal  aspect  in  a  human  embryo 
of  five  weeks,  Fig.  388,  Z,  has  somewhat  of  a  cask-shape.  The  anterior 


...-• 


M 


FIG.  388.— Part  of  Brain  of  His*  Embryo 


680  THE   FCETUS. 

end  adjoining  the  hemispheres  is  narrower  than  the  posterior  end 
adjoining  the  mid-brain ;  the  anterior  half  of  the  thalamencephalon 
slopes  inward.  Along  the  median  dorsal  line  is  a  ridge,  a,  which  is 
developed  as  a  fold  of  the  deck-plate  during  the  fifth  week ;  toward 
the  hemispheres  the  ridge  widens  out  and  disappears ;  the  continuation 
of  the  deck-plate  between  the  hemispheres  corresponds  to  the  tela 
choroidea;  toward  the  mid-brain  the  ridge  merges  into  a  median 
evagination  of  the  brain-wall ;  this  evagination  is  the  anlage  of  the 
pineal  gland,  p.  688 ;  there  are  soon  developed  the  two  ridges  which 
diverge  V-like  from  the  pineal  anlage  to  run  forward  along  the 
median  ridge,  and  which  are  destined  to  form  the  pars  habenularix 
(ganglia  habenulaB,  laminae  medullares,  and  pineal  stalk)  of  the  pineal 
lobe. 

Viewed  in  median  section,  Fig.  340,  the  median  fore-brain  is  seen 
to  have  a  great  downward  prolongation  which  begins  to  form  during 
the  fourth  week,  develops  rapidly  during  the  fifth  week,  and  persists 
throughout  life.  The  enlargement  may  be  designated  as  the  sub- 
thalamic  or  infundibular;  subthalamic  because  it  lies  below  the 
region  in  which  the  optic  thalami  arise,  infundibular  because  its 
apex  is  the  recessus  infundibuli.  As  seen  in  section  the  enlarge- 
ment has,  1,  a  posterior  wall,  M,  which  descends  at  nearly  a  right  angle 
to  the  axis  of  the  mid-brain ;  the  posterior  wall  is  convex,  and  it  is 
the  anlage  of  the  mammillary  tubercles;  2,  a  lower  wall  which 
includes  the  anlage  of  the  tuber  cinereum,  t.c.  of  the  infundibulum, 
and  of  the  optic  chiasma;  3,  an  anterior  wall  constituted  by  the 
lamina  terminalis.  At  the  angle  where  the  anterior  and  lower 
walls  meet,  the  recessus  opticus,  R.o,  leads  off  laterally  into  the 
hollow  stalk  (anlage  of  the  optic  nerve)  of  the  optic  vesicle.  Higher 
up  lies  the  foramen  of  Monro,  fin,  leading  into  the  cavity  of  the 
hemispheres,  H.  In  the  figure  there  is  seen  a  groove  which  runs 
from  the  recessus  opticus,  R.o,  to  the  mid-brain;  this  groove  marks 
the  division  line  between  the  dorsal  and  ventral  zones  of  His ;  it  per- 
sists in  part  throughout  life.  The  persistent  part  was  named  the 
sulcus  Monroi  by  Reichert  because  it  runs  later  from  the  lower  edge 
of  the  foramen  of  Monro,  the  foramen  extending  as  it  develops  much 
closer  to  the  recessus  opticus  than  it  does  in  the  early  stage  of  Fig.  )>40. 

In  older  stages  the  median  fore-brain  shows  many  minor  modifi- 
cations, but  its  fundamental  shape  and  division,  as  found  at  five 
weeks,  are  permanently  retained.  The  most  important  alterations 
are  due,  first,  to  thickening  of  the  walls,  which  is  especially  great  in 
the  region  of  the  optic  thalami ;  second,  to  the  fact  that  the  foramen 
of  Monro  does  not  enlarge  with  the  growth  of  the  brain,  and  there- 
fore becomes  relatively  small,  compare  Figs.  340  and  386. 

APPEARANCE  IN  CROSS  SECTIONS. — Fig.  389  is  a  section  of  the 
thalamencephalon  of  a  five  weeks'  embryo  nearly  at  right  angles 
to  its  axis.  In  the  median  line  is  the  deck-plate,  d.pl,  with  its 
three  folds  already  described;  the  division  between  the  dorsal, 
Th,  and  ventral,  s.  Th,  zones  of  His  is  well  marked  by  the  sulcus 
Monroi.  The  Bodenplatte  forms  the  mammillary  groove,  M a,  which 
is  bordered  by  two  eminences  internally;  the  eminences  are  the 
cross  sections  of  two  ridges,  which  border  the  groove  and  unite  with 
one  another  in  the  median  line  beween  the  tuber  cinereum  and  the 


THE   NERVOUS   SYSTEM. 


681 


mammillary  region  proper;  the  ridges  are  the  anlages  of  the  mam- 
mi  llary  tubercles. 

Fig.  390  represents  a  much  older  stage  and  serves  to  show  the 
thickening  of  the  walls  and  the  origin  of  the  choroid  plexus ;  the 
section  passes  through  the  foramen  of  Monro,  m,  and  the  optic  chi- 
asma,  ch;  and  the  plane  of  the  section  may  be  approximately  recog- 
nized from  Fig.  388.  Very  striking  is  the  great  thickening  of  the 
brain-walls  to  form 
the  anlage  of  the  cor- 
pus striatum,  st,  in 
the  hemispheres,  and 


sTh 


FIG.  381).  -Section  of  the 
Thalamencephalon  of  an  Em- 
I.I-VM  <>f  fiv.' \\Vi-ks  (His'  Sch). 
</./»/.  Deck-plat. •;  Th,  anlage 
of  tlialainus  «l<>rsal  zone  of 
His);  s.M.  sulc'iis  Monroi ; 
,s. Th.  pars  subthalamica  (ven- 
tral /.one  of  His);  md,  marn- 
ni i llary  groove  (Bodenplatte, 
on  either  si.l.-  of  which  appear 
the  inaininillurv  tubercles). 
After  W  His.  X  23  diams. 


Fio.  390.— Section  of  the  Fore-Brain  of  a  Sheep  Embryo  of  f7 
mm.  ?i,  Hippocampal  fold;  /,  falx;  J,  lateral  ventricle;  h, 
wall  of  hemisphere;  pi,  choroid  plexus;  st,  corpus  striatum; 
c,  pedunculus  cerebri;  th,  thalamus;  m,  foramen  of  Monro; 
s.  deck-plate;  f,  third  ventricle;  a,  orbito-sphenoid  cartilage; 
».  pharynx;  *u,  prae-sphenoid  cartilage;  ch,  optic  chiasrua; 
o,  optic  nerve.  After  Kolliker.  x  10  diams. 


of  the  optic  thalamus,  th,  and  the  pars  subthalamica  in  the  middle  part. 
The  deck-plate,  s,  closes  the  third  ventricle  £,  above.  The  medial 
wall  of  each  hemisphere  is  bent  in,  n,  owing  to  the  Bogenfurche, 
p.  <i(.)G.  The  wall  of  the  hemisphere  does  not  join  the  deck-plate,  s, 
directly  below  the  Bogenfurche,  but  changes  into  an  epithelial  mem- 
brane, which  forms  an  irregular  fold,  p/,  projecting  far  into  the  cav- 
ity, /,  of  the  hemisphere,  or  lateral  ventricle ;  this  fold  is  the  choroid 
plexus,  see  below. 

THE  DECK-PLATE. — The  entire  deck-plate  except  the  pineal  (and 
paraphysal)  parts  assumes  an  epithelial  character.  It  produces  the 
pineal  gland,  see  p.  688,  the  paraphysis,  see  p.  690,  and  the  choroid 
plexus,  and  persists  in  part  as  the  tela  choroidea.  The  pineal  gland 
and  paraphysis  are  so  far  independent  organs  that  they  are  treated 
in  separate  sections.  We  are,  therefore,  here  chiefly  concerned 
with  the  choroid  plexus. 

The  Choroid  Plexus. — When  the  hemispheres  begin  to  grow  out, 
the  deck-plate  between  tjiem  and  above  the  foramen  of  Monro  is 
convex,  but  it  soon  becomes  concave  and  during  the  fifth  week  the 
deck-plate  forms  a  fold  on  each  side  projecting  into  the  lateral  ven- 
tricle. The  space  between  the  two  hemispheres  is  occupied  by  mes- 
enchyma,  which  grows  into  the  lateral  fold  carrying  blood-vessels 


gg-2  THE   FCETUS. 

with  it ;  the  fold  is  the  anlage  of  the  choroid  plexus ;  its  relations 
are  well  shown  in  Fig.  391.  At  first  the  choroid  fold  contains  no 
connective  tissue,  the  ingrowth  of  mesenchyma  following  after  the 
fold  is  formed ;  the  fold,  therefore,  owes  its  origin  to  the  growth  of 
the  deck-plate.  Examined  in  a  side  view  the  fold  is  seen  to  be  thin, 
but  long ;  it  ends  abruptly  in  front,  but  disappears  posteriorily  more 
gradually  (His,  89.4,  695).  The  deck-plate  becomes  a  layer  of 
cuboidal  epithelium  covering  the  choroidal  fold,  and  merging  on  the 
one  hand  into  the  wall  of  the  hemisphere  and  on  the  other,  Fig. 
391,  into  the  median  part,  tela  choroidea,  of  the  deck-plate.  The 
tela  is  itself  an  epithelial  layer,  which  is  continuous  in  front  with 
the  lamina  terminalis,  behind  with  the  pineal  anlage.  During  its 
further  development  (cf.  Mihalkovics,  77.1,  114-117),  the  fold  in- 
creases in  length  and  diameter,  and  its  surface  is  thrown  up  into 
rounded  protuberances,  which  grow  into  irregular  processes.  The 
fold  takes  its  place  in  the  lower  part  of  the  lateral  ventricle,  lying 
close  against  the  basal  surface  (ganglia)  of  the  hemispheres  (Mihal- 
kovics, 77.1,  Taf.  1,  Fig.  10).  The  size  and  complexity  of  the 
choroid  plexus  are  correlated  with  the  degree  of  development  of  the 
hemispheres,  and  the  plexus  is,  therefore,  largest  and  most  specialized 
in  the  mammalia.  The  plexus  in  the  human  embryo  enlarges  more 
rapidly  than  the  lateral  ventricle  so  that  by  the  fourth  or  fifth  month 
it  quite  fills  the  lateral  ventricle,  but  after  that  period  the  plexus  lags 
somewhat,  and  there  is  gradually  produced  the  space  around  it  as 
found  in  the  adult. 

The  connection  of  the  choroid  fold  with  the  medullary  walls  of  the 
hemispheres  extends  during  embryonic  life  for  some  distance  back- 
ward from  the  foramen  of  Monro.  The  exact  history  of  this  modi- 
fication has  never  been  traced. 

LAMINA  TERMINALIS. — The  embryonic  history  of  the  lamina  ter- 
minalis was  long  imperfectly  understood,  but  it  has  been  cleared  up 
by  F.  Marchand's  investigations,  91.1,  on  human  embryos.  It 
may  be  regarded  either  as  a  prolongation  of  the  deck-plate,  or,  as 
suggested  by  His,  88.3,  as  the  result  of  the  union  of  the  dorsal 
zones  of  His  (Fliigelplatten)  in  front.  It  is  the  median  portion  of 
the  medullary  wall,  Figs.  340,  341,  in  front  of  the  recessus  opticus 
and  foramen  of  Monro;  it  unites  the  two  hemispheres,  being,  of 
course,  continuous  with  their  walls,  and  it  closes  the  third  ventricle 
anteriorly;  it  is  continuous  above  with  the  tela  choroidea,  Fig.  395, 
below  with  the  optic  chiasma  (or  anlage  thereof) .  At  five  weeks  it 
is  a  thin  plate,  Fig.  340,  of  about  the  same  thickness  as  the  deck- 
plate,  and  with  cells  but  little  if  at  all  differentiated . 

The  upper  part  of  the  lamina  terminalis  becomes  very  much  thick- 
ened, and  forms  (Mihalkovics,  77. 1,  122)  a  broad  band  of  triangular 
section  after  the  fourth  week,  uniting  the  two  hemispheres.  This 
band  is  the  anlage  of  the  septum  lucidum,  the  corpus  callosum,  the 
fornix,  and  the  anterior  commissure,  Fig.  391 ;  the  lower  apex  of  the 
triangle  is  the  anlage  of  the  anterior  commissure,  ca;  the  posterior 
vertical  border  of  the  fornix;  the  upper  horizontal  border  of  the 
corpus  callosum,  c.c,  and  the  remainder  of  the  area  is  the  anlage  of 
the  septum  pellucidum.  It  is  usually  described  as  resulting  from 
the  concrescence  of  the  two  hemispheres,  but  I  consider  it  simpler 


THE   NERVOUS   SYSTEM. 


683 


C.C 


th 


FIG.  391.— Brain  of  a  Human  Embryo  of 
about  three  Months  (according  to  '.Mar- 
chand,  four  months),  th,  Optic  thala- 
mus;  of,  Bogenfurche;  cc,  corpus  callo- 
sum; Sp,  septum  lucidum;  c.a,  anterior 
commissure;  Ol,  olfactory  lobe;  Chi,  op- 
tic ehiasma;  inf,  infundibulum;  Pons, 
DOBS  Varolii;  cW,  cerebellum;  nib,  mid- 
brain;  pin,  pineal  gland.  After  F.  Mar- 
chand.  •  I ' ._.  diams. 


and  more  natural  to  regard  it  as  a  thickening  of  the  lamina  termi- 
nalis,  which  it  is  morphologically.  The  anlage  may  be  well  seen  in 
a  median  longitudinal  section  of  the 
brain  of  a  cow  embryo  of  8  cm.  (Mi- 
halkovics,  I.e.,  Fig.  17)  or  of  a  human 
embryo  of  the  third  to  fifth  month, 
Fig.  <J(.U.  The  anterior  commissure 
acquires  its  fibres  before  they  appear 
in  any  other  part  of  the  lamina  termi- 
nal is.  and  early  become  separated  from 
the  fornix  and  septum  lucidum  a  short 
distance.  The  thickening  lies  below 
the  Bogenfurche,  6/,  and  in  front  of 
the  foramen  of  Monro.  In  it  the 
fibres  to  form  the  anterior  commis- 
sure and  the  fornix  have  been  ob- 
served to  appear  in  rabbit  embryos  of 
16  :;<>  mm.,  and  those  to  form  the  cor- 
pus callosum  in  rabbit  embryos  of  35- 
40  mm.  (Mihalkovics,  I.e.,  123,  124) 
in  pig  embryos  of  8  mm.  (Blumenau, 
91.1,0). 

The  fornix,   corpus  callosum.    and 
septum  lucidum  together  form  a  triangle,  which  after  its  formation 
expands  throughout  fostal  life.     The  anterior  apex,  where  the  fornix 

and  callosum  meet, 
grows  forward,  and 
the  posterior  apex,  cor- 
responding to  the  end 
or  splenium  of  the 
callosum,  grows  back- 
ward; the  corpus  cal- 
losum is  thus  not  only 
lengthened  but  carried 
backward,  Fig.  392, 
cc,  above  the  fora- 
men of  Monro  and  the 
optic  thalamus,  Th. 
The  development  of 
the  corpus  callosum 
also  extends  beyond 
the  anterior  apex ;  the 
part  below  the  apex  is 
short,  ro,  and  corre- 
sponds to  the  rostrum 

FIG.  392. —Brain  of  a  Human  Embryo  of  the  fourth  Month.  nf    Hp^rrirvH 

cc,  c..rpus   callosum;    sp,   septum    lucidum;    PI.  PI',  choroid  c 

plexus:  <-<»/t.  nt.  fommissura  mollis;  cal,  calcarine fissure ;  p.oc,  my 

parieto-occipital   fissure:  pin,  pineal  gland:   cbl,  cerebellum;  i    " 

md.ob,  medulla  oblongata;  Pons,  pons  Varolii;  op.  optic  nerve;  logical    point 
c.a.  commissura  anterior;    ro,  rostrum.     After  F.  Marchand. 
X  1^  diams. 


cc 


conj.m 


poc 


(mi 


md.ob 


f  Tom  a 

^  ^ 

Ot  V16W, 

i's  statement 

(Giornale  d.r.  Accad. 
M(jd.  Torino,  Nov. -Dec.,  1883),  that  the  corpus  callosum  is  covered 
by  a  thin  but  constant  layer  of  gray  matter,  is  very  significant. 
The  statement  has  been  verified  by  Blumenau,  91.1. 


684 


THE   FCETUS. 


The  septum pellucidum  (or  lucidumjis  developed  from  of  the  thick- 
ened lamina  terminalis  between  the  corpus  callosum  and  the  fornix. 
The  area  is  at  first  very  small,  but  rapidly  enlarges.  At  four  months 
a  small  cavity  appears  in  it,  Fig.  391,  /Sp,  which  enlarges  as  the  sep- 
tum grows  and  becomes  the  ventricle  of  the  septum  (ventriculus 
quint  us,  pseudoccele],  see  F.  Marchand,  91.1,21.  The  origin  of  the 
cavity  is  uncertain ;  it  has  no  connection  with  any  of  the  brain  cavi- 
ties proper;  Prof.  B.  G.  Wilder  writes  me  that  in  man  and  anthro- 
poids it  is  wholly  circumscribed  by  brain  tissue ;  it  is  much  narrower 
in  other  mammals,  but  the  pia  does  not  extend  into  it.  Marchand 
thinks  it  probably  arises  as  a  cleft  in  the  tissue. 

COMMISSURES  AND  FORNIX. — A  commissure  is  a  tract  of  trans- 
verse fibres  connecting  the  two  sides  of  the  nervous  system.  In  the 
mammalian  brain  three  such  tracts  are  known  to  arise  in  the  terri- 
tory of  the  first  vesicle;  they  are:  1,  the  anterior  commissure;  2, 
the  corpus  callosum;  3,  the  posterior  commissure.  The  anterior 
commissure  and  corpus  callosum  are  developed,  one  might  also  say, 
as  parts  of  the  septum  pellucidum  and  belong  morphologically  to  the 
lamina  terminalis.  The  fornix  may  be  defined  as  a  longitudinal 
commissure.  For  the  general  relations  of  the  commissures  and  sep- 
tum to  the  lamina,  see  above.  Osborn,  with  great  ability,  has 
traced  the  homologies  of  the  three  commissures  throughout  nearly 
the  entire  vertebrate  series,  and  has  shown,  86.1,  87.1,  that,  con- 
trary to  previous  belief,  the  corpus  callosum  is  not  confined  to  the 
mammalia,  but  is  present  in  birds,  reptiles,  and  amphibia,  and  prob- 
ably in  fishes,  and  further  that  in  amniota  and  amphibia  the  anterior 
commissure  comprises  always  two  divisions — an  olfactory  and  a 
temporal.  Mammals,  therefore,  are  distinguished  from  other  verte- 
brates, not  by  the  possession  of  the  corpus  callosum,  but  by  its  great 
size,  which  we  may  safely  correlate  with  the  great  size  of  the  mam- 
malian hemispheres.  The  typical  position  of  the  commissures  is 
shown  in  Fig.  393;  the  posterior  commissure,  P,  lies  behind  the 


H 


Pl.i     Pi    P  Q  Cbl     Pl.hr 


md.ob 


fm 


FIG.  393.  —Median  View  of  a  Frog's  Brain,  ol,  Olfactory  nerve ;  H,  hemisphere ;  PI.  i,  choroid 
plexus  of  fore-brain;  Pi,  pineal  gland;  p,  posterior  commissure;  Q,  mid-brain;  Cbl,  cerebellum; 
Pl.iv,  choriod  plexus  of  hind-brain;  fm,  foramen  of  Monro;  c,  corpus  callosum;  a,  anterior 
commissure;  op,  optic  nerve;  inf,  inf undibulum ;  hy,  hypophysis;  md.ob,  medulla  oblongata. 
After  H.  F.  Osborn. 

pineal  gland,  pi,  close  to  the  corpora  bigemina  (mid-brain) ;  the 
corpus  callosum,  c,  lies  close  to  the  foramen  of  Monro,  fm,  and 
the  anterior  commissure  is  situated  lower  down,  a,  in  the  lamina 
terminalis,  and  it  consists  of  two  bundles  of  fibres,  an  upper  larger 
pars  olfactoria  and  a  lower  smaller  pars  temporalis;  the  fibres 
of  the  temporal  bundle  are  distributed  to  the  temporal  portion  of  the 
so-called  mantle,  Fig.  394 ;  the  fibres  of  the  olfactory  portion  run  in 


THE   NERVOUS   SYSTEM. 


G85 


part  to  the  olfactory  lobes,  but  also  give  off  a  frontal  branch  bundle 
to  the  frontal  region  of  the  mantle.  The  mammalian  corpus  callosum 
consists  of  an  anterior  or  frontal  division  supplying  the  dorso-medial 
portions  of  the  mantle,  and  a  posterior  division,  the  commissura 
cornu  Ammonis,  supplying  the 
mantle  area  above  the  Ammon's 
horn  (H.  F.  Osborn,  87.1,  540). 

Th<>  development  of  the  commis- 
sure in  marsupials  (Osborn,  87.1, 
r>:>r>)  shows  that  the  homologies  es- 
tablished by  Osborn  are  correct. 
But  in  sheep  the  development  is  so 
far  modified  that  these  homologies 
are  less  clearly  brought  out.  The 
development  in  sheep  is  thus  de- 
scribed by  Osborn,  87.1,  535:  "In 
the  30  mm.  stage  the  hemispheres 
have  already  partially  united  in 
front  of  the  primitive  lamina  ter- 
minalis  forming  the  terminal  plate. 
The  anterior  commissure  now  ap- 
jM-ars  as  a  delicate  thread  of  fibres 
in  the  lateral  region  of  the  brain 
stem.  The  hippocampal  sulcus  is 
well  marked.  At  35  mm.  the  an- 


FIG.  394.— Section  through  the  Fore-Brain 


tenor    Commissure  extends   slightly    Th,    optic   thalamus;     V«,   third   ventricle. 

nearer  the  median  line.     In  an  em-  J 

oryo  of  37  mm.  the  terminal  plate  has  extended  considerably  for- 
ward. The  anterior  commissure  shows  a  division  into  the  pars 
olfactoria  and  temporalis,  while  in  the  median  line  its  fibres  be- 
gin to  unite  with  those  of  the  opposite  hemisphere.  This  union 
does  not  take  place  in  the  terminal  plate,  as  stated  by  Mihalkovics, 
but  in  front  of  it,  i.e.,  the  plate  does  not  form  the  ground  substance 
to  be  traversed  by  these  fibres.  On  the  other  hand  the  fibres  bridge 
the  fissure  which  is  gradually  closing  in  front  of  the  terminal  plate. 
Immediately  above  the  anterior  commissure,  on  either  side,  are  de- 
scending fibres  which  represent  the  first  stage  of  the  for  nix.  These 
appear  before  the  anterior  commissure  crosses  the  median  line.  This 
stage  corresponds  closely  to  that  figured  by  Mihalkovics,  77. 1,  Taf. 
VII.  Fig.  60.  In  the  next  stage  the  terminal  plate  has  extended  in 
front  of  the  anterior  commissure,  the  fornix  fibres  are  more  numer- 
ous, and  at  their  upper  limit  a  few  fibres  are  observed  extending 
toward  the  median  line ;  these  are  the  earliest  callosal  elements.  At 
49  mm.,  which  follows  a  considerable  interval  of  development,  the 
hippocampal  sulcus  is  very  deep  and  the  terminal  plate  is  much  more 
extensive.  In  its  lower  portion  the  anterior  commissure,  now  a 
compact  bundle,  extends  laterally  above  the  cerebral  peduncles.  The 
columns  of  the  fornix  are  well  defined,  and  between  them  in  the  upper 
portion  of  the  plate  pass  the  fibres  of  the  corpus  callosum.  A  care- 
ful study  of  these  fibres  shows  that,  like  those  of  the  anterior  com- 
missure, they  unite  with  each  other  in  front  of  the  terminal  plate. 
The  callosal  fibres  disappear  as  they  pass  around  the  hippocampal 


686 


THE    FOETUS. 


sulcus.  Above  this  sulcus  is  an  interval  in  the  inner  wall  of  the 
ventricle  in  which  no  fibres  can  be  observed,  but  in  the  roof  of  the 
ventricle  are  the  fibres  of  the  corona  radiata.  This  leads  me  to  doubt 
whether  the  fibres  extend  at  an  early  stage  from  the  corona  radiata 
into  the  corpus  callosum,  as  stated  by  Mihalkovics.  It  seems  rather 
that  this  is  a  subsequent  union.  This  stage  differs  considerably 
from  that  figured  by  Mihalkovics  as  the  initial  stage  of  the  corpus 
callosum. " 

The  posterior  commissure  has  been  but  little  studied  embryologi- 
cally.  Its  position  may  be  recognized  (Kolliker, "  Entwickelungsges. , " 
2te  Aufl.,  525)  in  a  sheep  embryo  of  29  mm.  as  a  slight  thickening 
of  the  dorsal  wall  of  the  fore-brain  close  to  the  mid-brain.  The 
fibres  of  this  commissure  appear  in  the  chick  the  latter  part  of  the 
fourth  day,  according  to  Mihalkovics,  77.1,  73. 

DORSAL  ZONE  OF  His  (Optic  Thalami). — The  dorsal  zone  of 
His  in  the  fore-brain  forms  the  hemispheres  and  in  the  median 
portion  produces  the  optic  thalami.  The  thalarni  may  be  defined  as 
thickenings  of  the  dorsal  zones  continuous  with  the  thickenings 
which  produce  the  corpora  striata  of  the  hemispheres.  It  will  be 
remembered  that  the  lower  limit  of  the  dorsal  zone  is  marked  by 
the  sulcus  Monroi.  The  development  of  the  thalamus  has  been  out- 
lined by  Kolliker  in  both  his  text-books;  the  early  stages  in  man 
(fourth  to  twelfth  week)  have  been  investigated  by  W.  His,  89.4, 
701,  731.  The  sulcus  Monroi  becomes  evident  during  the  fourth 
week  and  very  distinct  during  the  fifth ;  later  it  becomes  shallower, 
but  persists. 

At  the  beginning  of  the  fourth  week  the  thalamic  region  is  con-^ 
cave  toward  the  ventricle.  During  that  week  the  thickening  of  the 

walls  in  both  the  thala- 
mic and  sub-thalamic 
regions  begins,  and  by 
the  end  of  the  fifth  week 
the  wall  projects  in  both 
regions  convexly  into 
the  cavity  of  the  third 
ventricle.  The  thalamic 
thickening  does  not  ex- 
tend throughout  the  dor- 
sal zone  of  the  thala- 
mencephalon,  but  only 
in  a  circumscribed  re- 
gion. It  accordingly 
produces  a  large  tuber, 
Fig.  395,  Th,  the  long- 
continued  growth  of 
which  converts  the  third  ventricle  into  a  narrow  fissure.  The  tubers 
meet  toward  the  end  of  the  second  month  and  actually  unite  over  a 
small  area  across  the  median  line,  their  union  constituting  the  com- 
missura  mollis,  cm.  Mihalkovics,  77. 1,  71,  assigned  the  formation 
of  the  commissura  mollis  to  the  fifth  month,  and  this  date  is  confirmed 
by  F.  Marchand,  91.1,  310.  His  thinks  that  the  commissure  is 
formed  earlier.  Above  the  tuber  thalamicum  is  a  groove  named  the 


01 


Th 


FIG.  395.— Reconstruction  of  the  Brain  of  an  Embryo  of 
about  seven  and  one-half  Weeks  (His1  Zr).  Ol,  Olfactory 
nerve;  a,  Ammon's  groove;  t,  epiphysis;  Mb,  mid-brain; 
c.m,  commissura  mollis ;  Th,  optic  thalamus.  After  W.  His. 
X  lOdiams. 


THE   NERVOUS   SYSTEM.  687 

habenulce  by  His.  It  corresponds  to  the  external  ridge  de- 
scribed, p.  680,  as  running  obliquely  along  the  upper  surface  of  the 
thalainencephalon  to  the  pineal  anlage.  Below  the  tuber  is,  of  course, 
tin-  sulcus  Monroi  ;  the  two  grooves  are  united  behind  the  tuber,  where 


thov  are  also  joined  by  a  transverse  groove,  the  sulcus 
where  all  these  grooves  meet  there  is  a  slight  lateral  enlargement  or 
recess,  recessus  geniculi,  of  the  ventricle.  As  the  tuber  enlarges  tin- 
recessus  deepens  and  is  narrowed  so  that  at  two  months  and  a  half 
there  is  only  a  small  fissure  visible.  Later  even  this  fissure  disap- 
pears; its  walls  probably  give  origin  to  the  "centre  median"  of 
7.////.S-  (inediane  Sehhugelcentrum). 

The  part  of  the  dorsal  zone  between  the  thalamic  tuber  and  tlu> 
mid-brain  is  known  as  the  pars  retrothalamica;  it  includes  the  pul- 
vinar,  the  brachia  of  the  corpora  quadrigemina,  and  the  corpu> 
geniculatum. 

YKNTRAL  ZONE  OF  His.  —  This  comprises,  as  already  stated, 
the  region  of  the  thalamencephalon  below  the  sulcus  Monroi;  for 
this  part  Forel  ("Untersuch.  ub.  d.  Haubenregion,"  Arch.f.  Psych  i- 
«t  ri<>,  Bd.  VII.)  has  proposed  the  convenient  name  of  pars  snh- 
t/i<t/<iinica.  Concerning  its  embryonic  history  almost  nothing  is 
known.  It  becomes  very  thick  and  is  usually  described  as  part  of 
the  optic  thalamus. 

FLOOR  OF  THE  THIRD  VENTRICLE.  —  Along  the  floor  of  the  ven- 
tricle on  or  near  the  median  line  are  developed  the  following  struct- 
ures: «,  substantia  perforata  posterior;  6,  mammillary  tubercles;  c, 
tuber  cinereum;  d,  infundibulum  ;  e,  optic  chiasma;  /,  lamina  ter- 
ininalis,  but  this  last  does  not  properly  belong  to  the  floor.  What 
relation  the  Bodenplatte  bears  to  the  production  of  the  first  five 
structures  is  still  uncertain.  In  regard  to  this  development  little  is 
known. 

a.  Substantia  perforata  posterior  perhaps  really  all  belongs  to 
the  mid-brain.     It  becomes  distinct  during  the  fourth  month. 

b.  M<tinmillary  tubercles  (corpora  albicantia,  candicantia,  J\Iark- 
kugelchen)  begins,  Fig.  340,  ?/i,  as  a  single  relatively  large  convex 
projection  of  the  medullary  wall.     As  the  brain  enlarges  the  mam- 
millary region  grows  very  slowly  and  hence  becomes  relatively  small 
(W.  His,  89.4).      According  to  Mihalkovics,  77.1,72,  a  median 
groove  arises  early  in  the  fourth  month  dividing  the  region  into  two 
tubercles,  which  later  become  white  (owing  to  the  development  of 
medullated  nerve-fibres  ?). 

c.  Tuber  Cinereum.  —  This  is  part  of  the  floor  between  the  mam- 
millary tubercles  and  the  infundibulum  proper,  see  Fig.  340,  t.c, 
Concerning  its  development  no  details  are  known. 

d.  Infundibulum.  —  In  rabbit  embryos  of  12-16  mm.  and  in  hu- 
man  embryos  of  five  weeks  there  is  found    developing  a  small 
cylindrical  outgrowth  of  the  brain,  which  is  known  as  the  processus 
infundibuli.     The  outgrowth  takes  place  in  the  median  line  imme- 
diately in  front  of  the  tuber  cinereum  and  behind  the  optic  chiasma, 
Figs.  391  and  401,  Inf.     It  very  soon  comes  in  contact  with  the 
hypophysal  outgrowth  of  the  mouth,  and  is  ultimately  transformed 
into  the  posterior  lobe  of  the  pituitary  body  as  already  described,  p. 
574.     His'  observations,  89.4,  706,  on  the  human  embryo  confirm 


688 


THE    FCETUS. 


in  most  respects  Milhalkovics'  account  of  the  development  in  the 
rabbit. 

e.  Optic  Chiasma  and  Recessus. — The  optic  chiasma  and  tracts 
together  constitute  a  transverse  ridge-like  thickening  of  the  wall  of 
the  brain  to  allow  the  passage  of  the  nerve-fibres  of  the  optic  nerves. 
Laterally  the  ridges  merge  into  the  optic  nerve ;  the  recessus  opticus 
is  bounded  behind  by  the  ridges,  in  front  by  the  lamina  terminalis. 
The  optic  ridges  (Sehstreif  of  Mihalkovics,  77.1,78)  accordingly 
are  first  indicated  when  the  groove  of  the  recessus  opticus  develops, 
and  they  become  strongly  marked  as  the  optic  nerve-fibres  appear. 
In  the  chick  the  fibres  have  been  observed  in  the  latter  part  of  the 
fourth  day  passing  from  one  side  of  the  brain  through  the  optic 
ridge  to  the  optic  nerve  of  the  opposite  side,  Mihalkovics,  I.e.  The 
growth  of  the  fibres  is  centrifugal. 

The  recessus  opticus,  which  was  first  described  by  J.  Michel  in 
1872,  leads  to  the  optic  nerve,  being  a  transverse  groove,  Fig.  399, 
R.op.  It  is  more  marked  at  birth  than  in  the  adult,  but  may  be 
traced  throughout  life.  For  notices  of  the  scanty  literature  previous 
to  1877,  upon  the  chiasma  and  recessus,  see  Mihalkovics,  77.1, 
80-82. 

Pineal  Gland  (Epiphysis,  conarium,  pineal  or  parietal  eye, 
Zirbel^Zirbeldruse}. — The  pineal  gland  or  epiphysis  is  developed  as 
a  median  dorsal  evagination  of  the  medullary  wall  of  the  fore-brain  a 
short  distance  in  front  of  the  mid-brain;  between  it  and  the  mid- 
brain  is  situated  the  posterior  commissure,  p.  684.  Its  site  is  said 
by  A.  Goette,  75.1,  to  be  identical  in  Bombinator  with  that  of  the 
anterior  neuroporus,  or  point  where  the  medullary  groove  closes  last 

in  the  head ;  if  this  coincidence  is  true 
of  vertebrates  generally,  it  must  have 
some,  perhaps  important,  significance. 
The  development  of  the  epiphysis  in 
reptiles  and  lower  invertebrates  indi- 
cates that  it  was  primitively  a  median 
eye,  which  survives  as  a  rudiment, 
compare  below. 

The  pineal  evagination  appears  after 
the  development  of  the  brain  is  quite 
advanced  (chick,  end  of  fourth  day,  in 
the  rabbit  the  fourteenth  day,  in  white 
mice  of  9.5  mm.,  in  sheep  embryos  of 
3.5  mm.,  in  man  at  about  the  sixth 
week) ;  it  therefore  cannot — as  Mi- 
halkovics, 77.1,  95,  justly  observes 
against  A.  Goette,  75.1,  315-316— 
be  interpreted  as  resulting  from  the  connection  at  the  neuropore  of 
the  medullary  wall  with  the  epidermis,  for  the  two  layers  are  sepa- 
rated by  intervening  mesenchyma  in  amniote  embryos  long  before 
the  evagination  appears.  In  birds  the  evagination  points  forward, 
in  mammals  backward ;  this  difference  is  probably  due  to  the  greater 
development  of  the  corpus  callosum  forcing  the  pineal  gland  back  in 
mammalia.  Our  knowledge  of  its  development  in  mammals  and 
birds  we  owe  chiefly  to  Mihalkovics,  77.1,  94,  whose  results  have 


FIG.  3JG.  —  Brain  of  a 
Chick  Embryo,  fourth 
Day.  I,  First  II,  second 
cerebral  vesicle;  Ep,  epi- 
physis or  pineal  gland; 
H,  cerebral  hemisphere; 
L,  lens,  surrounded  by 
the  optic  vesicle;  of, 
otocyst ;  Md,  hind-brain. 
After  Duval. 


THK    NKKVOVS    SYSTEM.  689 

been  confirmed  by  Kraushaar's  observations  on  white  mice,  85.1. 
The  evagination  lengthens  out  until  it  nearly  reaches  the  epidermis; 
it  is  a  tube  or  sac  communicating  with  the  fourth  ventricle,  ending 
blindly,  and  having  walls  composed  of  cylinder  cells.  The  sac  next 
enlarges  at  its  upper  end,  and  the  wall  of  the  enlargement  after 
thickening  forms  buds  (chick  fifth  day,  rabbit  embryos  of  20-25  mm. ) , 
which  are  hollow  and  retain  in  the  chick  their  primitive  form  but  in 
mammals  the  hollow  buds  become  filled  with  proliferated  epithelial 
cells,  which  take  on  rounded  and  polygonal  forms  and  are  presumably 
degenerated  elements ;  the  cells  have  processes  and  lie  more  or  less 
separated  from  one  another. 

In  reptiles  the  epiphysis  assumes  a  more  complicated  structure, 
but  in  many  forms  its  differentiation  is  more  or  less  imperfect. 
When  carried  to  its  highest  known  development  the  pineal  sac  is 
differentiated  into  three  parts,  a  distal  eye-like  enlargement  close  to 
the  epidermis,  a  middle,  narrow  part  like  an  optic  nerve,  and  a  prox- 
imal larger  part,  as  shown  by  W  .B.  Spencer,  86.1,  whose  results 
have  since  been  verified  and  extended  by  Beraneck,  87.1,  Beard, 
88.2,  Francotte,  87.1,  88.1,  McKay,  89. 1,  Owsjannikow,  88.1, 
Ritter,  91.1,  Strahl  and  Martin,  88.1,  and  Wiedersheim,  86.1. 
A  synopsis  of  the  development  of  the  reptilian  epiphysis  is  given  by 
C.  K.  Hoffmann  in  Bronn's  "  Thierreich,"  VI.,  Abth.  III.,  1081-1993. 
The  distal  end  of  the  evagination  lies  near  the  epidermis ;  it  early 
enlarges  into  a  hollow  globe,  which  soon  flattens  out  somewhat;  the 
\vall  on  the  side  next  the  epidermis  thickens  and  assumes  a  lens-like 
character;  the  wall  on  the  opposite  side  is,  of  course,  united  with 
the  stalk  and  assumes  a  retinal  character.  Strahl  and  Martin,  /.c., 
observed  in  the  retinal  region  the  differentiation  of  the  Randschleier 
and  of  the  nuclear  layer,  and  the  presence  of  karyokinetic  figures 
next  the  cavity,  so  that  the  primary  stratification  is  the  same  as  in 
the  wall  of  the  brain  proper ;  later  pigment  granules  are  deposited  in 
the  part  of  the  retinal  layer  toward  the  lens,  and  nerve-fibres  can  be 
« )hsiTved  in  the  stalk.  There  can  be  little  question  that  the  structure 
in  question  is  a  true,  though  rudimentary  eye.  It  has  also  been 
observd  in  lampreys  and  amphibians. 

The  morphological  significance  of  the  pineal  body  is  still  under 
debate.  The  fact  that  it  forms  an  eye  in  Petromyzon  indicates  that 
the  optic  character  was  primitive,  but  it  appears  to  have  lost  that 
character  along  the  lines  of  descent  leading  to  the  teleosts  and  elas- 
mobranchs,  while  it  has  retained  it  along  the  lines  leading  to  the 
amphibians  and  reptiles,  becoming  in  them  more  or  less  rudimentary 
and  disappearing  altogether  in  the  birds.  As  the  pineal  eye  is  the 
distal  part  of  the  epiphysis  only,  and  is  wanting  in  mammals  (com- 
pare, however,  H.  F.  Osborn,  Science,  Jan.,  1886),  the  suggestion  is 
inevitable  that  the  pineal  gland  of  mammalian  anatomy  is  homolo- 
gous with  the  proximal  part  only  of  the  reptilian  epiphysis. 

Historical  Note. — The  first  suggestion  that  the  epiphysis  might 
represent  a  visual  organ  was,  so  far  as  known  to  me,  made  by  Rabl- 
Riickhard,  82.1;  it  was  renewed  by  Ahlborn,  84.1,  but  was  first 
definitely  verified  by  De  Graaf,  86.1,  whose  article,  together  with 
Baldwin  Spencer's  admirable  memoir,  86.1,  forms  the  basis  of  our 
present  knowledge  of  the  pineal  eye.  Ley  dig,  88.4,  90. 1,  attempted, 
44 


690  THE    FCETUS. 

but  unsuccessfully,  to  prove  that  the  pineal  eye  could  not  be  a  sense 
organ.  As  regards  the  development,  the  principal  authorities  are 
Mihalkovics,  77.1,  for  mammals,  Beraneck,  87.1,  and  Strahl  and 
Martin,  88.1,  for  reptiles. 

Paraphysis. — Under  the  name  of  paraphysis,  or  "  Stirnorgctn," 
Selenka,  90.1,  has  described  a  second  evagination  from  the  median 
dorsal  wall  of  the  fore-brain,  which  is  similar  to  the  epiphysis.  It 
is  further  forward,  being  between  the  orig-'n  of  the  hemispheres. 
Selenka  very  doubtfully  compares  it  with  the  median  auditory  organ 
of  ascidians,  as  the  epiphysis  has  been  compared  to  the  median  eye 
of  ascidians.  Selenka  has  observed  the  organ  in  sharks,  reptiles,  and 
marsupials.  In  reptiles,-  just  after  the  pineal  evagination  has  begun 
in  the  embryo,  there  appears  another  evagination  some  distance  in 
front  of  it  and  also  in  the  median  dorsal  line,  to  develop  the  para- 
physis. The  evagination  grows  backward  until  it  reaches  the  epi- 
physis ;  after  the  pineal  eye  is  cut  off,  it  shoves  itself  under  the  pineal 
eye,  but  without  uniting  with  it ;  the  end  of  the  paraphysis  is  en- 
larged and  forms  a  number  of  fine  hollow  buds ;  its  proximal  part 
or  stalk  is  round  or  oval  in  cross  sections ;  throughout  the  embryonic 
period  the  cavity  remains  in  communication  with  the  third  ventricle ; 
the  fate  of  the  organ  after  birth  is  unknown.  Unfortunately  Selenka 
gives  no  figures. 

The  paraphysis  has  been  observed  by  Charles  Hill,  91.1,  in  the 
embryo  of  7  mm.  of  Corregonus  (a  teleost)  to  grow  out  asymmetri- 
cally from  the  wall  of  the  brain  just  in  front  of  the  epiphysis;  it  is 
about  half  the  size  of  the  epiphysis. 

Cerebral  Hemispheres. — A  previous  section  is  devoted  to  the 
development  of  the  median  portion  of  the  fore-brain,  and  accordingly 
in  this  section  we  confine  ourselves  to  the  lateral  outgrowths  or 
hemispheres  of  the  fore-brain.  The  hemispheres  arise,  as  has  been 
described,  as  diverticula  of  the  dorsal  zone  of  His  in  the  anterior 
half  of  the  fose-brain,  and  therefore  they  can  never  develop  any 
structures  homologous  with  parts  arising  from  the  deck-plate,  the 
ventral  zones  of  His,  or  the  Bodenplatte.  The  choroid  plexus 
might  be  taken  as  an  exception  to  this  law,  but,  as  its  development 
teaches  us,  it  is  not  morphologically  part  of  the  hemispheres.  For 
convenience  the  cerebral  convolutions  are  considered  in  a  separate 
section,  p.  695. 

GENERAL  GROWTH. — The  hemispheres  of  the  human  embryo  of 
four  weeks  have  been  described,  p.  596.  They  continue  to  enlarge 
throughout  the  entire  foetal  period,  but  their  connection  with  the 
middle  portion  of  the  fore-brain  does  not  enlarge  correspondingly. 
There  is  but  little  (?  if  any)  enlargement  of  the  foramen  of  Monro 
after  the  fifth  week,  but  there  is  a  considerable  growth  of  the  walls 
of  the  foramen,  so  that  the  actual  size  of  the  structures  connecting 
the  hemispheres  with  the  median  fore-brain  increases  very  consider- 
ably, but  the  enlargement  of  the  hemispheres  is  still  more  rapid,  so 
that  they  become  and  remain  large,  pedunculate,  vesicular  lateral 
appendages,  and  project  beyond  the  median  fore-brain  forward, 
upward,  and  in  later  stages  backward  so  as  to  cover  the  mid-brain 
also.  The  enormous  expansion  of  the  hemispheres  is  one  of  the 
most  characteristic  features  of  the  amniote  embryo,  but  the  expan- 


THE   NERVOUS   SYSTEM. 


691 


sion  is  greater  in  mammals  than  in  reptiles,  in  man  than  in  any 
other  mammal.  The  size  of  the  hemispheres  in  the  adult  is  closely 
correlated  with  the  degree  of  mental  development  of  the  species.  The 
fundamental  relations  of  the  hemispheres  to  the  fore-brain  are  clear 
from  Fig.  388 ;  while  retaining  their  strictly  limited  connection  with 
the  anterior  part  of  the  median  fore-brain  by  means  of  the  medullary 
walls  bounding  the  foramen  of  Monro,  f.M,  the  hemispheres  are 
expanding  in  every  direction.  In  front  and  above  the  two  hemi- 
spheres have  walls  which  face  one  another  and  are  separated  by  a 
narrow  constantly  deepening  fissure,  Fig.  390,  /,  which  is  filled 
with  nirsrnrhymal  tissue  constituting  the  anlage  of  the  falx  cerebri 
(Hirnsichel).  Posteriorly  a  groove  separates  the  hemisphere  from 
the  Zwischenhirn.  In  a  lateral  view  the  hemisphere  shows  a  wide, 
shallow  depression  at  five  weeks, 
which  gradually  becomes  more 
marked,  Fig.  31)7,  and  is  ulti- 
mately transformed  into  the  fis- 
sure of  Sylvius.  Corresponding 
to  the  external  depression  there 
is  an  internal  projection  of  the 
wall  of  the  hemisphere  into  the 
cavity  of  the  lateral  ventricle; 
this  projection  is  the  first  indi- 
cation of  the  coffins  striatiun, 
which  arises  as  a  thickening  of 
the  wall  extending  not  only  over 
the  region  of  the  fossa  of  Syl- 
vius, but  aNo  past  the  foramen 
<>f  Monro,  t.>  be  continued  as  the 
thickening  of  the  thalamencepha- 
lir  wall,  which  produces  the  tha- 
lamus  opticus,  p.  G86.  We  thus 


obi 


FIG.  897.  —  Human  Embryo  of  about  four 
Months;  brain  in  situ.  Oc,  Occipital  lobe;  T. 
temporal  lobe;  S,  fissure  of  Sylvius;  F.  frontal 

have  a  hemisphere  the  floor  wall  NS^ra^68^erebellum;  °w'  medulla  obiongata. 
of  which  is  thickened  to  form  the 

anlage  of  the  so-called  basal  ganglia,  while  the  rest  of  the  wall  is  thin 
and  is  designated  as  the  mantle  (pallium) .  While  the  Sylvian  fossa  is 
appearing  the  anlage  of  the  olfactory  lobe  is  differentiated,  Fig.  341, 
R,  by  the  bulging  forth  of  the  lower  anterior  wall  of  the  hemisphere,* 
and  is  soon  marked  off  from  the  hemisphere  proper  by  a  distinct 
groove,  the  rhinal  fissure  of  comparative  anatomy.  We  find  that 
the  hemispheral  vesicle  is  now  divisible  into  three  primary  regions, 
which  all  persist  throughout  life ;  these  are : 

1.  The  mantle  (for  detailed  history,  see  p.  694). 

2.  The  basal  ganglia  (for  detailed  history,  see  p.  694-5). 

3.  Olfactory  'lobe  (for  detailed  history,  see  p.  703). 

The  mantle  outgrows  the  other  parts  and  forms  nearly  the  whole 
of  the  convoluted  surface  of  the  adult  brain.  During  the  fifth  week 
the  choroid  plexus  grows  into  the  lateral  ventricle,  compare  p.  681, 
and  thereafter  forms  a  conspicuous  structure,  but  it  is  not  part  of 
the  hemisphere  in  a  strictly  morphological  sense. 

*  Wm.  Turner.  HI. 2.  from  his  observations  on  the  comparative  anatomy  of  the  brain,  concludes 
that  the  olfactory  lobe  or  rhinencephalon  stretches  further  back  and  includes  the  lobus  hippo- 
campi. 


692  THE   FCETUS. 

As  stated  above,  the  upward  expansion  of  the  hemispheres  causes 
them  each  to  have  a  medial  wall,  bounding  the  fissure  in  which  the 
falx  cerebri  is  developed.  Along  this  wall  is  developed  a  fold,  which 
is  marked  by  an  external  groove  and  an  internal  ridge ;  the  groove  is 
the  Bogenfurche  of  German  embryologists,  Fig.  395,  a,  401,  a,  Fig. 
391,  bf;  and  is  in  part  equivalent  to  the  caliosal  fissure  or  groove  of 
the  adult,  while  the  internal  ridge  is  the  anlage  of  the  cornu  Am- 
monis.  The  Bogenfurche  begins  at  the  olfactory  lobe,  which  it 
crosses,  and  divides  it  into  an  anterior  and  posterior  part  (see  p.  703) ; 
the  Bogenfurche  then  curves  around,  Fig.  401,  a,  parallel  with  the 
edge  of  the  foramen  of  Monro.  It  begins  to  develop  anteriorly  and 
gradually  extends  further  and  further  backward,  until  it  is  a  long 
arching  groove,  terminating  in  the  temporal  lobe  (lobus  hippocampi) . 
It  must  be  remembered  that  the  posterior  end  of  the  groove  arises  in 
reality  by  itself  as  the  hippocampal  groove  (Ammonsfurche)  but  the 
two  ends  soon  join,  making  one  long  fissure  as  described  (W.  His, 
89.4,  697).  At  its  posterior  end  the  groove  forms  two  branches, 
each  corresponding  to  a  fold  of  the  brain  wall;  one  branch  is  the 
anlage  of  the  parieto-occipital,  the  other  of  the  calcarine  fissure. 
These  three  fissures  (Bogenfurche  and  its  two  branches)  and  the 
Sylvian  fissure  are  the  only  fissures  which  arise  as  folds  of  the  brain. 
Mihalkovics,  77.1,  has  proposed  for  them  the  distinctive  name  of 
Totalfurchen  (total  grooves).  All  other  fissures  (sulci)  are  merely 
depressions  of  the  cortical  surface,  not  folds  of  the  brain -wall. 
When  the  corpus  callosum  is  developed,  p.  683,  it  gradually  occupies 
by  its  enormous  expansion  most  of  the  space  under  the  Bogenfurche, 
so  that  the  fissure  (sulcus  corp.  callosi) ,  is  almost  hidden  above  the 
corpus  callosum  in  the  adult.  The  internal  ridge  corresponding  to 
the  Bogenfurche,  has,  of  course,  the  same  arched  course ;  it  begins 
at  the  olfactory  lobe,  curves  upward  and  backward  around  the  foramen 
of  Monro,  and  bending  downward  terminates  behind  the  corpus 
striatum  in  the  temporal  region.  Its  course  may  be  understood  from 
Fig.  388  and  Fig.  395.  As  to  the  fate  of  the  frontal  end  of  the 
ridge,  we  have  no  satisfactory  knowledge ;  the  posterior  end  is  the 
anlage  of  the  hippocampus,  the  ridge  corresponding  to  the  main 
groove  developing  into  the  hippocampus  major  (cornu  Ammonis) , 
and  the  ridge  corresponding  to  the  branch  (sulcus  calcarinus)  develop- 
ing into  the  hippocampus  minor  (calcar  avis) . 

The  three  lobes  (frontal,  temporal,  and  occipital)  of  the  adult  are 
very  gradually  evolved.  The  first  step  in  their  differentiation  is  the 
development  of  the  fossa  Sylvia3.  The  fossa  may  be  recognized  in  a 
human  embryo  of  five  weeks.  It  seems  to  owe  its  origin  to  the  fact 
that  the  brain-wall  forming  the  fossa  grows  principally  in  thickness 
to  produce  the  corpus  striatum,  while  the  mantle  grows  very  rapidly 
in  superficies;  it  follows  that  the  mantle  region  expands  and  projects 
beyond  the  thick- walled  fossa,  Fig.  397;  the  mantle  at  this  stage 
forms  a  vesicular  frontal  lobe,  F,  and  a  vesicular  post-Sylvian  lobe, 
each  with  thin  walls  and  each  including  a  portion  of  the  wide  lateral 
ventricle.  The  post-Sylvian  lobe  becomes  in  part  the  temporal  lobe, 
T,  but  it  also  expands  toward  the  cerebellum,  and  its  expansion 
forms  the  occipital  lobe,  Fig.  397,  Oc.  The  frontal  and  temporal 


THE   NERVOUS   SYSTEM.  693 

lobes  may  therefore  be  regarded  as  primary,  the  occipital  lobe  as  a 
secondary  or  later  acquisition. 

Each  lobe  includes  a  portion  of  the  lateral  ventricle ;  the  portion 
in  the  frontal  lobe  becomes  the  anterior  cornu ;  the  portion  in  the 
temporal  lobe  the  descending  cornu;  the  portion  (recessus  occipi- 
t  til  in)  in  the  occipital  lobe  the  posterior  cornu.  Now  the  Bogen- 
furche  extends  down  behind  the  fossa  of  Sylvius,  therefore  along  the 
medial  wall  of  the  temporal  lobe ;  hence  the  inner  ridge  correspond- 
ing to  the  Bogenfurche  projects  into  the  ventricular  cavity  of  that 
lobe;  now  the  ridge  is  the  anlage  of  the  hippocampus  major  (cornu 
Ammonis) ,  which  remains  throughout  life  a  ridge  projecting  into 
the  descending  cornu.  It  will  be  recalled  further  that  the  Bogen- 
furche has  a  branch,  the  calcarine  sulcus,  Fig.  392,  ca/,  which  runs 
on  to  the  medial  wall  of  the  occipital  lobe,  and  has  corresponding  to 
it  a  ridge  projecting  into  the  ventricular  cavity  of  that  lobe ;  this 
ridge  likewise  persists  throughout  life  and  is  the  hippocampus 
ni  I  nor  (calcar  avis)  of  descriptive  anatomy.  The  exact  history  of 
the  modifications  in  the  shape  of  the  lateral  ventricle  during  the 
fcetal  period  has  still  to  be  worked  out. 

The  fossa  of  Sylvius  undergoes  important  modifications  (compare 
Mihalkovics,  77. 1, 149).  At  the  end  of  the  second  month  the  hemi- 
sphere in  side  view  has  a  bean-like  shape,  the  hilus  facing  down- 
ward ;  the  fossa  is  situated  at  the  hilus.  At  three  months  the  fossa 
is  about  as  high  as  broad ;  during  the  fourth  and  fifth  months  it 
becomes  more  sharply  defined  and  has  a  marked  inclination  toward 
the  occiput.  The  floor  of  the  fossa  corresponds  to  the  corpus  striatum 
and  island  of  Reil;  the  brain- wall  constituting  the  floor  is  very 
much  thickened ;  the  external  surface  of  the  floor,  which  is  seen  when 
the  brain  is  viewed  from  the  side,  is  the  sorcalled  island  of  Reil. 
Morphologically  the  island  and  the  corpus  striatum  are  parts  of  the 
same  structure.  During  the  sixth  month  the  edges  of  the  fossa  be- 
gin to  spread  over  the  island  and  cover  it  in,  so  that  by  the  ninth 
month  it  is  entirely  buried,  and  can  be  seen  only  by  opening  the 
Sylvian  fissure. 

The  thickness  of  the  walls  of  the  hemisphere  apparently  increases 
throughout  the  second  to  ninth  month.  In  the  region  of  the  basal 
ganglia  the  thickening  takes  place  very  early  and  becomes  very 
great.  The  mantle  thickens  more  slowly  and  never  equals  the  basal 
ganglia  in  thickness. 

The  size  of  the  hemispheres,  as  a  whole,  increases  very  rapidly 
for  a  long  period,  so  that  at  birth  they  more  than  equal  all  the  rest 
of  the  brain  in  volume.  They  cover  first  the  thalamencephalon, 
later  the  mid -brain  also,  still  later  the  cerebellum  also.  Owing  to 
the  growth  of  the  cerebellum  after  the  fifth  month  it  is  less  com- 
pletely covered  by  the  hemispheres  at  the  end  than  during  the  mid- 
dle period  of  fcetal  life. 

FORAMEN  OF  MONRO. — The  foramen  of  Monro  is  at  first,  Fig. 
337,  a  rounded  opening,  which  soon  becomes  pointed  at  its  lower 
side.  As  to  its  actual  size  in  successive  stages  we  have  no  measure- 
ments ;  it  is  converted  into  a  fissure-like  opening,  and  is  commonly 
said  to  diminish  in  size,  but  I  think  it  probable  that  the  diminution 


694 


THE   FCETUS. 


is  relative  only,  not  absolute.  A  knowledge  of  the  foetal  history  of 
the  foramen  would  be  a  desirable  addition  to  Embryology. 

MANTLE  OR  PALLIUM. — The  mantle  comprises  all  that  part  of  the 
hemispheres  which  enters  into  the  formation  of  neither  the  olfactory 
lobes  (rhinencephalon)  nor  basal  ganglia  (Bodentheil,  Stammtheil) . 
Its  general  history  we  have  already  reviewed ;  the  development  of 
its  convolutions  is  treated  in  the  next  section ;  we  have,  therefore,  to 
present  only  what  little  is  known  of  the  histogenesis  of  the  cortex 
cerebri,  the  cortex  being  the  superficial  stratum  of  the  mantle. 

Histogenesis. — For  the  development  of  the  nerve-cells,  see  p.  624. 
We  have  to  add  here  what  little  is  known  concerning  the  develop- 
ment of  the  layers  of  the  cortex,  following  Vignal,  88. 1,  242,  who 
also  gives,  p.  232,  a  summary  of  previous  work.  In  a  rabbit  embryo 
of  fourteen  days  the  Randschleier  is  still  thin,  while  the  mantle  layer 

with  rounded  nuclei  and  the  inner 
layer  with  elongated  nuclei  have  both 

frown  very  much.  In  later  stages, 
ig.  398,  I  find  six  main  layers,  the 
homologies  of  which  with  the  layers, 
both  of  earlier  and  of  adult  stages, 
have  still  to  be  determined.  The 
outermost  layer  is  thin,  6,  and  con- 
tains very  few  nuclei ;  below  is  a 
broader  layer  with  the  nuclei  grouped 
chiefly  in  radial  lines,  5 ;  this  layer 
is  the  anlage  of  the  cortex  cerebri, 
sensu  strictu,  and  is  seen  to  consist 
of  three  strata ;  it  is  along  the  inner 
edge  of  this  layer  that  the  great  pyra- 
midal cells  arise  to  form  the  third 
layer  of  Meynert,  while  the  rest  of 
the  layer  produces  the  second  layer 
of  Meynert.  Now  if,  as  I  hold  to  be 
probable,  the  large  pyramidal  cells 

FIG.  398. -Section   through   the  Lateral    are    homologous    with    the    Purkinje 

Wall£f  the  Ce^e|)ral  Hemisphere  of  a  HU-  cells  of  the  cerebellum,  then  layers  5 

man  TT.mhrvn  r»f  frmr  TVTrmtVic        Minr>f  r"/->ll  n  -  .  "  * 

and  6  of  Fig.  398  are  derived  from 
the  original  Randschleier.  But  in  the 
present  state  of  our  knowledge  an- 
other interpretation  is  equally  possible — namely,  that  layers  1-4  are 
derived  from  the  inner  layer  of  the  embryo,  layer  5  from  the  mantle 
layer,  layer  6  from  the  Randschleier. 

For  a  comparison  of  the  layers  of  the  cortex  in  various  air-breath- 
ing vertebrates  see  Nakagawa,  90. 1. 

The  medullated  nerve-fibres  of  the  mantle  do  not  appear  until  after 
birth,  S.  Fuchs,  84.1,  181. 

BASAL  GANGLIA. — The  corpus  striatum  and  the  various  parts 
associated  with  it  arise  from  the  thickened  wall  of  the  fossa  of  Syl- 
vius. This  thickening  is  continuous  past  the  posterior  side  of  the 
foramen  of  Monro  with  that  thickening  of  the  dorsal  zone  of 
His,  which  produces  the  thalamus  opticus  of  the  median  fore-brain, 
p.  686.  The  part  of  the  thickening,  which  connects  the  corpus  stri- 


man  Embryo  of  four  Months.  Minot  Coll. 
31.  Each  dot  represents  a  nucleus,  1-6, 
successive  layers,  each  characterized  by 
the  distribution  of  its  nuclei. 


THE   NERVOUS   S VST KM. 


896 


H.i 


atttm  proper  with  the  median  fore-brain,  develops  into  part  of  tin-  so- 
called  peduncles  of  the  hemispheres  of  the  adult;  constituting  what 
is  termed  by  W.  His,  89.4,  700,  the  Streifenhugelstiel.  The  com- 
mencement of  the  thickening  may  be  observed  in  rabbit  embryos  of 
12-1:5  mm.  (Mihalkovirs,  77.1,  110),  in  the  human  embryo  at  four 
weeks ;  it  necessarily  coincides  with  the  first  formation  of  the  fissure 
of  Sylvius.  The  thickening  soon  becomes  (His,  89.4,  699)  a  con- 
siderable prominence;  it  is  broad,  forming  what  maybe  called  the 
floor  of  the  hemisphere ;  it  stretches  from  the  lamina  terminalis  and 
the  anlage  of  the  olfactory  lobe  across  the  fossa  of  Sylvius  and  behind 
the  foramen  of  Monro,  where  it  joins  the  anlage  of  the  optic  thala- 
mus.  Kven  at  the  beginning  of  the  fifth  week  traces  of  the  division 
of  the  corpus  into  three  limbs  can  be  detected ;  a  lower  limb  (hinterer 
SrlicnM  of  His)  runs  to  the  lamina  terminalis ;  on  the  upper  limb 
(  rorilrn-r  ,SV7/r ///,v7  of  His)  to  the  ante- 
rior olfactory  lobe,  and  a  middle  limb  to 
the  posterior  olfactory  lobe.  The  mid- 
dle and  lower  limbs  together  form  the 
r/v/.s  ntcd i«l(>,  the  upper  limb  the  r/-//.v 
/titcrtili'  of  descriptive  anatomy — com- 
pare  Fig.  :>'.i'.»,  which  well  illustrates  the 
primitive  form  and  subdivision  of  the 
arching  corpus  striatum.  Later,  in  the 
same  measure  as  the  hemisphere  ex- 
pands inward  the  cerebellum,  the  poste- 
rior part  of  the  corpus  striatum  grows. 
A  groove,  which  persists  into  late  foetal 
periods,  marks  the  division  between  the 
corpus  and  the  optic  thalamus;  the 
groove  ultimately  becomes  obliterated, 
and  the  tissue  which  fills  it  up  is  the 
anlage  of  the  stria  cornea  or  termina- 
lis {tcenia  ^  in  i  circular  is)  \  His  pro- 
poses, therefore,  to  designate  the  groove  as  the  sulcus  stria  cornece. 
The  origin  of  the  stria  cornea  (Hornstreif)  was  discovered  by  Mi- 
halkovics  77.1,  133. 

Cerebral  Convolutions. — We  must  divide  the  so-called  fissures 
which  produce  the  convolutions  (gyri)  of  the  brain  into  two  classes, 
the  primary  folds  and  the  secondary  fissures.  The  former  are  liter- 
ally folds  of  the  entire  brain -wall,  and  were,  therefore,  appropriately 
termed  "  Totalfurchen"  by  Mihalkovics,  77.1,  146,  who  first  clearly 
recongized  them  as  a  distinct  class  of  fissures.  The  latter  are  merely 
narrow  groove-like  depressions  of  the  surface  of  the  hemispheres. 

1.  PRIMARY  FOLDS. — These  are  the  fossa  Sylvi  and  the  Bogen- 
furche ;  the  latter  has  at  its  posterior  end  two  branches,  known  as 
the  calcarine  and  parieto-occipital  fissures  respectively.  To  these 
we  ought  possibly  to  add  the  fissura  collateralis,  p.  701,  which  is 
situated  on  the  lower  surface  of  the  temporal  lobe. 

The  fossa  of  Silvius,  as  already  stated,  p.  692,  is  at  first  a  wide, 
shallow  depression,  which  gradually  deepens.  The  wall  of  the 
depression  is  very  much  thickened  to  make  the  corpus  striatum ;  the 
outer  part  of  the  wall  makes,  of  course,  the  floor  of  the  fossa,  and 


B.ol 
R'.op 
399.— View  of  the  Hemisphere  of 


«•.  n-pus  striatum. or  thickening  by  which 
it  is  joined  to  the  pars  suhthalamica; 
cm,  cms  mediate  of  the  corpus  stria 
turn:  li.ol,  olfactory  lobe;  Ol.  ncrvis 
olfactorius ;  R.op,  recessus  opticus; 
R.i,  recessus  infundibuli.  After  W. 
His.  X  about  5  diams. 


696  THE   FCETUS. 

this  floor  becomes  the  island  of  Reil  in  the  adult.  The  growth  »nd 
expansion  of  the  mantle  causes  it  to  project  farther  and  farther, 
thereby  deepening  the  fossa.  At  four  months,  Fig.  397,  S,  it  is  very 
wide  and  ascends  backward  between  the  frontal,  F,  and  temporal, 

T,  lobes.  At  the  begin- 
ning of  the  fifth  month 
(Mihalkovics,  77.1, 
150)  the  fossa  has  be- 
9-  Jo  come  deeper,  longer,  and 

FIG.  400.— Outlines  of  the  Fissure  of  Sylvius  of  Human  Em-    more  oblique,  and  its  an- 
bryos  at  Successive  Lunar  Months.    After  G.  Mihalkovics. 

tenor  margin  is  broken 

by  an  angle,  Fig.  400,  5.  The  two  margins  gradually  approach  one 
another,  concealing  the  floor  of  the  fossa  or  island  of  Reil,  and  mean- 
while the  angular  notch  of  the  anterior  margin  becomes  more  marked. 
The  changes  continue  until,  as  shown  in  the  figure,  the  opening  of 
the  fossa  is  a  narrow  Y-shaped  cleft,  9,  leading  down  into  the  fossa 
proper  and  the  island  of  Reil.  The  walls  of  the  fossa  of  Sylvius, 
including  the  island,  acquire  secondary  furrows  during  the  ninth 
month.  The  part  of  the  margin  of  the  fissure  between  the  two  forks 
of  the  Y  is  sometimes  termed  the  operculum.  The  Bogenfurche^  or 
fissura  prima,  arises  very  early.  Its  anterior  part  appears  first,  be- 
ginning as  stated  above,  p.  692,  at  the  olfactory  lobe,  thence  passing 
along  the  medial  wall  of  the  hemisphere  in  a  curved  line,  which  may 
be  roughly  described  as  parallel  with  the  lamina  terminalis  and  tela 
choroidea,  Fig.  401,  a.  The  pos- 
terior part  of  the  Bogenfurche  ap- 
pears later;  it  corresponds  to  the 
Ammonsfurche  of  Mihalkovics, 
77.1,  145  (sulcus  hippocampi  of 
Huxley) ;  it  begins  on  the  medial 
wall  of  the  temporal  lobe,  and 
gradually  extends  upward  and  for- 
ward until  toward  the  latter  part 
of  the  second  month  it  joins  the 
anterior  part,  and  the  union  of  the 
two  produces  the  great  Bogen- 
furche  which  begins  at  the  olfac- 

tory    lobe,     runs     Widely     arching    torius;  In/,  infundibulum.    After  W.  His. 

along  the  medial  wall,  and  termi- 
nates at  the  lobus  hippocampi,  p.  691.  Corresponding  to  the  external 
groove  is  an  internal  ridge,  the  ridge  persists  in  the  posterior  part 
as  the  hippocampus,  but  its  fate  in  the  region  of  the  frontal  lobe  is  ob- 
scure. Below  the  ridge  is  a  strip  of  the  hemispheral  wall,  the  Rand- 
bogen  (gyrus  arcuatus)  of  F.  Schmidt,  62V1.  In  the  adult  a  large 
part  of  the  Randbogen  is  occupied  by  the  very  large  corpus  callosum, 
above  which  persists  the  Bogenfurche  as  the  callosal  groove.  The 
portion  of  the  Randbogen  immediately  behind  the  callosum  develops 
during  the  first  half  of  the  fifth  month  little  transverse  ridges  upon 
its  surface,  and  thereby  becomes  the  recognizable  anlage  of  the  gyrus 
dentatus  (Mihalkovics,  77.1,  147).  The  extreme  posterior  end  of 
the  Randbogen  is  bent  upon  itself,  hook-like,  and  is  easily  identified 
as  the  anlage  of  the  gyrus  uncinatus. 


THE   NERVOUS   SYSTEM. 


697 


From  the  posterior  part  of  the  Bogenfurche  run  out,  both  from 
nearly  the  same  point,  its  two  branches,  the  Jissura  calcarina  and 
the  Jissura  parieto-occipitalis,  Fig.  402,  a, 6,  which  both  appeal- 
while  the  occipital  lobe  is  growing  out,  the  calcarine  fissure,  a,  usu- 
ally, but  not  always  (His,  74.1,  114),  arising  before  the  parieto- 
occipital,  6,  which  last  first  develops  at  the  beginning  of  the  fourth 
month  (Mihalkovics,  77.1,  140).  Both  fissures  run  upward  and 
backward  on  the  medial  wall  of  the  hemisphere,  and  as  they  diverge 
they  enclose  a  space  between  them,  which  corresponds  to  the  so- 
called  cuneate  lobe  of  the  adult.  To  these  two  branches  of  the 
Bogenfurche  correspond  internal  ridges  (cf.  His,  74.1,  Fig.  113),  but 
the  ridg A  corresponding  to  the  parieto-occipital  fissure  is  subsequently 
obliterated  as  the  brain  wall  thickens,  while  that  corresponding  to 
the  calcarine  fissure  persists,  and,  as  indicated  by  its  name,  becomes 
the  calcar  avtS,  <>r  lu'iijHtraitt/nix  in i inn;  p.  693. 

During  the  seventh  month  the  parieto-occipital  fissure  extends 
beyond  the  medial  wall  on  to  the  external  wall  of  the  hemispheres, 
and  by  its  extension  establishes  the  life-long  boundary  between  the 
parietal  and  occipital  lobes.  The  anterior  boundary  of  the  parietal 
lobe  is  the  fissure  of  Rolando,  see  below. 

2.  SECONDARY  FURROWS. — These,  as  defined  above,  are  merely 
grooves  upon  the  surface,  not  folds  of  the  walls,  and  they  have,  there- 
fore, no  corresponding  internal  ridges  on  the  ventricular  side  of  the 
brain- wall.  We  may  conveniently  divide  them  into  main  or  essential 
fissures  and  accessory  or  unessential  fissures. 

The  MAIN  FISSURES  may  be  enumerated  as  follows : 


Calloso-marginal  or  splenial. 
Fissure  of  Rolando. 
Fissures  of  the  frontal  lobe. 

a.  praecentral. 

b.  superior  frontal. 

c.  inferior  frontal. 

d.  olfactory  or  rectus. 

e.  tri-radiate. 

/.  internal  frontal. 
Fissures  of  the  parietal  lobe. 
fi.  Intraparietal. 
b.  retrocentral. 


1.  Calloso-marginal  or  splenial.       5.  Fissures  of  the  temporal  lobe. 

2.  Fissure  of  Rolando.  a.  superior  temporal. 
:».  Fissures  of  the  frontal  lobe.  b.  inferior  temporal. 

c.  occipito-temporal  or  col- 
lateral (compare  6,  d) . 

6.  Fissures  of  the  occipital  lobe. 

a.  ascending  perpendicular. 

b.  superior  occipital. 

c.  inferior   occipital  (sagit- 

tal). 

d.  occipito-temporal     (com- 

pare 5,  c). 

7.  Fissures  of  the  island  of  Reil. 

a.  central. 

b.  praecentral. 

c.  postcentral. 

The  primitive  type  of  the  fissures  and  of  the  convolutions  between 
them  is  marked  in  the  adult  by  the  accessory  fissures,  which  join  the 
primary  fissures  or  arise  from  them,  and  also  by  secondary  bridges 
by  which  two  adjacent  convolutions  are  connected  with  one  another 
across  a  fissure. 

The  calloso-marginal  or  splenial  fissure,  Fig.  402,  e,  arises  about 
the  middle  of  the  fifth  month,  in  front  of  and  above  the  corpus  cal- 
losum,  cc,  by  the  fusion  of  two  or  three  shorter  fissures;  the  area  of 
the  hemispheral  mantle  between  the  calloso-marginal  fissure  and  the 


698  THE    FCETUS. 

corpus  callosum  is  the  gyrus  fornicatus.  Behind  the  main  fissure,  <% 
are  several  subsidiary  fissures  which  vary  considerably  in  different 
brains  in  both  number  and  arrangement;  they  appear  usually  to 
unite  with  the  calloso-marginal  fissure,  which  is  thus  prolonged  fur- 
ther back  above  the  corpus  callosum,  cc,  and  usually  the  added 


FIG.  402.— Brain  of  Human  Embryo  of  the  fifth  Month  after  Removal  of  the  Right  Hemis- 
phere, a,  Calcarine  fissure;  6,  parieto-occipital ;  c,  accessory  fissure  of  calloso-ruarginal :  <v. 
corpus  callosum;  th,  optic  thalamus;  d,  praecentral  superior;  e,  calloso  marginal;  si.  septum 
lucidum;/,  internal  frontal;  OJ,  olfactory  lobe;  op,  optic  nerve;  Inf,  inf undibulum ;  OC./H,  oculo- 
motor nerve;  Pons,  pons  Varolii;  Vi,  sixth  nerve ;  V,  trigeminus;  Md,  medulla  oblongata;  Cbl, 
cerebellum.  Natural  size.  (Compare  Fig.  404. ) 

secondary  fissures  cause  the  calloso-marginal  to  terminate  posteriorly 
with  an  upward  turn,  a  short  distance  behind  the  upper  end  of  the 
fissure  of  Rolando. 

The  development  of  the  fissure  of  Rolando  has  been  carefully 
studied  by  D.  J.  Cunningham,  90.1,  whose  account  is  as  follows: 
There  is  some  variability  in  the  time  at  which  the  fissure  makes  its 
appearance.  The  more  usual  time  is  the  last  week  or  ten  days  of 
the  fifth  month,  but  it  is  not  uncommon  to  meet  with  hemispheres 
well  on  in  the  sixth  month  of  development  with  no  sign  of  the  fissure. 
As  a  general  rule,  the  fissure  of  Rolando  is  developed  in  two  separate 
and  distinct  pieces,  Fig.  403,  Ro',  Ro" .  The  lower  portion  appears  in 
the  form  of  a  shallow  oblique  groove,  which  represents  the  lower  two- 
thirds  of  the  fully-formed  sulcus.  It  always  makes  its  appearance 
before  the  upper  piece.  Its  lower  end  is  placed  close  to  the  coronal 
suture — perhaps,  indeed,  it  may  lie  immediately  subjacent  to  the 
suture — while  the  upper  end  lies  further  back,  and  reaches  a  point 
midway  between  the  upper  margin  of  the  hemisphere  and  the 
Sylvian  fossa.  The  upper  piece  of  the  fissure  makes  its  appear- 
ance in  the  form  of  a  deep  pit  or  depression  between  the  upper  end 
of  the  lower  portion  and  the  margin  of  the  hemisphere.  An 
eminence  separates  the  two  portions  of  the  fissure  from  each 
other.  Soon,  however,  a  faint  furrow  runs  over  the  summit  of  this 
elevated  intervening  piece  of  the  cortex,  and  the  two  primitive  por- 
tions of  the  sulcus  are  partially  united  to  each  other.  As  develop- 
ment goes  on  the  more  complete  does  the  union  become,  and  the 
more  fully  is  the  intervening  eminence  borne  down  into  the  bottom  of 


THE    XKKVors    SYSTEM. 


§99 


the  fissure.  As  a  rule,  the  confluence  takes  place  rapidly,  but  in 
many  cases  the  process  appears  to  be  retarded.  Among  my  speci- 
mens I  have  several  hemispheres  which,  although  close  upon  the 
seventh  month,  show  still  a  complete  severance  of  the  two  constitu- 
ent elements  of  the  furrow.  But  the  portion  of  cerebral  cortex  which 
intervenes  between  the  t\v<>  parts  of  the  fissure  is  not  entirely  oblit- 
erated. It  disappears  from  the  surface,  it  is  true,  but  it  is  still  to 
l>e  discerned,  even  in  the  adult  brain,  in  the  bottom  of  the  fissure,  in 
that  shallowing  or  deep  annectant  gyrus  which  we  have  described 
at  the  junction  of  the  upper  and  middle  thirds  of  the  sulcus.  In 
some  rare  cases,  as  stated  by  Cunningham,  the  two  original  portions 
of  the  fissure  of  Rolando  remain  quite  distinct  throughout  life.  In 
these  the  intervening  bridge  of  cortex  remains  on  the  surface,  and  is 
not  pressed  down  by  the  fusion  of  the  upper  and  lower  divisions  of 
the  fissure.  We  have  noted  that  the  same  deep  annectant  gyrus 
may  be  observed  in  the  fissure  of  Rolando  of  the  chimpanzee  and 


Prc.s 


Ro' 


Sup.f 


Ext    /. 


Prc.i 


Fio.  403.— Ritfht  II»>inisph;Tc.  Natural  Size,  of  a  Foetus  of  nearly  spven  Months,  ip,  Inter- 
pariftal  tissuiv  partly  formed;  Ro',  upper,  Ro",  lower  piece  of  fissure  of  Rolando:  Prc.s,  supe- 
rior pra-rrntral  {issiire;  sup.f,  superior  frontal :  Prc.i,  inferior  praecentral ;  S,  Sylvian  fissure; 
Temp.g,  t.-iiiporalis  superior;  Tcmp.i,  teinporalis  inferior;  Ext,  external  perpendicular  fissure 
of  Bischoff.  After  D.  J.  Cunningham. 

orang.  We  may  assume,  therefore,  that  the  interrupted  form  of 
development  of  this  sulcus  holds  good  among  the  anthropoid  apes 
as  well  as  in  man.  With  regard  to  the  lower  apes,  we  have  no  evi- 
dence one  way  or  the  other.  The  development  of  the  fissures  in  the 
brain  of  the  ape  is  still  virtually  unknown ;  and  if  we  examine  the 
bottom  of  the  fissure  of  Rolando  and  the  other  primary  furrows  in 
a  low  ape,  we  find  a  uniform  depth  throughout,  and  an  absolute 
absence  of  deep  annectant  gyri.  It  is  dangerous  to  argue  from  the 
adult  condition  alone,  but  still  the  appearances  are  such  as  would 
lead  us  to  infer  that  the  continuous  and  not  the  disrupted  form  of 
development  of  the  primary  fissures  holds  good  among  the  lower 
apes.  The  lower  end  of  the  fissure  of  Rolando  is  sometimes  length- 
ened out  by  union  with  a  small  accessory  fissure  (fissure  of  Ober- 


700 


THE   FCETUS. 


staller,  "  Das  Stirnhirn,"  1890)  so  as  to  be  prolonged  to  the  fissure  of 
Sylvius.  The  inferior  genu  of  the  fissure  of  Rolando  appears  usu- 
ally about  the  seventh  month  and  always  before  the  superior  genu, 
in  the  lower  piece  of  the  fissure ;  the  superior  genu  is  developed  at  the 
junction  of  the  upper  and  lower  piece.  From  the  seventh  month 
onward  the  convolution  (posterior  central),  behind  the  fissure  grows 
more  rapidly  than  the  convolution  (ascending  frontal)  in  front  of  it. 

The  fissure  of  Rolando  was  first  so  named  by  Leuret  in  1839  ("  Anat. 
comp.  du  Systeme  nerveux  ") ;  in  Germany  it  is  usually  termed  the 
central  fissure.  It  is  the  now  accepted  division  between  the  frontal 
and  parietal  lobes.  Next  to  the  central  or  interhemispheral  fissure 
and  the  Sylvian  fissure  it  is  the  most  important  landmark  in  the 
topography  of  the  human  cerebrum.  It  is,  however,  not  a  primary 
or  essential  fissure  throughout  those  mammalia  having  convolutions. 
The  Fissures  of  the  Frontal  Lobe. — The  prce-central  arises  gen- 
erally toward  the  end  of  the  sixth  month,  and,  therefore,  some  time 

after  the  fissure  of  Rolando,  but  not 
invariably,  for  it  has  been  observed 
to  precede  the  fissure  of  Rolando,  see 
D.  J.  Cunningham,  90.1,  PL  I.,  Fig. 
1 ;  it  can  be  identified  by  its  position, 
it  lying  in  front  of  the  parietal  bone, 
which  covers  the  fissure  of  Rolando. 
It  runs  nearly  parallel  with  the  fis- 
sure of  Rolando,  and  arises  from  two 
pieces  (Cunningham,  /.c.,  p.  7,  com- 
pare Fig.  403,  Prc.i,  and  Prc.s), 
which  usually  remain  distinct  but 
are  sometimes  united.  The  develop- 
ment of  the  superior  and  inferior 
frontal  fissures  is  obscure.  If  the 
brain  be  viewed  from  below,  Fig. 
404,  the  lower  surface  of  the  frontal 
lobe  offers  at  five  to  six  months  three 
depressions,  which  I  have  found  to 
be  remarkably  constant.  One  of  these 
is  the  sulcus  rectus  or  olfactorius, 
in  which  the  olfactory  bulbus,  O/,  is 
lodged.  The  other  two  are  small; 
they  unite  later  with  one  another,  and  forming  branches  give  origin 
to  the  inappropriately  named  tri-radiate  fissure.  I  have  to  add 
here  what  may  be  called  a  new  fissure,  which  appears  not  to  have 
been  hitherto  generally  recognized,  although  so  far  as  my  uncompleted 
observations  go,  it  seems  very  constant  both  in  embryos  and  adults  ;* 
I  name  it  the  internal  frontal  fissure;  it  is  situated  on  the  medial 
wall  of  the  frontal  lobe.  Fig.  402,  /,  and  runs  approximately  parallel 
with  the  calloso-marginal  fissure;  it  divides  the  marginal  or  so- 
called  first  frontal  convolution  into  two  parts ;  if  the  conclusion  that 
the  internal  frontal  fissure  is  primary  and  constant  be  verified,  it  will 
be  necessary  to  subdivide  the  marginal  convolution  as  now  defined. 

*The  fissure  is  perfectly  shown  in  Dalton's  "Topographical  Anatomy  of  the  Brain,"  vol.   i., 
pis.  vi.  and  vii.,  and  in  several  well-known  text-books  it  is  clearly  figured. 


FIG.  404.— Underside  of  the  Brain  of  a 
Human  Embryo  of  the  fifth  Month.  CM, 
Cerebellum;  Me?,  medulla  oblongata;  in/, 
infundibulum ;  Op,  optic  nerve;  Ol,  olfac- 
tory lobe.  Natural  size.  (Compare  Fig. 
402.) 


THE    NERVOUS    SYSTEM.  701 

The  Fissures  of  tJte  Parietal  Lobe. — The  intra-parietal  fissure 
arises  (Mihalkovics,  77.1,  154)  as  two  limbs  during  the  sixth 
month;  one  limb  is  parallel  with  the  fissure  of  Rolando,  and  not  far 
behind  it ;  the  other  limb  has  a  more  longitudinal  course  and  lies 
not  far  from  the  median  plane ;  during  the  eighth  month  the  two 
limbs  unite.  During  the  seventh  month,  according  to  Mihalkovics, 
/.r.,  a  retro-central  fissure  appears  between  the  ascending  limbs  of 
the  intraparietal  ami  the  fissure  of  Rolando.  Along  the  median 
wall  of  the  parietal  lobe  extends  only  the  calloso-marginal  fissure, 
Fig.  40 '2,  e,  and  its  branches,  c. 

Thf  AY.S.S///V.S-  af  fhc  T<'in/Htr<i!  Lnhe. — During  the  sixth  month 
there  appears  the  siiftcrinr  temporal  fissure  (m  the  external  surface 
of  the  lobe  and  parallel  with  the  adjacent  margin  of  the  great  Syl- 
vian  fissure,  compare  Fig.  403,  Temp.  s.  Usually  somewhat  later 
appears  another  fissure,  the  inferior  tciujxtrftl,  immediately  below 
and  parallel  with  the  last  mentioned;  the  second  fissure,  Fig.  -in:}. 
Temp,  i.,  is  often  discontinuous.  On  the  lower  surface  of  the  lobe  is 
developed  during  the  sixth  month  also  the  great  OCcipito-temporal 
fissure,  thefissura  collaterally  of  Huxley;  this  fissure  varies  greatly 
in  length;  it  normally  extends  far  into  the  occipital  lobe,  hence  its 
name,  and  sometimes  runs  so  far  forward  as  to  border  the  gyrus 
hippocampi.  The  collateral  fissure  is  very  deep,  and  there  is  a  pro- 
jection on  the  inner  side  of  the  brain  corresponding  to  it,  and  which 
is  known  as  the  eminentia  collateral! s  of  Meckel ;  this  fissure  ought, 
perhaps,  to  be  classed  with  the  primary  folds,  p.  695.  The  collateral 
fissure,  according  to  D.  J.  Cunningham,  91.2,  344,  is  continued  for- 
ward in  the  middle  foetal  life  by  the  incisura  temporalis  and  the 
limiting  fissure  of  the  insula  Reilii ;  these  three  grooves  may  be  taken 
as  making  the  limits  of  the  temporal  lobe,  but  in  later  stages  tho 
originally  evident  relations  of  the  three  grooves  to  one  another  be- 
come obscured.  During  the  ninth  month  of  foetal  life  an  accessory 
transverse  fissure  on  the  under  side  of  the  temporal  lobe  unites  with 
the  limiting  fissure  of  the  insula,  and  therefore  in  the  adult  the  fissure 
appears  to  have  changed  its  primitive  course. 

The  Fissures  of  the  Occipital  Lobe. — This  lobe  has  three  surfaces, 
an  inner  or  medial,  an  external,  and  a  lower  or  cerebellar.  On  the 
medial  surface  the  lobe  is  bounded  anteriorly  by  the  parieto-occipital 
fissure,  Fig.  402,  6,  and  shows  the  calcarine  fissure,  a,  the  origin  of 
both  which  is  described  p.  697.  The  area  between  these  two  fissures 
is  the  cuneate  lobule  (Z wicket).  On  the  external  surface  the  first 
fissure  to  appear  is  a  small,  short  one,  Fig.  403,  Ext,  the  ascending 
perpendicular  of  Bischon°,68. 1,  447 ;  the  horizontal  limb  of  the  intra- 
parietal  fissure  extends  on  to  the  occipital  lobe,  and  probably  joins 
the  fissure  of  Bischoff;  the  prolonged  intraparietal  is  known  as  the 
superior  occipital  fissure.  Later  (eighth  month)  arises  lower  the 
longitudinal  inferior  occipital  (sulcus  sagittalis).  On  the  lower 
surface  during  the  sixth  month  appears  the  great  occipital  temporal 
fissure,  which,  as  stated  above,  also  belongs  to  the  temporal  lobe. 
The  data  of  this  paragraph  are  chiefly  from  Mihalkovics,  77. 1,  155. 

The  Fissures  of  the  Island  of  Reil. — The  best  account  of  the 
adult  fissures  of  the  insula  is  probably  that  of  Oberstaller  (Anat. 
Anzeiger,  1887,  p.  739).  He  finds  four  vertical  fissures:  the  first 


702  THE   FCETUS. 

and  second  prsecentral,  the  central,  and  the  post-central,  as  they  may 
be  called;  as  the  second  prse-central  is  small  and  insignificant,  it 
may  be  regarded  as  accessory.  Their  development  has  been  studied 
by  D.  J.  Cunningham,  91.2.  In  the  latter  half  of  the  fifth  month 
the  central  fissure  (sulcus  centralis  insulce)  becomes  evident  as  a 
faint  linear  furrow  which  runs  upward  and  backward  from  the 
lower  part  of  the  Sylvian  fossa ;  from  the  very  first  it  lies  accurately 
in  the  line  of  the  fissure  of  Rolando,  and  it  appears  at  the  same  date ; 
it  is  situated  much  nearer  the  hinder  end  of  the  insula  than  at  later 
stages,  owing  to  the  growth  of  the  posterior  part  of  the  insula.  The 
first  prce-central  fissure  is  developed  a  little  later,  but  as  a  general 
rule  before  the  end  of  the  fifth  month,  and  lies  accurately  in  line 
with  the  sulcus  prse-centralis  inferior  of  the  frontal  lobe ;  during  the 
last  month  of  foetal  life  its  upper  end  generally  moves  forward  to  a 
slight  extent  so  that  its  relation  to  the  frontal  prse-central  is  marked ; 
it  is  remarkable  that  for  a  certain  period  the  prse-central  fissure  is 
better  marked  than  the  central,  but  during  the  eighth  month  it  loses 
this  pre-eminence.  Guldberg  (Anat.  Anzeiger,  Oct.,  1887)  mistook 
the  prse-central  for  the  central  fissure.  The  post-central  fissure  is 
much  later  in  making  its  appearance.  As  a  rule,  it  does  not  show 
until  the  middle  of  the  sixth  month  or  even  later ;  its  development 
coincides  with  that  of  the  intraparietal  fissure,  the  line  of  which  it 
prolongs. 

The  remarkable  coincidence  of  three  main  fissures  of  the  island  of 
Reil  with  the  lines  prolonging  respectively  the  prse-central  inferior, 
the  fissure  of  Rolando,  and  the  intraparietal  necessarily  suggests 
that  the  insular  fissures  are  parts  of  the  same  fissures  as  those  of  the 
mantle  enumerated. 

THE  ACCESSORY  FISSURES. — Beside  the  main  fissures  the  human 
brain  has  a  large  number  of  short  fissures  of  an  irregular  and  vari- 
able character,  and  which  modify  and  mask  the  primary  fissures  to  a 
variable  extent.  These  accessory  fissures  appear  during  the  last 
month  of  fcetal  life,  for  the  most  part  as  branches  of  earlier  fissures, 
but  in  small  part  as  independent  grooves.  Whether  or  not  other 
fissures  are  developed  after  birth  I  do  not  know.  The  laws  govern- 
ing the  appearance  of  the  accessory  sulci  have  not  yet  been  ascer- 
tained. 

3.  TRANSITORY  FISSURES. — The  question  is  still  under  debate  as 
to  whether  there  are  in  early  stages  of  the  foetus  temporary  folds  or 
not.     Bischoff,  His,  and  others,  with  whom  I  am  strongly  inclined 
to  agree,  consider  the  irregular  folds,  which  are  often  to  be  observed 
on  the  surface  of  the  cerebrum  from  the  first  to  the  fourth  month,  as 
artificial  and   accidental.      On  the  other  hand,    Kolliker,    Ecker, 
Mihalkovics,   77.1,   144,  and  others,  consider  that  the  folds  are 
normally  present. 

4.  EVOLUTION  OF   THE  FISSURES. — It  is  well  known  that  there 
are  several  types  of  convolutions,  and  that  different  fissures  are 
typical  of  different  orders  of  mammalia.     It  is  probable  that  all  the 
fissures  (i.  e.,  secondary  furrows)  of  the  human  brain  were  evolved 
within  the  series  of  primates,  and  it  is  doubtful  whether  they  are 
any  of  them  homologous  with  the  fissures  in  other  mammalian  orders ; 
compare  Sir  Win.  Turner,  90.2. 


THE   NERVOUS   SYSTEM.  703 

5.  HISTORICAL  NOTE. — Our  knowledge  of  the  foetal  fissures  and 
convolutions  was  very  slight  until  Reichert  (*'  Der  Bau  des  mensch- 
lichen  Gehirns,"  1859,  76-90).  More  thorough  were  the  valuable 
memoirs  of  Bischoff,  68.1,  arid  Ecker,  83.1.  The  mechanical  fac- 
tors concerned  in  the  production  of  the  convolutions  have  been  dis- 
cussed by  His,  74. 1,  1 10-117.  D.  J.  Cunningham  has  made,  90. 1, 
91.2,  important  additions  to  our  knowledge  of  the  development. 
Of  the  anatomical  papers  on  the  convolutions  man}'  are  of  morpho- 
logical value;  among  them  Sir  Wm.  Turner's  address,  90.2,  is  of 
the  first  value  to  the  embryologist.  Of  the  knowledge  of  the  subject 
up  to  1877,  Mihalkovics  gives  an  admirable  summar3T,  77.1,  upon 
which  I  have  drawn  freely. 

Olfactory  Lobes  (Riechlappen).—The  following  account  is 
based  upon  the  researches  of  His,  89.4.  The  olfactory  lobe  arises 
by  differentiation  of  an  area  of  the  wall  of  the  primitive  hemisphere. 
The  differentiation  begins  in  the  human  embryo  during  the  fourth 
week  as  the  hemispheres  begin  to  enlarge,  and  affects  the  area  adjoin- 
ing the  median  lamina  terminalis,  compare  Fig.  339,  Ol.  The  olfac- 
tory area,  as  it  may  be  called,  expands  with  the  hemispheres,  and 
thus  soon  extends  well  forward  in  front  of  the  lamina  terminalis ;  it 
then  constitutes  a  slight  longitudinal  ridge  with  a  corresponding 
internal  groove,  along  tha  under  side  of  the  cerebral  hemisphere, 
Fig.  341.  The  area  now  appears  as  a  fold  of  the  hemispheral  wall. 
There  now  develops  the  primary  groove  (primare  Bogenfurche)  p. 
< -'i'*,  and  this  extends  not  only  in  an  arch  along  the  medial  surface 
of  the  hemispheral  wall  but  also  curves  on  to  the  olfactory  ridge,  and 
by  crossing  it  transversely  divides  the  ridge  into  an  anterior  and  a 
posterior  segment,  Fig.  399.  The  ridge  next  separates  from  the 
hemisphere,  so  as  to  be  converted  into  a  blind  tubular  diverticulum, 
which  remains  connected  at  its  posterior  end  with  the  hemisphere, 
and  we  now  have  an  olfactory  lobe,  which  has  a  central  cavity  in 
direct  communication  with  the  lateral  ventricle ;  the  lobe  has  two 
segments,  one  posterior  connected  with  the  brain,  the  other  anterior 
and  comprising  a  narrower  part  or  stalk,  and  an  enlarged  end ;  the 
stalk  is  the  anlage  of  the  tractus  olfactorius  and  triqonum;  the 
enlarged  end  is  the  anlage  of  the  bulbus  olfactorius.  The  posterior 
segment  becomes  the  posterior  olfactory  lobe,  a  part  of  the  brain 
which  has  been  long  imperfectly  recognized ;  it  comprises  the  pe- 
(Jmiculus  corporis  callosi  (or  gyrus  subcallosus  of  Zuckerkandl) , 
the  outer  and  inner  roots  of  the  olfactory  nerve,  and  the  substantia 
perforata  anterior. 

The  olfactory  ganglion  of  the  embryo  unites  with  the  bulbus 
olfactorius.  The  union  takes  place  during  the  latter  part  of  the  fifth 
week,  Fig.  405.  The  origin  of  the  olfactory  ganglion  is  described 
p.  037.  It  grows  upward,  and  as  during  the  fifth  week  the  end  or 
bulbus  of  the  olfactory  lobe  bends  toward  the  median  line,  Fig.  405, 
the  ganglion  lies  close  behind  the  bulbus,  in  the  groove  which  may 
be  regarded  as  the  prolongation  above  described  of  the  Bogenfurche 
or  primary  fissure  across  the  lobe.  The  ganglion  now  spreads  around 
the  bulbus,  and  unites  with  it,  as  may  be  seen  on  the  right  side  in 
Fig.  405,  and  forms  a  superficial  layer  over  the  surface  of  the  bulbus, 
which  thus  has  three  layers — the  outer  ganglionic  layer,  the  neuro- 


THE   FCETUS. 


t.c 


FIG.  405.— Section  of  the  Fore-brain 
of  a  Human  Embryo  of  nearly  five 
Weeks  (His1  Sch).  Ol,  anterior;  Ol', 
posterior  division  of  olfactory  lobe; 
Gl,  olfactory  ganglion  ;  c.s,  corpus 
striatum;  p.  s,  pars  sub-thalamica;  f.c, 
tuber  cinereum.  After  W.  His. 


glia  layer  corresponding  to  the  Randschleier,  and  the  inner  nucleated 
layer  corresponding  to  the  inner  layer  and  mantle  layer  of  the  spinal 
cord.  The  transformation  of  these  into  the  adult  layers  has  still  to 
be  worked  out.  Owing  to  the  fusion  of  the  ganglion  with  the  bulbus 
there  can  be  no  nerve -trunk  running  from  the  ganglion  to  the  brain; 

the  centrifugal  fibres  from  the  ganglio- 
nic  layer  run  off  in  bundles,  which  form 
a  plexus-like  network  on  their  way  to 
be  distributed  to  the  olfactory  epithe- 
lium covering  the  upper  turbinal  fold 
(obere  Muschel).  The  fusion  of  the 
ganglion  with  the  bulbus  explains  why 
the  olfactory  fibres  appear  in  the  adult 
to  arise  from  the  wall  of  the  lobe,  al- 
though not  medullary  nerve-fibres. 

During  the  second  month  the  hemi- 
spheres expand  so  rapidly  that  they 
carry  the  base  or  posterior  part  of  the 
olfactory  lobe  forward,  while  the  bulbus 
remains  attached  to  the  ganglion  so  that 
the  bulbus  at  the  end  of  the  second 
month  is  bent  back  and  lies  under  the 
posterior  segment,  but  during  the  third 
month  the  bulbus  bends  forward  again 
and  assumes  its  permanent  position. 
During  the  third  month  also,  the  anterior  half  of  the  lobe  lengthens 
out  and  becomes  clearly  differentiated  into  bulbus,  tractus,  and  trigo- 
num.  The  cavity  of  the  olfactory  lobe  becomes  obliterated  in  great 
part  before  adult  life,  but  exactly  how  or  when  is  not  known. 

Evolution  of  the  Head. — We  are  now  in  a  position  to  review 
briefly  the  factors  which  have  determined  the  differentiation  of  the 
head.  The  conception  that  the  head  is  composed  of  a  number  of 
segments  has  now  been  current  for  nearly  a  century.  For  a  long 
time  the  attempts  to  determine  the  number  of  cephalic  segments 
were  confined  to  the  study  of  the  skull,  following  Oken's  idea  that 
the  skull  is  composed  of  a  number  of  vertebra.  We  have  already 
seen,  p.  469,  that  all  such  attempts  were  necessarily  fruitless.  A 
great  advance  was  made  when  Gegeiibaur,  in  1872,  sought  to  deter- 
mine the  segmental  value  of  the  cranial  nerves,  compare  p.  469,  but 
the  correct  and  only  way  was  pointed  out  by  Balfour,  who  sought  to 
determine  the  number  of  actual  segments  in  the  embryonic  head, 
compare  p.  199.  Van  Wijhe  found  of  the  true  myotomes  at  least  nine 
in  shark  embryos,  and  Dohrn  has  found  in  a  very  young  stage  of 
the  torpedo  about  twice  that  number,  compare  p.  200  and  Fig. 
118.  It  has  thus  been  proved  that  the  head  is  a  segmented  region 
in  which  the  majority  of  the  segments  abort  very  early  in  the  embryo. 
The  next  step  must  be  to  ascertain  what  causes  have  resulted  in, 
and  what  effects  have  resulted  from,  the  disappearance  of  myotomes 
in  the  head.  The  first  thing  to  indicate  the  formation  of  the  head 
is,  in  the  embryo  of  all  classes  of  vertebrates,  the  dilatation  of  the 
medullary  tube  to  form  this  brain,  a  dilatation  which  crowds  the 
mesoderm  down  on  the  ventral  side  of  the  neural  tube.  I  think  also 


THE    NERVOUS    SYSTEM.  705 

that  the  enlargement  of  the  brain  is  the  direct  cause  of  the  formation 
of  the  head-bend,  and  that  probably  the  proamniotic  area  has  the  role 
t<  >  play  of  preventing  the  directly  forward  growth  of  brain,  because 
there  being  no  mesoderm  in  the  proamnion  the  entoderm  and  ecto- 
derm are  united  and  the  head  cannot  develop  across  the  area,  and 
consequent!}'  bends  to  allow  the  elongation  of  the  cerebral  vesicles. 
The  head-bend  still  further  crowds  the  myotomes,  compare  Fig.  118, 
and  it  is  to  this  crowding  that  the  abortion  of  the  myotomes  is  to  be 
attributed,  according  to  my  hypothesis. 

The  effects  of  the  abortion  of  the  cephalic  segments  have  been  to 
prevent  the  development  of  the  primordial  skeleton  into  separate 
vertebral  masses,  and  to  prevent  the  development  of  the  cranial 
nerves  on  the  type  of  those  of  the  spinal  cord.  It  must,  however,  be 
admitted  that  the  correlation  between  the  arrangement  of  the  nerves 
and  the  development  or  abortion  of  segments  is  very  obscure. 

Another  factor  in  the  evolution  of  the  head,  which  enters  into 
action  much  later,  is  the  development  of  gill  pouches  with  their  re- 
sultant modifications  of  the  gill-arches,  formation  of  the  branchial 
skeleton,  etc.  Noteworthy  is  the  fact  that  the  coelom  of  each  arch 
connects  a  myotome  with  the  splanchnocoele  (pericardial  cavity)  and 
is  apparently  homologous  with  the  nephrotome  of  a  rump  segment. 
If  this  homology  is  correct  we  must  describe  it  as  a  further  peculiar- 
ity of  the  head  that  its  nephrotomes  give  rise,  not  to  excretory  tubules, 
but  to  branchial  striated  muscles,  see  p.  478. 

A  third  factor  which  comes  into  play  still  later  is  the  annexation 
to  the  occipital  region  of  at  least  four  true  cervical  (hypoglossal) 
segments  with  their  vertebrae  and  nerves,  compare  p.  429  and  665. 

The  skull  plays  a  subsidiary  part  and  is  an  accessory  structure 
added  after  all  the  essential  morphological  characteristics  of  the  head 
are  present.  The  erroneous  notion  that  the  skeleton  is  the  frame- 
work upon  which  the  body  is  built  has  been  discarded  by  embryol- 
ogy. That  the  organs  of  special  sense  have  had  a  profound  influence 
on  the  head  during  its  evolution  cannot  be  doubted,  but,  while  we 
put  down  the  possession  of  the  olfactory,  visual,  and  auditory  organs 
as  essential  characteristics  of  the  head,  we  cannot  say,  so  far  as  we 
can  recognize  at  present,  that  they  have  influenced  the  constitution 
of  the  head  nearly  as  much  as  the  other  factors. 

We  must  for  the  present  define  the  head  as  the  anterior  region  of 
the  body,  in  which  the  medullary  tube  is  enlarged,  the  segments 
consequently  aborted,  and  the  skeleton  therefore  not  divided  into 
vertebrae,  nor  the  nerves  with  dorsal  and  ventral  roots  united; 
which  possesses  the  three  organs  of  special  sense;  in  which  the  gill- 
clefts  are  developed ;  and  which  has  increased  its  original  territory 
by  the  annexation  (at  least  in  amniota)  of  several  cervical  segments. 
45 


CHAPTER    XXVIII. 

THE  SENSE-ORGANS. 

IN  this  chapter  we  have  to  consider  a  large  number  of  structures. 
Concerning  the  lower  sense-organs,  touch,  taste,  etc.,  we  know 
almost  nothing;  concerning  the  olfactory  organ  a  little,  concerning 
the  eye  and  ear  a  good  deal — on  the  embryological  side,  of  course. 
We  have  further  to  emphasize  those  traces  which  have  been  discov- 
ered of  long  series  of  sense-organs,  of  which  the  nose,  eye,  and  ear 
are  probably  derivatives,  in  the  ancestors  of  vertebrates,  although 
in  all  known  vertebrates  most  of  these  series  have  become  rudimen- 
tary or  lost.  The  serial  sense-organs  I  designate  under  the  compre- 
hensive name  of  ganglionic  sense-organs.  There  are  probably  two, 
and  only  two,  series  along  each  side  of  the  body :  one  series,  the 
upper,  corresponds  to  the  lateral  line  of  comparative  anatomy,  the 
other  to  the  epibranchial  line.  The  olfactory,  visual,  and  auditory 
organs  are  probably  specialized  ganglionic  sense-organs.  The  organs 
of  touch,  taste,  etc.,  have  not  yet  been  shown  to  have  any  genetic 
relationship  to  the  ganglionic  sense-organs. 

Ganglionic  Sense-Organs.* — By  this  term  I  propose  to  desig- 
nate the  series  of  organs  formed  by  the  temporary  or  permanent  union 
of  the  sensory  ganglia,  described  in  the  previous  chapter,  with  the 
epidermis.  The  discovery  that  such  a  class  of  organs  exists,  and  that 
the  ear  probably,  the  eye  and  nose  possibly,  belong  to  the  class,  was 
due  to  Froriep,  85.1,  whose  article  marks  an  important  step  in  ver- 
tebrate morphology.  The  temporary  connection  of  certain  ganglia 
with  the  epidermis  was,  so  far  as  I  know,  first  discovered  by  J.  W. 
Van  Wijhe,  82. 1,  in  elasmobranch  embryos,  and  has  been  especially 
studied  by  Beard,  85.1,  whose  researches  have  proved  valuable 
and  suggestive,  though  his  publications  are  marred  by  premature 
and  too  diagrammatic  generalizations.  Beard  proposes  the  name  of 
branchial  sense-organs,  but  the  term  most  generally  used  is  seg- 
mental  sense-organs,  because  the  organs  are  believed  to  be  repeated 
in  each  segment.  The  term  adopted  here,  "ganglionic,"  is  purely 
descriptive,  and  involves  no  theory  as  do  the  two  others  just  men- 
tioned, and  moreover  serves  to  indicate  also  the  distinction  between 
the  two  main  classes  of  sense-organs. 

As  an  example  of  a  typical  ganglionic  sense-organ  of  an  embryo 
we  may  take  the  front  ganglion  of  a  young  elasmobranch.  This 
ganglion  belongs  to  the  third  cranial  or  oculo-motor  nerve.  Accord- 
ing to  Beard  it  grows  out  from  the  neural  crest  of  the  mid-brain 
shortly  before  the  closure  of  the  medullary  groove  at  that  point.  It 
soon  comes  in  contact  with  the  epidermis,  which  thickens  where  the 

*  Additional  details  are  given  in  connection  with  the  history  of  the  cephalic  nerves  in  the 
preceding  chapter,  p.  633. 


THE   SENSE-ORGANS. 


707 


01 


ganglion  touches  it.  The  thickened  epidermis  and  the  ganglion  then 
fuse,  Fig.  400,  the  former  becoming  depressed  so  as  to  make  a  shal- 
low pit.  The  boundary  between  the  two  tissues  becomes  indistinct. 
According  to  Beard  some  of  the  cells  are  spe- 
cialized later  to  form  what  he  terms  the  supra- 
branchial  nerve,  and  of  this  his  figure,  Fig.  40G, 
indicates  the  commencement,  Sp.n.  Similar 
fusions  have  been  demonstrated  in  the  amniota 
by  Froriep  in  the  case  of  the  ganglia  of  the 
facial,  glosso-pharyngeal,  and  vagus  nerves; 
the  site  of  the  fusion  is  for  each  of  these  ganglia 
directly  above  the  gill-cleft  to  which  the  nerve 
of  the  ganglion  belongs.  The  term  branchial 
sense-organ  has  reference  to  this  position,  wjiich 
is  assumed  by  Beard  to  be  typical  for  all  the 
organs  of  the  class. 

C.  Kupffer,  91.1,  has  shown  that  in  Petro- 
myzon  the  ganglia  form  two  series  of  unions 
with  the  epidermis,  and  maintains  that  at  each 
point  of  union  cells  are  budded  off  from  the  epi- 
dermis and  incorporated  in  the  ganglion  which 
is  so  enlarged.  Fig.  407  illustrates  the  ar- 
rangement of  the  ganglia  as  found  by  him  in 
a  young  Petromyzon  ( Ammoccetes  of  4  mm. ) .  The  five  great  ganglia 
of  the  head  (the  ciliary,  I. ;  trigeminal,  II. ;  acoustico-facial,  III. ; 
glosso-pharyngeal,  IV. ;  and  vagus,  V.)  are  each  connected  with  the 
epidermis  and  receive  cells  from  it ;  the  line  of  the  ganglia  is  pro- 
longed backward  by  the  lateral  line ;  if  the  line  of  the  ganglia  were 
continued  forward  it  would,  allowing  for  the  bend  of  the  head,  ter- 
minate in  the  nasal  pit,  JV,  or  anlage  of  the  olfactory  organ ;  the  ear 
(otocyst)  lies  directly  along  the  line  of  the  ganglia,  and  represents, 


J.at 


FIG.  406.— Horizontal  Sec- 
tion of  Oculomotor  Ganglion 
of  a  Young  Torpedo  Em- 
bryo. Ep,  Epidermis;  Sp.n, 
supra-branchial  nerve  an- 
lage, according  to  Beard; 
67,  ganglion.  Highly  mag- 
nified. After  John  Beard. 


FIG.  407. —Reconstruction  to  show  the  Cephalic  Ganglia  of  a  Petromyzon  Larva,  4mm.  long. 
_La£,  Lateral  nerve;  I-V,  cephalic  ganglia;  L.  lens  of  eye;  H,  fore-brain;  N,  nasal  pit;  F,  fold 
between  the  hypophysis  and  mouth;  M.  mouth  cavity;  1,  4,  6,  7.  12,  epibranchial  ganglia,  seri- 
ally numbered:  Or,  otocyst;  KG,  K7,  gill  pouches,  serially  numbered;  nch,  notochord.  After  C. 
Kupffer. 

as  has  been  long  known,  an  epidermal  area  in  contact  with  the 
acoustic  ganglion;  the  olfactory  area,  as  we  have  seen,  p.  637,  also 
throws  off  cells  and  produces  a  ganglion,  which  we  may,  hypothet- 
ically  at  least,  add  to  the  chain  of  ganglia  I.-V.  We  have  then  the 


708  THE   FOETUS. 

lateral  line  prolonged  forward  by  the  five  primary  cephalic  ganglia, 
and  by  the  olfactory  ganglion  the  entire  length  of  the  head.  It  is, 
I  think,  not  carrying  speculation  too  far  to  suggest  that  the  retina 
of  the  eye  represents  another  ganglionic  area,  which  has  been  dis- 
placed by  being  involved  in  the  invagination  to  form  the  fore-brain ; 
it  is  perhaps  not  superfluous  to  add  that  the  acceptance  of  this  specu- 
lation encounters  difficulties  which  cannot,  at  present,  be  removed. 
Each  pf  the  primary  ganglia,  Fig.  407,  I.-V.,  is  prolonged  down- 
ward and  joins  a  chain  of  epibranchial  ganglia,  which  are  connected 
together  longitudinally  with  one  another  and  are  much  smaller  than 
the  main  or  lateral  ganglia ;  every  epibranchial  ganglion,  also,  is  con- 
nected with  the  epidermis  and  receives  cells  from  it;  the  epibranchial 
chain  begins  immediately  behind  the  lens  of  the  eye  and  is  continued 
far  backward  above  the  mouth  and  then  above  the  gill-clefts.  Kup- 
ffer  figures  twelve  epibranchial  ganglion ;  the  first  is  immediately 
behind  the  lens  of  the  eye  and  is  united  with  the  ciliary  ganglion,  I. ; 
the  second,  third,  and  fourth  overlie  the  mouth,  the  third  being  con- 
nected with  the  trigeminal  ganglion,  II. ;  the  fifth  to  twelfth  ganglia 
lie  each  above  a  gill-cleft,  there  being  eight  gill-clefts  present  at  this 
stage ;  the  fourth  and  fifth  appear  to  be  connected  with  the  acoustico- 
f acial  lateral  ganglion,  III. ,  the  sixth  with  the  glosso-pharyngeal,  IV. , 
the  seventh  with  the  vagus  ganglion,  V. 

As  Kupffer  points  out,  Z.c.,  49,  we  have  to  do  in  Petromyzon  with 
very  primitive  conditions,  which  must  contribute  much  toward  the 
comprehension  of  the  morphology  of  the  ganglia  and  sense-organs 
of  the  higher  vertebrates.  There  can  be  little  doubt  that  the  lateral 
ganglia  and  sense-organs  as  one  series,  and  the  epibranchial  ganglia 
and  sense-organs  as  another  series,  are  common  to  all  vertebrates. 
As  already  stated,  it  seems  certain  that  the  ear,  probable  that  the 
olfactory  organ,  and  possible  that  the  eye  all  belong  to  the  lateral 
series,  and  there  can  be  little  doubt  that  the  organs  discovered  by 
Froriep,  and  now  generally  known  as  branchial  sense-organs,  are 
members  of  the  epibranchial  series.  I  deem  it  extremely  probable 
that  further  investigation  will  demonstrate  the  existence  of  both 
series  in  the  embryos  of  all  vertebrates. 

There  is  little  in  the  embryonic  organs  described,  beyond  the  union 
of  nervous  and  epidermal  tissue,  to  suggest  comparison  with  a  histo- 
logically  specialized  sensory  apparatus ;  nevertheless  we  may  safely 
interpret  both  the  lateral  and  epibranchial  structures  as  rudimentary 
sense-organs,  because  in  the  case  of  the  ear  and  nose,  as  described 
below,  such  a  union  constitutes  an  essential  stage  in  the  development, 
and  because  the  fact  that  the  organs  of  other  ganglia  abort  during 
embryonic  life  accounts  for  the  lack  of  the  histological  differentia- 
tion. The  number  and  fate  of  the  rudimentary  ganglionic  sense- 
organs  has  been  discussed  in  connection  with  the  history  of  the  sep- 
arate cranial  nerves.  The  problem  of  the  homologies  of  the  organs 
with  sense-organs  of  invertebrates  is  still  too  obscure  to  be  profitably 
discussed  here. 

Very  suggestive  in  this  connection  are  the  observations  of  H.  V. 
Wilson,  91.1,  244-253,  of  a  thickening  of  the  nervous  layer  of  the 
epidermis  on  either  side  of  the  head  in  the  bass  embryo  (Serranus 
atrarius) .  This  thickening  forms  a  long,  shallow  furrow,  which 


THE    SENSE-ORGAXS.  709 

subsequently  divides  into  three  parts,  of  which  the  first  becomes  a 
sense-organ  over  the  gill-cleft,  the  second  the  auditory  invagination, 
and  the  third,  the  anlage  of  the  sense-organs  of  the  lateral  line. 
This  peculiar  development  confirms  the  notion  that  all  these  organs 
belong  in  one  series,  but  the  appearance  of  a  continuous  thickening 
as  the  anlage  of  them  all  has,  as  yet,  been  observed  only  in  this  fish, 
and  may  not  indicate  a  corresponding  ancestral  condition.  Unfor- 
tunately Wilson  was  unable  to  make  out  anything  as  to  the  connec- 
tion of  the  sensory  plate  with  the  ganglia.  The  sense-organ  above 
the  gill-cleft,  though  differentiated,  is  a  larval  structure  only,  and 
disappears  in  the  adult. 

EVOLUTION  OF  THE  GANGLIONIC  SENSE-ORGANS. — Lenhossek, 
92.1,  has  shown  that  in  the  earth-worm  there  are  cells  scattered 
through  the  epidermis  which  give  off  fibres  which  run  to  the 
central  nervous  system,  and  there  like  vertebrate  sensory  fibres 
fork ;  one  fork  runs  head  ward,  the  other  tailward  within  the  cen- 
tral ganglionic  chain.  This  important  discovery  renders  it  prob- 
able that  sensory  ganglion  cells  and  sensory  cells  were  originally 
one,  and  that  the  ganglion  cells  of  vertebrates  are  nerve-sense  cells, 
which  have  migrated  from  the  epidermis.  The  ganglionic  sense- 
organs  in  this  way  are  traced  to  a  genetic  condition  arrested,  for  we 
may  assume  that  they  correspond  to  areas  in  which  the  nerve-sense 
cells  are  congregated,  and  that  part  of  the  cells  remain  in  the  epi- 
dermis, while  others  migrate  from  it  to  constitute  the  ganglion. 
Lenhossek 's  discovery  leads  him  to  the  further  hypothesis  that  the 
special  sense-cells  connected  with  a  nerve-fibre,  such  as  occur  in 
taste-bulbs,  the  olfactory  membrane,  and  the  organ  of  Corti,  are 
really  comparable  to  the  nerve-sense  cells  of  Lumbricus,  and  are 
true  neuroblasts  in  that  they  produce  the  nerve-fibres  connected  with 
them ;  hitherto  we  have  assumed  that  the  nerve-fibre  grew  to  the 
cell.  It  seems  to  me  that  Lenhossek's  hypothesis  is  likely  to  be  ver- 
ified with  revolutionary  results  for  our  conceptions  of  the  morphol- 
ogy of  the  nervous  system  and  sense-organs. 

The  Special  Sense-Cells. — I  wish  to  point  out  that  there  is  a 
remarkable  uniformity  in  the  specialization  of  the  sense-cells  in  the 
organs  of  taste,  smell,  sight,  and  hearing,  which  at  once  suggests 
that  they  are  all  derived  from  a  common  form.  The  cells  are  elon- 
gated and  have,  1,  a  lower  tapering  infra-nuclear  member,  which  is 
a  portion  of  the  protoplasmatic  body  of  the  cell,  and  is,  probably, 
always  connected  with  a  nerve-fibril;  2,  an  upper  supra-nuclear 
member,  which  is  also  part  of  the  protoplasmatic  cell  body  and 
stretches  to  the  surface  of  the  epithelial  layer  in  which  the  special 
sense-cells  are  situated ;  3,  a  projection  above  the  surface  of  the  epi- 
thelium ;  the  projection  is  different  in  character  from  the  protoplasm; 
it  differs  also  according  to  the  organ ;  the  projection  is  called  a  hair 
or  cilium  in  the  case  of  the  organs  of  taste,  smell,  and  hearing,  a  rod 
or  cone  in  the  case  of  the  eye. 

The  obvious  similarity  of  the  special  sense-cells  confirms,  I  believe, 
the  theory  that  the  special  sense-organs  are  modifications  of  ganglionic 
sense-organs,  which  in  the  ancestors  of  vertebrates  were  all  similar 
and  perhaps  served  a  general] zed  sensory  function .  Perhaps  the  sense- 
cell  s  are  also  nerve-sense  cells,  as  suggested  by  Lenhossek  (above). 


710  THE    FGETUS. 

Organs  of  Touch  and  Taste. — I  have  been  unable  to  find  a 
single  word  as  to  the  development  of  any  tactile  organs  by  any  writer. 

ORGANS  OF  TASTE. — The  development  of  the  organs  of  taste  has 
been  studied  by  A.  Lustig,  84.1,  Fr.  Herrmann,  85.1,  and  Fr. 
Tuckermann,  89.1,  89.2.  The  development  takes  place  quite  late; 
the  papilla3  (or  folds)  are  produced  first,  the  taste-bulbs  upon  them 
arising  later.  In  the  rabbit  the  formation  of  the  papilla3  begins 
with  the  third  week ;  in  man  Tuckermann  found  on  the  fretal  tongue 
five  circumvallate  papilla3  at  four  months,  six  at  five  and  a  half, 
eight  at  six  and  seven  months ;  at  four  months  the  development  of 
taste-bulbs  had  hardly  begun ;  at  six  months  the  bulbs  are  numerous 
and  the  papilla  have  become  lobate. 

Olfactory  Membrane.— The  development  of  the  nasal  pits  and 
their  enlargement  to  form  the  nasal  cavity  is  described,  p.  575 ;  the 
development  of  the  olfactory  ganglion  and  nerve  from  olfactory  epi- 
thelium is  described,  p.  637.  Concerning  the  further  history  of  the 
olfactory  membrane  and  the  genesis  of  its  sense-cells  nothing  is 
known. 

It  may  be  recalled  that  Blaue,  84.1,  has  recorded  that  in  vari- 
ous fishes  the  olfactory  cells  are  collected  in  groups,  having  a  very 
striking  similarity  to  both  the  taste-bulbs  and  the  sense-organs  of 
the  lateral  line  of  anamniota.  In  mammals  the  olfactory  cells  are 
not  so  grouped,  but  it  is  possible  they  may  be  so  in  the  embryo.  As 
the  organs  of  the  lateral  line  are  ganglionic  sense-organs,  Blaue's 
observations  offer  additional  evidence  for  interpreting  the  organ  of 
smell  as  likewise  a  ganglionic  sense-organ. 

DEVELOPMENT  OF  THE  EYE. 

The  Optic  Vesicles. — The  first  stages  of  the  optic  vesicles  as 
diverticula  of  the  fore-brain  have  been  traced  above,  p.  594.  The 
vesicles  form  the  retina,  the  choroid  coat,  and  the  optic  nerve  of  the 
adult  eye ;  the  differentiation  of  the  anlages  of  these  three  parts  forms 
the  subject  of  this  section. 

The  following  account  of  the  early  changes  in  the  shape  of  the 
optic  vesicles  in  the  human  embryo  is  based  on  His,  89.4,  085, 
who  has  also  traced,  68.1,  104,  132,  and  74.1,  100,  the  corre- 
sponding changes  in  the  chick.  The  vesicles  become  stalked  by  the 
fourth  week;  the  stalk,  Fig.  337,  springs  from  the  lower  edge  of  the 
fore-brain  (thalamencephalon)  just  in  front  of  the  infundibular 
region ;  the  base  is  broad,  but  very  rapidly  tapers  down  to  the  nar- 
row stalk  proper ;  the  end  of  the  vesicle  is  enlarged  and  the  enlarge- 
ment expands  upward  and  backward,  as  in  all  vertebrates.  The 
outer  and  lower  posterior  wall  of  the  vesicle  and  part  of  the  posterior 
wall  of  the  stalk  become  pushed  in,  and  thereby  the  vesicle  is  changed 
into  the  so-called  " optic  cup."  The  invagination  is  probably  due 
in  man,  as  in  the  chick  (Foster  and  Balfour,  "Elements,"  2d  ed.,  p. 
134)  and  in  the  rabbit  (as  I  have  observed),  to  the  contact  of  the  dis- 
tal wall  of  the  optic  vesicle  with  the  overlying  epidermis;  where  the 
contact  occurs  the  wall  of  the  vesicle  and  the  epidermis  becomes 
apparently  closely  united,  as  if  glued  together;  the  union  takes  place 
in  the  chick  the  end  of  the  second  day,  in  the  rabbit  the  end  of  the 


DEVELOPMENT   OF   THE   EYE. 


711 


ninth;    over  the  area  of  the  union  both  layers  become  thickened, 

Fig.  4os;  the  thickened  vesicular  area  is  the  anlage  of  the  retina, 

while  the  epidermal  thickening 

is  the  anlage  of  the  lens  of  the 

eye,   compare  Fig.  409,   .#,  L. 

The  lens  area  very  soon  begins 

to   be   pushed   in,  and   thereby 

the   retinal    anlage    is   carried 

back,  Figs.  401)  and  412,  toward 

tlic  posterior  wall  of  the  optic 

vesicle,  and  at  the   same  time 

the  cavity  of  the  vesicle  is  corre- 

spondingly reduced.     The  optic 

vesicle  now  has  t\vo  parts  dif- 

ferentiated,   the    thickened   in- 

vaginated    retinal  area,    Figs. 

ini»  and  4  1  •>,  /'.  and  the  thinner 

posterior  pigment  layer.     The 

optic   cup  and  lens  both  grow 

very  rapidly,  and  the   differen- 

tiation of  the  retinal  and  pig- 

ment layers  progresses  equally. 

In  the  rabbit  at  thirteen  days, 

Kig.  4nii,  I  find  the  hollow  optic 

nerve,    JV,    running    from    the 

brain,  to  the  eye;  its  walls  are 

continuous,  as  explained  below, 

with  both  the  pigment,  P,  and 

retinal   anlages;    the    pigment 

layer,   P,  is  thin,  and  pigment 

granules  have  begun  to  appear 

in  it;  it  extends  in  the  form  of 

a  wide  beaker  almost  to  theepi- 

TT»^        v  a       t    1 

CiermiS,  JLC,  Where  It  IS  reflected 

toward  the  lens  and  passes  over 
into  the  retinal  anlage,  R,  which 
represents  the  outer  wall  of  the  optic  vesicle.  The  retina,  .R,  has  be- 
come a  thick  wall,  in  which  we  can  distinguish,  as  in  the  wall  of  the 
medullary  tube  proper,  an  inner  wide  zone  with  numerous  nuclei  and 
an  outer  narrow  zone  with  nuclei  ;  the  outer  zone  lies  toward  the  lens  ; 
it  must  be  homologized  with  the  Eandschleier.  The  retina  as  a  whole 
forms  a  wide  cup,  which  is  almost  completely  filled  by  the  large 
lens,  L,  lodged  in  it.  Between  the  lens  and  retina  is  the  anlage  of 
the  future  aqueous  humor;  at  this  stage  the  anlage  is  merely  a 
small  ingrowth  of  vascular  mesenchymal  tissue,  tu.v. 

The  imagination  of  the  wall  of  the  optic  vesicle  is  not  confined  to 
the  retinal  area,  but  also  extends  along  the  stalk  (anlage  of  the  optic 
nerve).  Fig.  414  represents  a  section  of  the  invaginated  stalk. 
W.  His'  Fig.  11,  89.4,  makes  clear  the  arrangement  in  a  human 
embryo  at  about  four  weeks  ;  the  invagination  appears  as  a  fissure 
running  from  the  under  side  of  the  retinal  cup  and  then  curving  so 
as  to  extend  along  the  posterior  side  of  the  optic  stalk  ;  the  upper 


FIG.  406.—  Rabbit  Embryo  of  ten  and  one-half 
Days;  Section  of  Head.  Md.ob,  Region  of  the 
medulla  oblongata;  V,  blood-vessel  1;  R,  anlage  of 

anla?e  of  lens;  op'  optic  vesicle;  /6' 


12 


THE   FCETUS. 


border  of  the  fissure  is  the  Seitenleiste  of  His,  the  lower  border  the 
Basilarleiste  of  His ;  the  fissure  is  known  as  the  choroid  fissure;  it 
is  occupied  by  mesenchyma ;  and  there  is  developed,  probably,  early 
in  the  fifth  week  in  man,  a  blood-vessel,  which  runs  along  the  furrow 
to  branch  out  in  the  retinal  cup  between  the  retina  and  the  lens ; 
this  vessel  is  the  arteria  centralis  retince  or  arteria  hyaloidea. 
During  the  fifth  week  the  choroid  fissure  begins  to  close ;  the  closure 


R 


FIG.  409.— Rabbit  Embryo  of  thirteen  Days;  Section  of  the  Eye.     N,  Optic  nerve;  P,  Pigment 
layer;  .R,  retina;  EC,  epidermis;  jL,  lens;  tu.v,  tunica  vasculosa;  mes,  mesenchyma. 

commences  at  the  proximal  end  and  progresses  toward  the  retinal  end 
of  the  stalk ;  a  little  later  the  fissure  closes  at  the  lower  edge  of  the 
retina ;  there  is  thus  left,  Fig.  410,  Ch.f,  a  short  stretch  of  the  fissure 
open.  It  is  through  this  opening  that  the  arteria  centralis,  Art, 
enters  and  passes  on  to  the  hollow  of  the  retinal  cup ;  it  is  prolonged 
through  the  vitreous  humor,  and  there  breaks  up  into  numerous 
branches,  which  run  toward  the  posterior  surface  of  the  lens,  where 
the  terminal  branches  spread  out  to  produce  the  tunica  vasculosa 
enveloping  the  lens.  In  the  human  embryo  at  three  months  the 
central  artery  gives  off  a  cone  of  branches  with  no  main  stem  (or 
arteria  hyaloidea  proper)  which  run  through  the  vitreous  humor  to 
the  lens;  and  at  this  age  the  atrophy  of  the  vessels  has  begun  (O. 
Schultze,  1892,  in  "  Festschrift  zum  50jahr.  Doktorjub.  von  Kolliker"). 
At  five  to  six  months  most  of  the  branches  have  aborted,  and  the  main 


DEVELOPMENT   OF   THE   EYE.  713 

hyaloid  trunk  is  developed  as  a  continuation  of  the  arteria  centralis 

through  the  vitreous  humor.     During  the  last  month  of  foetal  life 

the  vessels  of  the  vitreous  humor  abort  completely,  and  the  only  trace 

of    their    existence  to   be 

preserved   is   the   canalis 

KyaloideuSi   which  corre-       /%=^\  I  Sk  St 

sponds  to  the  space  origin-     //^/^^  m  L^ 

allv  occupied  by  the  main     ((((  (^^^     \  W^^T 

.  J  .J  ilxJV^ 

stem  or  the  artery.      With 

the  disappearance   of   the 
artery  the  last  remnant  of 
the  ehoroid  fissure  closes. 
The     s(j.c(m<f(iry     optic 

,-nn  i<    +Via    t^T-m   ^rriWIrwWl  Fio.  410. —Reconstruction  from  His'  Embryo  Sch,  13.8 

(  up  18   tOe    teilll  employed  nun    (N;u.k,.niiillkMM.     L<    Lens;/?,   retina;   P,   pigment 

to    desiCTliate     the     double-  layer;  r//./.  Hmmidfissiuv;  6t,  optic  stalk :  Art,  arteria 

ni                *             3  *        i-t  centralis.    After  \\'.  His.     ~x  about  50  cliams. 

walled  cup  formed  by  the 

in vaginated  retina  and  the  pigment  membrane  covering  it.  The 
opening  of  this  cup  is  closed  by  the  lens,  and  so  remains  throughout 
life.  As  seen  in  Fig.  400,  the  lens  at  first  nearly  fills  the  cup,  but  as 
the  retina  and  pigment  layer  grow  rapidly,  the  optic  cup  enlarges 
and  becomes  the  anlage  of  the  ball  of  the  eye,  and  the  space  between 
the  lens  and  the  retina  is  increased  until  it  assumes  the  adult  dimen- 
sions; the  space  is  occupied  by  loose  immigrant  mesenchyma,  which 
forms  the  anlage  of  the  vitreous  humor.  As  the  eye  expands  the 
tissue  around  it  is  condensed  and  forms  an  envelope  of  connective 
tissue  inclosing  the  optic  cup  or  eyeball ;  the  envelope  is  the  anlage 
of  the  .sr/rra  and  choroid.  I  regard  it  as  probable  that  the  con- 
densation is  a  mechanical  result  of  the  expansion  of  the  optic 
cap. 

The  position  of  the  eye  is  at  first  lateral,  with  the  axis  turned 
slightly  forward;  in  the  course  of  its  further  development  (His, 
88.4,  689)  it  moves  more  and  more  from  its  original  site  downward 
and  forward.  Until  the  end  of  the  first  month  it  lies  near  the  side 
of  the  thalamencephaloii  and  higher  up  than  the  infundibular  pro- 
cess. During  the  fifth  week  it  gradually  descends  from  this  level, 
and  later  swings  around  more  and  more  toward  the  front,  and  by  the 
end  of  the  second  month  it  lies  below  the  olfactory  lobe.  During 
the  latter  half  of  the  second  month  the  two  eyes  have  their  axes  at 
an  angle  to  one  another  of  about  90  degrees ;  during  the  second  month 
the  angle  further  diminishes,  and  ultimately — the  exact  time  is  not 
known — the  axes  become  parallel  with  one  another.  The  insertion 
of  the  optic  stalk  is  from  the  start  eccentric ;  at  first  it  is  on  the 
lower  side  of  the  optic  cup,  but  as  the  eye  migrates  it  comes  to  lie 
on  the  inner  side  of  the  eyeball ;  it  remains  eccentric  throughout 
life.  At  no  time  does  the  insertion  of  the  stalk  (optic  nerve)  coin- 
cide with  the  position  of  the  macula  lutea,  as  has  been  erroneously 
assumed. 

By  referring  to  Fig.  409,  it  will  be  seen  that  the  edge  of  the  optic 
cup  lies  on  the  outer  surface  of  the  lens ;  this  is  always  the  case.  The 
orifice  of  the  cup  is  the  future  pupil  of  the  eye ;  it  is  a  circular  open- 
ing through  which  the  surface  of  the  lens  is  exposed.  As  the  eye 
grows  the  lens  enlarges,  but  the  orifice  of  the  cup  (or  pupil)  does  not 


14 


THE   FCETUS. 


become  larger  ;*  hence  there  comes  to  be  a  portion  of  the  optic  cup 
•which  rests  on  the  anterior  surface  of  the  lens,  Fig.  411,  Uv.  In  the 
region  of  the  optic  cup  around  the  edge  of  the  lens  and  on  the  front 
surface  of  the  lens,  both  layers  (retinal,  jR,  and  pigment,  p)  of  the 
cup  become  thin  and  very  closely  united,  so  that  from  an  early  stage 
their  development  progresses  as  if  they  were  one  layer ;  a  short  dis- 
tance from  the  lens  the  retinal  layer  thickens  and  extends  over  the 
rest  of  the  optic  cup  as  the  anlage  of  the  true  retina.  The  thin-walled 
portion  of  the  optic  cup  may  be  called  the  lenticular  zone;  the  por- 
tion of  the  zone  around  the  pupil  and  resting  on  the  lens  forms  the 
double  epithelial  pigment  layer  of  the  adult  iris  and  might  be  appro- 
priately designated  as  the  primitive  iris;  the  portion  of  the  zone 
around  the  lens,  or,  in  other  words,  between  the  edge  of  the  lens  and 
retina  proper,  early  becomes  thrown  into  folds,  which  give  rise  to  the 


FIG.  411.— Section  through  the  Iris  Region  of  the  Eye  of  a  Chick  of  thirteen  Days.  Ep,  Epi- 
dermis; cz7,  ciliary  muscle;  oil',  ciliary  ligament;  t>,  blood-vessel  (canal  of  Schlemm)  ;  c.  cornea; 
Aq,  aqueous  chamber ;  /,  iris;  Uv,  uvea;  I,  lens;  pro,  ciliary  process;  Z.z,  zomula  Zinnii;  pet, 
pectinate  ligament;  R,  retinal  layer;  p,  pigment  layer;  c/io,  choroid  layer.  After  Angelucci. 

ciliary  processes,  and  in  the  adult  it  persists  as  the  epithelial  pigment 
covering  of  the  ciliary  processes. 

It  must  be  expressly  stated  that  the  usual  description  of  the  devel- 
opment of  the  iris  by  a  growth  of  the  optic  cup  over  the  lens  is  erro- 
neous ;  the  walls  of  the  cup  expand  away  from  the  pupil ;  were  the 
usual  description  correct  the  pupil  of  the  embryonic  eye  would  have 
to  be  larger  than  the  iris  ~i  the  adult;  in  other  words,  larger  than  is 
the  whole  embryo,  when  the  pupil  is  first  developed. 

Lens. — The  lens  is  developed  from  the  ectoderm,  which  comes  in 
contact  with  the  outgrowing  optic  vesicles ;  the  distal  wall  of  each 
vesicle  becoming  closely  united  with  a  nearly  circular  area  of  the 
epidermis  at  the  side  of  the  head.  The  attached  epidermal  area 
tiickens  and  forms  the  anlage  of  the  lens,  Fig.  412,  L,  while  the 
attached  wall  of  the  optic  vesicle  also  thickens  and  forms  the  anlage 
of  the  retina,  R.  It  is  interesting  to  note  that  the  karyokinetic  fig- 
ures in  the  lens  anlage  are  toward  the  outer  surface,  and  those  of  the 
retinal  anlage  toward  the  cavity  of  the  optic  vesicle,  and  are  there- 

*  There  are  no  exact  measurements,  and  it  is  quite  possible  that  £he  pupillar  orifice  enlarges 
slightly,  but  not  at  all  in  proportion  to  the  lens. 


DEVELOPMENT   OF   THE   EYE. 


15 


fore  in  homologous  situations  in  both  ectodermal  layers.  The  lens 
area  now  becomes  invaginated,  Fig.  412,  and  may  easily  be  seen  in 
rabbit  embryos  of  the  eleventh  day,  a  chick  of  two  days,  or  the 
human  embryo  of  the  fourth  week,  as  a  small  pit  at  the  side  of  the 
head.  To  such  an  extent  does  the  involution  of  superficial  ectoderm 
take  place,  that  the  front  or  retinal  wall  of  the  optic  vesicle,  as  stated 
in  the  preceding  section,  is  pushed  close  up  to  the  hind  or  pigment 
wall,  and  the  cavity  of  the  vesicle  is  almost  obliterated,  Fig.  409. 
Meanwhile,  as  the  pit  deepens,  its  mouth  closes  over,  and  the  pit 
becomes  a  completely  closed  sac,  which  at  once  breaks  away  from 
the  overlying  epidermis,  which  forms  a  continuous  layer  in  front  of 
it,  all  traces  of  the  original  opening  being  lost.  The  closed  sac  is 
the  lens;  it  occupies  the  secondary  optic  cup,  Fig.  409,  and  later 
when  the  cup  expands  the  lens  closes  the  mouth  of  the  cup,  Fig. 
413.  At  this  stage  the  lens,  Z/,  is  a  rounded,  somewhat  flattened 
vesicle  with  thick  walls,  and  is  a  strictly  ectodermal  organ.  The 


tries 


EC 


FIG.  412.— Rabbit  Embryo  of  ten  and  one- 
half  Days;  Section  of  the  Lens  Anlage.  mes, 
Mesoderm;  P,  pigment  layer;  R,  retina;  L, 
lens;  EC.  ectoderm. 


Mes. 


FIG.  413.— Vertical  Section  of  the  Eye  of  a 
Chick  Embryo  of  the  third  Day.  EC,  Ectoderm, 
L,  lens:  Ret,  retina;  Cho,  choroid  layer;  Md, 
wall  of  brain ;  Mes,  mesenchyma.  x  108  diams. 


space  between  the  retina,  R,  and  the  lens,  L,  gradually  increases  to 
form  the  posterior  chamber  of  the  eye,  which  is  occupied  by  the  vit- 
reous humor,  p.  723.  In  the  chick  the  separation  of  lens  and  retina 
takes  place  before,  in  mammals  after,  the  differentiation  of  the  walls 
of  the  vesicular  lens  has  begun,  compare  Fig.  409  with  Fig.  413. 

The  next  step  is  the  thinning  of  the  outer  or  anterior  wall  of  the 
lens,  and  the  great  thickening  of  its  posterior  or  inner  wall.  The 
thickening  of  the  inner  wall  is  rapid,  and  soon  obliterates  the  original 
cavity  of  the  lens.  This  cavity  is  filled  in  birds  with  fluid,  but  in 
mammals  contains  scattered  cells,  which  break  down  and  disappear 
as  the  cavity  closes ;  these  cells,  I  think,  are  probably  part  of  the 


716  THE   FOETUS. 

epitrichial  layer  of  the  epidermis.  The  minute  structure  of  the  walls 
of  the  lens  in  its  vesicular  stage  is  not  known,  but  it  is  probably  an 
epithelium  of  cylinder  cells,  every  cell  stretching  through  the  entire 
thickness  of  the  wall,  but  the  nuclei  are  scattered  at  various  levels. 
The  anterior  wall  gradually  thins  out  and  is  converted  into  a  simple 
thin  layer  of  cuboidal  cells  with  round  nuclei,  and  is  known  in  descrip- 
tive anatomy  as  the  epithelium  of  the  lens.  The  posterior  wall,  Fig. 
409,  thickens  rapidly  by  the  growth  of  its  cells,  which  elongate  enor- 
mously, without,  however,  increasing  much  in  thickness,  thus  being 
metamorphosed  into  the  so-called  fibres  of  the  lens.  The  nuclei  of 
the  fibres  tend  to  occupy  a  middle  position,  hence  there  is  a  band  of 
nuclei  across  the  middle  of  the  thickened  wall,  as  shown  in  Fig.  40!). 
The  lens  fibre  is  merely  an  elongated  epithelial  cell,  and  as  such  it 
may  be  readily  recognized  in  the  adult.  The  fibres  change  their 
composition  so  as  to  be  better  fitted  for  the  optical  functions  of  the 
lens  than  protoplasmatic  cells  would  be,  but  how  the  protoplasm  of 
the  cells  is  metamorphosed  is  unknown.  The  fibres  all  stretch 
through  the  whole  thickness  of  the  wall,  but  become  bent  so  'as  to 
form  three  well-defined  systems  of  curves,  so  arranged  at  birth  that 
the  systems  on  the  front  of  the  lens  alternate  in  the  direction  of  the 
fibres  with  those  on  the  back  of  the  lens,  see  O.  Hertwig's  diagram 
("  Entwickelungsges.,"  3te  Aufl.,  Fig.  268).  At  the  edge  of  the  lens 
the  anterior  epithelium  is  continuous,  Fig.  411,  with  the  thickened 
posterior  wall  or  layer  of  lens  fibres,  and  there  is  a  gradual  transition 
between  the  two. 

The  growth  of  the  lens  is,  of  course,  largely  due  to  the  growth  of 
the  fibres,  but  it  is  supposed  that  cells  are  added  at  the  edges  of  the 
lens  from  the  anterior  to  the  posterior  wall,  and  converted  into  fibres, 
thus  adding  new  fibres.  So  far  as  known,  there  is  no  proliferation 
of  the  fibres  themselves.  About  two-thirds  of  the  total  growth  of 
the  lens  is  accomplished  before  birth.  Huschke  is  stated  to  have 
found  the  average  weight  of  the  lens  to  be,  at  birth,  123  milli- 
grammes, in  the  adult  190. 

CAPSULE  OF  THE  LENS. — Around  the  lens  of  the  adult  is  found 
an  anhistic  membrane,  known  as  the  capsule  of  the  lens.  The 
membrane  is  presumably  homologous  with  the  anhistic  layer  found 
under  the  ectoderm  elsewhere  and  which  is  permanent  in  the  amnion, 
p.  334.  We  have  little  knowledge  of  the  history  of  the  capsule  of 
the  lens  in  the  embryo,  except  that  it  grows  in  thickness  and  con- 
tains no  cells.  Kolliker  ("  Entwickelungsges. ,"  2te  Aufl. ,  G36)  regards 
the  capsule  as  the  product  of  the  lens,  but  it  more  usually  is  regarded 
as  a  specialized  part  of  the  matrix  of  the  surrounding  mesenchyma. 

TUNICA  VASCULOSA  LENTIS.* — The  lens  early  becomes  surrounded 
by  a  special  mesenchymal  membrane  richly  vascularized  by  branches 
of  the  arteria  centralis,  p.  712,  Fig.  410,  which  reaches  the  lens  from 
behind,  and  by  branches  of  the  arterial  circulus  iridis,  which  reach 
the  lens  about  its  equator.  As  the  lens,  being  an  epithelial  structure, 
contains  no  vessels  itself,  its  rapid  growth  on  the  embryo  is  proba- 
bly dependent  on  supply  from  the  tunica  vasculosa.  The  vessels 
radiate  out  from  the  central  artery  over  the  inner  wall  of  the  lens, 

*  The  principal  authority  is  O.  Schultze,  "Festschrift  zum  50jahrigen  Doktorjubilaum  von 
Kolliker,"  1892. 


DEVELOPMENT    OF   THE    EYE.  717 

and,  branching  as  they  go,  pass  around  the  edge  of  the  lens  and 
branch  in  loops  on  the  anterior  surface  (see  Kolliker,  "Entwicke- 
lungsgesch.,"  2te  Aufl.,p.  050).  The  network  is  particularly  fine 
and  close  about  the  equator  of  the  lens  (O.  Schultze) ;  it  will  be  re- 
membered that  it  is  principally  at  the  equator  that  the  growth  of 
the  lens  is  supposed  to  take  place.  The  veins  are  small  vessels  which 
pass  off  in  more  or  less  radial  directions  from  the  edge  of  the  lens 
and  join  the  venaB  vorticosaB  of  the  choroid  coat.  Until  O.  Schultze's 
investigations  the  veins  were  practically  unknown.  The  tunica 
vasculosa  also  extends  across  the  pupil,  but  toward  the  close  of  foetal 
life  the  vessels  abort  under  the  pupil,  which  thereafter  is  bordered 
by  characteristic  vascular  loops  (see  Kollirrcr,  "Entwickelungsges.," 
2te  Aufl.,  Fig.  409). 

In  descriptive  anatomy  three  names  are  employed,  each  for  a  part 
of  the  tunica  vasculosa;  at  the  back  of  the  lens  it  is  the  membrana 
(•ujtxultirix;  at  the  front  of  the  lens  in  the  centre  the  membrana 
/)nj}i//aris,  and  around  the  centre  (i.  e.,  beneath  the  iris)  the  mem- 
brana capsulo-pupillaris  (cf.  Kolliker,  /.c.,  p.  049) .  All  these  names 
ought  to  be  discarded.  If  the  membrana  pupillaris  persists  there 
results  atresia  pupilke  congenita.  The  pupillary  membrane  is 
wanting  in  birds  (Angelucci,  81.1,  150). 

The  tunica  attains  its  greatest  development  in  man  during  the 
seventh  month  and  usually  disappears  before  birth,  but  the  time  of 
its  disappearance  seems  to  be  variable. 

Optic  Nerve. — The  hollow  optic  stalk  develops  into  the  optic 
nerve,  first  by  becoming  solid,  second  by  acquiring  nerve-fibres.  It 
becomes  solid  by  the  growth  of  its  own  walls  and  the  gradual  oblitera- 
tion of  its  cavity  thereby.  It  acquires  nerve-fibres  from  the  thala- 
mencephalon  and  from  the  retina,  the  former  set  of  fibres  growing 
centrifugally,  the  latter  centripetally.  Formerly  it  was  assumed 
that  ttfe  optic  nerve-fibres  arose  in  loco  from  the  cells  of  the  nerves 
(see  for  example  Hiltner,  85.1),  but  there  have  been  no  actual  obser- 
vations to  support  the  assumption.  It  is  possible  that  the  nerve, 
being  part  of  the  medullary  tube,  develops  neuroblasts,  but  it  is  cer- 
tain that  most  of  the  fibres,  if  not  all,  come  from  the  brain  and 
retina,  the  largest  contingent  from  the  brain.  Falchi,  87.2, 
searched  for  neuroblasts  in  the  optic  nerve  of  cow  embryos,  but 
found  none. 

The  choroid  fissure  permits  the  wall  of  the  optic  stalk  to  remain 
directly  continuous  with  the  retina,  as  already  explained.  The  optic 
stalk  consists  of  a  basal  or  inner  part,  and  an  outer  or  distal  part, 
along  which  latter  alone  the  choroid  fissure  extends.  Fig.  414  rep- 
resents a  transverse  section  of  the  optic  nerve  as  obtained  from  a 
sagittal  section  of  a  rabbit  embryo  of  thirteen  days.  In  the  fissure, 
as  described,  p.  712,  the  arteria  centralis  retinaB  is  developed.  The 
length  of  the  choroid  fissure  varies,  it  being  longer  in  mammals  than 
in  birds,  and  longer  in  man  than  in  certain  other  mammals. 
Throughout  the  length  of  the  optic  stalk  the  central  cavity  is  oblit- 
erated ;  the  obliteration  begins  next  the  brain  and  progresses  toward 
the  retina ;  it  is  completed  in  the  chick  by  the  seventh  day  (Mihal- 
kovics,  77. 1,  79),  in  man  probably  by  the  third  month;  in  man  the 
closure  begins  during  the  seventh  week  (W.  His,  89.4,  090).  In 


718 


THE   FCETUS. 


the  distal  part  of  the  stalk  the  choroid  fissure  also  becomes  closed, 
but  much  later  than  the  central  cavity.  By  these  changes  the  hol- 
low stalk  is  converted  into  a  solid  cylindrical  cord  continuous  with 
the  retina. 

The  tissue  of  the  stalk,  while  its  cavity  is  disappearing,  changes 
into  neuroglia;  in  the  chick  during  the  fifth  day  (Mihalkovics,  77. 1, 
70)  appears  a  clearer  layer  round  the  outside,  with  nerve-fibres  in  it; 


mes 


FIG.  414.— Section  of  the  Distal  Portion  of  the  Optic  Nerve  of  a  Rabbit  Embryo  of  thirteen 
Days,  mes,  Mesoderm;  Op,  outer  wall  of  optic  nerve:  v,  blood-vessel:  /,  choroid  fissure. 
X  113  diams. 

this  layer  is,  perhaps,  homologous  with  the  Randschleier,  p.  613,  of 
the  central  nervous  system.  After  the  nerve  has  become  solid  Kol- 
liker  ("  Grundriss,"  2te  AufL,  299)  finds  the  following  structure :  The 
cells  are  placed  radially  and  form  a  delicate  network,  the  meshes  of 
which  are  extended  longitudinally,  and  contain  numerous  bundles 
of  fine  nerve-fibres ;  and  there  are  also  cells  arranged  in  longitudinal 
rows,  which  share  with  the  radial  cells  in  forming  the  network. 
The  nuclei  of  the  radial  cells  in  cow  embryos  are  oval  and  nucle- 
olated  (Falchi,  87.2). 

The  nerve-fibres  of  the  opticus  begin  to  appear  in  the  chick  the 
fifth  day.  Mihalkovics  and  Kolliker  have  shown  that  fibres  arise  in 
the  wall  of  the  thalamencephalon  and  grow  in  a  bundle  toward  the 
median  ventral  line  following  the  tractus  opticus,  p.  688.  As  each 
bundle  continues  to  grow  in  its  original  direction  they  cross  one 
another,  and  each  enters  the  nerve  of  the  opposite  side  and  grows 
along  it  toward  the  retina ;  the  crossing  of  the  fibres  constitutes  the 
optic  chiasma;  from  the  mode  of  development  it  is  evident  that  all 
the  fibres  from  one  side  must  cross  to  the  nerve  of  the  opposite  side. 
The  progress  of  the  centrifugal  fibres  has  yet  to  be  accurately  fol- 
lowed. The  retina  (p.  719)  contains  in  the  embryo  true  neuroblasts, 
which  send  off  centripetal  fibres  through  the  optic  nerve  to  the  brain. 
Froriep,  91.1,  has  observed  in  an  embryo  of  Torpedo  ocellata,  in 
Balfour's  stage  M,  retinal  neuroblasts,  sending  fibres  into  the  optic 
nerve  about  one-sixth  of  the  length  of  the  nerve  toward  the  brain 
and  before  any  other  nerve-fibres  are  present.  This  observation  raises 
the  question  whether  or  not  the  centripetal  fibres  are  developed  in 
other  vertebrates  also,  before  the  centrifugal.  The  origin  of  optic 


DEVELOPMENT   OF   THE   EYE.  719 

i 

nerve-fibres  from  the  retina  was,  I  believe,  first  suggested  by  W. 
Miiller,  74.1,  :.}?;  the  suggestion  has  been  confirmed  by  the  obser- 
vations of  His,  90.1,  on  mammals,  and  by  those  of  Keibel,  89.1, 
on  reptiles. 

Bernheimer,  89. 1,  has  studied  the  progressive  development  of  the 
sheaths  of  tlie  optic  nerve-fibres,  and  reached  interesting  conclusions 
by  this  means  as  to  the  course  of  the  fibres. 

The  optic  nerve  enlarges  both  in  length  and  diameter,  its  enlarge- 
ment being  due  to  the  multiplication  of  its  cells  and  the  growth  of 
its  nerve-fibres.  It  is  probably  owing  to  its  enlargement  that  the 
neighboring  mesenchyma  around  becomes  condensed  and  forms  a 
connective-tissue  envelope  around  the  nerve.  Concerning  the  histo- 
genesis  of  this  envelope  we  know  only  that  it  becomes  differentiated 
into  two  layers — an  inner  highly  vascular  layer  continuous  on  the  one 
hand  with  the  pia  mater  of  the  brain,  on  the  other  with  the  choroid 
of  the  eye,  and  an  outer  fibrillar  layer  continuous  with  the  dura 
mater  of  the  brain  and  the  sclera  of  the  eye. 

Retina. — If  we  consider  the  structure  of  the  retina,  compared 
with  that  of  the  embryonic  brain,  I  think  the  same  three  primary 
layers  can  be  recognized  as  in  the  dorsal  and  ventral  zones  of  the 
central  nervous  system,  see  p.  GIG.  Next  to  the  pigment  layer  is 
the  membrana  limitans  externa,  which  is  the  boundary  of  the  ret- 
inal layer  proper  toward  the  brain  cavity,  which  in  the  eye  is  repre- 
sented by  the  fissure  between  the  pigment  layer  and  the  retina  proper, 
Fig.  401),  R.  The  projection  of  the  rods  and  cones  across  this  fissure 
and  into  the  pigment  layer  is  secondary,  as  explained  below.  The 
limitans  exterua  is,  therefore,  the  homologue  of  the  limitans  interna 
of  the  brain  and  spinal  cord.  The  cells  with  their  nuclei  next  this 
membrane  correspond  to  the  inner  neuroglia  layer,  which  in  the 
spinal  cord  becomes  the  lining  epithelium,  in  the  brain  the  inner 
white  matter  plus  the  ependjTma,  and  in  the  retina  the  outer  nuclear 
(or  granular)  layer  and  perhaps,  also,  the  inner  reticular  layer  and 
the  inner  nuclear  layer.  These  layers  of  the  retina  might,  there- 
fore, collectively  be  called  the  ependymal  layer.  The  nerve-fibre 
layer  is  to  be  homologized  with  the  Randschleier  (p.  613).  The 
middle  part  of  the  retina  between  the  inner  nuclear  layer  and  the 
internal  nerve-fibre  layer  is  comparable  to  the  mantle  layer  or  gray- 
matter  layer  of  the  medullary  wall ;  it  includes  the  inner  reticular 
(or  molecular)  layer  and  the  ganglion-cell  layer  of  descriptive  anat- 
omy. The  homologies  drawn  are  probably  correct,  but  they  can  be 
definitely  accepted  only  if  verified  by  a  fuller  knowledge  of  the  devel- 
opment of  the  retina  than  we  possess  at  present. 

The  first  step  in  the  histogenesis  of  the  retina  proper  is  the  differ- 
entiation of  the  narrow  inner  zone  (i.e.,  toward  the  lens),  which 
contains  no  nuclei,  and  a  wide  outer  zone  (i.e.,  toward  the  pigment 
layer)  with  numerous  nuclei  in  many  layers ;  this  stage  may  be  seen 
in  rabbit  embryos  of  4-5  mm.  (Lowe,  78.1,  602).  The  narrow 
zone  I  identify  with  the  Randschleier  (p.  613)  of  the  spinal  cord,  and 
the  wider  nucleated  zone  with  the  mantle  and  inner  neuroglia  layer 
of  the  axial  medullary  tube. 

The  second  step  is  the  subdivision  of  the  wide  nucleated  zone 
into  two  layers  of  about  equal  thickness  and  distinguished  by  the 


720  THE   FOETUS. 

character  of  their  nuclei ;  the  nuclei  of  the  outer  are  smaller,  more 
oval,  and  stain  more  deeply  than  those  of  the  inner  layer.  This 
stage  may  be  observed  in  a  rabbit  embryo  of  20  mm.  or  human  of 
38  mm;  it  has  been  described  and  figured  crudely  by  Lowe,  78.1, 
604,  and  accurately  by  Falchi,  87.2.  I  interpret  the  two  layers  as 
representing  respectively  the  inner  neuroglia  layer  (ependymal 
layer)  and  the  mantle  layer  (gray  matter)  of  the  brain.  The  outer 
layer  with  smaller  nuclei  is  to  be  regarded  as  the  anlage  of  the  outer 
nuclear  layer,  and  later  produces  the  rods  and  cones.  The  inner 
layer  undergoes  further  modification. 

The  third  step  is  the  differentiation,  1,  of  the  inner  layer  (just 
described  and  homologized  with  the  mantle  layer)  into  two  distinct 
layers :  the  inner  reticular  layer  and  the  ganglion-cell  layer ;  2,  of 
the  outer  layer  (just  described  and  homlogized  with  the  inner  neurog- 
lia layer  of  the  brain)  into  three  distinct  layers :  the  outer  nuclear 
layer,  the  outer  reticular  layer,  and  the  inner  nuclear  layer.  The 
five  layers  are  partially  distinct  in  the  rabbit  at  birth,  although  their 
differentiation  is  then  still  far  from  completed ,  but  are  clearly  marked 
out  in  a  human  embryo  of  215  mm.  (Falchi,  87.2,  Fig.  3,  p.  387-380). 
The  fourth  step  is  the  development  of  the  rods  and  cones,  which 
was  superbly  investigated  by  Max  Schultze  in  1800,  66.1,  230-247. 
Until  quite  an  advanced  period  the  membrana  limitans  externa  of 
the  retinal  layer  proper  remains  smooth.  There  then  appear  numer- 
ous small  projections  over  the  surface  of  the  membrana  limitans ;  the 
projections  are  rounded  in  form,  and  are  of  two  sizes,  Fig.  415;  the 

larger  ones  are  *ne  anlages  of  the  rods,  the 
smaller  ones  °f  the  cones,  the  latter  being 
much  the  most  numerous  in  the  chick. 
The  young  rods  and  cones  are  at  first  hemi- 
spherical  in  shape,  and  each  is  an  outgrowth 
of  an  elongated  sense-cell,  the  nucleus  of 
which  is  situated  in  the  outer  nuclear  layer ; 
according  to  Falchi,  87.2,  the  nucleus  of 
the  cell  is  lodged  in  the  rods,  at  least  when 
Fie.  415. -Surf ace  view  of  the  they  begin  to  form.  The  rods  and  cones 

Membrana  Limitans  Externa  with    -i      1-1        -\  A  4-      4-       4-1^ 

the  Developing  Rods  and  cones  of    both  elongate  and   penetrate  the   pigment 
iSSSdpSSMaJSJ:  kyer,  cf.  infra    in  which  they  are  com- 
mor.  After  Max  Schultze.   x  400-   pletely  imbedded.      Ine  rods  and  cones  de- 
velop first  their  inner  members,  and  as  they 

grow  longer  their  tips  assume  the  character  of  outer  members.  In 
the  chick  about  the  eighteenth  day  there  appear  in  the  cones  first 
very  small  red  oil  globules,  then  yellow  ones.  It  is  probable  that  in 
the  chick  after'  hatching  the  rods  and  cones  grow  only  in  size,  not  in 
number.  It  should  be  noted  that  Babuchin,  65. 1,  states  that  in  the 
frog  the  rods  and  cones  at  first  differ  from  one  another  but  little. 
The  development  of  the  rods  and  cones  begins  in  the  chick  the  seventh 
to  tenth  day ;  they  are  present  in  man  and  ruminants  at  birth,  though 
smaller  in  size  than  in  the  adult ;  in  rabbits  and  cats,  and  probably 
in  other  mammals  born  blind,  they  are  not  present  until  after  birth. 
Falchi,  87.2,  387,  has  observed  the  rods  just  commencing  in  a  hu- 
man embryo  of  215  mm. 

The  cells  in  the  retina  become  differentiated  into  two  main  classes, 


DEVELOPMENT   OF   THE   KYK. 


T21 


nerve-cells  and  supporting  or  neuroglia  cells.  With  the  former  I 
include  the  cells  which  abut  permanently  against  the  membrana 
liniitans  externa,  and  which  by  producing  the  rods  and  cones  become 
the  sensory  cells  of  the  adult.  As  to  the  exact  series  of  changes 
through  which  the  cells  pass,  our  information  is  scanty.  The  series 
of  histogenetic  changes  do  not  progress  uniformly  throughout  tin4 
retina,  but  are  more  rapid  toward  the  optic  nerve,  less  rapid  toward 
the  1<  m,  or,  better  said,  toward  the  ciliary  body. 

The  ivtina  proper  grows  more  rapidly  than  the  remaining  parts  of 
the  eye,  and  therefore  is  thrown  into  folds.  The  folds  begin  to 
appear  in  the  human  embryo  during  the  third  month.  According 
to  Kolliker  ("  Gruimriss,"  2te  Aufl.,  206)  the  first  fold  arises  below  the 
entrance  of  the  optic  rerve  and  numerous  other  folds  are  added  later. 
Toward  the  end  of  foetal  life  all  the  folds  gradually  disappear,  and 
at  birth  lli»i  retina  is  again  smooth. 

The  macula  lutea  is  developed  after  birth. 

PIGMENT  LAYER. — The  outer  lamina  of  the  secondary  optic  cup, 
Fig.  412,  P,  very  early  becomes  a  simple  cuboidal  epithelium;  pig- 
ment granules  develop  in  this  layer  in  the  rabbit  about  the  thirteenth 
day,  Pig.  4()9.  The  pigmented  epithelium  comes  to  lie  close  against 
the  limitans  externa  of  the  retinal  layer  proper.  When  the  rods  and 
cones  develop  they  grow  into  the  layer  and  become,  as  it  were,  bur- 
ied in  pigment;  the  pigmented  epithelium  becomes  thicker  as  the 
rods  and  cones  become  longer,  and  remains  throughout  life  a  dis- 
tinctly epithelial  membrane.  Its  function  is  supposed  to  be  to  op- 
tically isolate  the  rods  and  cones  from  one  another. 

BLOOD-VESSELS  OF  THE  RETINA. — The  following  paragraph  is 
based  011  O.  Schultze's  admirable  memoir  on  the  blood-vessels  of 
the  fcetal  eye  ("'  Festschrift 
zum  SOjahr.  Doktorjubi- 
laum  von  Kolliker,"  1892), 
where  references  to  the 
previous  observations  may 
be  found.  A  layer  of  mes- 
enchymal  cells  is  developed 
quite  late  (pig  embryo  of 
90  mm.,  in  man  after  the 
third  month)  over  the  sur- 
face of  the  retina  toward 
the  vitreous  humor;  the 
cells  arrange  themselves  in 
a  very  distinct  network 
and  are  then  hollowed 
out  to  form  blood-vessels. 
The  vascularization  begins 
next  the  optic  nerve  and 
spreads  toward  the  lens, 
with  the  result  of  forming 
a  layer  of  vessels  (mem- 
brana vasculosa  retina) 
which  may  be  injected,  and  then  presents  a  highly  characteristic  ap- 
pearance, Fig.  4iG.  Red  blood  plastids  develop  in  the  network;  the 
46 


Art 


Ve 


FIG.  416.  — Injected  Vascular  Membrane  of  the  Retina  of 
the  Eye  of  a  Pig  Embryo,  16  cm.  long.  Art,  Artery; 
Fe,  vein.  After  O.  Schultze. 


722  THE   FCETUS. 

vessels  were  observed  in  a  pig  embryo  of  175  mm.  to  have  grown 
from  the  membrane  into  the  retina.  The  network  is  not  connected 
with  the  arteria  centralis  retinae,  but  with  vessels  which  enter  around 
the  periphery  of  the  optic  nerve. 

Lenticular  Zone. — The  term  is  defined  p.  714.  The  lenticular 
zone  of  the  secondary  optic  cup  forms  the  parts  beyond  the  ora  serrata, 
viz.,  the  ciliary  processes  and  the  uvea.  The  opening  of  the  optic- 
cup  is  the  pupil,  Fig.  413,  and  in  early  stages  is  just  filled  by  the  lens ; 
at  the  stage  of  Fig.  412,  the  two  layers  of  the  optic  cup  are  essen- 
tially uniform  in  character  throughout  their  extent ;  later,  while  the 
optic  cup  and  lens  are  enlarging,  the  character  of  the  walls  of  the 
optic  cup  changes,  and  in  a  circular  zone  around  the  pupil  both  the 
pigment  layer  and  the  retinal  layer  of  the  cup  become  simple  cuboidal 
epithelium ;  the  thin-walled  portion  of  the  optic  cup  is  what  I  have 
named  the  lenticular  zone,  cf.  Fig.  411.  The  pigment  layer  of  the  zone 
very  early  acquires  pigment  granules  (in  the  rabbit  by  the  thirteenth 
day)  and  thereafter  changes  but  little  histologically.  The  retinal 
layer  begins  to  thin  out  in  cow  embryos  of  about  30  mm.,  in  rabbit 
embryos  about  the  sixteenth  day,  and  it  quite  slowly  assumes  the 
form  of  a  cuboidal  epithelium.  The  lenticular  zone  increases  in 
width,  bat  of  its  rate  of  growth  I  find  no  record  published ;  as  it  be- 
comes wider,  we  see  that  one  portion  of  it  overlies  the  lens,  though 
separated  from  the  lens  by  the  tunica  vasculosa  lentis ;  and  another 
portion,  which  adjoins  the  true  retina,  does  not  rest  on  the  lens.  The 
portion  overlying  the  lens  is  the  anlage  of  the  uvea  of  the  iris,  Fig.  417, 
Uv;  the  other  portion  is  the  anlage  of  the  ciliary  processes.  The  two 
epithelia  of  the  lenticular  zone  become  closely  adherent  to  one  another, 
and  in  their  further  development  act  as  if  constituting  one  layer. 

THE  UVEA  is  the  name  usually  applied  to  the  lenticular  zone  in  the 
adult,  and  may  be  defined  as  the  double  epithelial  layer  covering  the 
choroid  processes  and  the  inner  surface  of  the  iris,  Fig.  417,  Ur. 

THE  CHOROID  PROCESSES,  Fig.  417,  arise  toward  the  end  of  the 
second  month,  or  early  in  the  third,  as  folds  of  the  uvea  around  the 
edge  of  the  lens ;  the  folds  are  filled  with  vascular  mesenchyma  and 
gradually  increase  in  height ;  they  are  well  developed  by  the  fourth 
month;  in  the  fifth  month  Kolliker  ("  Entwickelungsges.,"  2te  Aufl., 
680)  found  the  processes  0. 12-0. 18  mm.  high  and  0. 10-0. 12  mm.  wide. 
The  pigment  of  the  uvea  is  much  darker  in  the  embryo  over  the  cho- 
roid processes  than  elsewhere. 

Sclera  and  Choroid. — The  primitive  eyeball  consists  of  the 
optic  cup  and  lens,  and,  as  it  expands,  the  connective  tissue  around 
it  becomes  condensed,  forming  a  mesenchymal  envelope,  out  of 
which  the  sclera  and  choroid  coats  are  gradually  evolved.  The 
sclera  (sclerotic  coat)  may  be  homologized  with  the  dura  mater,  the 
choroid  with  the  pia  mater. 

The  sclera  is  developed  from  the  outer  part  of  the  mesenchymal 
envelope,  and  is  thickened  by  accretions  from  the  surrounding 
mesoderm  as  the  eye  enlarges ;  during  foetal  life  the  sclera  has  no 
definite  external  boundary  and  is  comparatively  thin ;  at  what  period 
the  connective-tissue  fibrillse  in  it  begin  to  develop  I  do  not  know. 

The  choroid  or  vascular  layer  is  developed  from  the  inner  part  of 
the  mesenchymal  envelope,  and,  indeed,  may  be  said  to  begin  before 


DEVELOPMENT   OF   THE   EYE.  723 

condensation  of  the  connective  tissue  has  begun  around  the  eye, 
because  a  capillary  network  appears  veiy  early,  making  a  special 
vascular  layer  over  the  pigment  layer  of  the  optic  cup — this  stage 
may  be  seen  in  a  cow  embryo  of  23  mm.  (Kolliker,  "Entwicke- 
lungsges.,"  2te  Aufl.,  Fig.  422).  This  primitive  vascular  tunic  is 
continuous  with  the  tunica  vasculosa  lentis,  p.  716.  Concerning 
the  histogenesis  of  the  choroid,  I  have  found  no  satisfactory  obser- 
vations. 

Vitreous  Humor. — By  this  name  is  designated  the  mesenchy- 
mal  tissue  which  fills  the  cavity  of  the  optic  cup  between  the  retina 
and  the  lens.  The  tissue  appears  very  early,  growing  into  the  optic 
cup  through  the  choroid  fissure,  and  accompanying  the  blood-vessels, 
which  form  the  vascular  tunic  of  the  retina- and  lens;  the  tissue  at 
first  contains  typical  anastomosing  mesenchymal  cells  with  a  large 
amount  of  basal  substance  between  them  (rabbit  of  thirteen  days) . 
KVibel  affirms,  86.1,  that  no  mesenchyma  except  the  blood-vessels 
grows  in,  but  my  sections  show  conclusively  that,  as  concerns  rabbit 
embryos,  he  is  in  error.  In  the  chick,  on  the  other  hand,  the  cells 
and  blood-vessels  are  both  absent  (Angelucci,  81.1). 

As  to  the  histogenesis  of  the  vitreous  humor  our  knowledge  is  very 
unsatisfactory.  It  probably  consists  principally  in  an  enormous 
development  of  the  basal  substance,  with,  perhaps,  ultimate  abortion 
of  the  mesenchymal  cells.  The  space  originally  occupied  by  the 
stem  of  the  central  artery  persists  and  is  called  the  hyaloid  canal. 
Over  the  surface  of  the  vitreous  humor  is  developed  a  homogeneous 
layer  without  cells,  known  as  the  hyaloid  membrane,  which,  there- 
fore, covers  the  retina,  the  ciliary  processes,  and  the  lens.  In  the 
ciliary  region  it  becomes  so  much  thickened  that  the  processes  are, 
so  to  speak,  entirely  imbedded  in  it.  The  thickened  hyaloid  mem- 
hraue  of  the  ciliary  region  constitutes  the  suspensory  ligament 
(zonnfd  Zinnii}  of  the  lens;  it  differs  from  other  parts  of  the 
membrane  in  that  it  develops  radiating  connective-tissue  fibrils.  The 
fibrils  (Angelucci,  81.1,  157)  appear  in  the  chick  about  the  ninth 
day  and  in  cow  embryos  of  about  00  mm. ;  the  number  of  fibrils  is  at 
first  small,  but  increases  afterward  very  much.  A  h}-aloid  mem- 
brane is  also  developed  over  the  outer  or  anterior  surface  of  the  lens 
and  is  continuous  with  tho  suspensory  ligament.  The  lens  is  thus 
completely  covered  by  a  hyaloid  layer,  which  is  known  in  the  adult 
as  the  capsule  of  the  lens. 

The  wandering  cells,  which  are  found  in  the  adult  vitreous  humor, 
are  at  first  not  present,  but  immigrate  later — when,  I  do  not  know— 
although  they  can  be  distinguished  in  quite  early  stages.  They  take, 
of  course,  no  share  in  the  production  of  the  blood-vessels. 

Anterior  Mesenchyma  of  the  Eye. — The  lens  at  first  lies 
close  against  the  epidermis.  Later  the  mesenchyma  grows  in  be- 
tween and  forms  a  layer  of  some  thickness;  a  cavity  (anterior 
chamber  of  the  eye)  which  is  at  first  fissure-like  appears  in  the 
mesenchymal  layer,  and  divides  it  into  an  inner,  thinner  sheet  next 
the  lens,  and  an  outer,  thicker  sheet  next  the  epidermis ;  the  inner 
sheet  includes  part  of  the  tunica  vasculosa  of  the  lens  and  the  con- 
nective tissue  of  the  iris ;  the  outer  sheet  the  connective  tissue  of 
the  cornea.  The  cells  around  the  cavity  assume  an  epithelial  char- 


724  THE    FCETUS. 

acter,  epithelium  of  the  anterior  chamber,  which  covers  the  outer 
surface  of  the  iris  and  the  inner  surface  of  the  cornea. 

The  ingrowth  of  the  anterior  mesenchyma  begins  in  the  chick 
during  the  fourth  day,  in  the  rabbit  the  fourteenth  day ;  that  is  to 
say,  not  until  the  thickening  of  the  posterior  wall  of  the  lens  is  well 
advanced.  According  to  Kessler,  77.1,  a  homogeneous  layer  is 
formed  between  the  lens  and  epidermis  before  the  cells  penetrate 
there;  he  names  the  homogeneous  layer  cornea propri a,  and  con- 
siders it  a  product  of  the  epithelium,  but  Kolliker  ("  Entwickelungs- 
ges.,"  2teAufl.,  669)  points  out  that  it  is  more  naturally  to  be  regarded 
as  mesodermal  basal  substance.  The  cells  of  the  neighboring  mes- 
enchyma gradually  make  their  way  into  the  homogeneous  layer  and 
form  at  first  (chick,  sixth  day)  a  single  layer  between  the  lens  and 
ectoderm ;  thereafter  the  number  of  layers  of  cells  gradually  increases. 
Meanwhile  the  branches  of  the  arteria  centralis  retina  spread  out 
and  pass  on  to  the  anterior  surface  of  the  lens,  thus  converting  the 
innermost  part  of  the  mesoderm  in  front  of  the  lens  into  the  anterior 
portion  of  the  tunica  vasculosa  of  the  lens.  The  remaining  and 
thicker  portion  of  the  mesodermic  layer  between  the  lens  and  epider- 
mis is  the  anlage  of  the  connective  tissue  of  the  cornea. 

The  next  step  is  the  production  of  the  anterior  chamber  of  the  eye, 
which  arises  as  a  narrow  fissure  between  the  tunica  vasculosa  and 
the  corneal  anlage  (Kolliker,  "  Entwickelungsgeschichte, "  2te  Aufl., 
671).  In  mammals  there  appear  first  (cow  embryos  90mm.)  a  series 
of  small  spaces  between  the  papillary  membrane  and  the  cornea 
proper,  and  these  spaces  subsequently  fuse  into  a  continuous  fissure 
(Angelucci,  81. 1,  161).  I  have  observed  the  continuous  fissure  in  a 
rabbit  embryo  of  sixteen  days.  It  extends  at  first  only  to  the  edge 
of  the  pupil,  but  it  soon  develops  beyond  the  edge  (rabbit  eighteen 
days)  until  it  overlies  the  whole  of  the  uvea ;  by  this  means  the  iris 
is  formed ;  the  iris  is,  so  to  speak,  a  circular  shelf  of  mesenchymal 
tissue  bounding  the  pupil,  and  itself  bounded  externally  by  the  cavity 
of  the  anterior  chamber  and  covered  internally  by  the  uvea,  p.  722. 
Concerning  the  growth  of  the  anterior  chamber  wre  lack  precise  ob- 
servations. It  is  to  be  regarded  as  a  serous  cavity,  and  the  aqueous 
humor  as  a  serous  fluid  filling  the  cavity. 

In  the  chick  the  tunica  vasculosa  of  the  lens  does  not  extend  across 
the  pupilla;  the  first  layer  of  mesenchymal  cells  which  grows  in 
between  the  lens  and  epidermis  at  once  forms  a  thin  epithelium  (or 
so-called  endothelium),  and  the  space  between  this  layer  and  the  lens 
becomes  the  anterior  chamber;  the  layer  itself  becomes  the  inner 
layer  of  the  cornea),  Angelucci,  81.1. 

CANAL  OF  SCHLEMM. — This  term  is  applied  to  small  persistent 
vessels,  Fig.  417,  v,  on  the  inner  side  of  the  cornea  where  it  joins 
the  iris.  Angelucci,  81.1,  163,  has  observed  that  these  vessels 
appear  early  (pig  23  mm.)  and  persist  in  birds  and  mammals 
throughout  life. 

Cornea. — The  cornea  consists  of  two  layers :  1,  the  layer  of  meso- 
derm bounding  externally  the  anterior  chamber  of  the  eye;  2,  the 
epidermis  overlying  this  area. 

The  mesoderm  is  a  layer  of  mesenchyma  which  increases  in  thick- 
ness and  in  the  number  of  its  cells.  The  cells  next  the  anterior  cham- 


DEVELOPMENT   OF   THE   EYE. 


'25 


her  assume  an  epithelioid  character  and  finally  become  a  true  cuboidal 
•epithelium.  The  remaining  cells,  which  are  widely  separated  by 
basal  substance,  become  flattened  out;  they  are  commonly  termed 
cornea!  ror/;//.sv/r.s  in  the  adult;  bundles  of  connective-tissue  fibrils 
are  developed  in  the  basal  substance — just  when  is  uncertain. 
Against  the  epithelial  lining  of  the  anterior  chamber  and  against  the 
corneal  epidermis  is  developed  a  hyaloid  membrane  similar  to  that 
formed  by  the  vitreous  humor ;  these  membranes  have  been  named  re- 
spectively eldxh'cd  interim  (or  membrane  of  Descemet)  andelastica 
externa.  As  neither  membrane  contains  any  elastic  tissue,  both 
names  are  to  be  regretted.  Ifolliker  failed  to  find  either  hyaloid 
membrane  of  the  cornea  in  rabbits  of  twenty  days  (see  His,  "  Ent- 
\\  ickelungsges.,"  2te  Aufl.,  073)  and  it  is  probable  that  they  are  both 
developed  lato,  contrary  to  Kessler's  opinion.  The  corneal  mesoderm 
contains  blood-vessels  during  foetal  life  and  in  man,  at  least,  at  birth 
(Kolliker,  I.e.). 

The  corneal  epithelium  (epidermis)  develops,  so  far  as  known,  like 
the  epidermis,  but  its  development  is  arrested  at  what  I  have  called 
the  amphibian  stage,  that  is  to  say,  there  are  several  layers  of  cells, 
but  the  superficial  cells  are  not  flattened  out  and  there  is  no  stratum 
corneum.  Kolliker  (u  Entwickelungsgesch.,"  2te  Aufl.,  698)  has  ob- 
served in  rabbit  embryos  that  just  before  the  eyelids  meet  (eight- 
eenth day)  the  uncovered  part  of  the  corneal  epithelium  is  thickened, 
and  that  this  thickening  disappears  when  the  eyelids  unite. 

Iris  and  Ciliary  Muscle. — The  iris  results  from  the  extension 
of  the  anterior  chamber  of  the  eye ;  it  may  be  described  as  a  circular 


Uv 


R 


FIG.  417.— Section  through  the  Iris  Region  of  the  Eye  of  a  Chick  of  thirteen  Days.  Ep,  Epi- 
dermis; c//,  ciliary  muscle ;  oil',  ciliary  ligament;  F,  blood-vessel  (.canal  of  Schlemm):  C,  cornea; 
Aq,  aqueous  chamber;  /,  iris;  Ui\  uvea;  L,  lens;  pro,  ciliary  process;  Z.Z,  zomula  Zinnii ;  pcf, 
pectinate  ligament;  R,  retinal  layer;  p,  pigment  layer;  c/io,  choroid  layer.  After  Angelucci. 

shelf  of  mesoderm  covered  on  its  outer  side  by  the  mesodermic  epi- 
thelium lining  the  anterior  chamber  of  the  eye,  and  covered  on  its 
inner  side  by  the  uvea,  Fig.  417,  Uv.  The  mesoderm  of  the  iris,  1, 
is  directly  continuous  with  the  choroid  envelope,  c/zo,  of  the  eye,  and 
differs  in  character  from  the  mesenchyma  of  the  cornea  and  sclera, 
and  it  is  to  be  regarded  as  a  prolongation  of  the  choroid  layer,  cho. 
The  choroid  layer,  Fig.  417,  cho,  thickens  considerably  as  we 


726 


THE   FCETUS. 


approach  the  ciliary  processes,  pro,  from  the  retinal  side.  The 
thickening  subdivides  into  two  layers,  the  choroid  proper,  cho,  and 
the  ciliary  layer,  til',  cil,  pet;  the  ciliary  layer  is  at  first  alike  in 
structure  throughout  its  extent,  but  very  soon  the  part  of  this  layer 
nearest  the  ciliary  processes  and  the  iris  changes  in  character,  the 
tissue  becomes  looser,  the  cells  move  apart,  and  spaces  appear  be- 
tween them ;  the  cells  lengthen  out,  assume  a  more  fibrous  character, 
and  constitute  the  ligamentum  pectinatum  of  anatomy ;  the  spaces 
correspond  to  Fontana's  canals. 

The  remainder  of  the  ciliary  layer  is  converted  into  the  ciliary 
muscle ;  the  part  of  the  anlage  next  the  pectinate  ligament  becomes 
the  ligament,  cil ';  the  part  farther  from  the  lens  becomes  the  muscle 
proper,  cil;  the  boundary  between  the  two  parts  last  mentioned  is 
approximately  indicated  by  the  position  of  the  canal  of  Schlemm, 
Fig.  417,  V  (compare  p.  724).  Angelucci,  81.1,  162,  observed  the 
fibres  of  the  ciliary  muscle  to  be  transversely  striated  in  the  chick 
the  last  day  of  incubation. 

The  Eyelids  arise  quite  early  (cow  embryo  of  23  mm.,  rabbit  of 
sixteen  days)  as  two  folds  of  the  integument,  a  little  above  and  below 
the  cornea,*  and  they  grow  toward  one  another  until  they  actually 
meet  and  unite,  Fig.  418.  Each  fold  consists  of  undifferentiated 
mesenchyma  and  is  covered  on  both  sides  by  the 
epidermis.  The  folds  cover  not  only  the  cornea 
proper,  but  also  a  certain  space  around  it ;  this 
space  is  the  future  conjunctiva.  As  the  lids 
approach  one  another  the  epidermis  along  the 
edge  of  each  fold  thickens  (cow  embryo  of  35 
mm. ;  rabbit  embryo  of  nineteen  days) .  When 
the  edges  of  the  folds  meet,  their  epidermal 
thickenings  unite  and  all  trace  of  any  boundary 
disappears,  as  shown  by  Bonders  (Graefe's  Ar- 
chiv,  IV.,  291)  and  Schweigger-Seidl,  66.1, 
228 ;  it  is  said  that  the  lids  were  formerly  sup- 
posed to  be  simply  adherent,  but  in  reality  they 
actually  grow  together.  The  union  of  the  lids 
takes  place  in  man  early  during  the  third  month, 
in  the  rabbit  the  twentieth  day,  according  to 
Kolliker  ("Entwickelungsgesch.,"  2te  Aufl., 
698).  The  union  of  the  lids  is  probably  inherited 
from  reptilian  ancestors,  since  in  certain  reptiles 
the  union  is  permanent.  The  union  persists  in 
man  until  a  short  time  before  birth,  when  the 
eyelids  permanently  separate ;  the  separation  is, 
I  think,  probably  effected  by  the  breaking  down 
of  the  cells  in  the  centre  of  the  epithelial  layer 
uniting  the  eyelids.  The  eyelids  do  not  open  in  dogs  and  rabbits 
until  after  birth. 

The  histological  differentiation  of  the  eyelids  begins  after  they  are 
soldered  together.  The  epidermis  on  the  outside  produces  hairs. 
The  concresced  epithelium  of  the  edges  produces  large  hairs  (eye- 
lashes) and  sebaceous  glands.  The  latter  develop  in  a  similar 

*  Well  shown  in  Fig.  429  of  Kolliker's  "Entwickelungsgesch.,"  2te  Aufl.,  1879. 


FIG.  418.— United  Eye- 
lids of  a  Human  Embryo 
of  about  four  Months, 
seen  in  Vertical  Section, 
a,  United  surfaces;  m, 
muscle;  /i,  hair. 


l)i;\  IILOI'.MKNT   OF   THE   KAK.  fafi 

manner  to  the  sebaceous  glands  of  the  skin,  p.  502,  but  subsequently 
acquire  a  large  size  and  are  known  in  the  adult  as  the  Meibomian 
<//<indx.  The  free  ends  of  the  eyelashes  are  imbedded  in  the  epithe- 
lium between  the  lids  until  the  eyes  open.  The  mesoderm  develops 
three  layers:  an  outer,  continuous  with  the  dermis  of  the  neighbor- 
ing skin ;  an  inner,  continuous  with  the  connective  tissue  of  the  con- 
junctiva; and  a  middle,  in  which  muscle  fibres  are  developed,  Fig. 
4  1  s.  As  I  observe  the  muscles  of  the  eyelids  to  be  continuous  with 
the  platysma  of  the  head,  it  is  probable  that  they  are  modifications  of 
a  part  of  the  platysma. 

Membrana  Nictitans  (Third  eyelid,  plica  semilunaris,  Nick- 
haut). — The  third  eyelid  is  well  developed  in  birds,  etc.,  but  is  rudi- 
mentary in  man.  Concerning  its  development,  nothing  accurate  is 
known. 

Tear  Gland  (Lachrymal  gland,  Thranendruse).— The  tear  gland 
arises  in  man  during  the  third  month  as  a  solid  downgrowth  of  the 
epithelium  of  the  conjunctiva  on  the  nasal  side  of  the  eyeball  and 
close  to  the  upper  lid,  and  almost  at  once  forms  solid  branches;  the 
M>! id  anlage  afterward  becomes  hollow  (Kolliker,  "  Entwickelungs- 
gvs.,"  2te  Aufl.,  009).  The  formation  of  the  tear  gland  begins  in  the 
v-hick  the  eighth  day  (Remak,  50.1,  1)2). 

The  formation  of  the  lachrymal  duct  is  described  p.  580. 

Evolution  of  the  Vertebrate  Eye. — This  subject  is,  as  yet, 
by  no  means  ripe  for  discussion,  for  we  have  not  only  no  definite 
clew  to  the  homologies  of  the  vertebrate  eye  with  any  invertebrate 
eye,  but  also  no  collation  of  our  knowledge  of  the  eye  sufficient  to 
trace  the  comparative  anatomy  of  the  eye  within  the  vertebrate 
series.  In  regard  to  the  evolution  of  the  eye  within  the  vertebrate 
series  see  W.  Miiller,  74.1. 

There  are  two  hypotheses  as  to  the  origin  of  the  vertebrate  eyes : 
one,  that  they  are  derived  from  a  single  median  eye;  this  is  the 
hypothesis  of  those  who  put  foremost  the  affinities  of  vertebrates 
through  Amphioxus  with  the  tunicates,  which  are  animals  with  a 
single  median  eye;  the  other,  that  they  are  derived  from  the  paired 
eyes  of  annelids.  The  first  hypothesis  has  recently  found  an  advo- 
cate in  Howard  Ayers,  90.1,  228,  but  he  offers  little  in  support  of 
his  opinion  beyond  his  longing  to  establish  a  complete  homology 
between  Amphioxus  and  true  vertebrates.  The  second  hypothesis  is 
a  corollary  of  Semper 's  theory  that  vertebrates  were  evolved  from 
annelids,  and  because  that  theory  has  become  more  probable  as 
our  knowledge  has  increased,  it  follows  that  the  second  hypothesis 
has  also  gained  in  probability.  For  an  able,  though  speculative,  dis- 
cussion of  the  way  in  which  the  hypothesis  can  be  worked  out,  see 
J.  von  Kennell,  91.1. 

Anton  Dohrn's  hypothetical  speculations,  86.2,  as  to  the  phylo- 
genesis of  the  eye,  are  not  likely,  it  seems  to  me,  to  prove  of  perma- 
nent value. 

DEVELOPMENT  OF  THE  EAR. 

Morphologically  the  vertebrate  ear  consists  of  two  entirely  distinct 
parts:  1,  the  auditory  organ  proper,  the  otocyst,  or  so-called  mem- 
branous labyrinth;  2,  the  accessory  parts,  the  meatus  auditorius 


728  THE   FCETUS. 

externus,  the  tympanum  and  ear  bones,  and  the  Eustachian  tube,  to 
which  we  may  add  the  external  ear,  or  so-called  concha.  The  devel- 
opment of  these  two  parts  is  very  distinct ;  the  membranous  laby- 
rinth arises  as  an  invagination  of  the  ectoderm ;  the  ear  passages 
and  ossicles  arise  by  modifications  of  certain  of  the  branchial  arches 
and  clefts  of  the  embryo;  the  concha  again  has  an  independent 
development.  Accordingly  we  take  up  in  order  the  history  of  the 
otocyst,  of  the  auditory  passages  and  ear  bones,  and  of  the  external 
ear. 

The  membranous  labyrinth  is  developed  from  a  simple  otocyst, 
which  is  at  first  a  spheroidal  sac  of  epithelium,  and  arises  as  an 
invagination  of  the  ectoderm  (epidermis)  just  over  the  first  visceral 
or  branchial  arch.  In  the  history  of  the  labyrinth  it  is  convenient 
to  distinguish  the  following  stages:  1,  origin  of  the  otocyst;  2,  first 
appearance  of  the  recessus  labyrinthis  vestibuli ;  3,  commencement 
of  the  semicircular  canals ;  4,  outgrowth  of  the  cochlea ;  5,  separation 
of  the  sacculus  from  the  vestibule.  During  all  these  changes  the 
otocyst  or  labyrinth  is  a  closed  sac  or  cavity,  with  a  continuous  epi- 
thelial lining.  The  process  of  differentiation  may  be  considered 
twofold:  1,  the  gradual  conversion  of  the  simple  otocyst  into  a  very 
complex  one ;  2,  the  specialization  of  certain  areas  of  the  epithelium 
(maculaB  acusticaB),  connected  with  the  acoustic  nerve. 

1.  Origin  of  the  Otocyst. — The  ear  arises  as  a  lateral  pit, 
lying  somewhat  dorsally  and  opposite  about  the  middle  of  the  medulla 
„  oblongata,  and  just 

•"  T5  above  the  first  gill- 

fyTTT'Sfflv  vr  cleft.  The  pit  is  an 

»T  $M/\m  R'>^W^!^  invagination  of  the 

UK        ff%T  mm     outer  serm  la^er  <ec- 

0    tfeiil^ IJI]          toderm)  and  is  at  first 

Vift§>:^^- ' :- £w\          wide  open.    This 

HcT  i|l/^i  PlSS^P^i          stage  has    been    ob~ 

X40  McC---^^i^v^^|:W  served   by  His   in  a 

^^^^-^I^X  2. 0      human  embryo  of  2 . 4 

FTG.  419.— Sections  of  Human  Embryos  showing  the  Otocyst;  T      j_i '        i  •    i     j_i     '  ^ 

A,  embryoof  2.4  mm.  ;  B,  embryo  of  4mm.    ot,  Otocyst;  JV,  ner-  In  the  CI11CK  tne  nrst 

vous  system,  Md,  mandibular  arch;   2,  hyoid  arch;   ch,  chorda  oio-n      of     tViA     fnfnr-A 

dorsalis.     After  W    His.  blgn 

auditory   organ  is  a 

local  thickening  of  the  ectoderm,  usually  after  thirty  hours'  incu- 
bation; this  thickened  area  is  afterward  invaginated  and  forms 
the  lining  of  the  otocyst.  In  fact,  the  difference  between  the  newly 
arisen  auditory  vesicle  and  the  ectoderm,  in  respect  of  the  thickness 
of  the  two  epithelia,  is  very  striking,  and  the  character  of  the  otocyst 
epithelium  is  very  important,  because  it  exhibits  an  analogy  between 
it  and  the  rudimentary  ganglionic  sense-organs.  As  suggested  by 
Froriep,  there  is  probably  a  true  serial  homology  in  this  case ;  and 
the  ear  is  one  of  a  series  of  organs  extending  along  the  lateral  line, 
none  of  which,  except  the  ear  and  nose,  persist  in  mammals  save 
during  early  embryonic  stages,  cf.  p.  708.  In  short,  the  derivation 
of  the  complex  membranous  labyrinth  of  man  from  the  specialization 
of  one  of  a  long  series  of  general  sense-organs  in  lower  ancestral 
forms  is  extremely  probable. 


DEVELOPMENT   OF   THE   EAR. 


729 


The  mouth  of  the  pit  very  soon  closes  over,  and  the  invagination 
becomes  a  closed  sac,  which  quickly  loses  its  connection  with  the 
ectoderm,  making  a  separate  spheroidal  vesicle,  the  otocyst,  Fig.  419, 
B,  of.  The  sac  is  lined 
by  a  quite  thick  epithe- 
lium, which  contains 
tlu>  nuclei  scattered  at 
various  levels,  Fig.  420, 
and  is  built  up  appar- 
ently of  slender  colum- 
nar rrlls,  bellied  out 
A v here  each  cell  contains 
its  nucleus.  As  yet  the 
embryonic  connective 
tissue  (mesoderm)  has 
•formed  110  envelope 
around  the  epithelium, 
but  later  the  cells  about 
tin*  vesicle  condense 
around  it  and  constitute 
a  sub-epithelial  mem- 
brane. The  epithelium 
retains  its  cylindrical 
form  over  and  imme- 
diately on  the  borders  of 
all  the  areas,  where  the 
sensory  hair-cells  or  so- 
called  auditory  cells  are 
developed;  over  all  the 

remaining  portions  it  FjQ  430. -Horizontal  Section  of  the  Otocyst  of  a  Chick  of 
Ultimately  thins  OUt,  be-  the  third  Day.  Of,  Otocyst;  Ep,  epidermis;  Br,  walls  of  the 

c,  ,n,  ing  efther  a  cuboidal  brain  <medu'la) :  *  »'  blood-ves' 

or  a  pavement  epithelium.  In  fishes  there  are  seven  ;  in  amphibians, 
reptiles,  and  birds  eight,  in  man  only  six,  of  these  areas  of  sensory 
cells.  It  is  desirable  to  call  attention  to  this  thinning  out,  because 
it  is  usual  to  find  it  stated  that  a  thickening  arises,  when,  in  reality, 

it  is  the  thinning  of  adjacent  parts  which 
effects  the  differentiation,  and  though 
there  may  be  an  absolute  thickening  also, 
yet  the  thinning  round  about  is  the  prin- 
cipal factor. 

2.  Recessus  Vestibuli. — The  oto- 
cyst next  loses  its  spherical  form  by  the 
development  of  a  prolongation  on  the 
dorsal  side,  in  consequence  of  which  it 
acquires  somewhat  of  a  pear  shape.  The 
upper  tapering  end  is  the  commencement 
of  the  recessus  vestibuli  or  aquceductus 
endolymphaticus,  Fig.  421,  Eec.  The 
acoustico-facial  ganglion  is  in  actual  con- 
tact, as  stated  p.  646,  with  the  anterior  wall  of  the  otocyst,  and  it  is 
probable  by  analogy  that  the  otocystic  epithelium  gives  off  cells, 


FIG.  421.  —Left  Otocyst  of  a  Human 
Embryo  of  about  four  Weeks;  A, 
from  the  inner,  B,  from  outer  side. 
i;«\  K.-c.-ssus  vestibuli,  V,  vestibu- 
lar region;  C  cochlear region,  After 
W  His,  jun.  X  30  diains. 


730 


THE    FCRTUS. 


which  join  the  ganglion ;  compare  the  history  of  the  olfactory  gan- 
glion, p.  637.  The  lower  portion  soon  changes  its  configuration, 
and  in  a  human  embryo  of  about  four  weeks  was  found  by  Kolliker 
("  Entwickelungsges.,"  2te  Aufl.),  to  have  a  new  rounded  protuber- 
ance behind  and  a  little  outside  the  base  of  the  recessus,  which 
marks  the  situation  of  the  future  vestibule ;  some  traces  of  the  semi- 
circular canals  were  already  indicated ;  the  lower  end  of  the  pear 
was  somewhat  elongated  preparatory  to  the  outgrowth  of  the  coch- 
lea. The  otocysts  at  this  time  lie  near  the  middle  of  the  hind-brain, 

Figs.  171,  D,  and  338,  of; 
as  seen  in  cross  sections, 
Fig.  422,  of  the  head  the  oto- 
cysts are  pear-shaped  and 


S.e 


chs 


Md 


FIG.  422. —Transverse  Section  of  the  Head  of  a  Rabbit 
Embryo  of  ten  and  one-half  Days.  IV,  Fourth  ventricle ; 
Ep,  ependyma;  7?v,  recessus  vestibuli;  Vt  vestibule  of 
otocyst;  Ve,  vein;  Md,  mandible. 


FIG.  423.— Left  Otocyst  of  a  Hu- 
man Embryo  of  about  five  Weeks, 
seen  from  outside  and  below.  N.  r, 
Saccus  endolymphaticus ;  cs,  upper ; 
ci,  lower;  chs,  horizontal  semicir- 
cular canal;  Ut,  utriculus;  .SVrr.sac- 
culus;  cc/i,  cochlea.  After  W.  His, 
Jr.  X  30  diains. 


closely  appressed  to  the  dorsal  zones  of  His  of  the  medulla  oblongata. 
The  recessus  vestibuli  rapidly  enlarges,  and  its  upper  end  becomes  di- 
lated, Fig.  423,  to  form  the  saccus  endolymphaticus,  /S.e,  the  narrower 
portion  develops  into  the  ductus  endolymphaticus  of  authors.  The 
ductus  subsequently  becomes  greatly  elongated,  and  reaches  through 
the  whole  pars  petrosa,  so  that  the  saccus  lies  within  the  skull  in 
the  dura  mater.  Kolliker  (in  his  "  Entwickelungsgeschichte, "  pp. 
744,  745)  gives  a  fragmentary  series  of  measurements  of  the  recessus 
in  mammalian  embryos  of  various  ages. 

3.  The  semicircular  canals  arise  next  from  the  walls  of  the 
primitive  vestibule,  and  rapidly  acquire  great  prominence,  while  the 
cochlea  grows  out  slowly.  Each  canal  first  appears  as  a  narrow  fold, 
Fig.  371,  A.sc,  P.sc,  Ek.  sc,  a  wide  but  thin  evagination.  In  the  mid- 
dle of  each  evagination  the  opposite  walls  meet  and  coalesce,  leaving 
only  the  rim  of  the  original  flat  pouch;  this  rim  is  the  permanent 
semicircular  canal.  N".  Riidinger,  88.2,  asserts  that  each  semi- 
circular canal  arises  from  two  buds,  w<hich  elongate  as  blind  tubes,  and 


DEVELOPMENT   OF   THE   EAR.  731 

the  tubes  uniting  form  a  complete  caiial.  The  later  investigations 
of  R.  Krause,  90.1,  and  W.  His,  Jr.,  89.1,  confirm  the  older,  not 
Riidinger's  view.  The  canals  do  not  all  develop  synchronously; 
the  upper  vertical  canal  is  first  differentiated  (R.  Krause,  90.  1,  300), 
next  the  lower  vertical  canal,  and  last  the  outer  or  horizontal  canal. 
\V.  His  found  in  a  human  embryo  of  five  weeks  that  the 
three  evaginationfi  were  present,  but  only  the  two  vertical  canals 
had  become  rings,  Fig.  423.  The  further  development  of  the  canal 
consists  in  the  gradual  assumption  of  the  adult  form  and  size,  the 
ampull-e  appearing  quite  early,  Fig.  4-25.  The  embryonic  connective 
tissue  about  the  organ,  as  a  whole,  is  gradually  converted  into  carti- 
lage and  ultimately  ossifies.  The  connective  tissue  (mesoderm) 
immediately  about  the  otocyst  has  a  different  history,  which  may 
be  readily  followed  in  connection  with 
the  study  of  the  semicircular  canals, 
and  hence  may  be  mentioned  now. 
In  Fig.  4>.).">.  the  epithelial  semicircu- 
lar ranal,  /y/,  /,  is  seen  surrounded  by 
a  cartilage,  r,  but  separated  from  it 
1  >y  a  thick  layer  of  gelatinous  tissue, 
f/,  and  the  fibrous  perichondrium  (fu- 
ture1 periosteum)  ,  /.  Later,  the  layer, 
'/,  is  separated  into  a  thin  subepi- 
thelial  layer,  which  persists,  and  a 
main  or  gelatinous  layer  proper, 
which  atrophies,  thus  leaving  the 

\.                                 ,    .  ,                   T  FIG.    434.  —  Transverse    Section    of    the 

peri-lymphatlC  Space  a  DOUt  the  Canal.  Semicircular  Canal  of  an  Embryo  Rahi.il 

Tlid  crolatirwme  l*i\-«»r    fon<i<t«    r»f    an  of  twenty-four  Days,     m,  /,  Epithelium  of 

1  lie  gelatmOUS  la}  er    COnSlfetS   Or    an-  the  canal.   ^  gelatinous  tissue:    c,  carti- 


, 

Connective-tissue      Cells,      la*e:    /•    fibrous    perichondrium.      After 
.    ,  ,.  T7-..1T1  T        •!      Kolliker.     x  41.5  diains. 

with,  according  to  Kolliker,  a  liquid 

matrix.  The  meshes  of  the  network  gradually  increase  in  size,  until 
finally  only  a  few  threads  are  left,  thereby  establishing  the  condition 
in  the  adult.  As  far  as  known,  the  whole  of  the  peri  -lymphatic 
spaces  are  formed  in  this  manner,  including,  of  course,  the  scala 
tympani  and  the  scala  vestibuli  of  the  cochlea. 

The  ampulkc  of  the  semicircular  canals  appear  quite  early  as 
enlargements  of  the  canals  and  develop  each  a  macula  acustica, 
which  is  stated  by  Kolliker  to  be  found  in  older  embryos  covered 
with  a  delicate  cuticula  of  considerable  thickness,  the  membrana 
•eetoria  of  Hasse,  the  cupula  terminalis  of  Lang. 

4.  The  cochlea  is  the  third  part  to  grow  out  from  the  primordial 
otocyst,  Fig.  4-2:5,  cell  :  the  commencing  outgrowth  may  be  observed 
in  a  human  embryo  of  five  weeks,  a  sheep  embryo  of  1C  mm.,  pig 
of  IS  mm.,  rabbit  of  10  mm.  It  arises  from  the  lower  end  of  the 
vesicle  and  grows  downward,  inward,  and  forward,  Fig.  423,  as  a 
canal  somewhat  flattened  in  one  diameter  and  therefore  oval  in 
transverse  section.  The  epithelial  cochlear  canal  lengthens  very 
much,  and,  as  it  lengthens,  curves  more  and  more,  Fig.  425  ;  on  its 
concave  upper  side  appears  the  commencement  of  the  future  ganglion 
spirale,  compare  p.  646.  The  cochlear  canal  is  the  anlage  of  the 
wain  media  of  the  adult.  In  the  stage  of  Fig.  423  it  closely  resem- 
1  >les  the  condition  found  in  adult  monotremes,  and  also  the  lagena 


732 


THE   FOETUS. 


FIG.  425.— Left  Otocyst  of  a  Human  Embryo  of 
about  two  Months.  Ant,  Anterior,  Ex,  external, 
post,  posterior  semicircular  canal;  Cms,  crus 
commune;  c.reun,  canal  is  reuniens;  Aq,  aquae- 
ductus  vestibuli ;  Ut,  utriculus ;  Sac,  sacculus ; 
9och,  cochlea.  After  W.  His,  Jr.  x  20  diams, 


(cochlea)  of  birds.  By  further  elongation  and  coiling  the  canal 
gradually  assumes  the  final  shape  of  the  scala  media.  In  man  there 
is  one  complete  coil  by  the  eighth  week,  Fig.  425,  and  by  the  twelfth 

week  all  the  coils  are  formed. 
Baginsky,  86.1,  has  observed 
that  in  very  young  rabbit  em- 
bryos there  are  numerous  karyo- 
kinetic  figures  in  the  walls  of 
the  cochlear  canal;  later  (em- 
bryos of  50-60  mm.)  they  can 
no  longer  be  found;  they  con- 
tinue longest  at  the  apex ;  these 
observations  show  that  the  canal 
grows  throughout  its  extent  and 
not  merely  at  its  apex. 

Histogenesis  of  the  Cochlea. 
— Our  knowledge  of  this  subject 
rests  principally  upon  the  elabo- 
rate researches  of  Bottcher, 
89.1,  which  have  been  con- 
firmed and  supplemented  by 
Gottstein,  Kolliker  ("  Entwick- 
elungsgeschichte,"  2te  Aufl.), 
Pritchard,  79.1,  Baginsky, 
86.1,  and  others.  The  histo- 
logical  development  of  the  coch- 
lea is  the  same  throughout  its  entire  length,  but  progresses  most  rap- 
idly at  the  base,  or  the  stretch  nearest  the  vestibule.  I  append  here  the 
complete  history  of  the  cochlea.  The  first  change  in  the  epithelium 
is  in  the  height  of  the  cells,  those  upon  the  upper  side  thin  out ;  in 
other  words,  that  portion  of  the  epithelium  decreases  in  thickness ; 
it  remains  a  perfectly  simple  columnar  epithelium,  Fig.  426,  Ep,  and 
forms  the  lining  of  one  side  of  Reissner's  membrane,  and  the  outer 
wall  of  the  scale.  The  lower  portion  of  the  epithelium  which  remains 
thicker  forms  the  crista,  the  sulcus,  and  Corti's  organ.  The  two 
divisions  of  the  epithelium  are  not  sharply  separated,  but  pass  grad- 
ually into  each  other. 

The  second  change  is  that  the  loss  in  thickness  of  the  epithelium 
is  continued  on  the  under  side,  or  the  wall  next  the  scala  tympani,  so 
as  to  leave  two  thick  epithelial  ridges,  which  are  of  very  unequal 
dimensions.  The  larger  ridge  lies  nearest  the  columella  and  be- 
comes the  thick  lining  of  the  sulcus  spiralis.  It  very  early 
acquires  a  thick  cuticula,  the  beginning  of  the  meinbrana  tectoria. 
A  very  different  view  is  announced  by  Howard  Ayers,  91.2,  who 
states  that  he  has  ascertained  that  the  membrana  tectoria  is  really 
composed  of  very  long  hairs,  which  spring  from  the  cells  of  the 
organ  of  Corti,  and  that  it  is,  therefore,  in  no  sense  a  cuticular 
structure.  The  smaller  ridge  lies  nearer  the  ligamentum  spirale, 
and  is  metamorphosed  into  the  organ  of  Corti,  including  the  sup- 
porting cells,  the  inner  and  outer  hair  cells,  and  Corti's  rods.  Very 
soon  after  the  two  ridges  are  distinctly  formed,  the  lamina  spiralis 
begins  to  grow  up  between  the  sulcus  or  broad  inner  ridge,  and 


DEVELOPMENT   OF   THE   EAR. 


733 


the  axis  of  the  cochlea  to  develop  into  the  crista.  The  epithelium  on 
the  crista  is  thus  maintained  with  its  upper  surface  even  with  that 
of  the  sulcus.  Over  both  parts  stretches  the  cuticula,  Fig.  4\M'»,  /;//, 
which  gradually  thickens  into  the  fully  developed  tectorial  membrane, 
which  has  been  hitherto  supposed  to  have,  at  no  time,  any  histo- 
genetic  connection  with  the  organ  of  Corti — compare  the  reference 
t<>  Avers'  view  above — although  it  grows  out  so  far  as  to  overhang 
it.  The  membrane  always  remains  firmly  attached  to  the  crista, 
but  is  loosely  united  to  the  epithelium  of  the  sulcus  internus,  and  in 
the  adult  it  is  probably  entirely  separated  from  the  sulcus  and  at- 
tached only  to  the  crista.  From  some  unknown  cause  the  lower 
boundary  of  the  epithelium  of  the  crista  becomes  indistinguishable. 
The  cells  in  the  sulcus  apparently  assume  an  oblique  position,  so 
that  in  sections  there  seem  to  be  several  layers  of  cells.  Middendorf 
and  others  have  been  misled  to  describe  a  stratified  (mehrschichtiges) 
epithelium  in  the  sulcus. 

The  small  ridge  or  anlage  of  the  organ  of  Corti,  Fig.  426,  1-7, 
is  made  up  of  four  sets  of  cells ;  each  set  is  disposed  in  a  longitudinal 
row  following  the  spinal  curve  of  the  cochlea.  The  first  row,  or 
that  nearest  the  sulcus,  sul,  is  composed  of  a  single  line  of  cells,  the 
future  inner  hair  cells.  The  second  row  is  composed  of  two  lines 
of  cells,  1,  2,  the  future  rods  of  Corti;  in  early  stages,  as  shown  in 


., 


SUl 


G00 


N 


v.sp* 


FIG.  426.— Transverse  Section  of  Scala  Media  Cochleae  of  a  Rabbit  Embryo  of  55  mm.  Ep, 
epithelium  of  Reissner's  membrane;  mf,  membrane  tectoria;  $r/Z,  sulcus;  Jv,  nerve:  v.sp,  vas 
spirale;  1-6,  cell  rows  of  Corti 's  organ;  7,  Deiter's  cells;  8,  outer  cells.  After  Baginsky. 

the  figure,  the  cell  next  the  inner  hair-cell  is  considerably  larger 
than  its  fellow,  but  later  their  relative  sizes  are  reversed  (Baginsky, 
86.1) ;  the  third  row  includes  three  main  lines  of  cells,  3,  4,  5,  the 
outer  hair  cells;  and  the  fourth  row,  6,  has  several  lines  of  cells, 
which  become  the  supporting  cells.  The  further  differentiation  of 


734 


THE   FCETUS. 


v.sp 


the  four  rows  is  followed  best  in  transverse  sections  of  the  ridge,  and 

in   the   following  description  reference  is  made  to  the  appearance 

seen  in  such  sections. 

The  inner  cell  slopes  toward  the  centre  of  the  ridge,  has  a  broad  base, 

a  narrower   top,  and  a  single  clear,  oval  nucleus  toward  its  basis, 

Fig.  427,  1.  It  becomes 
the  inner  hair-cell,  with 
a  distinct  nucleus  of  its 
own,  a  somewhat  coarsely 
granular  protoplasm,  and 
a  horseshoe  of  hairs  upon 
its  free  upper  surface, 
and  a  tapering  base, 
which  last  is  probably 

FIG.   427.— Section  through  Cortrs  Organ  of  the  Lower  -,         -.a 

Coil  of  the  Cochlea  of  a  Rabbit  Embryo  of  75  mm.     m.  has,  Connected    Wltll    a    liei'Ve- 

Membrana   basilaris;    1,  inner   hair  cell;   2,  Corti's  cells.  fiV»ro        W"hem     +Vif>     anrli 
Other  letters  as  in  Fig.  426.    After  B.  Baginsky.  re-        VV  n 

tory  cells  of  Corti  s  organ 

are  viewed  from  the  surface,  the  hairs  are  seen  to  mark  out  a  horse- 
shoe on  the  top  of  each  cell.  The  open  end  of  the  horseshoe  always 
faces  inward,  i.e.,  toward  the  columella.  The  base  of  the  cell  also 
acquires  one,  or,  according  to  Bottcher,  two  nuclei ;  the  cell  becomes 
finely  granular,  and  is  finally  incorporated  in  Waldeyer's  "  Korner- 
schicht."  Baginsky,  86. 1,  29,  maintains,  probably  rightly,  that  the 
two  nuclei  below  the  inner  hair-cell  belong  to  distinct  cells,  and  are 
not  derived  from  the  inner  hair-cell ;  he  compares  them  to  the  so- 
called  Deiter's  cells  between  the  bases  of  the  outer  hair-cells. 

The  second  and  third  cells  broaden  at  their  bases,  where  lie  their 
spherical  nuclei.  The  bases  widen  out  rapidly  (immediately  after 
birth  in  dogs)  until  the  two  cells  form  a  triangle  in  section;  the 
width  of  the  base  of  the  triangle  exceeds  its  height.  Bottcher,  69.1, 
supposed  that  this  triangle  was  a  single  cell  with  two  nuclei ;  that 
Bottcher  was  in  error  was  shown  by  B.  Baginsky,  86.1,  26. 
Meanwhile  the  two  nuclei  place  themselves  near  the  two  lower  angles 
of  the  cells.  Next,  the  cells  lose  their  finely  granular  appearance 
and  become  striated  (rabbit  embryo  of  75  mm.),  first  along  the  inner 
side  of  the  inner  rod-cell,  or  next  the  inner  hair-cell ;  second,  along 
the  outer  side  of  the  outer  rod-cell,  or  next  the  outer  hair-cells.  The 
striated  lateral  portions  of  the  two  cells  form  the  two  Corti's  rods, 
sensu  strictu.  A  triangular  space  between  the  rods  and  the  base- 
ment membrane  is  soon  hollowed  out,  thus  forming  the  tunnel  under 
the  arching  rods.  The  protoplasm  of  the  cells  is  next  reduced  to 
small  nucleated  masses,  one  at  the  base  of  each  rod.  The  further 
development  takes  place  principally  by  the  growth  of  the  rods,  until 
they  assume  their  ultimate  shape  and  size.  Recent  investigations 
have  added  little  to  the  account  of  the  structure  of  Corti's  rods, 
given  by  Waldeyer  in  Strieker's  "Handbuch,"  1872,  pp.  931-034. 

The  third  band,  which  is  three  cells  wide,  Fig.  427,  3,  4,  5,  forms  the 
outer  hair-cells.  Like  the  other  cells,  they  acquire  two  nuclei,  a  larger 
oval  one  above,  and  a  smaller  one  below.  This  was  first  observed 
by  Pritchard.  The  two  parts  around  the  two  nuclei  early  become  sep- 
arated into  an  upper  cell  (Corti's  cells  or  absteigende  Horzellen)  and 
a  lower  cell  (Deiter's  cells  or  aufsteigende  Horzellen),  7.  The  base 


DEVELOPMENT    OF   THE   EAR.  ?;J5 

of  the  upper  cells  is  at  first  rounded  off,  but  subsequently  a  fine  pro- 
cess extends  down  to  the  membrana  basilaris,  and  the  base  tapers 
gradually  into  the  process.  The  cells  become  slenderer,  and  acquire 
an  oblique  position  about  the  time  of  birth.  The  rod  (Stcibclicn  or 
Haupthaar)  and  the  horseshoes  of  hairs  (cf.  xnj>ra)  are  developed 
upon  the  free  ends  of  the  cells  during  the  later  stages  of  foetal  life. 
The  lower  cells  taper  at  their  upper  ends,  which  are  continued  each 
by  a  fine  process.  They  were  supposed  by  Waldeyer  and  others  to 
be  united  in  the  adult  with  the  upper  cells,  thus  forming  twin  cells, 
which  have  been  most  fully  described  by  Lavdowsky  and  Nuel. 
The  development  of  these  twin  cells  is  by  no  means  clearly  under- 
stood yet.  The  upper  and  lower  cells  appear  distinctly  separate 
in  new-born  and  young  animals.  The  upper  cells  enlarge  at  the 
expense  of  the  lower.  The  nucleus  becomes  smaller  and  is  placed 
near  the  top  of  the  cell.  The  rod  (Haupthaar)  disappears.  The 
horseshoe  of  hairs  opens  toward  the  Corti's  rods,  as  can  be  best  seen 
in  silver  preparations.  The  hairs  are  more  like  short  rods,  vitreous, 
with  rounded  ends,  and  are  parts  of  the  cell,  not  of  the  membrana 
reticularis.  The  basal  process  of  the  upper  cell  is  inclosed  by  (Lav- 
dowsky) or  fused  with  (Nuel)  the  body  of  the  lower  cell.  The  tops 
of  the  upper  cells  (Corti's  absteigende,  or  Stabchenzellen,  Lavdow- 
sky) occupy  the  rings;  the  tops  of  the  processes  of  the  lower  cell 
<  HV u py  the  phalanges  of  the  membrana  reticularis.  The  lower  part 
of  the  united  cells  appears  as  their  common  body,  and  contains  the 
lower  nucleus.  The  nerve-fibre  unites  with  the  cell  at  the  side  near 
the  lower  nucleus.  The  twin  cells  end  below  by  a  single  basal  pro- 
cess. The  above  account  is  mainly  from  Lavdowsky.  Nuel  agrees 
with  him  in  the  main,  but  the  latter's  paper  I  know  only  from  the 
abstract  in  Hoffmann's  and  Schwalbe's  Jahresbericht. 

Connected  with  the  third  row,  or  outer  hair-cells,  are  various 
structures,  which  are  probably  to  be  grouped  under  the  general  head 
of  intercellular  formations.  Of  these  the  most  important  are  the 
"  Stutzf aseru  "  (supporting  fibres)  and  the  incnibrana  reticularis. 
The  latter  is  generally  regarded  as  the  exposed  edges  of  the  intercel- 
lular substance,  the  rings  and  phalanges  being  the  spaces  where  the 
free  end  of  the  hair-cells  are  exposed.  The  "  Stutzf asern"  form  a 
network  underneath  the  tunnel,  and  also  a  finer  network  between 
the  outer  hair-cells.  They  were  dimly  recognized  by  Bottcher, 
clearly  seen  by  Nuel,  and  elaborately  described  by  Lavdowsky. 

The  fourth  row  of  cells,  Fig,  427,  6,  undergoes  no  striking  differ- 
entiation; it  decreases  in  height  from  the  hair-cells  outward,  *so  that 
the  row  merges  gradually  into  the  low  cells  of  the  zo na  pectinata. 
Klein  states  that  in  the  guinea-pig  the  supporting  cells  do  not  form, 
as  is  usually  the  case,  a  simple  continuation  of  the  last  row  of  the 
outer  hair-cells,  but  ride  upon  the  sides  of  the  hair-cells. 

Underneath  the  organ  of  Corti  is  developed  the  membrana  basi- 
Ifiris.  A  large  space  is  developed  in  the  mesenchyma  underneath 
the  organ ;  this  space  is  the  scala  tympani,  and  is  apparently  a  lymph- 
chamber.  Between  the  scala  tympani  and  the  organ  of  Corti,  there 
remains  a  sheet  of  connective  tissue,  which  contains  the  vas  spirale, 
Fig.  427,  v.sp,  and  is  the  anlage  of  the  membrana  basilaris.  The 
cells  next  the  epithelium  of  the  scala  media  flatten  out,  their  nuclei 


736 


THE   FCETUS. 


elongate  and  take  radial  positions,  Fig.  427,  m.bas,  thus  marking 
out  a  subepithelial  layer  from  the  loose  connective  tissue  below ;  the 
looser  tissue  gradually  disappears;  the  denser  subepithelial  layer 
becomes  the  permanent  membrana,  in  which  we  can  distinguish 
three  layers:  a  thin  homogeneous  basement  membrane  next  Corti 's 
organ,  a  homogeneous  nucleated  layer,  and  a  lowest  fibrillar  layer. 
The  spiral  vessel  underlies  the  rods  of  Corti ;  the  basilar  membrane 
as  described  is  developed  only  beyond  the  vessel ;  that  is,  underneath 
the  outer  hair-cells.  Embryologically  speaking,  the  so-called  inner 
zone  (habenula  tecta)  is  not  a  part  of  the  true  basilar  membrane 
(B.  Baginsky,  86.1,  31-34). 

According  to  the  preceding  summary,  the  cochlea  is  a  tubular 
extension  of  the  lower  sicfe  of  the  primitive  ectodermal  otocyst ;  upon 
one  side  of  this  tube  are  two  ridges;  a  larger  one,  which  forms  a 
thick  cuticula,  the  membrana  tectoria,  and  a  smaller  one  which, 
through  complicated  differentiations,  becomes  the  organ  of  Corti. 
The  nerves  grow  to  the  hair-cells. 

5.  Sacculus  and  Utriculus.— The  separation  of  the  sacculus 
has  been  studied  principally  by  Bottcher.  There  is  first  developed  a 
constricted  tube,  the  canalis  reuniens,  Fig.  428,  b,  between  the  base 

of  the  cochlea  and  the  central 
otocystic  cavity.  Afterward 
appears  a  ring-shaped'  constric- 
tion,^/, around  the  main  cavity 
(primitive  vestibule,  Kolliker), 
by  which  it  is  divided,  in  most 
mammals,  into  two  cavities  con- 
nected by  a  narrow  canal,  into 
which  opens  the  recessus  laby- 
rinthi  (ductus  endolymphati- 
cus  of  Hasse) ;  hence  the  reces- 
sus appears  to  have  two  legs, 
derived  from  the  canal ;  one  leg 
leading  into  the  upper  secondary 
cavit}r,  the  adult  utriculus, 
Fig.  423,  428,  and  the  other 
into  the  lower  cavity,  the  adult 
sacculus  rotundus,  Fig.  425, 

— i    ^^^^^'\/  as  428.    These  relations,  as  well  as 

4      ^\v^aM     ^^><^  ^e  Ofoer  essential  dispositions 

of  the  parts  of  the  labyrinth, 
are  sometimes  all  recognizable 

FIG.  428.— Section  through  the  Jnternal  Earjof  a    in  a  single  fortunate  Section,  as 

in  Fig.  428,  or  better  in  a  model, 
Fig..  425,  reconstructed  from 
sections.  In  man,  however,  the 
relations  are  somewhat  different 
in  that  the  ductus  opens  directly  into  the  sacculus  (Fig.  429,  de). 
The  developmental  process  resulting  in  this  disposition  has  not  yet 
been  followed  out. 

The  maculce  acusticce  of  the  sacculus  and  utriculus  arise  as  cir- 
cumscribed areas  where,  as  before  stated,  the  epithelium  remains 


Sheep  Embryo,  28  mm.  (After  Bottcher).  D.M, 
Dura  mater;  R.V,  recessus  vestibuli :  H.B.  V,  pos- 
terior  vertical  canal ;  {/,  utriculus ;  H.  B,  horizontal 
canal;  a./,  the  constriction  between  sacculus,  S.R, 
and  utriculus;  6,  canalis  reuniens;  ec,  cochlea; 
K.K,  and  K.B,  cartilage;  c/i,  chorda  dorsalis. 


DEVELOPMENT   OF   THE   EAR. 


737 


thick,  and  is  differentiated  into  auditory  cells  of  elongated  forms, 
with  hairs  on  the  free  ends. 

Of  the  otoliths  the  development  is  unknown.  Kolliker  merely 
s.-iys,  they  "appear  as  quite  small  punctifonn  bodies,  and  remain  a 
long  time  in  that  form,  until  they  finally  increase  in  size  and  gradu- 
ally assume  a  crystalline  form  "  (u  Entwickelungsgeschichte, "  1879, 
p.  loo). 

The  development  of  the  definite  form  of  the  inner  ear  is,  as  we 
from  the  investigations  of  Retzius,  nearly  complete  by  the  end 


\\ 


/ 


IK;.  -129. — Isolated  Right  Membranous  Labyrinth  of  Human  Embryo  ot  six  Months  seen 
from  in  front  and  outside.  Magnified  about  eight  diameters,  ca,  Anterior  semicircular  canal; 
•\tiTnal  semicircular  canal;  c.p,  posterior  semicircular  canal;  aa,  anterior,  ae,  exterior, 
<i/>.  posterior,  ampulla?;  cr,  crista  ampullae;  rae,  ranulus  ampulla)  communis;  mtt,  macula 
utriruli;  /.  nrrvus  facialis;  ms,  macula  acustica  sacculi ;  /.  lagena  cochlearis;  m&,  membrana 
hasilaris:"/.s.  ligamentum  spirale;  >-t>,  ranulus  basilaris  of  the  cochlear  nerve;  esc,  canalis  sac- 
culo-oochlearia  or  reuniens  Henseni.;  t'6,  Vorhofsblindsack  of  Retzius;  rap,  nerve  of  the  pos- 
terior ampullae;  or',  crista  acustica  of  the  same;  ss,  sinus  utriculi  superior;  de,  ductus  endo- 
lymphaticus,  with  its  internal  opening,  cus;  rec,  recessus  utriculi.  After  Retzius. 

of  the  sixth  month  of  foetal  life,  as  shown  by  the  accompanying  Fig. 
-1  •.".),  which  represents  the  isolated  right  labyrinth  of  a  six  months' 
human  embryo,  seen  from  in  front  and  the  outside.  In  the  figure 
the  most  conspicuous  parts  are  the  semicircular  canals,  the  cochlea, 
and  the  nerves  stained  dark  by  the  osmic  acid  with  which  the  prep- 
aration had  been  treated.  The  cochlea  is  a  long  spiral,  commencing 
with  a  central  blind  end,  Z,  and  making  two  and  one-half  turns,  and 
continuing  off  tangentially  toward  the  posterior  ampulla,  ap,  to  end 
in  a  small  blind  pouch,  vb,  theVorhofsblmdsack  of  Retzius.  At  the 
base  of  the  pouch  springs  a  small  canal,  esc,  canalis  sacculo-coch- 
learis  (canalis  reuniens  Henseni) ,  which  affords  direct  communi- 
cation with  the  sacculus.  In  the  cochlea  (as  shown  in  the  figure) 
we  can  distinguish  the  ligamentum  spirale,  l.s,  the  membrana  basi- 
47 


*738  THE   FCETUS. 

laris,  mb,  and  the  branches,  rb,  of  the  cochlear  nerve.  The  three 
semicircular  canals — anterior,  ca;  external,  ce;  and  posterior,  cp — 
together  with  their  respective  ampullae,  aa,  ae,  ap,  are  easily  iden- 
tified. The  anterior  and  posterior  canals  have  a  common  stem,  ss, 
which  leads  into  the  wide  utrtculus,  u;  from  the  utriculus  a  second 
canal  leads  into  the  posterior  ampullae,  ap;  finally  from  the  upper 
portion  of  the  utriculus  arises  a  wide  coecal  evagination,  rec,  the 
recessus  utriculi,  the  development  of  which  has  not  been  yet  fol- 
lowed out,  so  far  as  I  am  aware.  The  canalis  reuniens,  esc,  leads 
into  the  sacculus  rotundus,  which  has  on  one  side  a  large  macula 
acustica,  ms,  and  on  the  other  communicates  with  the  ductus  endo- 
lymphaticus,  de,  of  which  only  the  commencement  is  shown  in  the 
figure ;  in  reality  it  extends  clear  through  the  pars  petrosa,  and  ter- 
minates in  the  dura  mater  with  a  blind  enlargement.  It  is  note- 
worthy that  the  ductus  opens  into  the  sacculus  rotundus,  and  not, 
as  in  many  mammals,  into  the  canal  between  the  sacculus  and  utri- 
culus. The  last-mentioned  canal  may  be  seen  in  part  between  the 
points  lettered  mu  and  ms  in  the  figure.  From  this  description  it 
is  evident  that  the  labyrinth  is  merely  an  otocyst  of  extremely 
complex  form,  and  is  still  a  closed  epithelial  sac,  •  continuous 
through  all  its  parts.  The  acoustic  nerve  reaches  the  neighborhood 
of  the  labyrinth  in  company  with  the  n.  facialis,  which,  of  course, 
passes  on  beyond.  The  acoustic  nerve  divides,  first,  into  two 
branches :  one,  the  posterior,  rac  and  mu,  and  the  other,  anterior,  a, 
which  supplies  the  cochlea  and  also  gives  off  a  few  small  branches 
to  the  macula  acustica  sacculi,  ms,  and  a  more  considerable  branch, 
rap,  to  the  posterior  ampullae. 

The  labyrinth  has  only  six  sensory  areas ;  two — namely,  the  macula 
neglecta  and  the  papilla  acustica  lagenae — being  wanting,  though 
present  in  amphibia,  reptiles,  and  birds.  The  six  present  in  man 
are:  1,  2,  3,  in  the  three  ampullae;  4,  in  the  recessus  utriculi;  5,  in 
the  sacculus;  6,  in  the  scala  media  cochleae  (the  organ  of  Corti). 

The  auditory  Passages  are  developed  from  the  first  gill-cleft 
of  the  embryo.  It  will  be  remembered,  see  p.  264,  that  the  clefts  are 
not  open,  as  in  lower  vertebrates,  but  closed  by  a  thin  membrane. 
This  membrane  is  the  rudiment  of  the  tympanal  membrane;  the 
portion  of  the  gill-cleft  within  it  becomes  the  Eustachian  tube  and 
the  cavity  of  the  drum,  which  are  accordingly  lined  throughout  life 
by  an  epithelium  derived  from  the  entoderm ;  the  inner  division  of 
the  first  gill-cleft  has  been  named  the  tubo-tijmpanal  canal.  The 
portion  of  the  gill-cleft  outside  the  membrane  is  lined  by  ectoderm 
and  becomes  the  meatus  auditorius  externus.  That,  contrary  to 
the  assumption  of  older  writers,  the  tympanum  and  external  meatus 
never  communicate,  even  in  early  stages,  was  first  discovered  by 
D.  Hunt,  79.1.  Some  recent  writers,  e.  g.  Urbantschitsch,  73.1, 
and  N.  Kastschenko,  87.1,  0,  have  maintained  that  the  auditory 
passages  are  not  derived  from  the  first  gill-cleft,  but  they  appear  to 
me  to  offer  no  justification  of  this  singular  view,  which  has  been,  in 
fact,  set  aside  by  F.  P.  Mall,  88.1. 

In  the  chick  (according  to  Mall,  /.  c.)  during  the  third  day  of  incu- 
bation an  ectodermal  involution  is  formed  from  the  dorsal  part  of 
the  first  external  branchial  groove.  This  involution  lies  in  direct 


DEVELOPMENT   OF   THE   EAR.  739 

apposition  with  the  dorsal  part  of  the  first  internal  branchial  pocket, 
and  blends  with  the  facial  nerve.  During  the  fifth  day  of  incubation 
the  connection  between  the  facial  nerve  and  the  ectoderm  is  severed, 
and  a  new  outgrowth  (canalis  tubo-tympanicus),  from  the  outer  part 
of  the  first  internal  branchial  pocket,  takes  its  place.  This  new  out- 
growth first  extends  outward,  upward,  and  forward,  but  through  the 
erection  of  the  head  its  direction  is  changed  to  outward,  upward,  and 
backward.  It  forms  the  tympanic  cavity.  In  mammals  the  develop- 
ment of  the  tympanal  cavity  is  essentially  the  same;  it  arises  as  a 
blind  dilatation  of  the  end  of  the  entodermal  portion  of  the  first  gill- 
cleft.  The  dilatation  at  first  forms  only  a  thin,  flattened  cavity,  which 
for  some  time,  at  least  in  the  human  embryo,  is  only  potentially  pres- 
ent, because  the  opposite  epithelial  walls  grow  together  and  obliterate 
the  actual  lumen.  In  the  human  embryo  at  three  months  the  tym- 
panal cavity  is  still  very  small,  Fig.  430,  Ty,  and  immediately  over- 


coch 


Md        Mk       Eut 

FIG.  430.— Section  through  the  Region  of  the  Ear  of  a  Human  Embryo  of  three  Months  (Minot 
Coll.  No.  41).  Sc,  Semicircular  canals;  /.ot%  position  of  the  foramen  ovale;  F,  primitive  ves- 
tibule; eoc/i,  cochlea;  Eu. t,  eustachian  tube ;  Mk,  Meckel's  cartilage;  Md,  mandible;  m.ex, 
meatus  extern  us ;  Ty,  tympanum;  ma/,  malleus. 

lies  the  inner  end  of  the  solid  plug  of  epithelium,  m.ex,  representing 
the  meatus  externus;  immediately  above  the  tympanum  lies  the 
malleus,  mal,  or  upper  end  of  the  cartilage  of  Meckel.  The  same 
figure  shows  the  auditory  labyrinth  lying  in  the  cartilaginous  peri- 
otic  capsule,  the  precursor  of  the  os  petrosum ;  between  the  cochlea, 
coc/i,  and  the  semicircular  canals,  Sc,  lies  the  primitive  vestibule, 
F,  the  wall  of  which  comes  close  to  the  outer  surface  of  the  periotic 
capsule,  at  a  point,  /.  cw,  where  the  f enestra  ovalis  is  to  be  developed ; 
close  to  this  point  arises  the  anlage  of  the  stapes;  it  will  be 
observed  in  the  figure  that  there  is  a  considerable  space  around 
the  ear  bones  between  the  f  enestra  ovalis,  f.ov,  on  the  one  hand, 


740  THE   FCETUS. 

and  the  auditory  passages,  m.ex,  Ty,  on  the  other;  this  space  is  filled 
with  embryonic  connective  tissue.  After  birth  the  connective  tissue 
atrophies,  while  in  the  same  measure  the  tympanal  cavity  expands 
around  the  bones  of  the  ear  (malleus,  incus,  and  stapes) ,  so  that  these 
ossicula  apparently  lie  within  the  tympanal  cavity ;  but  they  are,  of 
course,  covered  by  the  tympanal  epithelium  or  entoderm,  and  are, 
therefore,  morphologically  outside  the  cavity,  just  as  the  intestine 
is  outside  the  peritoneal  cavity.  As  mentioned  above,  the  inner  end 
of  the  meatus  externus,  m.ex,  lies  immediately  against  the  tym- 
panal cavity,  Ty;  the  two  passages  are  separated  by  a  plate  composed 
of  two  layers  of  epithelium ;  this  plate  is  the  closing  membrane  of 
the  gill-clefts,  and  also  the  anlage  of  the  tympanal  membrane ;  mes- 
enchyma  is  found  between  the  two  epithelial  layers  in  the  adult,  but 
when  it  penetrates,  I  do  not  know.  The  enlargement  of  the  mem- 
bane  depends  chiefly  upon  the  expansion  of  the  tympanum  around 
the  malleus,  in  part  also,  doubtless,  upon  the  actual  growth  of  the 
membrane;  it  is  said  to  measure  at  three  months,  2.0  X  1.25  mm. ; 
at  five,  7.0  X  5.5mm.;  at  nine,  9.75  X  8.5  mm.  (compare  Kolliker, 
"Entwickelungsgesch.,"  2te  Aufl.,  751). 

The  inner  end  of  the  tubo-tympanal  canal  is  transformed  into  the 
tuba  Eustachii.  It  becomes  small  in  diameter,  and  has  a  small 
opening  into  the  pharynx  just  behind  the  root  of  the  soft  palate,  Fig. 
323 ;  it  widens  out  gradually  into  the  tympanum.  Its  lumen  is  oblit- 
erated for  a  time,  presumably,  simply  by  concrescence  of  the  epithelial 
walls.  The  cartilage  of  the  tuba  appears  during  the  fourth  month, 
as  a  plate  of  hyaline  cartilage  on  the  medial  side  of  the  upper  end  of 
the  tube  (Kolliker). 

The  meatus  auditorius  externus  is  at  first  shallow,  but  gradually 
deepens,  becoming  a  long  horizontal  tube ;  the  diameter  of  this  tube 
very  early  diminishes,  and  it  soon  loses  its  lumen,  Fig.  430,  by  the 
concrescence  of  the  epithelium;  the  occlusion  continues  till  after 
birth.  The  wax-glands  appear  during  the  fifth  month,  and  are  de- 
veloped, according  to  Kolliker,  after  the  type  of  the  sweat-glands. 
A  special  bone  arises,  as  the  so-called  annulus  tympanicus,  around 
the  margin  of  the  tympanum,  and  subsequently  extends  itself  out- 
ward around  the  meatus;  the  ring,  however,  is  incomplete  on  the 
lower  anterior  side,  and  so  remains  for  several  years  after  birth. 

The  fenestra  rotundus  and  the  fenestra  ovalis  are  spots  where 
the  tissue  between  the  labyrinth  and  the  tympanum  is  so  much 
reduced  that  only  a  thin  membrane  is  left  over  each  spot. 

The  Bones  of  the  Ear  are  the  malleus,  incus,  and  stapes.  The 
development  of  the  first  two  is  described  p.  444. 

The  stapes  (compare  also  p.  44G)  develops  from  the  connective 
tissue  near  the  fenestra  ovalis.  Staderini's  careful  observations, 
91.1,  show  that  in  very  early  stages  the  external  jugular  vein  runs 
past  the  tympanum;  immediately  below  it  lies  the  facial  nerve, 
between  which  and  the  tympanum  is  situated  a  small  branch  (ar- 
teria  stapedialis)  of  the  carotid  artery ;  the  mesenchyma  around  this 
artery  becomes  condensed  (embryos  of  pig,  of  15  mm.)  and  the 
condensed  tissue  is  the  anlage  of  the  stapes,  and  subsequently  ossifies, 
according  to  H.  Rathke,  from  three  centres.  The  artery  atrophies 
in  man,  leaving  the  perforated  bone,  but  persists  in  many  other 


DEVELOPMENT   OF   THE   EAR. 


741 


mammals.  Staderini  seems  to  me  to  settle  the  debate  as  to  the  ori- 
Lcin  of  the  stapes,  and  to  show  that  it  is  to  be  regarded  as  an  ossifica- 
tion of  the  fenestra  ovalis,  not  as  a  modification  either  in  whole  or 
in  part  of  the  visceral  skeleton  (mandibular  or  hyoid  cartilages). 
This  view  is  confirmed  by  F.  Villy,  90.1,  178,  who  states  that  in 
the  frog  the  stapes  is  formed  independently  of  the  branchial  carti- 
lages, "  as  a  chondrification  in  the  capsular  membrane  closing  the 
fenestra  ovalis,  at  a  period  when  the  remainder  of  the  capsule  is 
well  developed,  and  not  long  before  the  tadpole  begins  to  assume  the 
frog's  form." 

The  External  Ear. — W.  His  has  traced  out  very  fully  the 
history  of  the  form  of  the  external  ear  ("Anat.  Menschl.  Embry- 
onen,"  Heft  III.,  •>! \-->->l).  Before  the  end  of  the  first  month  there 
appears  around  the  external  opening  of  the  first  gill-cleft  a  series  of 


FIG.  431.— Development  of  the  Human  External  Ear.  A,  Embryo  of  one  month;  B,  six 
wt'fk*:  ('.  »'itfht  w.-.-ks:  D,  ten  weeks;  E,  fourteen  weeks.  The  six  primitive  tubercles  are  num- 
bered 1  to  0:  tli.-  primitive  ridge  is  marked  c;  1  is  the  tragus;  4,  theanthelix;  5,  the  antitragus; 
6,  taenia  lobularis;  2,  3,  and  c  form  the  helix. 

six  tubercles,  Fig.  431,  A;  two  in  front,  on  the  hind  edge  of  the  first 
visceral  (or  the  mandibular)  arch ;  one  above  the  cleft,  and  three 
behind  it.  Similar  tubercles  have  been  observed  by  G.  Schwalbe, 
91. 1,  in  the  embryos  of  birds  and  reptiles.  A  little  later  a  vertical 
furrow  appears  down  the  middle  of  the  hyoid  arch  in  such  a  way  as 
to  mark  off  a  little  ridge,  Fig.  431,  A,  c,  which  joins  on  to  tubercle,  3, 
and  descends  behind  tubercles  4  and  5.  The  second  stage  is  reached 
1  > y  the  growth  of  all  the  parts ;  the  fusion  of  tubercles  2  and  3  and 
the  growth  of  the  ridge  down  behind  tubercle  5  to  become  continuous 
with  6.  After  these  changes,  it  is  not  difficult  to  identify  the  parts, 
Fig.  431,  B.  1  is  the  tragicum;  2  and  3,  together  with  the  arching 


742  THE   FCETUS. 

ridge,  represent  the  helix;  4  is  the  anthelix;  5,  the  anti-tragicum; 
6,  the  tcenia  lobularis.  The  deep  pit  bounded  by  1,  2,  3,  4,  and  5 
is  the  fossa  angularis.  During  the  latter  part  of  the  second  month 
the  ear  changes  its  proportions  somewhat,  becoming  more  slender ; 
tubercle  2  projects  farther  backward  toward  the  helix,  making  the 
separation  between  it  and  the  tragicum  more  marked,  and  also 
rendering  the  fossa  angularis  more  irregular. 

The  third  stage  begins  with  the  third  month.  The  upper  and 
posterior  part  of  the  concha  arises  from  the  surface  of  the  head  and 
gradually,  but  rapidly,  bends  over  forward,  so  as  to  completely 
cover  the  anthelix,  B,  4,  and  the  upper  portion  of  the  fossa  angularis, 
Fig.  431,  c.  It  is  during  this  stage  that  in  mammals  the  assump- 
tion of  the  pointed  form  of  the  ear  commences.  For  a  discussion  of 
the  development,  significance,  and  frequency  of  the  pointed  form  of 
the  ear  in  man,  see  G.  Schwalbe's  admirable  papers,  89.1,  91.2. 
The  antiversion  lasts  only  a  short  period,  probably  not  much  over  a 
fortnight.  The  ear  now  unfolds  and  shows  the  anterior  tubercle 
still  more  projecting  than  before,  Fig.  431,  D,  and  the  upper  part  of 
the  fossa  angularis  very  much  reduced. 

The  fourth  stage  commences  with  the  fourth  month.  The  tuber- 
culum  anterior  encroaches  still  more  upon  the  fossa  angularis,  and 
reduces  the  lower  part  of  it  also  to  a  fissure,  hence  the  tuberculum, 
2,  itself  almost  touches  the  anthelix,  4,  and  the  anti-tragicum,  5. 
There  now  appears  a  ridge  which  grows  out  from  the  second  tubercle 
and  unites  it  with  the  anthelix,  Fig.  431,  C,  Cr.h,  and  divides  the 
upper  part  of  the  fossa  from  the  lower,  which  latter  becomes  the 
opening  of  the  meatus.  Shortly  after  the  first  ridge  a  second  ap- 
pears, which  unites  the  second  tubercle  with  the  anti-tragicum,  Fig. 
431,  E,  Cr.s.  Finally  the  sixth  tubercle  becomes  pendent  and 
appears  distinctly  as  the  tcenia  lobularis.  These  changes  are 
completed  by  the  end  of  the  fifth  month.  The  further  development 
is  very  gradual  and  is  partly  post-natal.  Of  the  two  ridges,  the 
first  formed  is  permanent,  and  is  the  crus  or  spina  helicis,  while 
the  second  (crus  supra- tragicum,  His)  becomes  nearly  obliterated; 
the  subdivision  of  the  tragicum,  already  indicated  in  Fig.  431,  E,  1, 
becomes  more  marked;  the  concha  enlarges,  and  its  cavity  grows 
more  evident.  By  these  and  other  subsidiary  changes,  the  adult  ear 
is  developed.  The  differences  in  the  ears  of  adults  are  mainly  the 
product  of  secondary  modifications. 


CHAPTER  XXIX. 
THE  ENTODERMAL  CANAL. 

THE  first  stages  of  the  entodermal  canal  are  described  in  Chap- 
ters IV.  and  V.,  its  earliest  differentiation  as  the  archenteron  in 
Chapter  XII.  We  have  now  to  take  up  the  differentiation  of  the 
various  entodermal  organs  after  the  formation  of  the  gill-clefts. 

For  convenience  I  prefix  a  list  of  all  the  organs  or  parts  derived 
from  the  entodermal  canal.  They  are : 

1.  Gill-clefts.  10.  Duodenum. 

2.  Pharynx  and  tonsils.  11.  Yolk-sac. 

3.  Thyroid  gland.  12.  Small  intestines. 

4.  Thymus  gland.  13.  Ccecum. 

5.  Larynx,  trachea,  and  lungs.       14.  Vermix. 

6.  (Esophagus.  15.  Colon. 

7.  Stomach.  16.  Rectum. 

8.  Liver.  17.  Allantois. 

9.  Pancreas.  18.  Schwanzdarm. 

Of  these  there  have  been  already  described — 1,  the  gill-clefts;  11, 
the  yolk-sac;  17,  the  allantois,  and,  18,  the  Schwanzdarm. 

In  this  chapter  is  presented,  first,  the  history  of  the  alimentary 
tract;  second,  the  history  of  the  respiratory  organs  (i.  e.,  of  the  above 
list,  5,  larynx,  trachea,  and  lungs) . 

I.  THE  ALIMENTARY  TRACT. 

Pharynx  or  Branchial  Region. — That  part  of  the  archen- 
teron in  which  the  gill-clefts  are  situated  becomes  the  pharynx  of 
the  adult.  The  entodermal  pouches  of  the  gill-clefts  undergo  pro- 
found modifications.  The  pouch  of  the  first  or  hyo-mandibular  cleft 
becomes  the  tubo-tympanal  canal,  compare  p.  738.  The  pouch  of 
the  second  cleft  becomes  broad  and  shallow,  and  gives  rise  to  the 
tonsils,  p.  745.  The  remaining  pouches,  so  far  as  I  know,  have  no 
recognizable  traces  on  the  surface  of  the  adult  pharynx,  though  their 
epithelial  walls  are  concerned  in  the  development  of  the  thyroid  and 
thymus  glands. 

The  pharyngeal  cavity  early  becomes  continuous  with  the  mouth 
cavity  by  the  rupture  of  the  oral  plate,  p.  262. 

The  change  of  shape  in  the  pharynx  has  never  been  traced,  nor 
have  we  any  definite  knowledge  of  the  histological  development  of 
its  walls.  In  the  adult  it  resembles  the  oesophagus  histologically. 

Its  posterior  limit  is  marked  by  the  opening  of  the  trachea.  It 
is,  therefore,  a  relatively  small  tract  in  the  adult,  although  in  the 
embryo,  when  the  gill-clefts  arise,  it  constitutes  nearly  half  the 


744 


THE    FCETUS. 


archenteron.     So  too,  among  vertebrates,  as  we  ascend  the  series, 
we  find  that  the  relative  importance  of  the  pharynx  diminishes. 

From  the  floor  of  the  pharynx  are  developed  the  tongue  and  the 
epiglottis;  the  tongue  is  treated,  p.  592,  in  connection  with  the 
mouth-cavity;  the  epiglottis  is  treated,  p.  778,  in  connection  with 
the  larynx. 

Cervical  Sinus  (Sinus  prce-cervicalis  of  His). — Although  the 
cervical  sinus  is  an  ectodermal  structure,  yet  its  formation  is  due  to 
modifications  of  the  gill-arches,  and  therefore  its  history  may  bo 
presented  conveniently  in  connection  with  that  of  the  pharynx.  E. 
Dursy,  69. 1,  112,  gives  the  earliest  accurate  description  of  the  cer- 
vical sinus  known  to  me.  He  observed  that  in  a  cow's  embryo  of 
11  mm.  the  third  and  fourth  branchial  arches  are  much  smaller  than 
the  others  and  constitute  a  triangular  area  depressed  below  the  level 
of  the  surrounding  external  surface ;  the  apex  of  the  triangle  points 
toward  the  ventral  side.  The  corresponding  stage  in  man  is  found 
in  embryos  of  9-10  mm.,  compare  Fig.  219,  cs.  By  the  growth  of 
the  caudal  margin  of  the  second  branchial  arch  the  depressed  area 
becomes  further  invaginated ;  Dursy  compared  the  second  arch  to  the 
operculum  (KeimendeckeT)  of  fishes — a  comparison  originally  sug- 
gested by  H.  Rathke  in  1825,  25. 1.  His  ("  Anat.  Menschlicher  Em- 
bryonen,"  III.  28,)  also,  86.3,  has  traced  the  invagination  of  the 
third  and  fourth  gill-arches  in  the  human  embryo,  resulting  in  the 

formation  of  a  deep  fissure  on  each 
side  of  the  neck  somewhat  toward 
the  ventral  surface,  Fig.  432 ;  ow- 
ing to  its  position  toward  the  ven- 
tral side  His  named  the  fissure 
prce-cervical  sinus ;  Rabl  mistook 
the  prefix  to  mean  head  ward  of  the 
neck,  and  accordingly  made  an 
acrimonious  attack,  86.1,  upon 
His  for  saying  that  the  sinus  was 
not  connected  with  the  neck. 
Rabl's  blunder  was  corrected  by 
His,  86.3,  428.  His  has  shown 
that  the  fourth  arch  is  turned  in 
first,  and  that  the  third  arch  is 
turned  in  a  little  later ;  the  sinus  is 
so  narrow  that  the  arches  come  in 


FIG.  432. — Reconstruction  of  the  Pharyngeal 
Region  of  a  Human  Embryo  of  11.5  mm.  (His1 
Rg).  jY,  Nasal  pit;  gl,  processus  globularis; 
hy,  hypophysis;  &i,  sinus cervicalis;  Lu,  lung; 
Md,  mandible;  II,  second  branchial  arch. 
After  W.  His.  X  10  diams. 


contact  with  the  opposite  wall ;  the 
ectoderm  of  the  arches  concresces 
with  that  of  the  caudal  side  of  the 
sinus,  the  opening  of  which  is  thus 
obliterated.  The  sinus  is  now 
an  epithelial  cord  connected  with  the  epidermis  on  the  one  hand, 
and  on  the  other  with  two  spaces  lined  with  ectoderm :  one  space 
corresponds  to  the  ectodermal  furrow  of  the  second  gill-cleft,  the 
other  to  the  ectodermal  furrow  of  the  third  gill-cleft.  All  trace  of 
the  second  furrow  is  soon  obliterated  (compare  Fig.  434),  but  the 
remnant  of  the  third  furrow  persists  longer  and  lies  in  close  prox- 
imity to  the  anlage  of  the  thymus,  Fig.  434.  His,  "  Anat.  menschl. 


THE   ALIMENTARY    TRACT. 


745 


Embryonen,"  III.,  104,  regarded  the  buried  remnant  of  the  third 
ectodermal  branchial  furrow  as  the  anlage  of  the  thymus.  In  1886, 
he  still  adhered  to  this  opinion  in  an  article,  86.3,  which  gives  the 
fullest  history  of  the  sinus  we  have  yet,  but  after  the  entodermal 
origin  of  the  thymus  had  been  demonstrated  in  various  types,  His 
ivuorkcd  the  question,  and  in  a  brief  paper,  89.2,  withdrew  his 
earlier  opinion. 

So  far  as  known,  the  cervical  sinus  entirely  disappears,  but  its 
abnormal  persistence  may  account  for  certain  cysts  occurring  patho- 
logically in  the  neck. 

Tonsils.  — The  tonsils  are  developed  from  the  second  gill-cleft.  In 
a  n  embryo  of  four  or  five  months,  the  shallow  pouch  which  represents 
this  cleft  is  found  bounded  in  front  by  the  arcus palatoglossus,  which 
N  a  survival  of  part  of  the  second  branchial  arch,  and  is  partly 
covered  by  the  uvula,  which  is  continued  on  to  the  wall  of  the  phar- 
ynx  as  a  fold,  the  plica  triangularis  of  His  ("  Anat.  menschlicher 
Embryonen,"  Heft  III.,  82),  which  bounds  the  pouch  on  the  dorsal 
side.  The  pouch  is  lined  by  the  mucous  membrane  (entoderm  plus 
meseiichyma)  of  the  pharynx. 

The  histogenesis  of  the  tonsils  has  been  made  the  subject  of  a  long 
memoir  by  E.  Retterer,  88.1,  who  maintains  that  the  epithelium 
commingles  with  the  connective  tissue,  forming  a  special  angiothe- 
lial  tissue  of  double  origin.  P.  Stohr, 
91.1,  and  Gulland  (Lab.  Kept.  R.  Coll. 
Phys.  Edinburgh, III.,  1891)  have  shown 
that  Retterer's  view  is  erroneous.  Ac- 
cording to  Stohr,  the  tonsil  has,  at  three 
months,  a  stratified  epithelium  resting 
on  mesenchyma  without  leucocytes. 
At  four  months  the  tonsillar  fissures 
begin  to  branch,  and  the  epithelium 
presents  buds,  some  of  which  are  the 
solid  anlages  of  glands,  while  others 
are  the  commencements  of  branches  of 
the  tonsils.  The  formation  of  solid  ton- 
sillar buds  continues  not  only  through 
the  embryonic  period,  but  also  for  a 
year  after  birth.  The  solid  buds  grad- 
ual!}' become  hollow  by  a  change  in  the 
central  cells,  which  assume  a  corneous 
appearance  and  gradually  contract  into 
a  mass  in  the  centre  of  the  bud,  Fig. 
4:}:5,  c.  Meanwhile,  the  cavity  of  the  tonsil  extends  into  the  upper 
part  of  the  bud,  until  it  communicates  with  the  space  containing  the 
degenerated  mass,  which  is  then  expelled.  The  epithelium  is  at  all 
periods  separated  from  the  mesoderm  by  a  distinct  endothelial  base- 
ment membrane,  Fig.  433,  b.m,  nevertheless  it  is  penetrated  by  leu- 
cocytes during  the  fourth  month,  Fig.  433,  /  I.  Up  to  the  time  of 
birth  the  number  of  the  immigrant  cells  in  the  epithelium  gradually 
increases ;  indeed  they  may  become  so  numerous  that  the  epithelium 
and  basement  membrane  are  scarcely  recognizable.  In  the  mesen- 
chyma there  are  connective-tissue  fibrilla?  at  three  months,  and  at 


FIG.  433.— From  a  Section  of  a  Tonsil 
of  a  Human  Embryo  of  five  Months. 
c,  Corneous  central  mass;  6.w,  base- 
ment membrane ;  l.l,  leucocytes.  After 
P.  Stohr. 


746 


THE    FCETUS. 


that  stage  there  are  also  leucocytes  scattered  about,  but  the  infiltra- 
tion is  diffuse.  As  the  number  of  leucocytes  increases,  they  show 
an  increasing  tendency  to  form  groups — the  anlages  of  follicles — but 
it  is  not  until  after  birth  that  the  follicles  become  well  defined  with 
distinct  germinating  centres.  The  leucocytes  are  probably  derived 
from  the  blood  by  migration  to  the  walls  of  the  blood-vessels  in  loco. 
The  Thymus  is  developed  from  the  entoderm  of  the  third  gill- 
cleft,  as  a  thickening,  which  remains  after  the  cleft  aborts.  That 
the  thymus  is  of  exclusively  entodermal  origin  in  all  birds  and  mam- 
mals is  extremely  probable,  though  not  quite  certain.  Froriep, 
91.2,  64,  asserts  that  in  sharks  the  thickening  is  identical  with 
that  which  forms  the  epibranchial  organ,  a  view  that  interprets  the 
thymus  as  ectodermal.  The  form  of  the  third  gill-cleft  in  young 
embryos  is  described  p.  264.  In  a  human  embryo  from  the  begin- 


FIG.  434.— Section  through  the  Third  Gill-Cleft  of  a  Human  Embryo  from  the  beginning  of 
the  third  Week.  II,  III,  IV,  Second,  third  and  fourth  branchial  arch ;  Sp,  remnant  of  ectoder- 
mal groove  between  the  second  and  third  arches;  IX,  ganglion  of  the  glqsso-pharyngeus ;  3,  third 
entodermal  pouch;  Ao3,  third,  Ao4,  fourth  aortic  arch;  Ep,  epiglottis;  4,  fourth  entodermal 
pouch;  nl,  nervus  laryngeus  superior;  XII,  hypoglossus;  Ip,  His1  infundibulum  prsecervicale ; 
F,  Fundus  of  cervical  sinus.  After  W.  His. 

ningof  the  fifth  week,  His,  89.2,  found  the  entodermal  pouch  of  the 
third  gill-cleft  open,  Fig.  434,  3;  the  entoderm  in  the  distal  part  of 
the  cleft  is  somewhat  thickened,  and  is  in  immediate  contact  with 
the  ectoderm  of  the  cervical  sinus.  In  a  pig  embryo  of  11  mm. 
Born,  83.1,  288,  finds  the  lower  part  of  the  entodermal  pouch  still 
open,  but  in  the  dorsal  apex  the  epithelium  has  grown  and  obliterated 
the  cavity.  In  a  pig  embryo  of  13  mm.  Born,  p.  29,  finds  the  dorsal 
and  distal  end  of  the  third  pouch  enlarged,  and  the  rest  transformed 
into  a  very  narrow  canal  by  which  the  end  is  connected  with  the 
pharynx  proper.  In  a  cow  embryo  of  12mm.,  Froriep,  85.1,  23, 
found  a  very  similar  condition,  but  the  lumen  of  the  canal  was 
beginning  to  disappear.  In  the  rabbit  at  thirteen  days,  Piersol, 
88. 1, 175,  and  Fig.  24,  finds  the  distal  dorsal  dilatation  of  the  pouch 
very  marked ;  its  walls  are  greatly  thickened,  but  the  central  cavity 
still  persists ;  the  canal  to  the  pharynx  has  become  a  solid  epithelial 
cord.  The  connection  of  the  pouch  with  the  pharynx  is  soon  lost, 
and  the  third  entodermal  gill-pouch  may  be  then  designated  as  the 
independent  anlage  of  the  thymus.  This  anlage  is  an  elongated  sac 


THE   ALIMENTARY   TRACT.  747 

with  thickened  epithelial  walls;  it  occupies  an  oblique  dorso- ventral 
line;  its  dorsal  end  is  especially  enlarged  and  corresponds  to  the 
future  head  of  the  thymus.  Born,  p.  297,  found  the  connection  of 
th«»  thymus  with  the  pharynx  severed  in  a  pig  embryo  of  20  mm. 
F.  P.  Mall,  8^.1,  16-28,  has  followed  the  development  of  the  thymus 
in  the  chick,  and  found  it  essentially  identical  with  that  in  mam- 
mals ;  the  thickening  of  the  entodermal  walls  begins  the  fourth  day ; 
the  fifth  day  the  thymus  separates  from  the  pharynx  and  becomes 
an  elongated  body,  situated  at  about  the  same  level  as,  and  nearly 
parallel  with,  the  pharynx  and  overlying  the  third  and  fourth  aortic 
arches.  The  manner  in  which  the  thymus  changes  its  form  and 
position  is  clear  from  the  reconstructions  in  Fig.  436,  thm,  and  there- 
fore requires  no  special  description. 

The  lumen  of  the  anlage,  though  long  persistent,  is  gradually 
obliterated  until  it  completely  disappears  (pig  of  35  mm.);  in  a  pig 
embryo  of  25  mm.  the  ventral  end  of  the  thymus  is  developing  lat- 
eral buds,  and  in  an  embryo  of  35  mm.  the  whole  organ  is  budding 
(Born,  83.1,  306).  A  similar  condition  is  found  in  the  rabbit  at 
sixteen  days,  in  man  about  the  twelfth  week  (Kolliker,  "  Grundriss," 
2teAufL,  370,  371). 

Histogenesis. — Kolliker  ("Entwickelungsgesch.,"  2te  AufL,  878) 
records  for  the  rabbit,  that  between  the  twentieth  and  twenty- third 
da}~s  the  cells  of  the  thymus  become  smaller  and  their  outlines  disap- 
pear, so  that  the  organ  appears  to  be  an  accumulation  of  small  round 
nuclei.  At  about  the  same  period  blood-vessels  and  connective  tissue 
grow  into  the  epithelial  anlage.  After  the  penetration  of  the  vessels 
the  differentiation  of  the  cortex  and  medulla  is  recognizable ;  in  car- 
mine preparations  the  cortex  is  the  darker  part.  According  to 
Stieda,  81.1,  the  concentric  bodies  of  the  adult  thymus  are  derived 
from  the  epithelium  (entoderm) . 

The  remarkable  changes  in  the  thymus  after  birth  are  outlined  in 
all  the  principal "  Anatomies. "  For  details  see  especially  Afanassiew, 
77.1. 

Historical  Note. — L.  Stieda,  81.1,  Discovered  in  1881  that  the 
thymus  gland  arises  in  intimate  connection  with  a  gill-cleft.  Kolli- 
ker in  1884  recorded  ("Grundriss,"  2te  AufL,  369)  that  the  primitive 
anlage  of  the  gland  was  an  epithelial  mass.  G.  Born  in  an  essay  of 
great  excellence,  83.1,  demonstrated  that  the  gland  is  developed 
from  the  entodermal  lining  of  the  third  gill-cleft.  Born's  result  has 
been  confirmed  by  C.  Rabl,  86.1,  Fischeles,  85.1,  De  Meuron, 
86.1,  F.  P.  Mall,  87.1,  88.2,  Froriep,  85.1,  47,  91.2,  64,  and 
Prenant,  91.2.  His,  85.3,  "  Anat.  menschl.  Embryonen,"  III.,  at 
first  maintained  that  the  thymus  arose  from  the  ectoderm  of  the 
cervical  sinus,  but  having  made  further  observations  finally  reached 
the  same  conclusion  as  Born,  and  showed,  89.2,  that  in  man  the 
thymus  is  derived  from  the  third  entodermal  pouch.  Kastschenko, 
87.1,  believed  that  the  thymus  was  partly  ectodermal,  partly  ento- 
dermal, an  opinion  which  is  incompatible  with  our  present  knowledge. 

Thyroid  Gland. — The  thyroid  gland  is  developed  from  three 
anlages,  one  median  and  two  lateral,  which  unite  and  undergo  a 
common  differentiation.  We  take  up:  1,  the  median  anlage;  2,  the 
lateral  anlages ;  3,  their  union;  4,  their  differentiation ;  5,  homologies. 


748 


THE   FCETUS. 


md 


ms 


1.  The  Median  Anlage. — This  is  an  evagination  of  the  floor  of 
the  pharynx  between  the  bases  of  the  first  and  second  branchial 
arches ;  it  lies  in  the  median  line  behind  the  tuberculum  impar,  p. 
592,  and  the  furcula,  or  the  two  parts  of  the  tongue.  In  the  human 
embryo,  as  v/e  learn  from  His  ("Aiiat.  menschl.  Embryonen,"  II., 
04-72,  97-102),  the  evagination  is  a  small  pouch  beginning  to  expand 
sideways  in  an  embryo  of  5  mm. ;  in  an  embryo  of  10  mm.  (cf.  Fig. 
335,  m.tli)  the  lateral  expansion  has  increased  very  much  and  there  is 
a  distinct,  though  narrow,  median  duct,  the  opening  of  which  upon 
the  surface  of  the  tongue  corresponds  to  the  foramen  caecum;  the 
duct  itself  is  known  as  the  ductus  thyreoglossus.  The  anlage  now 
consists  of  a  bilateral  epithelial  vesicle,  connected  by  a  slender,  hol- 
low pedicle  with  the  surface  of  the  tongue.  The  duct  persists  up  to 

the  eighth  week,  gradually 
elongating  as  the  thyroid  and 
the  tongue  separate.  The  duct 
usually  obliterates  completely 
or  partially,  but  it  sometimes 
persists  more  or  less  intact 
throughout  life.  The  abortion 
of  the  duct  begins  usually  dur- 
ing the  fifth  week,  and  when 
the  anlage  of  the  hyoid  bone 
reaches  the  median  line,  it  is 
situated  directly  in  the  path  of 
the  duct,  a  topographical  rela- 
tion of  pathological  impor- 
tance (W.  His,  91.1).  The 
abortion  begins  with  the  clos- 
ure of  the  lumen  of  the  duct ; 
the  solid  cord  gradually  dimin- 
ishes in  size  and  becomes  fragmented  as  resorption  progresses,  but  the 
upper  portion  near  the  surface  of  the  tongue  retains  its  thickness  for 
a  time  at  least.  Kanthack,  in  an  article  of  slight  value,  91.1,  has 
denied  without  justification  the  existence  of  the  thyroid  duct.  The 
vesicular  portion  of  the  median  anlage  expands  quite  rapidly,  Fig. 
435,  ms,  and  lies  nearly  at  the  level  of  the  third  aortic  arch,  3,  or 
internal  carotid,  and,  indeed,  is  from  the  beginning  in  close  prox- 
imity to  the  larynx.  In  embryos  of  9-10  mm.  it  is  a  narrow,  long 
transverse  body,  the  lateral  ends  of  which  curve  dorsal  ward,  and 
which,  with  the  duct,  form  a  figure  somewhat  like  an  inverted  T. 

The  development  in  other  mammals,  so  far  as  known,  is  closely 
similar  to  that  in  man.  Thus  in  the  rabbit,  Piersol,  88.1,  182, 
found  the  anlage  to  appear  the  end  of  the  ninth  day  (embryo  of  3.3 
mm.) ;  the  epithelium  of  the  thyroid  evagination  at  once  thickens 
and  the  anlage  becomes  solid  the  tenth  day ;  the  twelfth  day  the  abor- 
tion of  the  duct  begins;  and  after  the  separation,  not  before,  as  in 
man,  the  lateral  outgrowth  of  the  anlage  begins.  In  the  pig,  G.  Born, 
83.1;  in  the  chick,  Seesel,  78.1,  and  F.  Mall,  87.1;  in  Amphibia, 
A.  Gotte,  75.1;  in  Petromyzdn,  W.  Muller,  71.3,  73.1,  and  A. 
Dohrn,  86. 1,  87.2,  have  studied  the  median  anlage  of  the  thyroid, 
which  may  now  be  said  to  be  a  structure  common  to,  and  therefore 


FIG.  435. — Reconstruction  of  the  Pharyngeal  Re- 
gion of  a  Human  Embryo  of  9. 1  mm.  (His1  Rn). 
1,  2,  3,  4,  5,  Aortic  arches;  car,  carotid;  Ep,  epi- 
glottis; Tg,  tongue;  hy,  hypophysis;  md,  lower 
aw;  ms,  median  anlage  of  thyroid;  Tm,  thymus; 
,  lateral  anlage  of  thyroid.  After  W.  His.  x  20 
diams. 


THE   ALIMENTARY    TRACT. 


74'.» 


characteristic  of,  all  vertebrates.  The  references  just  given  might 
easily  l>e  multiplied. 

G.  Born,  83.1,  301,  found  that  in  the  pig  the  median  anlage 
( ( munences  its  histological  differentiation  and  is  penetrated  by 
blood- vessels  before  it  is  joined  by  the  lateral  anlages.  In  man  the 
differentiation  is  much  less  advanced  when  the  union  occurs. 

•,!.  Thf  lateral  un  hujes  are  derived  from  the  epithelium  (entoderm) 
of  the  fourth  gill-clefts.  The  fourth  entodermal  pouch  develops  a 
ventral  prolongation  (human  embryo  of  10  mm.,  Fig.  435,  Is).  His 
("  Auat.  mcnschl.  Embryoneu,"  III.,  97)  draws  a  distinction  between 
the  diverticulum  and  the  pouch,  but  upon  what  grounds  is  not  clear 
to  me.  In  an  embryo  of  12.5  mm.  (Nackenlange)  His,  I.e.,  98,  found 
the  diverticulum  a  closed  vesicle  entirely  separated  from  the  phar- 
ynx; the  vesicle  curved  forward  and  was  just  beginning  to  form  a 
few  round,  hollow  buds,  and  may  now  be  designated  as  the  lateral 
thyroid  anlage.  The  median  anlage  at  this  stage  is  situated  further 
t«»\vard  the  mouth  and  the  ventral  side.  In  an  embryo  of  13.8  mm. 
( Xackenlange)  the  lateral  anlages  have  moved  neaier  the  median, 
and  take  such  a  position  that  they  prolong  the  median  anlage  for- 
ward and  upward  on  each  side. 

In  a  pig  embryo  of  13  mm.  Prenant,  91.2,  211,  observed  the 
ventral  diverticulum  of  the  fourth  pouch  still  connected  with  the 


Ihnx 


FIG.  43(5.— Reconstructions  to  show  the  Development  of  the  Thyroid  Gland  in  the  Pig.  A, 
Embryo  of  15  mm.  ;  B,  of  1C  mm:  C,  of  20  mm;  D,  of  22.5  mm.  Ph,  Outline  of  pharynx;  m.th, 
median  thyroid;  thm,  thymus;  I. th,  lateral  thyroid;  g>J,  glottis;  Ao3,  third  aortic  arch  After 
G.  Born.  X  about  20  diams 

pharynx,  and  records  a  similar  condition  for  a  bat  embryo  of  6  mm. 
and  a  sheep  embryo  of  14  mm.  Piersol,  88.1,  182,  found  the  cor- 
responding stage  in  a  rabbit  embryo  of  the  eleventh  day,  and  states 
that  the  anlage  remains  "  for  a  relatively  long  time"  connected  with 


750  THE    FCETUS. 

the  pharynx  by  an  epithelial  cord.  Born  does  not  state  clearly  when 
the  lateral  anlages  separate  in  the  pig  from  the  pharyngeal  epithe- 
lium, but  apparently  the  separation  occurs  in  embryos  of  about  15 
mm.,  compare  Born,  83.1,  299. 

3.  Union  of  the  Three  Anlages. — This  was  discovered  by  Born, 
83. 1 ,  299.     It  takes  place  in  the  pig  when  the  embryo  is  from  20  to  22 
mm.  long;  the  median  anlage  is  at  this  time  a  network  of  epithelial 
cords  and  considerably  larger  than  the  lateral  anlages,  which  have 
gradually  changed  their  position,  Fig.  436,  A,  B,  u,  I),  until  they 
have  come  to  lie  against  the  lateral  ends  of  the  median  part,  C ;  with 
these  ends  the  lateral  parts  then  unite  and  soon  acquire  the  same 
reticulate  structure  as  the  median  portion,  and  there  remains  no  evi- 
dence of  the  triple  origin  of  the  gland. 

In  man  the  union  takes  place  probably  during  the  seventh  week— 
the  exact  time  has  not  been  recorded.  The  lateral  anlages  are  rela- 
tively larger,  and  the  median  anlage  less  differentiated  before  the 
union  in  man  than  in  the  pig.  As  to  the  process  of  union  in  other 
mammals  I  find  no  precise  data. 

4.  Differentiation. — In  a  pig  embryo  of  15  mm.  (Born,   38.1, 
301)  the  median  thyroid  is  a  transverse  band  of  epithelium,  around 
which  the  mesenchyma  is  beginning  to  form  a  capsule.     The  epi- 
thelial band  is  beset  with  buds,  which  grow  in  such  a  way  that  the 
band  soon  becomes  a  network  of  epithelial  cords,  Fig.  430 ;  the  cords 
are  solid  with  a  superficial  layer  of  distinct  high  cylinder  cells  with 
elongated  nuclei,  and  surrounding  a  granular  nucleated  mass  with- 
out distinct  cell  boundaries.     At  the  same  stage  the  lateral  thyroid 
is  merely  an  epithelial  vesicle,  at  the  ventral  end  of  which  the  walls 
are  thickened.     After  the  fusion  of  the  three  parts  one  can  still  rec- 
ognize (pig  embryos  of  26  mm.)  the  lateral  portions,  because,  though 
now  similar  in  structure  to  the  middle  portion,  the  epithelial  cords 
are  thicker  and  the  meshes  between  them  smaller  than  in  the  middle 
part.    In  an  embryo  of  37  mm.  (Born,  /.c.,  305)  the  gland  has  become 
an  oval  body  inclosed  in  a  smooth  capsule  of  connective  tissue. 

His  ("Anat.  menschl.  Embryonen,"  III.,  102)  records  that  in  a 
human  embryo  of  the  eighth  week  the  formation  of  the  hollow  acini 
had  begun;  the  acini  were  lined  by  epithelial  cells  with  each  an 
outer  granular  zone  containing  the  nucleus,  and  an  inner  zone  of 
clearer  appearance.  The  outer  zone  stains  more  deeply  than  the 
inner.  Wolfler,  71.1,  has  claimed  that  the  hollow  epithelial  acini 
are  formed  by  the  degeneration  of  the  central  tissue  of  the  solid  cords, 
and  in  this  conclusion  he  is  supported  by  Lustig,  91.1,  but  whereas 
Wolfler  maintained  that  the  differentiation  begins  in  the  centre  of 
the  organ  and  progresses  toward  the  periphery,  Lustig  asserts  that 
the  differentiation  goes  on  throughout,  so  that  mature  and  immature 
acini  may  be  found  in  every  part  at  once.  Fig.  437  represents  a 
section  of  the  human  foetal  thyroid  at  about  four  months.  It  will 
be  noticed  that  many  of  the  acini  are  still  solid. 

At  two  months  the  gland  in  man  consists  of  two  lobes  connected 
by  a  narrow  isthmus  (Miiller,  71.3,  447).  Miiller,  I.e.,  also  gives 
some  details  of  the  growth  of  the  acini  up  to  the  period  of  puberty, 
as  well  as  good  observations  on  the  foetal  gland  in  various  verte- 
brates. 


THE   ALIMENTARY   TRACT. 


'51 


5.  Homologies. — The  mammalian  thyroid  gland  is  shown  by  its 
development  to  be  a  double  organ.  The  median  part  is  alone  homol- 
oy<  HIS  with  the  so-called  thyroid  gland  of  other  vertebrates,  while 


FIG.  437.  —A   Section  of  the  Thyroid  Gland  of  a  Human  Embryo  of  about  four  Months.    B,  a 
single  acinus  more  highly  magnified. 

its  later  portions  are  presumably  homologous  with  supra-pericardial 
bodies;  see  Piersol,  88.1,  1S3,  also  Van  Bemmelen,  86.1,  89.2, 
K.  Maurer,  85.1,  87.1,  etc. 

Historical  Note. — That  the  thyroid  gland  arose  from  the  pharynx, 
and  commenced  as  a  thickening  of  the  entoderm,  was  discovered  by 
Remak,  50.1,  81 — 82.  This  discovery  was  confirmed  by  Goette's 
observations  on  the  chick,  67.1,  and  on  Bombinator,  75.1,  (;»'.;. 
W.  Miiller's  investigations,  71.3,  73.1,  added  considerably  to  our 
knowledge  of  the  median  anlage  in  various  classes,  and  led  him  to 
homologize  the  thyroid  evagination  with  the  hypobranchial  groove 
or  endostyle  of  tunicates  and  Amphioxus.  This  homology  has  found 
an  earnest  defender  in  Anton  Dohrn,  86.1.  Seesel  gave,  77.1,  a 
more  accurate  description  of  the  anlage  in  the  chick,  and  it  was  also 
studied  in  man  by  His  ("  Anat.  menschl.  Embryonen,"  I.,  56),  and 
in  the  rabbit  by  Kolliker  ("Entwickelungsgesch.,"  2te  Aufl.,  871). 
In  1881  L.  Stieda,  81.1,  discovered  the  lateral  anlages,  and  trac:d 
them  to  a  connection  with  one  of  the  gill-clefts ;  the  same  discovery 
was  made  the  same  year,  but  independently,  by  Wolfler,  81.1,  who 
gives  an  extensive  review  of  the  previous  literature.  Stieda  and 
Wolfler  overlooked  the  median  portion.  Born's  thorough  investi- 
gation, 83.1,  finally  cleared  away  the  uncertainty  by  tracing  out 
with  rare  precision  the  exact  role  of  each  part  of  the  triple  anlage. 
Born's  results  have  since  been  abundantly  confirmed  by  His,  "  Anat., 
Embryonen,"  III,  91.1,  Von  Kolliker  ("  Grundriss, "  2te  Aufl.,  369), 
Froriep,  85.1,  De  Meuron,  86.1,  Piersol,  88.1,  F.  P.  Mall,  87.1, 
88.2,  and  A.  Prenant,  91.2,  204-220. 


752 


THE   FCETUS. 


The  oesophagus  is  developed  from  the  short  piece  of  the  vor- 
derdarm,  p.  261,  between  the  pharynx  and  the  stomach,  Fig.  441,  oe. 
During  the  fourth  week  it  begins  to  lengthen  out,  and  by  the  end  of 

the  fifth  week  has  become  a  cylindri- 
cal  tube  of  considerable  length,  Fig. 
444,  C.  As  regards  its  further  his- 
tory  we  have  little  exact  information. 
I  have  observed  that  during  the 
fourth  to  sixth  month  it  has  usually 
four  well-marked  ridges  formed  by 
its  mucous  membrane,  and  that  below 
the  larynx  these  ridges  are  so  ar- 
ranged as  to  give  the  cavity  of  the 
oesophagus,  as  seen  in  cross  sections, 
the  outline  of  a  Greek  cross,  which 


.  -01 

L.Mx 
...Ma 


.  _-_Vom 


-_CL 


FIG.  439. —Transverse  Section  of  the  (Esophagus 
of  a  Human  Embryo  of  four  Months  (Minot  Coll, 
No.  35).  Ep,  epithelium;  conn,  connective  tissue  of 
mucosa ;  MC,  circular  muscular  coat ;  ML,  longitu- 
dinal muscular  coat. 


was  observed  by  Kolliker  ("  Entwick- 
elungsges.,"  2te  Aufl.,  853).  At  four 
months  the  inner  circular  muscular 
coat,  Fig.  439,  M C,  and  the  outer 
longitudinal  muscular  coat,  ML,  are 
clearly  differentiated. 

The  epithelium*  of  the  oesophagus 
at  four  months,  Fig.  440,  consists  of 
numerous  layers  of  clear  cells,  but  at 
certain  points  some  of  the  cells  at  the 
free  surface  have  a  granular  proto- 
plasmatic appearance,  a  somewhat 
cylindrical  form,  and  bear  a  crown  of  cilia.  The  presence  of  cilia  in 
the  human  embryo  of  eighteen  to  thirty-two  weeks  was  discovered 
by  E.  Neumann,  76.1. 


FIG.  438.— Reconstruction  of  His'  Em- 
bryo B ;  the  Head  is  drawn  as  if  erected. 
Ol,  Nasal  pit;  A£r,  maxillary  process;  Ma, 
mandibular  arch;  Hy,  hyoid  arch;  3,  4, 
branchial  arches;  Li,  liver;  V.om,  om- 
phalo-mesaraic  vein;  W,  Wolfflan  body; 
In,  intestine;  CL,  cloaca;  Bl,  allantois; 
Vi,  vitelline stalk;  St,  stomach;  Lu,  lung; 
La,  larynx.  After  W.  His. 


*  Demon,  85. 1,  has  also  studied  the  foetal  oesophagus,  but  at  the  time  of  writing  I  have  been 
unable  to  consult  his  dissertation. 


THK    ALIMENTARY    TRACT. 


753 


Balfour  ("Works,"  III.,  61)  records  that  in  shark  embryos  the 
cavity  of  the  oesophagus  is  entirely  obliterated  about  the  time  the 
fourth  gill-cleft  is  formed,  and  so  remains  for  a  long  period;  the  ob- 


Fio.  440.—  Highly  magnified  View  of  a  small  portion  of  the  Epithelium  of  Fig.  439. 

literation  is  effected  by  the  growth  of  the  entodermal  epithelium. 

That  the  entodermal  canal  is  for  a  time  in  teleost  embryos  a  solid  cord 

lias  been  already  stated,  and  accordingly  we  find  in  them  the  O3soph- 

au'iis  without  a  lumen  during 

certain   stages,    cf.    Mclntosh 

and   Prince,    90.1,   771.     De 

Meuron,  86.2,  states  that  the 

obliteration   can   be   observed 

in  anura,  just  after  the  larva 

hatches;   in   lizards,    and   the 

chick  embryo  of  five  and  one- 

half   days;  in  lizards  the  ob- 

literation is  incomplete.     W. 

Opitz,  87.1,  states  that  part 

of  the  lumen  is  closed  in  the 

human  embryo,  and  concludes 

from  that  fact  that  the  amnio- 

tic  fluid  cannot  be  swallowed 

by  the  foetus. 

Stomach.—  The  first  trace 
of  the  stomach  appears  in  a 
human  embryo  of  five  or  six 
days  as  a  slight  dilatation  of 
the  entodermal  canal,  Fig. 
441,  st,  between  the  oesopha- 
gus, oe,  and  the  liver,  /;  the 

Stomach  at  this  Stage  is  in  the 

rnorlian    rklano  and  nvorlifKi  tho 
median    plane  ana  Overlies  tne 

Septum  tranSVerSUm.       The  dl- 

fetation  continues  to  increase 

rhiTMncr  flip  Ax/hnlfk  frpfal  nprinrl 

)a' 

The   stomach  very  early   mi- 

grates into  the  abdominal  cavity  below  the  liver,  Fig.  444,  A,  B,  C, 

there  being  a  corresponding  elongation  of  the  oesophagus. 

48 


FIG.  441.—  Reconstruction  of  Fors  Embryo.  V, 
Fore-brain;  H,  hemisphere;  ftp,  hypophysis;  J9, 
^andible;  Vr,  median  thyroid;  m,  m&-£rain;  Hb\ 
hind-brain:  ch,  notochord;  1.  2,  3,  4,  gill-pouches; 


WoUBan  body;  Wd,  Wolfflanduct;  ri,  vitelline  duct  ; 
Al,  allantois;  rf,  cloaca.     After  Fol. 


In  conse- 


'54 


THE   FCETUS. 


quence  of  this  migration  the  stomach  acquires  a  mesentery,  which 
on  its  dorsal  side  is  known  as  the  mesogastrium,  on  the  ventral  side 
as  the  lesser  omentum ;  the  mesogastrium  is  the  anlage  of  the  greater 
omeiitum  or  epiploon.  During  its  migration  the  stomach  also  be- 
comes asymmetrical  in  shape  and  position,  Fig.  444,  0 ;  in  that  fig- 
ure, which  is  taken  from  a  five  weeks'  embryo,  the  adult  form  of  the 
stomach  is  clearly  indicated ;  the  figure  also  shows  that  the  greater 
curvature  belongs  to  the  dorsal,  the  lesser  curvature  to  the  ventral 
side  of  the  stomach.  Finally  during  its  migration  the  stomach  also 
revolves  around  its  own  axis  so  that  its  left  surface  is  turned  front- 
ward and  its  right  surface  backward,  see  Fig.  445,  st,  and  more- 
over the  cephalic  end  of  the  stomach  is  on  the  left  side,  the  caudal 
or  pyloric  end  on  the  right  side.  In  the  change  of  position  of  the 
stomach  we  find  the  explanation  of  the  origin  of  the  omentum  by  the 
folding  of  the  mesogastrium,  and  also  of  the  connection  of  the  ventral 
mesentery  or  lesser  omentum  with  the  lesser  curvature  or  primitive 
median  ventral  line  of  the  stomach  on  the  one  hand,  and  the  liver  on 
the  other. 

The  revolution  of  the  stomach  around  its  own  axis  explains  the 
asymmetrical  position  of  the  vagus  in  the  adult,  for  the  embryonic 
left  side  innervated  by  the  left  vagus  becomes  the  "  anterior"  sur- 
face, according  to  the  descriptive  anatomy  of  the  adult. 

HISTOGENESIS. — Our  knowledge  of  the  development  of  the  gastric 
glands  rests  chiefly  on  the  admirable  memoir  of  Toldt,  80. 1 ,  who 
also  reviews  the  scanty  results  of  his  predecessors.  The  entoderm 
of  the  stomach  consists  in  young  embryos  (cat  30-130  mm.)  of  a 
cylinder  epithelium,  which  gradually  increases  in  thickness  until 
the  formation  of  the  peptic  glands  begins  (cat  embryos  of  00-70 
mm.,  human  embryos  tenth  week).  Groups  of  cells  arrange  them- 
selves in  miniature  glands,  which  are  contained  entirely  within  the 
thickness  of  the  epithelium;  that  is,  they  do  not  project  into  the 


mes 


¥10.  442. —Epithelium  of  the  Greater  Cur- 
vature of  the  Stomach  of  an  Embryo  Cat  of  85 
mm.  Ep.  epithelium;  gl,  anlage  of  peptic 
gland.  After  C.  Toldt. 


FIG.  443.— Peptic  Glands  from  the  greater 
Curvature  of  Stomach  of  a  Human  Embryo 
from  the  end  of  the  eighth  lunar  month.  Ep, 
epithelium:  al,  branching  gland.  After  C. 
Toldt. 


mesoderm .;  each  gland,  Fig.  442,  gl,  when  fully  marked  out,  con- 
sists of  a  small  central  cavity  and  a  wall  of  finely  granular  cuboidal 
cells,  and  is  separated  from  the  neighboring  glands  by  the  unaltered 
high  cylinder-cells.  This  stage  is  described  for  a  rabbit  embryo  of 
42  mm.  by  E.  Salvioli,  90.1,  73.  The  glands  grow  down  into  the 
mesoderm  (cat  embryos  of  85  mm.),  and  one  can  soon  distinguish  an 
upper  portion  or  duct  lined  by  high  cylinder-cells  and  a  lower  gland- 


THE   ALIMENTARY   TRACT.  755 

ular  portion  with  a  cuboidal  epithelium,  Fig.  443.  The  gland  proper 
forms  terminal  and  later  lateral  buds  also,  so  that  each  duct  acquires 
several  branches,  Fig.  443.  The  formation  of  new  gland  anlages 
ceases  when  the  budding  begins,  but  the  glands  continue  to  multiply, 
owing  to  the  division  of  the  ducts.  At  seven  months  the  foetal 
stomach  has  about  seven  glands  to  each  duct,  and  this  proportion  is 
kept  until  birth ;  but  after  birth,  owing  to  the  continued  division  of 
the  ducts,  the  proportion  is  diminished ;  thus  Toldt  found  at  ten 
years  an  average  of  six  glands  for  each  duct ;  at  fifteen  years  five 
glands ;  in  the  adult  only  three.  The  peptic  cells  (parietal  or  del- 
unmrphous  cells,  Belegzellen)  arise  by  differentiation  of  single  gland- 
cells  ;  the  differentiation  begins  by  the  accumulation  of  coarse  gran- 
ules (z3rmogen?),  at  first  in  the  outer  part,  later  through  the  whole  cell; 
these  glandular  cells  first  become  recognizable  about  the  time  the 
branching  of  the  glands  commences  (in  man  toward  the  end  of  the 
fourth  month).  The  number  of  peptic  cells  increases  both  by  divi- 
sion of  the  cells  and  the  metamorphosis  of  the  original  cells.  As 
the  peptic  cells  are  differentiated  they  take  up  their  position  on  the 
outside  of  the  gland.  After  the  sixth  month  pepton  may  be  obtained 
from  the  stomach.  H.  Sewall,  78.1,  asserted  that  the  peptic  cells 
immigrated  from  the  mesoderm,  an  error  which,  as  Toldt  has 
shown,  was  due  to  incomplete  observation. 

The  ntn<'<niN  <//umlx  (Toldt,  80.1,  119)  appear  about  the  same 
time  (cat  embryo  of  50  mm.)  as  the  peptic,  as  evaginations  of  the 
epithelium,  which  are  lined  throughout  by  cylinder — not  by  cuboidal 
— cells.  Later  the  glands  become  branched.  Kolliker  ("  Entwicke- 
lungsgesch . . "  -,'te  Aufl.,  854)  observed  that  the  gastric  glands  measure 
during  the  fifth  month  from  0.13-0.22  mm. ;  during  the  sixth  0.42- 
O.T1  mm. 

The  spaces  between  the  gland-openings  become  somewhat  promi- 
nent during  the  third  month,  and  these  prominences  have  been 
described  as  villi  by  Kolliker  and  others,  but  there  are  no  sufficient 
grounds  for  maintaining  that  there  are  any  true  gastric  villi  at  any 
period.  The  pseudo-villous  appearance  is  most  marked  toward  the 
pylorus,  and  persists  at  least  through  the  fifth  month.  A  little  later 
than  the  villi  there  appear  also  on  the  inner  surface  of  the  stomach 
longitudinal  ridges  which  vary  in  number  from  12  to  15. 

During  the  fourth  month  the  inner  circular  muscle  layer  and  the 
outer  longitudinal  layer  become  well  marked  (Kolliker,  "  Entwicke- 
lungsgesch.,"  2te  Aufl.,  85:}).  * 

Intestine. — The  intestine  includes  the  whole  of  the  entodermal 
canal  from  the  stomach  to  the  anus.  Four  entodermal  organs  are 
appended  to  it,  the  liver,  pancreas,  yolk-sac,  and  allantois  or  bladder. 
Win  MI  first  formed  it  is  a  short,  straight,  median  tube,  to  the  ventral 
side  of  which  are  apppended  the  yolk-sac  and  allantoic  diverticulum, 
compare  Chap.  XII.  The  intestinal  canal  very  early  begins  to 
elongate,  and  continues  to  do  so  throughout  fcetal  life;  while  elon- 
gating it  also  increases  gradually  in  diameter.  I  know  no  measures 
of  the  growth  of  the  intestine.  A  consequence  of  its  growth  is  that 
it  has  to  form  coils,  which  finally  produce  important  anatomical 
changes.  The  posterior  portion  of  the  intestine  increases  more  than 
the  rest  in  diameter  and  becomes  the  large  intestine  (colon  and  rec- 


756 


THE    FCETUS. 


turn) .     From  the  anterior  end  of  the  colon  grows  out  the  coscum, 
and  from  the  coecum  the  appendix  vermiformis. 

GENERAL  GROWTH. — The  elongation  and  twisting  of  the  intestine 
in  the  embryo  is  indicated  by  Fig.  444.     It  has  been  carefully  worked 


by  W.  His  ("  Anat.  menschl.  Embryonen,"  III.,  12-25).  In  an  em- 
bryo of  4.2  mm.  the  stomach  is  barely  indicated,  A;  the  neck  of  the 
yolk-sac,  Yks,  is  very  wide;  nearly  the  whole  space  between  the 
yolk-stalk  and  the  stomach  is  occupied  by  the  hepatic  anlage,  li.  In 
an  embryo  of  7  mm.,  Fig.  444,  B,  the  stomach  has  elongated  and 


THE   ALIMENTARY    TRACT. 


757 


begun  to  descend  into  the  abdominal  cavity ;  the  yolk-stalk  is  con- 
siderably smaller,  Yks;  between  it  and  the  stomach  the  entodermal 
canal  has  lengthened ;  near  the  stomach  are  appended  the  pancreas, 
P,  and  the  liver,  Li.d;  the  intestinal  canal  below  the  yolk-sac  has 
also  lengthened  out,  so  that  the  intestine  as  a  whole  describes  a  long 
loop  toward  the  ventral  side,  to  be  there  attached  to  the  yolk-sac,  see 
also  Fig.  17;  as  the  stomach  is  situated  entirely  on  the  left  side,  it 
follows  that  the  loop  is  asymmetrical,  the  upper  limb  of  the  loop 
lying  more  to  the  right,  while  the  lower  limb  lies  more  to  the  left. 
The  asymmetry  is  more  evident  in  later  stages,  Fig.  444,  C.  The 
upper  limb,  together  with  part  of  the  lower  limb,  forms  the  small 
intestine ;  the  division  between  large  and  small  intestine  does  not 
coincide  with  the  insertion  of  the  yolk-stalk.  The  cephalic  limit  of 
the  large  intestine  is  first  given  in  embryos  of  about  12  mm.  by  a 
small  diverticulum,  Fig.  445,  B,  Coe,  the  anlage  of  the  coecum, 
compare  also  Fig.  444,  C,  Coe ;  the  whole  of  the  canal  on  the  caudal 
side  of  the  coecum  increases  in  diameter  and  forms  the  large  intes- 
tine, Fig.  445,  B,  col.  The  small  intestine  now  lengthens  rapidly 
for  a  long  period,  and  forms  coils 
below  the  level  of  the  coecum, 
Fig.  445,  B;  at  the  same  time 
the  large  intestine,  col,  also 
lengthens,  but  more  slowly,  and 
its  coecal  end  is  carried  over  to 
the  left  side  toward  the  cardiac 
end  of  the  stomach,  with  the  re- 
sult that  the  small  intestine  has 
to  cross  ventrad  of  the  large  in- 
testine from  right  to  left.  The 
crossing  of  the  two  intestines  in- 
troduces considerable  complexity 
into  the  arrangement  of  the  mes- 
entery, as  explained  in  the  next 
section.  At  the  stage  we  have 
now  reached,  Fig.  445,  the  stom- 
ach, st,  is  relatively  large,  and 
has  essentially  its  adult  form,  but 
it  still  lies  almost  wholly  on  the 
left  side;  its  pyloric  end  is  to  the 
right  of  the  median  line;  from 
the  pylorus  springs  the  duodenum  or  beginning  of  the  small  in- 
testine; it  runs  toward  the  median  line  nearly  parallel  with  the 
greater  curvature  of  the  stomach ;  the  liver  duct,  Li,  and  pancreas, 
/v/y/,  are  both  connected  with  the  upper  end  of  the  duodenum ;  the 
pancreas  lies,  as  stated,  p.  767,  in  a  transverse  position  between  the 
duodenum  and  the  stomach.  The  small  intestine  makes  several  coils 
and  terminates  on  the  right  side  of  the  body  by  joining  the  colon;  in 
Fig.  445,  B,  however,  the  end  of  the  colon  lies  on  the  left,  but  this 
is  unusual  and  was,  perhaps,  a  case  of  partial  reversus  situs  vis- 
cerum. 

As  the  large  intestine  grows,  its  coecal  end  descends  toward  the 
pelvis  on  the  right  side,  and  it  may  then  be  subdivided  into  the  four 


Pan 


col 


FIG.  445.  —Two  Front  Views  of  the  Entodermal 
Canal;    A,   Embryo  Sch.  1  of  His;    B,  His1  em- 
,   Sch.    2.     T>\  r~ 
;  St,  stomach; 

oe, 


758 


THE    FCETUS. 


S.in 


V. 


parts  recognized  in  descriptive  anatomy,  to  wit:  1,  2,  3,  the  ascend- 
ing, transverse,  and  descending  colons ;  and  4,  the  rectum. 

CCECUM  AND  APPENDIX  VERMIFORMIS. — The  coecum  arises  as 
an  outgrowth  of  the  ileal  end  of  the  large  intestine ;  it  appears  in 
human  embryos  of  10-12  mm.  The  appendix 
appears  as  a  blind  outgrowth,  Fig.  446,  F,  of 
the  coecum.  At  six  months,  Fig.  446,  it  is  long 
and  slender,  with  a  narrow  free  mesentery  and 
is  relatively  much  better  developed  than  in  the 
adult,  and  also  is  less  sharply  marked  off  from 
the  coecum  proper. 

INTESTINAL  HERNIA. — By  this  term  we  may 
designate  the  normal  temporary  extrusion  of  the 
intestinal  canal  into  the  umbilical  cord.  So  far 
as  I  can  now  recall  this  extrusion  has  been  ob- 
served only  in  man.  In  human  embryos  of  10 
mm.  the  part  of  the  intestine  attached  to  the 
yolk-stalk  begins  to  enter  the  umbilical  cord, 
and  thereafter  the  length  of  the  intestine,  which 
leaves  the  body  cavity  proper  and  lodges  in  the 
coelom  of  the  yolk-stalk,  increases  until,  per- 
haps, the  tenth  week.  Thereafter  it  is  gradu- 
ally withdrawn  into  the  abdomen.  The  cause 
of  this  temporary  umbilical  hernia  is  believed 
to  be  the  strain  produced  by  the  yolk-sac ;  at- 
tention is  directed  to  it  in  the  descriptions  and 
of6 T££Sf  Emb^o  figures  of  embryos  in  Chapter  XVIII. 
of  about  six  Months  (Minot  HiSTOGENESis. — The  intestinal  canal  consists 

Coll. ,  No.  65).      s  in,  Small        ,     ,  -i  -i      £    ,-,        r>      i  ,-\        (•  •  i 

intestine;  F,  vermiform  ap-  at  the  end  or  the  tirst  month  ot  an  inner  layer 
terydian(TarisSgf "Irom^the  °^  entoderm  and  an  outer  layer  of  mesoderm ; 
small  coecum;  i. in,  large  the  former  becomes  the  epithelium  of  the  villi 

intestine.     Natural  size.  i       i        j       j.-u       i    j.  • 

and  glands,  the  latter  gives  rise  to  the  connec- 
tive tissue  of  the  villi,  mucosa,  submucosa,  etc. ,  and  also  to  the  two 
muscular  layers  and  to  the  peritoneal  covering.  The  epithelium  is 
a  high  cylinder  epithelium  like  that  throughout  the  undifferentiated 
entodermal  canal.  The  mesoderm  is  a  thick  layer  of  mesenchyma 
covered  externally  by  the  cuboidal  epithelium  (mesothelium) ,  which 
lines  the  coelom,  except  that  the  anal  end  of  the  intestine  (future  rec- 
tum) has  no  mesothelium  because  it  lies  beyond  the  coelom. 

At  two  months  I  find  the  villi  and  glands  of  the  small  intestine 
beginning  their  development,  Fig.  447,  and  all  the  layers  of  the 
mesoderm  sufficiently  differentiated  to  be  recognized.  The  stratifi- 
cation of  the  intestinal  mesoderm  can  be  recognized  in  a  cat  embryo 
of  25  mm.  according  to  Patzelt,  83.1,  146.  The  villi,  Vi,  are  short, 
thick,  and  few  in  number,  but  additional  villi  are  developing  between 
those  already  present ;  the  entoderm  has  altered  its  primitive  char- 
acter very  slightly ;  the  epithelial  glands  are  to  grow  out  between 
the  bases  of  the  villi.  The  villi  also  appear  throughout  the  large 
intestine,  but  are  obliterated  (Kolliker  "Grundriss,"  2te  Aufl.,  360) 
there  by  the  upward  growth  of  the  glands,  while  in  the  small  intes- 
tine the  villi  enlarge  and  persist  throughout  life.  C.  von  Langer, 
87.1,  54-56,  studied  the  mesodermal  cores  of  the  villi  and  found 


THE   ALIMENTARY    TRACT. 


759 


msth 


conn 


1m 


c.m 


Vi 


Ent. 


them  well  developed  in  nut 

the  large  intestine  dur- 
ing the  fourth  month, 
partially  aborted  at 
birth,  and  completely 
aborted  one  montli  af- 
ter birth.  The  meso- 
thelium,  msth,  has  be- 
gun to  thin  out  to  con- 
vert itself  into  the 
peritoneal  epithelium, 
but  the  connective-tis- 
sue layer  of  the  perito- 
neum is  not  yet  re- 
cognizable. The  two 
muscular  layers,  ////, 
c///,  are  marked  out  by 
the  elongation  of  the 
mesenchymal  cells  to 
form  smooth  muscle- 
fib^  « .  The  submu- 
cosa,  conn,  consists  of 
dense  undifferentiated 
mesenchyma  ;  its 
thickness  about  equals 
that  of  the  entoderm, 
Kit  /,  or  that  of  the  two 
muscular  layers,  ////, 
<•///,  taken  together. 

The  entoderm  often  contains  leucocytes.  After  the  second  month 
it  gradually  loses  its  embryonic  character.  Over  the  villi  of  the 

small  intestine,  Fig.  448, 
it  becomes  a  beautiful 
cylinder  epithelium 
with  basally  placed  nu- 
clei, which  all  lie  nearly 
at  one  level,  in  marked 
contrast  to  their  earlier 
distribution.  The  villi 
themselves  are  more  or 
less  cylindrical  in  form 
with  the  free  ends 
rounded.  In  the  sec- 
tions, which  I  have  ex- 
amined, the  entodermal 
villi  are  only  partially 
filled  with  mesoderm — 
a  peculiarity  which  I 
am  inclined  to  regard  as 
normal,  not  as  artificial. 

FIG  448  —Section  of  the  Small  Intestine  of  a  Human  Em-  The  qlands  begin  to 
bryoof  three  Months  (Minot  Coll.  No.  41).  gl,  Gland;  Ft,  vil-  •  porlv  ,'r,  f}1P  third 
lus;  mu,  mucosa;  msc,  muscularis;  msf,  mesenterial  insertion,  arise  early  in  1116 


FIG.  447.  —Section  of  the  Small  Intestine  of  a  Human  Embryo 
of  sixty -three  to  sixty -eight  Days  (Minot  Coll.,  No.  1:38.)  ms£, 
Mesentery;  msth,  mesothelium;  conn,  connective  tissue  of  sub- 
mucosa ;  Zm,  longitudinal  muscles ;  cm.  circular  muscles ;  Ft, 
villus;  Ent,  epithelial  entoderm.  x  144  diams. 


gl 


760  THE    FCETUS. 

month  (in  the  rabbit,  when  the  embryo  is  about  45  mm.,  Barth, 
68.1,  131).  They  are  hollow  outgrowths  of  the  entoderm  (Barth, 
I.e.;  Patzelt,  83.1),  extending  into  the  mesoderm ;  for  a  considerable 
period  they  remain  short  as  compared  with  the  villi,  see  Fig.  448. 
The  development  of  the  glands  of  the  small  intestine  has  been  im- 
perfectly studied ;  Barth,  68. 1, 133,  states  that  the  glands  of  Brunner 
may  be  recognized  by  their  branching  in  rabbit  embryos  of  70  mm. 
The  glands  of  the  large  intestine  have  been  studied  by  Patzelt, 
83.1,  principally  in  the  pig  and  rabbit  embryos,  which  he  found 
more  favorable  than  human  embryos;  the  entoderm  in  cat  em- 
bryos of  33  mm.  contains  small  groups  of  short  granular  cells, 
with  oval  nuclei  with  nucleoli ;  these  groups  are  gland  anlages,  and 
are  easily  recognized  by  their  pale  nuclei ;  the  anlages  are  separated 
from  one  another  by  lines  of  cells  with  longer  nuclei,  which  stain 
more  darkly  with  hsematoxylin ;  but  in  embryos  of  50  mm.  and  older 
all  the  nuclei  stain  nearly  alike.  The  villi  of  the  large  intestine  are 
temporary ;  they  have  been  shown  by  C.  von  Langer,  87. 1,  54-56,  to 
be  united  by  ridges  running  between  their  bases ;  the  ridges  subdi- 
vide the  surface  into  little  areas,  and  in  each  area  lie  several  glands ; 
in  the  human  foetus  the  ridges  are  still  present  at  term,  but  disappear 
in  the  course  of  the  first  month  after  birth.  The  gland  anlages  grow 
slowly — in  the  cat  at  birth  they  are  only  0. 23  mm.  Both  the  anlages 
and  the  young  glands  multiply  by  division,  which  begins  at  the 
lower  end  of  the  gland  and  spreads  to  its  mouth.  Patzelt  found  in 
a  section  of  the  large  intestine  of  cat  embryos  of 

33  mm.,    6-8    glands.  95  mm.,    45-50    glands. 

50    u        14-17       "  101    "         54-57 

60    "        16-19       "  114    "          67-70 

68    "        21-23       "  140    "        110-120      " 

82    "       40-42       " 

The  first  beaker-cells  of  the  large  intestine  appear  on  the  villi  (cat 
embryos  of  60  mm.),  they  rapidly  increase  in  number,  so  that  in  the 
cat  most  of  the  entoderm  consists  of  beaker-cells  both  over  the  villi 
and  in  the  glands. 

GROWTH  OF  THE  INTESTINAL  ENTODERM. — The  gland  anlages 
and  later  the  fundi  of  the  glands  are  the  centres  of  growth  for  the 
intestinal  epithelium,  as  first  suggested  by  Pfitzner's  observation  that 
the  karyokinetic  figures  occur  chiefly  in  the  glands,  not  generally 
over  the  epithelium  (Arch.  f.  mikrosk.  Anat.,  XX.,  137),  but  the 
definite  recognition  of  the  fact  is  due  to  Patzelt,  83.1,  165.  The 
multiplication  of  cells  in  the  glands  of  the  intestine  and  stomach  is 
confined  in  the  adult  to  the  fundi  of  the  glands.  That  the  bottom  of 
each  gland  is  a  separate  centre  of  growth  was,  I  think,  first  suggested 
byW.  Flemming,  85.2,  and  has  since  been  fully  demonstrated  by 
the  researches  of  Bizzozero  and  Yasale,  85.1,  Heidenhain,  88.1, 
26-28,  Bizzozero,  88. 1,  89.2,  and  E.  Salvioli,  90. 1.  I  consider  that 
the  notion  of.  discrete  centres  of  growth  in  epithelia,  with  its  corol- 
lary of  translation  of  the  cells  from  their  place  of  origin,  is  an  impor- 
tant advance  in  our  conceptions.  It  is  probable  that  other  glands 
also  grow  in  the  embryo  as  in  the  adult,  but  no  direct  observations 
on  this  point  have  yet  been  made. 


THE   ALIMENTARY   TRACT.  761 

The  Liver. — The  early  development  of  the  liver  has  been  de- 
scribed p.  208,  and  its  situation  in  the  septum  transversum  explained. 
O.  Hertwig  (**  Lehrb.  d.  Entwickelungsgesch.,"  3teAufl.)  describes 
the  liver  as  being  primitively  lodged  in  the  ventral  mesentery — an 
error  of  statement  for  which  I  cannot  account. 

The  liver  of  all  vertebrates  consists  of  two  parts :  1,  a  branching 
system  of  epithelial  gall-ducts,  and  2,  a  network  of  hepatic  cylinders. 
The  two  parts  are  morphologically  distinct.  The  gall-ducts  are 
surrounded  by  connective  tissue,  and,  as  is  well  known,  are  accom- 
panied by  the  branches  of  the  portal  vein  and  hepatic  artery.  The 
hepatic  cylinders  are  separated  from  one  another  only  by  endothelial 
blood-vessels.  The  essential  primitive  features  of  the  hepatic  cylin- 
ders are  illustrated  by  Fig.  449;  every  cylinder,  hp,  is  an  epithelial 
tube  with  a  small  central  lumen  and 
covered  by  an  endothelium,  which 

is  easily  recognized  by  its  flattened,  r®rv^^^^^ 

darkly  stained  nuclei;  the  endothe- 
lium is  the  wall  of  a  blood-vessel 
or  channel,  bl.  The  hepatic  cylin- 
ders by  branching  and  uniting  form 
a  network,  all  the  meshes  of  which 
are  entirely  occupied  by  blood-ves- 
sels. In  sharks,  Fig.  449,  each 
cylinder  comprises  in  its  cross  sec- 
tion usually  eight  to  ten  cells,  and 
is  almost  completely  bathed  in 
blood.  In  amphibia  the  cylinders 
are  smaller;  they  comprise  only  four 

to  five  cells  in   cross  section    and  ^..p^^  aSectionof  theLiver 

their  lumen  is  very  small,  and  the  of  an  Acanthias  Embryo  of  29  mm.  /»/>,  He- 
blood-channels  between  them  are  §fatmscvlinders;  w*  blood-^annels-  *  ™ 
relatively  diminished.  In  mammals 

each  hepatic  cylinder  comprises  merely  two  epithelial  cells ;  the  lumen 
is  reduced  to  a  minute  canal  (the  gall  capillary) ;  the  cylinders  anas- 
tomose with  one  .another  very  frequently  and  at  very  short  intervals; 
and  finally  the  blood-vessels  between  the  cylinders  become  smaller  for 
the  most  part  than  the  cylinders.  In  mammals  we  have  further  the 
hepatic  cylinders  gathered  into  radiating  groups ;  the  groups  are  the 
lobules  of  descriptive  anatomy.  In  most  text-books  the  mammalian 
hepatic  cylinders  are  referred  to  as  "radiating  rows  of  liver  cells." 
If  the  fundamental  notions  just  recapitulated  are  kept  in  mind  the 
following  paragraphs  can  be  better  understood. 

The  liver  commences,  as  stated  p.  268,  as  a  diverticulum  of  the 
entodermal  canal  extending  into  the  septum  transversum.  This 
single  median  diverticulum  may  be  designated  as  the  Amphioxtis 
stage,  since  a  similar  diverticulum  in  the  cyclostome  is  regarded, 
probably  correctly,  as  the  homologue  of  the  primitive  hepatic  anlage 
of  true  vertebrates.  The  single  diverticulum  develops  to  a  consider- 
able size  in  shark  and  amphibian  embryos,  but  in  amniota  it  forms 
two  branches  almost  immediately  (chick  fifty- five  to  sixty  hours, 
rabbit  eleventh  day),  so  that  it  is  usual  to  describe  the  amniote  liver 
as  arising  from  two  diverticula.  The  evaginations  are,  of  course, 


702  THE   FCETUS. 

lined  by  entoderm;  they  are  situated  immediately  behind  the  heart, 
and  embrace  between  them  the  two  vitelline  veins  forming  the  roots 
of  the  ductus  venosus.  In  the  chick  the  right  pouch  is  from  the 
first  longer,  but  of  smaller  diameter  than  the  left  (Foster  and  Bal- 
four,  "  Elements,"  2d  ed.,  179).  In  the  rabbit,  according  to  Kolliker 
("Grimdriss,"  2te  Aufl.,  372),  the  left  pouch  appears  the  tenth,  the 
right  the  eleventh  day.  In  the  human  embryo  of  3  mm.  His,  81.1, 
found  the  hepatic  diverticulum  single. 

In  the  primitive  form  of  vertebrate  development  (Petromyzon  and 
amphibians)  the  hepatic  diverticulum  extends  into  a  mass  of  eiito- 
dermal  yolk-cells,  so  that  it  has  from  the  start  several  layers  of  ento- 
dermal  cells  around  its  cavity.  The  cells  form  a  mass  which,  as 
described  by  W.  T.  Shore,  91.1,  179-183,  separate  off  (in  the  frog, 
at  least)  from  the  rest  of  the  yolk,  the  cells  themselves  multiplying 
and  changing  into  liver-cells.  They  constitute  a  thick,  solid  wall 
around  the  hepatic  diverticulum;  channels  appear  in  the  solid  walls, 
and  these  channels  acquire  endothelial  linings,  and  blood  enters 
them ;  the  yolk-cells  between  the  blood  spaces  gradually  develop  into 
hepatic  cylinders.  These  changes  can  be  favorably  studied  in  a 
frog's  tadpole  six  or  seven  days  after  hatching. 

In  amniota  there  is  an  early  separation  of  the  liver  anlage  and 
yolk-sac,  and  the  former  has  thin  walls  when  it  arises.  W.  T. 
Shore,  9 1 . 1 ,  184,  states  that  in  the  chick  the  walls  of  the  diverticulum 
begin  to  thicken  almost  immediately  by  the  proliferation  of  the  cells, 
and  in  the  thickened  mass  channels  appear, "  there  take  place  irrup- 
tions, as  it  were,  of  capillary  blood-vessels  from  the  vitelline  vein 
into  the  solid  mass  of  proliferated  hypoblast  (i.e.,  entoderm),  break- 
ing it  up  into  more  or  less  branched  rods  of  cells  (second  half  of  the 
third  day)."  In  most  text-books  the  hepatic  entoderm  is  described  as 
sending  out  solid  buds  between  which  the  blood-vessels  arise,  but  it 
is  doubtful  whether  such  a  description  is  accurate.  I  strongly  incline 
to  accept  Shore's  view  that  the  solid  anlage  is  broken  up  by  the  for- 
mation of  blood-vessels  in  it.  If  Shore  is  right  we  can  understand 
why  the  hepatic  cylinders  form  a  network.  So  far  as  known  the 
hepatic  cylinders  are  at  first  solid  and  do  not  acquire  their  lumen 
until  later.  In  the  later  stages  of  incubation  the  liver  has  the  color 
of  the  yolk.  In  a  chicken  just  hatched  the  liver-cells  contain  oil 
drops. 

In  mammals  the  development  of  the  liver  is  similar  to  that  in  the 
chick.  The  walls  of  the  primitive  diverticulum  thicken,  become 
permeated  by  blood-vessels,  and  so  divide  into  hepatic  cylinders,  Fig. 
450.  The  cylinders  are  at  first  solid  and  quite  irregular  in  shape 
and  size,  Fig.  450,  Tip,  and  the  blood-channels,  bl,  are  very  large. 
The  differentiation  of  the  cylinders  in  the  human  embryo  has  been 
studied  by  Toldt  and  Zuckerkandl,  76. 1.  They  found  the  cylinders 
to  have  a  lumen  in  a  four  weeks'  embryo;  from  this  age  till  the  end 
of  foetal  life  the  C3^1inders  contain  two  forms  of  cells:  1,  large  poly- 
hedral cells,  resembling  those  of  the  adult  organ ;  2,  smaller  round 
cells,  the  nuclei  of  which  stain  darkly ;  the  two  forms  are  mingled 
irregularly ;  the  smaller  cells  entirely1  disappear  after  birth  and  are 
presumably  only  a  young  stage  of  the  liver-cell.  It  is  not  until 
some  time  after  birth  that  the  cylinders  assume  the  adult  mammalian 


THE   ALIMENTARY    TRACT. 


'63 


type ;  they  become  longer  and  slenderer,  not,  however,  by  a  change  in 
the  size  of  the  liver-cells,  but  by  a  rearrangement  of  the  cells,  such 
that  the  number  of  cells  in  a  cross  section  of  a  cylinder  is  gradually 
reduced  to  two;  the  cylinders  after  this  change  are  zig-zag,  but 
soon  straighten  out.  The  metamorphosis  takes  place  irregularly,  so 
that  several  stages  can  be  seen  under  the  microscope  in  one  field  of 
view.  As  regards  the  development  of  gall-ducts,  we  have  no  definite 


msth 


FIG.  450.— Section  through  the  Liver  of  a  Rabbit  Embryo  of  thirteen  Days,     lip,  Hepatic  cylinder; 
,„'!»,  vascular  enclothelium;  bl,  blood -channels ;  msth,  mesothelium.     x  227  diams. 

knowledge.  We  may  surmise  that  they  arise  as  evaginations  of  the 
primitive  diverticulum  and  are  always  distinct  from  the  hepatic 
cylinders. 

LOBULES. — Toldt  and  Zuckerkandl,  76.1,  have  investigated  the 
changes  in  the  blood-vessels  in  the  human  liver.  In  a  four  weeks' 
embryo  the  vessels  are  all  large,  compare  Fig.  450,  but  by  the  eighth 
or  ninth  week  the  main  efferent  and  afferent  stems  are  recognizable. 
For  the  history  of  the  metamorphosis  of  the  large  veins  passing 
through  the  liver  see  p.  545.  During  the  third  or  fourth  month  the 
vascular  territories  of  the  portal  and  hepatic  veins  become  distin- 
guishable, for  the  branches  of  the  two  veins  distribute  themselves  so 
as  always  to  be  separated.  There  now  appear,  scattered  through  the 
liver,  islands  of  tissue  with  abundant  fine  ramifications  of  the  hepatic 
vein ;  each  island  is  the  anlage  of  a  group  of  lobules,  and  is  sur- 
rounded by  portions  of  the  liver  containing  the  branches  of  the  postal 


764 


THE   FCETUS. 


vein.  The  portal  system  cuts  into  the  island,  so  as  to  divide  it 
gradually,  while  it  expands,  into  lobules,  and  these  primary  lobules 
are  similarly  subdivided  until  the  permanent  lobules  are  established. 
The  lobules  enlarge  after  the  production  of  new  lobules  has  ceased. 

GROWTH. — The  liver  enlarges  very  rapidly,  compare  Figs.  179, 
222,  444,  and  451,  and  consequently  has  to  project  from  the 
septum  transversum  into  the  abdominal  cavity  more  and  more.  It 
forms  two  lobes,  one  each  side  and  connected  across  the  median  line ; 
between  the  two  lobes,  Fig.  451,  r.li  and  LLi,  is  situated  the 


Md 


FIG.  451,— Section  of  a  Rabbit  Embryo  of  thirteen  Days  through  the  Region  of  the  Fore  Limbs 
and  Liver.  Md,  Spinal  cord;  Ao,  aorta;  F.c,  cardinal  vein;  Lu,  lung;  R.li,  right  liver;  Ve, 
great  vein  of  liver;  1. Li,  left  liver. 

great  vein,  Fe,  of  the  liver;  as  that  vein  is  constituted  partly 
by  the  umbilical  vein,  it  is  attached  to  the  ventral  body-wall  of 
the  embryo.  In  a  rabbit  of  thirteen  days  both  lobes  are  well  de- 
veloped and  project  beyond  the  level  of  the  umbilical  vein,  but  in  the 
median  line  the  liver  is  entirely  on  the  cephalic  side  of  the  vein.  In 
longitudinal  median  sections  this  shows  very  clearly,  as  does  also 
the  fact  that  the  liver  is  an  appendix  of  the  septum  transversum. 
While  the  liver  is  expanding  the  stomach  migrates  into  the  abdom- 
inal cavity ;  after  that  migration  we  find  the  stomach  connected  by 


THE   ALIMENTARY   TRACT.  765 

a  thin  membrane,  or  ventral  mesenten',  with  the  median  dorsal  line 
of  the  liver;  the  membrane  extends  forward  to  the  septum  trans- 
versum  and  joins  it ;  the  membrane  is  the  anlage  of  the  omentum 
in  in  us;  concerning  its  development  we  possess  no  accurate  informa- 
tion beyond  the  fact  that  it  arises  after  the  first  differentiation  of 
the  liver  and  stomach,  and  is  a  new  structure  produced  as  the  stom- 
ach and  liver  descend  into  the  abdominal  cavity.  Similarly  we  find 
on  the  ventral  side  of  the  liver  there  is  developed  a  mesenterial  mem- 
brane by  which  the  liver  is  bound  to  the  median  ventral  line  of  the 
somatopleure;  this  membrane  is  the  anlage  of  the  suspensory  liga- 
ment; posteriorly  it  extends  at  least  to  the  umbilical  vein,  anteriorly 
to  the  septum  transversum,  with  which  it  is  continuous.  The  liver 
now  has  the  following  attachments :  1,  by  the  omentum  minus  to  the 
lesser  curvature  of  the  stomach ;  2,  by  the  suspensory  ligament  to 
the  median  line  of  the  body  and  the  inferior  surface  of  the  ventral 
part  of  the  septum  transversum  (or  future  diaphragm) ;  3,  to  the 
dorsal  part  of  the  septum  transversum.  The  connection  with  the 
septum  transversum  is  both  primitive  and  permanent,  so  that  in  the 
adult  the  liver  may  be  described  as  an  appendage  to  the  diaphragm. 
But  whereas  in  early  embryonic  stages  the  attachment  of  the  liver 
occupies  nearly  the  entire  septum,  in  later  stages  the  septum  develops 
over  a  considerable  expanse,  so  that  the  attachment  becomes  rela- 
tively smaller,  Fig.  455,  and  is  confined  to  the  dorsal  region  of  the 
septum  or  diaphragm.  The  area  of  attachment  finally  becomes  round, 
with  two  lateral  prolongations;  the  round  part  is  the  coronary  liga- 
mciit,  while  the  prolongations  are  the  lateral  ligaments  of  descrip- 
tive anatomy. 

As  the  liver  grows  in  the  septum  it  is,  of  course,  covered  by  meso- 
thelium,and  as  it  enlarges  and  becomes  a  more  independent  projection 
it  retains  its  mesothelial  envelope,  Fig.  450,  msth.  Later  a  layer  of 
mesenchyma  is  developed  between  the  liver-cells  and  the  mesothe- 
lium,  and  the  two  mesodermic  layers  together  constitute  the  perito- 
neum. As  to  the  histogenesis  of  the  hepatic  peritoneum  we  have 
no  accurate  information.  From  the  mode  of  development  of  the 
liver  it  is  evident  that  first  the  mesothelium,  and  later  the  perito- 
neum, covering  the  liver  must  be  directly  continued  on  to  the  liga- 
ments of  the  liver,  the  diaphragm,  and  the  lesser  omentum. 

For  illustrations  of  the  growth  and  position  of  the  human  foetal 
liver  see  Figs.  153,  170,  259,  284,  303,  305,  319.  During  the  second 
month  it  becomes  of  relatively  enormous  size;  so  that  during  the 
third  month  it  extends  far  into  the  hypogastric  region  and  fills  out 
the  greater  part  of  the  abdominal  cavity.  After  the  fifth  month  the 
intestines  and  other  viscera  overtake  the  liver,  but  at  birth  the  liver 
makes  two  thirty-sixths  of  the  total  weight,  as  against  one  thirty- 
sixth  in  the  adult.  Immediately  after  birth  the  liver  diminishes  in 
size  (Kolliker,  "Entwickelungsges.,"  2te  Aufl.,  889).  The  right  lobe 
of  the  liver  is  probably  always  larger  than  the  left ;  after  birth  its 
predominance  increases. 

Another  important  factor  of  the  development  of  the  liver  is  the 
atrophy  of  the  hepatic  cylinders  in  certain  parts,  as  discovered  by 
Toldt  and  Zuckerkandl,  76.1.  They  have  observed  this  atrophy 
near  the  lateral  and  suspensory  ligaments,  next  the  gall-bladder  and 


7<iO  THE    FCETUS. 

in  the  neighborhood  of  the  umbilical  vein.  When  the  atrophy  be- 
gins the  liver-cells  become  finely  granular,  opaque,  and  lose  their 
outline ;  the  protoplasm  breaks  down  and  disappears ;  the  nuclei  per- 
sist a  little  longer.  Changes  occur  also  in  the  gall-ducts  of  the 
atrophying  regions. 

FUNCTIONS. — I  cannot  do  more  than  allude  to  the  manifold  and 
important  functions  of  the  foetal  liver.  For  its  sanguinif active  role, 
see  Chapter  X.  For  a  general  discussion  of  its  physiology,  see  W. 
Preyer,  "  Specielle  Physiologie  des  Embryo."  For  speculations  upon 
the  relation  of  its  functions  to  its  mode  of  development,  see  W.  T. 
Shore,  91.1,  who  also  makes  suggestive  remarks  as  to  the  evolution 
of  the  liver.  In  regard  to  the  glycogenic  function  of  the  liver  in  the 
embryo,  see  especially  Claude  Bernard  (C.  R.  Acad.  Sci.  Paris, 
XLVIIL,  77-86). 

The  gall-bladder  arises  in  the  chick  during  the  fifth  day  as 
an  evagination  of  the  right  primary  diverticulum.  Kolliker  observed 
it  in  the  human  embryo  during  the  second  month,  and  saw  folds  on 
its  inner  surface  during  the  fifth  month. 

Pancreas. — In  amphibia,  there  are  three  pancreatic  evagina- 
tions ;  one  dorsal,  and  two  symmetrically  placed  on  the  ventral  side 
close  to  the  ductus  choledochus ;  the  triple  anlages  were  first  dis- 
covered by  A.  Goette,  75.1,  in  Bombinator,  and  have  since  been 
studied  in  Triton,  Siredon,  Rana,  and  Bufo,  byE.  Goppert,  91.1, 
113-118. 

In  the  chick  the  pancreas,  as  described  by  Foster  and  Balfour 
("  Elements,"  2d  ed.,  181),  arises  during  the  fourth  day, "in  the  form 
of  an  almost  solid  outgrowth  from  the  dorsal  side  of  the  intestine, 
nearly  opposite,  but  slightly  behind  the  hepatic  outgrowths.  Its 
blind  end  becomes  somewhat  enlarged,  and  from  it  numerous  diver- 
ticula  grow  out  into  the  passive  s]flanchnic  mesoblast.  As  the  duc- 
tules  grow  longer  and  become  branched,  vascular  processes  grow  in 
between  them,  and  the  whole  forms  a  compact  glandular  body  in  the 
mesentery  on  the  dorsal  side  of  the  alimentary  tract.  The  primitive 
outgrowth  elongates  and  assumes  the  character  of  a  duct.  On  the 
sixth  day  a  new  similar  outgrowth  from  the  duodenum  takes  place 
between  the  primary  diverticulum  and  the  stomach.  This,  which 
ultimately  coalesces  with  its  predecessor,  gives  rise  to  the  second 
duct,  and  forms  a  considerable  part  of  the  adult  pancreas.  A  third 
duct  is  formed  at  a  much  later  period. 

In  mammals  only  the  single  pancreatic  evagination  was  known 
until  recently.  Its  development  in  man  is  thus  described  by  O.  Hert- 
wig  ("Entwickelungsgesch.,"  3te  Aufl.,  280)  :  The  dorsal  anlage  ap- 
pears a  little  later  than  the  hepatic  diverticulum ;  it  has  been  observed 
by  W.  His  in  embryos  of  8  mm.  as  a  small  diverticulum,  Fig.  444, 
B,  P,  which  grows  into  the  dorsal  mesentery,  sending  out  meanwhile 
hollow,  branching  buds,  Fig.  444,  C,  P,  and  thus  becoming  by  the 
sixth  week  an  elongated  gland,  which  extends  so  as  to  lie  in  the 
mesogastrium  or  future  omentum,  and,  therefore,  between  the  greater 
curvature  of  the  stomach  and  the  vertebral  column.  The  pancreas, 
therefore,  changes  its  position  as  the  omentum  develops ;  thus  at  six 
weeks  it  lies  parallel  with  the  longitudinal  axis  of  the  body ;  there- 
after it  revolves  so  that  its  anterior  end  moves  to  the  left,  as  the 


THE   ALIMENTARY   TRACT. 


767 


omentum  develops,  until  the  gland  occupies  its  permanent  transverse 
position,  and  the  so-called  head  of  the  gland  lies  in  the  bend  of  the 
duodenum,  while  the  so-called  tail  is  near  the  spleen  and  the  left 
kidney.  The  duct  of  the  pancreas  is  at  first  in  front  of  the  bile  duct, 
but  during  foetal  life  it  shifts  and  first  approaches  and  then  joins  the 
ductus  choledochus. 

Stoss  in  a  preliminary  notice,  91.1,  states  that  in  mammals  he 
has  found  the  dorsal  and  double  ventral  pancreatic  anlages  (sheep 
embryos  of  4  mm.  about  seventeen  to  eighteen  days).  The  two 
anlages  unite  (sheep  of  15  mm.) ;  the  duct  of  the  ventral  anlage  is 
the  duct  us  Wirsingianus,  of  the  dorsal  anlage  the  ductus  isv///- 
torliii.  In  sheep  and  man  the  ventral  duct  is  preserved;  in  the 
horse  and  dog  both  ducts;  in  cattle  and  the  pig  probably  the  dorsal 
duct  only.  In  sheep  the  lumen  of  the  dorsal  duct  is  obliterated  in 
embryos  of  70  mm.,  and  in  embryos  of  00  mm.  only  the  ventral  duct 
can  be  found. 

AJS  regards  the  relations  of  the  pancreas  to  the  peritoneum:  the 
entodermal  portion  of  the  pancreas  being  situated  in  the  mesogas- 
trium,  it  is,  of  course,  covered  on  both  sides  by  peritoneum  and  may 
be  said  to  be  attached  to  the  wall  of  the  abdomen  by  a  mesentery  <  >f 
its  own,  although  the  pancreatic  mesentery  is  only  a  part  of  the 
mesogastrium  (C.  Toldt,  89.1).  The  pancreatic  mesentery  aborts 
during  the  fifth  month,  and  the  pancreas,  losing  its  movability, 
becomes  directly  attached  to  the  dorsal  abdominal  wall. 

The  histogenesis  of  the  pancreas  is  still  to  be  investigated.  In  a 
human  embryo  of  four  months,  Fig.  452,  the  alveoli  show  clearly 


FIG.  452.— Section  of  the   Pancreas  of  a  Human  Embryo  of  four  Months  (Minot  Coll.,  No.  35). 
The  dark  masses  are  groups  of  alveoli. 

and  lie  in  groups — drawn  dark  in  the  figure — which  are  widely  sep- 
arated from  one  another  by  young  connective  tissue.  The  ducts  are 
lined  by  a  cuboidal  epithelium ;  the  cells  of  the  alveoli  are  small, 
containing  very  little  protoplasm,  but  each  having  a  well-developed 
spherical  granular  nucleus. 

Mesentery  and  Omentum. — To  understand  the  development 
of  the  mesentery  it  is  necessary  to  recall  the  fact  that  the  ventral 
portion  of  the  coalom,  or,  in  other  words,  the  splanchnocoele,  is  consti- 
tuted by  a  pair  of  cavities  (pleuro-peritoneal  spaces),  which  are 
separated  from  one  another  throughout  the  body  by  a  median  parti- 
tion or  mesentery,  Fig.  453,  A,  mes;  in  this  partition  is  lodged  the 
entodermal  canal,  ent;  the  partition  consists  of  mesenchy^ia  and  is, 
of  course,  covered  on  both  sides  by  mesothelium,  msth.  The  con- 


768 


THE    FOETUS. 


nection  of  the  mesentery  with  the  somatopleure  along  the  median 
ventral  line  is  lost  for  the  most  part  very  early,  but  the  stomach  is 


FIG.  453. —Two  Diagrams  to  illustrate  Morphological  Relations  of  the  Vertebrate  Mesentery; 
A,  earlier,  B,  later  condition,  md,  Medullary  tube;  nch,  notochord;  Ao,  aorta;  mes,  mesen- 
tery; msth,  mesothelium;  Coe,  coelom;  Ent,  entoderm. 

always  connected  by  a  ventral  mesentery  (dmentum  minus)  with  the 
ventral  body  wall.  The  partial  disappearance  of  the  ventral  mesen- 
tery establishes  the  condition  indicated  by  Fig.  453,  B;  the  entoder- 

mal  tube,  together  with  the 
mesoderm  around  it,  constitutes 
the  alimentary  canal,  which  is 
suspended  by  a  dorsal  perma- 
nent mesentery  from  the  median 
line ;  the  coelom,  coe,  of  one  side 
communicates  below  the  intes- 
tine with  the  coelom  of  the  op- 
posite side.  In  other  words,  by 
the  disappearance  of  the  ventral 
mesentery  the  paired  splanch- 
nocceles  have  fused,  and  there  is 
henceforth  a  single  abdominal 
cavity. 

In  the  cephalic  region  of  the 
abdomen,  however,  the  primi- 
tive complete  separation  of  the 
coelom  of  the  two  sides  persists. 
As  the  stomach  and  liver  de- 
scend from  the  septum  transver- 
sum,  or  primitive  diaphragm, 
tailward  into  the  abdominal 
cavity,  we  find  that  the  mesen- 
terial  partition  grows  with  them 
and  is  never  aborted  either  on  the  ventral  or  dorsal  side.  Four  or- 
gans are  lodged  in  this  partition,  Fig.  454,  the  spleen,  Spl,  pancreas, 


pnstk 
Li 


FIG,  454.  —Diagram  to  illustrate  the  Relations  of 
the  Mesentery.  Md,  Spinal  cord ;  nch,  notochord ; 
Ao,  aorta ;  W,  Wolffian  ridge ;  Spl,  spleen ;  mes, 
mesogastrium ;  pan,  pancreas;  Ent,  entoderm; 
Coe,  coalom;  om.m,  omentum  minus;  msth,  meso- 
thelium; Li,  liver;  si,  suspensory  ligament. 


THE   ALIMENTARY    TRACT. 


pan,  stomach,  Ent,nu(\  liver,  Li.  Each  of  these  organs  produces  a 
thickening  of  the  partition,  and  therefore  causes  the  mesothelium  on 
l><>th  surfaces  to  bulge  laterally.  Later,  when  a  special  connective- 
tissue  layer  is  developed  under  the  mesothelium,  we  have  the  perito- 
neum produced,  and  this  peritoneum  covers  the  partition  and  the 
four  organs  contained  in  the  partition.  The  part  of  the  partition  in 
which  the  spleen  and  pancreas  are  lodged,  and  by  which  the  stomach, 
Ent ,  is  connected  with  the  median  dorsal  line,  is  the  future  greater 
omentum,  mes;  the  portion  between  the  stomach  and  liver  is  the  fu- 
ture omentum  minus,  om.in,  while  the  portion  joining  the  liver,  Li, 
to  the  ventral  somatopleure  is  the  future  suspensory  ligament,  si,  of 
descriptive  anatomy. 

Besides  these  names  there  are  also  employed  mesogastrium  for 
th<>  embryonic  greater  omentum,  and  mesocolon  for  the  portion  of 
the  mesentery  connected  with  the  large  intestine. 

The  condition  just  described  is  reached  by  the  human  embryo 
during  the  fourth  week.  Concerning  the  mode  of  disappearance  of 
the  ventral  mesentery  I  can  recall 
no  exact  observations,  nor  do  I 
know-  of  any  satisfactory  descrip- 
tions of  the  early  stages  of  the  par- 
t  it  i<>n  in  which  the  stomach,  etc., 
{ire  lodged.  We  are,  therefore, 
forced  to  content  ourselves  for  the 
present  with  the  preceding  dia- 
grammatic explanation.  The  dia- 
gram. Fig. -J.v>.  will  serve  to  render 
both  the  preceding  account  and  the 
sul (sequent  changes  clearer.  The 
diagram  is  fairly  correct,  except  in 
representing  the  stomach  in  the 
median  line,  for  as  soon  as  the 
stomach  descends  it  takes  an  asym- 
i  net  deal  position,  p.  754.  It  will 
be  evident  upon  glancing  at  the 
diagram  that  the  mesogastrium, 
///.sf/,  mesentery,  ///,sf,  and  meso- 
colon, >//.sr,  are  merely  different 
regions  of  the  same  membrane, 
and  that  the  spleen,  spl,  pancreas, 
I m  n,  stomach,  st,  and  liver,  Li, 
are  located  in  one  complete  mesen- 
terial  partition,  so  that  in  the  region  of  the  stomach  and  the  liver,  in 
order  to  get  from  the  left  splanchnoccele  to  the  right  splanchnoccele, 
we  must  pass  around  the  liver  on  the  caudal  side ;  the  septum  trans- 
versum,  st,  prevents  our  passing  across  on  the  cephalic  side. 

The  further  changes  in  the  relations  of  the  mesentery  depend 
chiefly  on  two  factors :  first,  the  elongation  and  coiling  of  the  stom- 
ach and  intestines ;  second,  the  formation  of  secondary  adhesions  of 
certain  parts  of  the  mesentery  with  other  parts  and  with  the  abdom- 
inal wall. 

The  development  of  the  stomach  and  intestine  has  been  described 
49 


VI 


col 


Soni 


FIG.  455.— Diagram  of  the  Human  Mesentery 
in  its  Primitive  Relations.  Au,  Auricle; 
peric,  pericardium;  s.t,  septum  transversum ; 
Li,  liver:  .s7,  suspensory  ligament]  dd,  duode- 
num; V>,  vein:  IV.  vitelline  stalk;  cc,  cse- 
cum  ;  som,  somatopleure ;  Oe,  oesophagus ; 
OHI.  in,  omentum  minus;  spl,  spleen;  st,  stom- 
ach; msg,  mesogastrium;  pan,  pancreas; 
mst,  mesentery;  msc,  mesocolon;  col,  colon. 


770 


THE    FCETUS. 


st 


Coe 


Om 


msg- 


•Om' 


pp.  753,  755.  The  result  of  the  primary  twisting  upon  the  mesen- 
tery is  illustrated  by  Fig.  450.  Owing  to  the  deflection  of  the  stom- 
ach to  the  left,  and  of  its  revolution  around  its  axis,  by  which  its 
median  dorsal  line  or  greater  curvature  becomes  lateral,  the  meso- 
gastrium, msg,  is  folded  so  as  to  form  a  pouch  that  projects  toward 
the  left  side;  the  pouch  is  the  anlage  of  the  great  omentum,  Om;  it 
opens  toward  the  right  side,  its  opening  being  the  foramen  of  Wins- 
low;  the  inner  surface  of 

A  the  pouch  is  formed  by  the 

right  surface  of  the  meso- 
gastrium, the  outer  by  the 
left.  The  cavity  of  the 
pouch  may  be  termed  the 
omental  cavity  (Setzbeu- 
tel);  F.  P.  Mall,  91.2, 
terms  it  the  gastric  diver- 
ticulum  ;  in  descriptive  an- 
atomy it  is  known  as  the 
lesser  peritoneal  space. 
From  the  lesser  curvature 
of  the  stomach  extends  the 
ventral  mesentery  or  lesser 
omentum ;  an  inspection  of 
the  diagram,  Fig.  45(3,  will 
show  that  it  extends  the 
pouch  of  the  omentum  to- 
ward the  right.  A  section 
of  a  human  embryo  in 
which  the  omental  cavity 
is  just  beginning  to  form  is  figured  by  F.  Mall,  91.3,  474.  The 
duodenum  is  situated  near  the  dorsal  side  of  the  body  cavity  and  has, 
even  in  the  young  embryo,  only  a  short  mesentery ;  as  development 
progresses  the  duodenum,  after  making  its  pyloric  bend,  comes  to  lie 
in  a  nearly  transverse  direction  close  to  the  dorsal  abdominal  wall ; 
its  mesentery  obliterates,  and  thereafter  the  duodenum  forms  merely 
a  slight  projection  covered  by  mesothelium  (and  later  by  peritoneum) , 
compare  Fig.  457,  A.  Finally,  owing  to  the  intestines  forming  a 
great  loop  to  the  right,  the  large  intestine  crosses  the  body  011  the 
ventral  side  of  the  duodenum ;  the  mesentery  meanwhile  remains  at- 
tached along  the  median  dorsal  line,  but  its  ventral  border  elongates 
with  the  intestine ;  and  further,  the  manner  in  which  the  loop  is  de- 
veloped brings  the  right  surface  of  the  mesentery  to  face  ventralward 
(or  "forward,"  according  to  human  descriptive  anatomy)  and  the 
left  surface  to  face  dorsal  ward. 

The  additional  changes  are  indicated  by  the  two  diagrams  after 
O.  Hertwig,  Fig.  457,  A,  B.  The  star  (*)  is  placed  in  the  omental 
cavity.  In  A  the  liver,  /,  is  attached  to  the  dorsal  part  of  the  dia- 
phragm, zf;  the  stomach,  mg,  occupies  a  transverse  position,  arid  is, 
therefore,  seen  in  cross  sections;  along  what  was  primitively  its 
median  ventral  line  is  attached  the  lesser  omentum,  kn,  by  which 
the  stomach  is  connected  with  the  liver.  Along  the  greater  curva- 
ture, gc,  of  the  stomach  is  inserted  the  mesogastrium,  yn\  f//r,  or, 


vi 


FIG.  456. —Diagrams  to  illustrate  the  History  of  the  Hu- 
man Mesentery;  A,  earlier,  B,  later  condition,  msg, 
msg'  Mesogastrium  or  omentum ;  St,  stomach ;  Cce,  coe- 
cum;  Vi,  vitelline  duct;  msc,  mesocolon;  R,  rectum;  Om, 
Om',  omentum  or  mesogastrium;  dd,  duodenum;  verm, 
vermiform  appendix.  After  O.  Hertwig. 


THE   ALIMENTARY   TRACT. 


771 


as  we  may  now  call  it,  the  greater  omentuin ;  it  has  grown  so  much 
that  it  forms  a  fold,  which  is  beginning  to  hang  over  the  transverse 
colon,  ct;  the  fold  is  destined  to  grow  still  further,  as  indicated  by 
the  dotted  line,  yn*;  the  pancreas,  p,  lies  in  the  omentum  close  to 
the  dorsal  wall  of  the  abdomen.  The  duodenum,  du,  is  already  closely 
united  with  the  dorsal  wall.  The  transverse  mesocolon,  msc,  springs 
from  the  wall  of  the  abdomen  between  the  pancreas,  p,  and  duo- 
denum, du;  the  reason  for  this  apparent  anomaly  will  be  understood 
by  referring  to  Fig.  456.  Below  the  duodenum,  du,  springs  the 
mesentery,  mes,  of  the  small  intestine,  dd.  In  B  the  omental  fold 
has  extended,  gn3,  far  down  in  front ;  the  mesocolon,  msc,  has  united 


mg 


gn* 
ct 


FIG.  457.  —Two  Diagrams  to  illustrate  the  History  of  the  Mesentery.  A,  earlier,  B,  later  stages. 
Tti"  diagrams  represent  median  vertical  sections.  The  black  line  on  the  right  of  each  diagram 
ri-pivs.'Mts  the  aorta.  For  explanation  of  lettering  see  text. 

with  the  part  of  the  omental  fold  nearest  it,  and  there  results  a  single 
membrane  of  double  origin,  by  which  the  colon  is  suspended ;  it  is 
this  membrane  which  is  known  in  the  descriptive  anatomy  of  the 
adult  as  the  mesocolon ;  the  adult  mesocolon,  therefore,. includes  the 
true  mesocolon  and  part  of  the  mesogastrium.  As  a  further  result 
of  the  secondary  adhesion,  we  note  that  the  omentum,  gn\  appears 
to  spring  from  the  transverse  colon,  ct.  Both  the  pancreas,  p,  and 
duodenum,  dn,  now  occupy  their  permanent  (or  so-called  retro-peri- 
toneal) positions. 

Admirable  descriptions  of  the  condition  of  the  mesentery  at  suc- 
cessive stages  in  the  human  embryo  are  given  by  C.  Toldt  in  his 
classic  memoir,  79.1,  and  further  valuable  observations  on  the 
adhesions  are  recorded  by  Toldt  in  his  second  article,  89.1. 

HISTOGENESIS. — From  its  mode  of  formation  it  is  evident  that  the 
mesentery  is  primitively  a  sheet  of  mesenchyma  covered  on  both 
sides  by  mesothelium.  The  differentiation  of  this  simple  membrane 
has  been  carefully  traced  by  C.  Toldt,  79. 1,  42-50.  At  four  weeks 
the  mesenchymal  cells  are  very  much  crowded — there  being  but  little 
basal  substance — and  they  have  but  little  protoplasm ;  some  of  them 
are  beginning  to  assume  the  spindle  shape ;  the  mesothelium  varies 
somewhat,  being  here  a  cuboidal,  there  a  cylinder  epithelium.  At  six 


772  THE    FOETUS. 

weeks  more  of  the  mesenchymal  cells  are  spindle-shaped,  and  the  mes- 
othelial  cells  are  beginning  to  flatten  out;  they  are  thinner  and  wider 
and  their  nuclei  protrude.  At  eight  weeks  the  mesothelium  has 
essentially  the  endothelial  type,  which  it  retains  thoughout  life.  At 
eight  weeks  the  mesenchymal  cells  next  the  mesothelium  on  each 
side  commence  to  form  a  special  recognizable  layer,  which  is  per- 
fectly distinct  by  the  end  of  the  third  month ;  this  layer  is  four  to 
six  cells  thick  and  contains  no  vessels ;  together  with  the  overlying 
mesothelium  it  constitutes  the  peritoneal  membrane  of  descriptive 
anatomy.  Between  the  two  peritoneal  membranes  lies  the  looser 
mesenchyma,  corresponding  to  the  membrana  propria  mesenterii 
of  Toldt,  and  in  which  are  distributed  the  blood-vessels  and  nerves, 
and  later  (fifth  month)  the  lymphatic  glands  and  fat-cells.  The 
mesentery  thus  comprises  five  layers,  all  of  which  can  be  well  seen  in 
embryos  of  the  fourth  month.  The  development  of  the  connective- 
tissue  fibrils  in  the  omentum  has  been  previously  described,  p.  400. 
The  fat-cells  do  not  attain  their  typical  development  until  the  end  of 
the  eighth  month,  though  their  differentiation  begins  during  the 
fifth,  when  the  anlages  of  the  lymph-glands  also  appear. 

The  mesodermic  layer  of  the  peritoneum  is  always  very  thin,  but 
Toldt,  79.1,  46,  distinguishes  in  it  toward  the  end  of  foetal  life 
three  sub-layers,  viz. :  1,  next  the  mesothelium  with  fine  elastic 
net- work;  2,  middle  sub-layers  with  coarser  elastic  network;  3,  sub- 
serous  layer  of  looser  texture  uniting  the  peritoneum  to  the  mem- 
brana propria. 

MESHES  OF  THE  OMENTUM. — After  birth  the  omentum  becomes 
pierced  with  numerous  holes.  A  few  months  after  birth  (C.  Toldt, 
79.1,  49)  there  can  be  seen  numerous  scattered  spots  where  the 
membrane  is  thinner  and  contains  fewer  connective-tissue  fibrils 
than  elsewhere ;  these  spots  lie  more  or  less  remote  from  the  blood- 
vessels. At  these  spots  the  holes  are  formed  and  are  at  first  always 
very  small.  The  formation  of  the  omental  perforations  may  be  fol- 
lowed in  children  of  from  a  few  weeks  to  four  years  old.  If  the 
omentum  of  a  child  a  few  months  old  is  stained  with  nitrate  of  silver 
there  will  appear,  between  the  mesothelial  cells,  spots  colored  by  the 
silver ;  then  other  spots  similarly  colored,  but  larger  and  light  in  the 
centre ;  and  finally  still  larger  ones  in  which  the  light  centre  has 
become  a  hole,  Toldt,  Z.C.,  Fig.,  17.  Toldt  regards  the  holes  as  the 
result  of  the  distention  of  the  membrane,  and  the  silver  marks  just 
described  as  indicating  the  pulling  apart  of  the  endothelial  cells ;  the 
blood-vessels  and  fat-cells  around  them  serve  to  maintain  the  thick- 
ness of  the  membrane  between  the  holes.  Ranvier,  74. 1,  sought  to 
attribute  the  origin  of  the  holes  to  leucocytes  forcing  their  way 
between  the  omental  tissues,  but  Toldt  has  shown  that  this  explana- 
tion does  not  hold  good. 

HISTORICAL  NOTE. — The  foundations  of  our  knowledge  of  the 
embryonic  mesentery  were  laid  by  J.  F.  Meckel,  17. 1,  and  Johannes 
Muller,  30.1.  But  little  was  added  until  the  investigations  of  C. 
Toldt,  whose  two  memoirs,  79.2,  79.1,  constitute  the  classic  au- 
thority on  the  subject.  Lockwood's  article,  84. 1,  did  not  add  much, 
and  contains  important  errors,  as  pointed  out  by  Toldt,  89. 1. 


THE    RESPIRATORY    TRACT. 


X24- 


II.     THE  RESPIRATORY  TRACT. 

The  respiratory  organs  arise  as  a  single  evagination  of  the  ento- 
dermal  canal  on  the  ventral  side  of  the  caudal  end  of  the  pharynx. 
The  evagination  branches,  each  branch  develops  into  a  lung;  the 
main  stem  becomes  the  trachea,  and  the  opening  of  the  stem  into  the 
pharynx  forms  the  larynx.  Accordingly  we  take  up:  1,  the  pul- 
monary anlage;  2,  the  lungs;  3,  the  trachea;  4,  the  larynx  and 
epiglottis. 

Pulmonary  Anlage.— The  first  trace  of  the  outgrowth  of  the 
exitodenna]  canal  to  form  the  lungs  is  an  increase  of  the  vertical 
diameter  of  the  canal  in  the  cesophageal  region.  This  increase  re- 
sults from  the  development  of  what  may  be  called  the  pulmonary 
</roor<\  which  is  a  furrow  lined  with  entoderm,  and  begins  just 
behind  the  fourth  gill-cleft  and  extends  to  the  stomach,  Fig.  458. 
The  groove  is  shallow  toward  the  pharynx 
and  deepens  toward  the  stomach,  ending 
abruptly  as  a  rounded  projection.  The  en- 
toderm lining  the  pulmonary  groove  is 
thicker  than  that  lining  the  oesophagus 
above  it  (W-  His,  87.3,  90).  This  stage 
may  be  seen  in  a  human  embryo  of  3. 2  mm., 
or  in  chick  of  sixty  to  seventy-two  hours. 
The  pulmonary  groove  is  narrower  than  the 
cesophageal  division  from  which  it  springs, 
hence  in  a  cross  section  of  this  stage  the  en- 
todermal  canal  has  in  the  region  of  the 
oesophagus  the  outline  of  an  inverted  pear. 
The  groove  now  deepens  and  its  gastric  end 
grows  out  farther,  Fig.  458,  Lu,  and  the 
esophagus  between  the  end  of  the  groove 
and  the  stomach  begins  to  lengthen.  Pres- 
ently the  blind,  free,  lower  end  of  the  groove 
widens  out,  and,  as  it  grows,  forks;  each 
fork  is  the  anlage  of  a  lung,  and  has  the 
form,  Fig.  438,  of  a  short  rounded  pouch 
situated  laterally.  In  front  views  the  two 
pouches  are  easily  found ;  in  a  side  view  one 
hides  the  other.  The  median  portion,  by 
which  the  pouches  are  connected  with  the 

pulmonary  groove,  is  the  anlage  of  the  trachea.  As  is  well  shown  in 
Fig.  444,  A,  B,  C,  the  oesophagus,  lungs,  and  trachea  all  grow  rap- 
idly ;  the  branching  of  the  lungs  begins  in  embryos  of  seven  milli- 
metres, B.  In  embryos  of  seven  millimetres  the  fundamental  parts 
are  all  marked  out,  except  that  the  dilatation  of  the  upper  end  of  the 
trachea  follows  later  (embryos  of  13  mm.  Fig.  444,  C,  La).  The 
elongated  opening  of  the  pulmonary  groove  is  the  future  glottis,  and 
in  front  of  it  is  the  anlage  of  the  epiglottis,  Fig.  444,  C,  Ep;  the 
median  cylindrical  tube,  Fig.  445,  Tr,  is  the  trachea,  and  its  two 
branches  are  the  lungs. 

The  situation  and  topographical  relations  of  the  pulmonary  anlage 
are  very  important,  because  they  explain  numerous  anatomical  facts. 


FIG.  458.— Ou' line  9f  the  Ento- 
dermal  Canal  of  His'  Embryo 
Li*.  Hy,  Hypophysis ;  Ch,  noto- 
chord;  Lu,  anlage  of  lung;  Li, 
liver;  Yks,  yolk-sac;  Al,  duct 
of  allantois;  W,  Wolffian  duct. 
After  W.  His. 


774  THE   FCETUS. 

At  first  the  anlage  directly  overlies  the  heart  and  the  septum  trans- 
versum, including  the  liver,  as  illustrated  by  Fig.  259.  It  will  be 
remembered,  see  p.  480,  that  the  coelom  extends  above  the  septum 
transversum  on  either  side  of  the  oesophagus,  making  during  early 
stages  a  free  communication  between  the  abdominal  and  pericardial 
cavities.  In  transverse  sections  one  sees  at  once  that  the  lungs 
project  into  the  ccelomatic  passage  (or  future  pleural  cavity)  above 
the  liver  and  septum  transversum.  The  manner  in  which  the  pleu- 
ral cavities  are  finally  shut  off  is  described  p.  482.  It  is  essential 
to  note  that  the  lungs  arise  on  the  dorsal  side  of  the  heart  and  liver. 
The  lungs  (and  pleural  cavities)  only  gradually  expand  forward  on 
the  right  and  left  of  the  heart. 

In  the  above  description  heed  is  given  only  to  the  epithelial  or 
entodermal  portion  of  the  pulmonary  anlage.  The  epithelium  is, 
however,  surrounded  by  mesc-lerm,  which  makes  a  thick  layer.  In 
sections  the  rudimentary  lung  is  readily  seen  to  consist  of  a 
ring  of  epithelium  composed  of  high  cylinder  cells ;  the  epithelium 
is  inclosed  by  a  thick  layer  of  mesenchyma,  and  so  far  as  the  lung 
projects  into  the  coelom  it  is,  of  course,  covered  by  mesothelium. 
The  mesothelium  of  the  adult  is  known  in  descriptive  anatomy  as 
the  epithelium  of  the  pleural  membrane. 

Lungs. — The  lungs  arise,  as  described  in  the  previous  section,  as 
two  nearly  symmetrical  diverticula  of  the  pulmonary  anlage  and 
immediately  above  the  auricle  of  the  heart  (human  embryo  of  4 
mm.).  The  diverticula  lengthen  out  and  grow  dorsalward  011  either 
side  of  the  oesophagus  close  to  the  cephalic  end  of  the  stomach,  Fig. 
444,  B,  and  there  form  branches,  C,  all  of  which  at  first  extend  dor- 
salward. These  branches  of  the  entodermal  diverticulum  are  inclosed 
in  a  thick  covering  of  mesoderm ;  the  two  layers  thus  associated  con- 
stitute the  embryonic  lung.  The  organ  as  a  whole  projects  into  the 
coelom  above  the  septum  transversum ;  its  ccelomatic  surface  is,  of 
course,  covered  by  mesothelium. 

The  branching  entodermal  tube  forms  the  so-called  bronchial  tree, 
the  entoderm  itself  persisting  as  the  lining  epithelium  of  the  bronchi, 
bronchioles,  infundibula,  and  air-cells.  The  development  of  the 
branches  during  early  stages  has  been  traced  by  W.  His,  87.3,  in 
the  human  embryo,  and  less  thoroughly  by  A.  Robinson,  91.1,  in 
rats  and  mice.  The  following  account  refers  to  man.  The  right 
diverticulum  is  somewhat  larger  than  the  left  and  extends  further 
back — peculiarities  which  are,  perhaps,  connected  with  the  changes 
accompanying  the  asymmetrical  development  of  the  heart.  At  six 
weeks  the  asymmetry  of  the  lungs  is  more  marked,  for  the  right 
diverticulum  is  much  longer  and  has  three  primary  branches  bud- 
ding forth,  while  the  left  lung  has  only  two ;  each  of  these  primary 
branches  corresponds  to  a  lobe  of  the  adult  lung,  hence  the  right 
lung  has  three  lobes,  the  left  lung  two.  Morphologically,  however, 
the  upper  and  anterior  branches  of  the  right  lung  (and  therefore  the 
lobes  they  produce)  are  equivalent  to  the  single  upper  branch  of  the 
left  lung.  Each  branch  elongates  and  branches,  and  the  branches 
branch,  and  so  on,  Fig.  459 ;  every  branch  is  short  and  has  a  rounded 
and  somewhat  enlarged  end ;  as  new  branches  are  added  those  pre- 
viously formed  become  stems  and  increase  in  diameter.  The 


THE   RESPIRATORY   TRACT. 


'75 


branching  occurs  in  a  highly  characteristic  manner,  for  the  stem 
always  forks,  but  the  forks  develop  unequally,  one  (terminal  bud) 
growing  more  rapidly  and  becoming  practically  the  continuation  of 
the  main  stem,  while  the  other  (lateral  bud)  appears  as  a  lateral 
branch.  Speaking  in  general,  it  may  be  said  that  the  ventral  fork 
serves  as  the  stem,  cf.  Fig.  444,  C,  Lu.  In  consequence  of  this 
method  of  growth  the  adult  lung  consists  of  main  stems  with  lateral 
branches,  as  we  learned  through  the  able  investigations  of  Aeby 
("DerBronchialbaum,"  etc.,  Leipzig,  1880).  But  it  is  erroneous  to 


Oc 


FIG.  459.  —  Three  Views  of  the  Lungs  of  a  Human  Embryo  of  10.5  mm.  (His'  Embryo  N).  A, 
Seen  from  the  right  side;  B,  s.-.-n  tn»n  in  front;  C,  seen  from  the  left  side;  the  figures  are  re- 
constructions from  sections.  Oe,  (Esophagus;  TV, trachea;  Art,  pulmonary  artery;  F.p,  pul- 
monary vein;  I..  II.,  III.,  primary  branches  of  the  bronchus,  Z/',  upper:  L",  lower  lobe  of  the 
lung.  After  W.  His.  x  30  diams. 

suppose,  as  did  Aeby,  that  the  system  of  growth  is  strictly  mono- 
pmlial,  it  being  in  reality  a  modified  dichotomous  system.  The 
branches  all  arise  by  terminal  forking,  never  as  outgrowths  from  the 
side  of  a  stem.  In  cross  sections  the  lung  has  a  triangular  outline; 
one  apex  is  the  point  of  attachment  and  contains  the  main  bronchus ; 
the  three  sides  we  may  designate  as  dorsal,  lateral,  and  ventral ; 
the  branches  of  the  bronchial  tubes  arrange  themselves  so  that  we 
can  distinguish  those  toward  the  ventral  from  those  toward  the 
dorsal  side,  while  the  terminations  of  the  tubes  in  the  embryo  lie,  for 
the  most  part,  toward  the  lateral  side  of  the  lung.  Later  the  lungs 
revolve  forward,  and  the  ventral  surface  becomes  medial  or  cardial ; 
the  lateral  side  corresponds  to  the  costal  surface.  Fig.  459  shows  the 
bronchial  ramifications  of  an  embryo  of  10.5  mm. ;  they  have  been  de- 
scribed in  detail  by  His,  /.c.,  98.  The  same  primary  branches  appear 
in  both  lungs,  and  they  occupy  essentially  symmetrical  relations  as 
regards  the  veins ;  examined  in  detail,  however,  the  two  lungs  are 
not  perfectly  symmetrical.  The  arteries,  on  the  other  hand,  are 
entirely  asymmetrical;  the  right  artery,  A,  Art,  passes  in  front 
of,  but  the  left  artery  passes  behind,  the  first  branch;  this  relation 
persists  throughout  life,  and  led  Aeby  to  designate  the  first  right 
bronchus  as  eparterial  arid  all  the  other  bronchi  hyparterial; 
Aeby — an(j  His  seems  to  accept  his  view — inferred  that  the  right 
lung  contained  a  bronchus  not  represented  in  the  left  lung.  I  think, 
however,  that  this  view  is  untenable  and  that  the  right  and  left  first 


770  THE    FCETUS. 

branches,  I.,  are  homologous ;  the  difference  between  the  two  sides 
is  due  to  the  precocious  development  on  the  right  side,  and  to  sec- 
ondary modifications  of  the  arteries ;  the  relation  of  the  veins  to  the 
bronchi  confirms  the  interpretation  here  advanced.  His'  account  of 
the  development  appears  to  me  to  flatly  contradict  Aeby's  conclu- 
§ion.  The  peculiar  course  of  the  right  pulmonary  artery  is  probably 
due  to  the  abortion  of  the  fifth  aortic  arch  on  the  right  side,  and  the 
consequent  transfer  of  the  origin  of  the  artery  to  the  left  side ;  if 
this  suggestion  is  correct  there  should  be  in  reptiles  no  eparterial 
bronchus. 

The  further  ramifications  of  the  bronchi  begin  as  short,  rounded 
buds  forking  off  at  the  end  of  the  branches,  as  may  be  easily  seen  in 
sections  through  the  foetal  lung,  Fig. 46 2 ;  hence  the  primary  branches 
are  permanent,  and  by  their  enlargement  give  rise  to  the  main 
bronchi.  Fig.  460  represents  four  views  of  the  lungs  of  an  embryo 


FIG.  460.— Lungs  of  a  Human  Embryo  of  five  Months    (Minot  Coll.,  No.  71).     A,  From  left 
side ;  B,  from  right  side ;  C,  from  behind  ;  D,  from  below. 

of  five  months,  and  is  intended  to  show  the  homology  of  the  two 
primary  lobes  on  each  side;  the  upper  (and  anterior)  lobe  of  the 
right  side,  B,  being  partially  subdivided. 

HISTOGENESIS. — The  entodermal  bronchial  tubes  are  at  first  widely 
separated  from  one  another ;  the  space  between  them  is  filled  with 
mesenchyma.  The  tubes  themselves  have  at  first  a  high  cylinder 
epithelium,  Fig.  461,  with  oval  granular  nuclei,  and  have  only  a 
small  lumen,  but  by  their  growth  the  mesoderm  is  condensed  around 
them,  forming  a  special  envelope,  Fig.  461,  which  ultimately  enters 
into  the  composition  of  the  bronchial  wall.  The  smooth  muscle-fibres 


THE   RESPIRATORY   TRACT. 


'77 


conn 


FIG.  461.— Cross  Section  of  the  Bronchial 
Tube  of  a  Human  Embryo  of  sixty-thn-.-  to 
sixty-eight  Days  (Minot  Coll.,  No.  138). 


were  observed  in  the  bronchial  wall  in  sheep  embryos  of  120  mm. 
by  L.  Stieda,  78.1,  111.  As  development  progresses  the  ramifica- 
tions of  the  bronchial  tubes  arise  more  rapidly  than  the  growth  of 
the  inesenchyma,  so  that  the  amount 
of  connective  tissue  between  the 
branches  gradually  diminishes,  Fig. 
462,  until  at  birth  only  thin  parti- 
tions are  left  between  the  adjacent 
air-spaces.  The  epithelium  remains 
in  its  embryonic  stage  (i.e.,  a  high- 
cylinder  epithelium)  in  the  bronchi ; 
in  the  bronchioles  it  becomes  a  cu- 
bnidal  epithelium,  in  the  infundi- 
bula  (Alueolargange)  and  alveoli  a 
very  thin  layer  (pavement  epithe- 
lium, eiidothelium).  This  differen- 
tiation I  find  is  established  as  the 
buds  are  formed;  thus  the  first  buds 
are  lined  by  cylinder  epithelium  and  form  bronchi ;  later  buds  are 
lined  with  cuboidal  epithelium,  Fig.  462,  Alv,  and  form  bronchioles; 

toward  the  close  of  foe- 
tal life  the  buds  appear, 
which  are  converted 
into  the  inf  undibula  and 
alveoli,  and  these  are 
lined  with  flattened  epi- 
thelium, as  discovered 
in  sheep  embryos  of  250 
mm.  by  L.  Stieda,  77. 1, 
113.  The  common  as- 
sumption that  the  flat 
alveolar  epithelium  is 
not  present  until  the 
lungs  are  stretched  by 
the  first  breath  is  erro- 
neous. 

Trachea. — E  x  c  e  pt 
upon  the  early  develop- 
ment of  the  trachea  as 
part  of  the  pulmonary 
anlage  (p.  463)  I  have 
found  no  observations. 
In  sections  of  the  tra- 
chea of  a  four  months' 
embryo,  Fig.  463,  the 
epithelium  is  a  high 
•  cylinder  epithelium  as 

FIG.  462. -Section  through  the  Lung  of  a  Human  Embryo  m  early  Stages  and  also 
of  the  fourth  Month  (Minot  Coll.,  No.  6).  Br,  Bronchiole;  m  |he  adult,  Since  the 
C,  connective  tissue;  Alv,  termim  entoderm  of  the  trachea 

and  bronchi  preserves  the  embryonic  type  throughout  life;  the  epi- 
thelium is  ciliated ;  glands  are  present  and  lined  with  mucous  cells. 


778 


THE   FCETUS. 


A  six  months'  specimen  (Minot  Coll.,  No.  8)  shows  the  glands  well 

advanced,  as  are  also  the  tracheal  cartilages. 

Larynx. — The  larynx  is  essentially  the  portion  of  the  trachea 

opening  into  the  oesophagus.  It  is  best  regarded  as  the  metamor- 
phosed pulmonary  groove ;  the  groove 
early  becomes  marked  off  from  the 
oesophagus  or  pharynx  by  two  ridges, 
one  on  each  side ;  the  ridges  approach 
one  another  in  front  and  devaricate 
posteriorly;  they  are  the  anlages  of 
the  vocal  chords.  Kolliker  found  the 
anlages  of  the  arytenoid  cartilages 
the  sixth  week,  but  chondrification 
does  not  take  place  in  the  larynx  un- 
til the  eighth  or  ninth  week.  The 
annular  and  arytenoid  cartilages  are 
disproportionately  large  in  early 

FIG.  463.— Epithelium  and  Gland  of  the    cfarrpa    wViilp    thp    fhvrrvirl    r^vtilao-Pst. 
Trachea  of  a  four  Months'  Embryo  (Minot    ^'^g^,   WI1.  lliyrc 

coll.,  NO.  35.    x  about  270  diams.    (The  develop  more  tardily.     For  a  certain 

nuclei  in  the  eland  should  be  of  the  same  •    j     ±1         ^  £    j.i         £ 

size  as  those  in  the  epithelium.)  period   the   larynx   of   the   foetus   is 

completely  closed  by  the  concrescence 

of  its  lining  epithelium,  a  fact  which  was  first  recorded  by  Roth, 
80.1,  and  since  by  Kolliker  and  Putelli,  88.1;  Strazza,  88.1,  has 
published  some  observations  on  the  development  of  the  laryngeal 
muscles. 

EPIGLOTTIS. — In  a  human  embryo  of  4.25  mm.  His  has  found 
("  Anat.  menschlicher  Embryonen,"  III.,  06)  that  the  elongated  open- 
ing of  the  larynx  lies  just  behind  the  fourth  branchial  arch  and  is 
bounded  by  two  slight  ridges  which  meet  in  front,  but  fade  out  be- 
hind. HJe  has  also  found  that  the  ridges  are  the  anlages  of  the 
median  epiglottis  and  of  the  lateral  ary-epiglottic  folds.  In  an 
embryo  of  10  mm.,  Fig.  335,  the  epiglottis,  Epg,  is  well  developed. 
As  to  the  further  growth  and  the  histogenesis  of  the  organ  little  is 
known,  though  a  few  details  are  given  by  Ganghofner,  80. 1. 


HARVARD  MEDICAL  SCHOOL,  BOSTON, 
March  llth,  1892. 


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abbreviations  are  used  ;  the  remaining  abbreviations  are  the  usual  ones  and 
therefore  do  not  require  explanation  : 

ABBREVIATIONS. 

A.  A.— Anatomischer  Anzeiger. 
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Q.  J.— Quarterly  Journal  of  Microscopical  Science. 

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80.1.  Glands,  stomach,  Wien.  Sitzb.  Ixxxii.  3  Abth.  57-128.  89.1.  Mes- 
entery. D.'irkschr.  .Math.  :saturw.  Kl.  Akad.  Wien.  Ivi.  i-46.  Toldt,  C. 
and  Zuckerkandl,  E.  76.1.  Liver.  Wien.  Sitzb.  Ixxii.  3  Abth.  241-295. 
Tonge,  M.  70.1.  Semilunar  valves.  Phil.  Trans,  clix.  387-411.  Tourneux, 
F.  87.2.  Male  vagina.  C.  R.  N>c.  Hiol.  Paris.  ser.  8.  iv.  807-812.  88.1. 
cloaca,  etc.  Journ.  FAnat.  xxiv.  503-517.  88.3.  Organ  of  Rosenmuller, 
Robin's  .Journ.  Anat.  xxiv.  109-1  HO.  89.1.  Genital  tubercle.  Journ.  de 
I'Anat.  xxv.  ^D-'jr,:1,.  90.3.  Rectum.  C.  R.  Soc.  Biol.  Paris.  ser.  9.  ii.  207- 
211.  Tourneux,  F.  et  Hermann  G.  87.3.  Medullary  vestiges.  Journ.  de 
Anat.  Physiol.  -xxiii.  498-529.  Tourneux,  F.  et  Legay,  Ch.  84.1.  Uterus 
and  vagina,  Robin's  Journ.  Anat.  1884.  330-380.  Trinchese,  S.  86.1. 
Muscle-plates.  Arch.  Ital.  de  Biol.  vii.  376-379.  Tuckerman,  F.  89.1. 
Ta>i«-  bulbs.  Journ.  Anat.  and  Physiol.  xxiii.  559-582.  89.2.  Tastr- 
Imlbs.  Journ.  Anat.  and  Physiol.  xxiv.  130-131.  Turner,  Wm.  73.1.  Pla- 
centa, man.  Journ.  of  Anat.  Physiol.  vi.  2  s^r.  120-133.  76.1.  Lectures 
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Vignal,  W.  83.1.  Nerve  fibres.  Arch,  de  Physiol.  513-535.  83.2. 
Nerve  fibres,  growth.  Arch,  de  Physiol.  xv.  536-548.  84.1.  Medulla, 
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Wagner,  B.  35.1.  Germinative  vesicle.  Mullens  Arch.  1835.  373-377. 
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A.  A.  ii.  345-368. 


INDEX. 


ABDOMINAL    and    pleural    cavities, 

M-paration  of  the,  484 
Alulueens  nerve,  04 1 
Achromatic  threads,  Ho 
J'7/.v  -n*t  rang,  134 
Acroblast,  154 
After-birth,  the,  304 
Aff>  nlnnn,  260 
Aft'  rh < i lit  in  sauropsida,  190 
Ahlfeld's  ovum,  289 
Aire  germinatwe,  271 

n/.vf///f//>e,  differentiation  of  the, 

272 

Ala  vespertilionis,  origin  of,  244 
AleriThal  ova,  61 
Alinu'iitary  tract,  743 
Allantoic  fluid,  355 
Allantois,  the,  353 

conversion  of,  into  the  bladder, 
514 

histology  of,  353 

in  mammals,  354 

in  man,  354 

origin  of,  'Jo  7 

union  of,  with  the  chorion,  376 
AUantoishOcker,  259 
Allantois-stalk,  354 

partial  closure  of,  in  embryo  of 
twenty-seven  days,  387 

see  also  Bauchstiel  and  umbilical 

cord 

AllaiitniNWulst,  259 
Alveolar gttnge,  777 
Ameloblasts,  586 
A  tn nionsfurche,  692,  696 
Amnion,  the,  333 

and  chorion,  union  of,  337 

definition  of,  333 

differentiation  of,  334 

evolution  of,  344 

histology  of,  333 

origin  of,  281 


Amnion,  progressive  history  of,  35 

retrogressive  history  of,  ^ 
Amnionstrang,  284 
Amniotie  cord,  284 

fluid,  337 

Amphiaster  of  the  ovum,  63,  95 
Amphibia,  concrescence  in,  120 

development  of  the  primitive  seg- 
ments in,  195 

medullary  groove  of,  177 

mesoderin  of,  145 

primitive  axis  in,  129 
Amphioxus,    differentiation    of   the 

mesoderin  of,  209 
Anal  canal,  189 

membrane,  260 

plate,  190 

Annulus  tyinpanicus,  740 
Anus  of  Rusconi,  121,  129 

primitive,  259 
Aorta,  division  of  the,  531 

dorsal,  and  its  branches,  539 

pulmonary,  538 
Aortic  arches,  534 

evolution  of  the,  538 

in  branchiate  vertebrates,  537 
Aortic  valves,  534 

Aortic  wall,  development  of  the,  537 
Appendix  verniiformis,  758 
Aqueductus  endolymphaticus,  729 

Sylvii,  678 

Aqueous  humor,  724 
Archenteric  cavity,  254 

extension  of  the,  271 
Archenteron,  115 

comparison  of  mammalian  and 
amphibian,  269 

definitive,  127 

early  development  of  the,  254 

separation  of  the,  from  the  yolk- 
sac,  255 
Arches,  aortic,  534 


794 


INDEX. 


Arches,  branchial,  265,  267 

branchial,  muscles  of  the,  477 

gill,  205,  267 

mandibular,  578 

visceral,  265,  267 

Archianiphiaster  of  the  ovum,  63 
Archiblast,  120,  153 
Archipterygium,  450 
Archistoine,  113 
Arcs  branchiaux,  265,  267 
Area  embryonalis,  271 

germinativa,  271 

gland,  564 

interposita,  525 

opaca,  134,  254,  271 

pellucida,  254,  271 

pellucida  of  a  hen's  egg,  131 

placentalis,  374 

vasculosa,  197,  274 

v^sculosa,  differentiation  of  the, 
272 

vitellina,  272 
Arm,  skeleton  of  the,  456 
Arsenoblasts,  78 
Arterial    system,  transformation  of 

the,  534  * 

Arteries,  intersegmental,  540 
Atrioventricular  valves,  533 
Attraction,  sphere  of,  94 
Auditory  passages,  738 
Auricles,  division  of  the,  528 

formation  of  the,  521 
Auricular  canal,  division  of  the,  529 
Aussenfalte,  vordere,  132 
Axial  cord,  134 
Axis,  development  of  the,  430 

primitive,  118 

BARS,  hyoid,  445 

mandibular,  444 

thyro-hyoid,  447 
Bartholinfs  glands,  development  of, 

516 
Basal  ganglia,  694 

layer  of  the  epidermis,  550 

substance,  mesenchymal,  166 
Basement  membranes,  421 
Basilarleiste,  712 
Bauchganglienkette,  570 
Bauchstiel,  33,  354,  356 

partial  closure  of,  in  embryo  of 

twenty-seven  days,  387 
Beethaare,  561 


Beigel's  ova,  289,  308 
Belegknochen,  461 
Belegzellen,  755 

Bildungsgewebszellen,     transforma- 
tion of  leucocytes  into,  208 
Birds,  concrescence  in,  124 

primitive  axis  and  streak  of,  131 
Bladder,  development  of  the,  514 
Blastocyst,  see  Blastoderinic  vesicle 
Blastodermic  rim,  117 
Blastoderinic  vesicle,  105 

homologies  of,  107 

homologies  of  the  mammalian, 
141 

mammalian,  135 

with  one  complete  layer,  135 

with  primitive  streak,  139 

with  primitive  streak  and  head 
process,  140 

with  two  layers,  137 
Blastophore,  80 
Blastopore,  127 

secondary,  190 

yolk,  124 

Blastoporic  canal,  128 
Blastula,  111 

stage  of  segmentation,  97 
Blatt,  animates,  of  Von  Baer,  167 

vegetatives,  of  Von  Baer,  167 
Blood,  one-celled,  223 

origin  of  the,  211 

plastid,  224 

two-celled,  223 
Blood-cells,  red,  215,  218 
Blood-corpuscles,morphology  of  the, 

223 

Blood-germ,  the,  212 
Blood-islands,  213 
Blood-plates,  origin  of  the,  223 
Blood-vessels,    growth  of,  into   the 
embryo,  214 

origin  of  the,  211 

primitive,  218 

transformations  of  the,  534 
Blutkeim,  212 
Bodenplatte,  606 
Body-cavity,  197 
Bogenfurche,  692,  696 
Bone,  growth  of,  410 

origin  of,  407 

origin  of  the  marrow  of,  420 
Bones,  dermal,  422,  461 

frontal,  development  of,  463 


INDEX. 


795 


Bones,  interparietal, development  of, 

463 

jugal,  development  of,  464 
lachrymal,  development  of,  463 
malar,  development  of,  464 
maxillary,  development  of,  464 
nasal,  development  of,  463 
of  the  human  skull,  homologies 

of  the,  4<M 

palatine,  development  of,  463 
parietal,  development  of,  463 
pne-maxillary,  development  of, 

4(14 

pterygoid,  development  of,  464 
splenial,  or  splint,  461 
squamosal,  development  of,  463 
tympanal,  development  of,  464 
typical  dermal,  in  anmiota,  462 

dang,  539 
cloacal,  516 

lion rrelet  entodermo-mtellin,  133 

Brain,  unlade  of  the,  178 
definition  of  the,  593 
development  of  the,  593 
fore-,  178,  595,  610 
hind-,  178,  595,  598 
mid-,  178,  595,  598,  610,  677 

Branchial  arches,  265,  267 

arches,  muscles  of  the,  477 
clefts,  263 
nerves,  636 
region,  743 
sense-organs,  706 
skeleton,  443 

Branchiate  vertebrates,  aortic  arches 
in,  537 

Braun's  cords,  513 
third  canal,  190 

Breus'  ovum,  288 

Broad  ligament,  development  of  the, 

4!)'.) 

Bronchi,  hyparterial,  775 
Bronchial  tree,  774 
Bronchus,  eparterial,  775 
Bruch's  embryo,  308 
Jiriif/;>-nk*rn,  zavltiger,  671 
11  rii <•];<- n I;  r n  in m ling,  599 

in  embryo  of  thirty-one  days,  389 
]1  in  i  del,  ovales,  661 
Bundle,  oval,  661 
Bimlaeh,  cords  of,  661 
Bui>a  Fabricii,  260 

parietalis,  482 


Bursal  cavities,  development  of,  421 

CADUCA,  see  Decidua 
Calcar  avis,  693 
Canal,  anal,  189 

blastoporic,  128 

Braun's  third,  190 

central,  of  the  spinal  cord,  659 

entoderuial,  743 

hyaloid,  723 

medullary,  178 

neural,  179 

neurenteric,  188 

notochordal,  126 

notochordal,    differentiation    of 
the,  182 

of  Schlemm,  724 

true  neurenteric,  188 

tubo-tympanal,  738 
Canalis  cranio-pharyngeus,  574 

hyaloideus,  713 

reunions  Henseni,  737 
Canals,  Fontana's,  726 

Gartner's,  503 

semicircular,  730,  738 
Cap,  cephalic,  282 
Capsule  of  the  lens,  716,  723 
Capsules,  periotic,  438 

supra-renal,  485 
Capuchon  caudale,  283 

c6phalique,  282 
Carotids,  internal,  development   of 

the,  538 

Carpus,  ossification  of  the,  457 
Cartilage,  appearance  of  the  matrix 
of,  404 

degeneration  of  ossifying,  406 

elastic,  406 

ensiform,  434 

fibro-,  406 

growth  of,  405 

mature  hyaline,  405 

MeckeFs,  445 

origin  of,  403 

ReichertX  445 

xyphoid,  434 

young  hyaline,  404 
Cauda  equina,  658 
Caudicantia,  687 
Cavit^  somatique,  197 
Cavities,  bursal,  development  of,  421 

coelomatic,  in  the  head,  199 

mesenchymal,  420 


796 


INDEX. 


Cavities,  pleural,  expansion,  of  the, 

483 
separation   of    the  pleural    and 

abdominal,  484 
separation    of    the    pleural  and 

pericardial,  482 
synovial,  development  of,  421 
Cavity,  amniotic,  284 
archenteric,  254 

archenteric,  extension  of  the,  271 
niandibular,  200 
of  the  primitive  segment,  202 
omental,  770 

oral,  development  of,  567 
pleuro-peritoneal,  197 
pne-mandibular,  200 
primitive  body,  150 
segmentation,  97 
sub-germinal,  115 
yolk,  115 
Cells,  Corti's,  734 
daughter,  44 
decidual,  12 
Deiter's,  734    ' 
dome,  550 

dumb-bell,  of  the  ovum,  56 
ectodermal,  97  et  seq. 
egg-,  48 
enamel,  586 

entodermal,  97  et  seq.,  254 
epidermic,  548 
fat-,  development  of,  417 
germinating,  611 
lutein,  67 
mother,  44 

nerve,  development  of,  624 
nerve,  origin  of,  611 
of  the  Graafian  follicle,  53 
of  the  seminiferous  tubules,  42 
of  the  vitelline  epithelium,  347 
ovic,  48 
parent,  43 
peptic,  755 

pigment,  origin  of,  419 
plasma,  possibly  regressive  stages 

of  fat-cells,  419 
primitive  mesodermic,  149 
red  blood- 215,  218 
Sertoli's,  42 
sexual,  249 
special  sense-,  709 
vasoformative,  218 
young  neuroglia,  611 


Cement,  dental,  589 
Centrolecithal  ova,  61 
Centrosoma,  94 
Cephalic  cap,  282 

ganglia,  603 

nerves,  general  morphology  of, 

633 
Cerebellum,  672 

histogenesis  of  the,  674 
Cerebral  convolutions,  695 

flexures,  600 

hemispheres,  690 

nerves,  morphology  of  the,  633 

vesicles,  178,  593 

Cerebrum,  peduncles  of  the,  678 
Cervical  nerves,  629 

sinus,  744 

Chamber,  anterior,  723 
Cheek-plate,  568 
Cheiropterygium,  450 
Chiarugi's  embryo,  304 
Chiasm,  optic,  688,  718 
Chondrocranium,  434 

atrophy  of  parts  of  the,  441 

ultimate  history  of  the,  438 
Chorda  dorsalis,  181 
Chordae  funiculse,  362 
Chorion  and  amnioii,  union  of,  337 

blood-vessels  of  the,  331 

evolution  of  the,  331 

frondosum,  318 

general  description  of  the,  317 

histology  of  the,  322 

laeve,  318 

primitive,  281 

progressive  history  of  the,  35 

retrogressive  history  of  the,  28 

the  human,  317 

true,  286 

union  of  the  allantois  with,  376 
Chorionic  fluid,  318 

vesicle,  317 

villi,  318 
Choroid,  713,  722 

plexus,  681 

processes,  722 
Chromatin,  the    essential  factor  in 

the  function  of  heredity,  90 
Cicatricula,  99 
Ciliary  ganglion,  640,    642 

muscle,  725 

Circulatory  system,  origin  of  the,  211 
Clavicle,  development  of  the,  454 


INDEX. 


'97 


Cleavage,  see  Segmentation 
Clefts,  branchial  or  gill,  263 

Clitoris,  origin  of  the,  518 

Cloaca,  -J.VJ 

clnifitni  Hu'xndrrniique,  482 

Cochlea,  7:51 

C'i'cum.  758 

Co'lenteron,  113 

Coelom,  111 

extension  of  the,  271 

extra-embryonic,  extension  of 
the,  280 

formation  of  the,  150 

of  the  head,  199 

primitive  divisions  of  the,  192 

theory  of  the  mesoderm,  155 

unsegmented,  197 

ventral.  1!»7 

Commissure,  ganglionic,  603 
Commissures,  cerebral,  084 
( 'oiiarium.  i'>v^ 
Concrescence,  115 

in  birds,  124 

in  bony  fishes,  117 

in  elasmobranchs,  118 

in  mammals,  124 

in  marsipobranchs,  ganoids,  and 
amphibians,  120 

in  sauropsida,  122 

law  of,  115 

significance  of,  12." 

summary  of,  125 
Conjunctiva.  ?;M> 
Connective-tissue   fibrils,    origin   of, 

899 

Continuity,  germinal,  87 
Convolutions,  cerebral,  695 
Cord,  amniotic,  284 

genital,  501 

genital,  formation  of  the,  491 

spinal,  607,  see  Spinal  cord 

sympathetic,  630 

umbilical,  356,  see  Umbilical  cord 
Cords.  Kraun's,  513 

medullary,  development  of,  249 

of  Burdach,  661 

vocal,  778 
Coriuni,  5">:> 
Cornea,  724 
Cornu  Ammonis,  692 
Corona  radiata  of  the  discus  prolige- 

rus,  53,  5! » 
Corpora  albicantia,  687 


Corpora  quadrigemina.  678 
CO/YAV  iniKnitiin',  501 
Corpus  albicans,  68 

callosum,  683 

cavern  osum,  518 

hemorrhagicum,  66 

luteum  of  menstruation,  67 

luteum  of  pregnancy,  68 

spongiosum,  518 

striatum,  691 
Corpuscles,  Malpighian,  509 

red  blood-,  origin  of,  221 

white  blood-,  origin  of,  221 
Corti,  organ  of,  733 
Corti'scells.  731 
Coste's  embryo,  300 
Cotyledons  of  the  placenta,  366 
Couronne  polaire,  95 
Co  wper's  glands,  development  of,  516 
Cranial  nerves,  morphology  of  the, 

633 
Cranium,  development    of    the,   434 

ossification  of  the,  439 
Crescent,  anterior,  of  the  area  pellu- 

cida,  132 

Crest,  neural,  601 
Crura  cerebri,  695 
Cumulus  proligerus  of  the  Graafian 

follicle,  52 

Cut  is,  development  of  the,  553 
Cuvier,  duct  of,  275,  542 
e,  607,  662 


Darmfaserblatt  of  the  inesoderm,  152 
Darmpforte,  vordere,  261 
Decidua,  changes  in  the,  at  parturi- 
tion, 21 

graviditatis,  6,  26 

menstrualis,  4,  26 

reflexa,  8 

reflexa,  fate  of  the,  19 

se  rot  in  a,  8 

serotina   at    the    end    of    seven 
months,  17 

subchorialis,  as  described  by  Kol- 
liker  and  others,  18 

vera,  8 

Deckkjiochen,  461 

Deeklamelle,  graue  moleculttre,  675 
Deck-plate  of  the  fore-brain,  681 
Deckplatte  of  His,  606 
Deckschicht,  106,  108.  135,  139 

modification  of,  in  rodents,  141 


798 


INDEX. 


Deiter's  cells,  734 

Dental  follicle,  origin  of  the,  584 

papilla,  587 

processes,  588 

ridge,  578 

shelf,  583 
Dentine,  588 

Dentition,  double,  of  mammals,  589 
Dermal  bones,  461 
Dermal  teeth  of  sharks,  581 
Dennis,  development  of  the,  553 
Descemet,  membrane  of,  725 
Deutoplasm  of  the  ovum,  49 
Development,  human,  general  out- 
line of,  28 
Diaderm,  111 
Diaphragm,  development  of  the,  485 

rudiment  of  the,  269 
Differentiation,  160 

histological,  164 
Dilatations,  161 

Diphyodont,  definition  of  term,  582 
Disc,  germinal,  99 
Discus  proligerus,  99 

of  the  Graafian  follicle,  52 
Diverticula,  161 
Dome  cells,  550 
Dorsal  flexure,  the,  313 
Dotterbildung,  Nerd  der,  53 
Dotterhaut,  58 
Dotternabel,  124 

Dottersaekepithel,  cells  of  the,  347 
DrUsen,  zusammengesetzte,  163 
Drilsenfeld,  564 
Duct,  lachrymal,  580 

milk,  565 

Mtlllerian,  230,  244,  253,  503 

of  Santorini,  767 

of  Wirsung,  767 

pronephric,  230,  234,  253 

segmental,  230,  234,  253 

Wolfflan,  230,  234,  253,  502 
Ductus  arteriosus,  539 

Botalli,  539 

Cuvieri,  275,  542 

Cuvieri,  changes  in  the,  544 

thyreoglossus,  748 

EAR,  bones  of  the,  740 

development  of  the,  727 
external,  741 

Ecker's  ovum,  307 

Ectental  line,  97 


Ectoderm    and    entoderm,   differen- 
tiation of,  110 
differentiation  of  the,  323 
formation  of  the,  97  et  seq. 
Ectodermal  organs    of    the    human 

body,  160 

Egg,  see  Embryo,  Foetus,  and  Ovum 
Ehrenritter's  ganglion,  649 
Einzeldrilsen,  103 

Elasmobranchs,  concrescence  in,  118 
mesoderm  of,  144 
primitive  axis  in,  130 
Elastic  tissue,  origin  of,  401 
Embryo,  170 
amniote,  278 
attachment  of  the,  374 
cross-section  of  a  typical,  279 
determination  of  the  age  of,  384 
form  of  the,  277 
growth  of  the,  381 
human,    progressive    history    of 

the,  35 
human,  retrogressive  history  of 

the,  28 

measuring  the  length  of,  384 
mesoblastic,  128 

primitive  type  of  vertebrate,  277 
secondary  type  of  vertebrate,  277 
transition  from  the,  to  the  foetus, 

391 

see  also  Foetus,  Ova,  and  Ovum 
Embryos,    classification    of    known 

human,  by  stages,  286 
of    known    ages,   characteristics 

of,  384 
for  those  of  His  and  others,  see 

under  Ova 
Enamel,  deposit  of,  587 

organ,  development  of,  584,  585 
Enddarm,  the,  260 
Endocardkissen    of    the    auricular 

canal,  530 
Endothelherz,  226 

Endothelkissen  of  the  auricular  ca- 
nal, 530 

EndstUck  of  a  spermatozoon,  41 
Ensiform  cartilage,  434 
Enterocoele,  169 
Enteron,  113 

Entoderm  and  ectoderm,  differenti- 
ation of,  110 
cells  of  the,  254 
formation  of  the,  97  et  seq. 


INDEX. 


799 


Entoderm,  intestinal,  growth  of  the, 

TOO 
separation  of  notochordal  band 

from,  182 
Entodermal  canal,  713 

organs  of  the  human  body,  160 


Envelopes,    fcetal,    progressive    his- 
tory of  the,  35 

foetal,    retrogressive    history    of 

the,  28 

Kpcncephalon,  599 
Epfiidyiiui,  616 
/:>>  /•',//  in'i-hi€al>  517 
Epibranchialis  nerve,  651 
Epicoele,  ir,'.» 
Epidermal  system,  548 
Epidermis,  formation  of  the,  548 
Epididymis,  development  of  the,  500 
Epiglottis,  778 
Epiphysis  cerebri,  688 
Episternum,  454 

Epistropheus,  formation  of  the,  430 
Epithelium,  differentiation  of,  165 

germinal,  247 
KpitnVlmim,  550,  552 
Kpoiiychiuiii,  555 
Epob'phoron,    development    of   the, 

500 

Ki-xntzhaare,  561 
Eustachian  tube,  740 
Eye,  anterior  mesenchyina  of  the,  723 

development  of  the,  710 

evolution  of  the  vertebrate,  727 

parietal  or  pineal,  688 
Eyelashes,  726 
Eyelid,  726 

third,  727 

FABRICIUS,  bursa  of,  260 
Face,  development  of  the,  567 
Facial  apparatus,  position  of  the,  467 
Fallopian  tube,  504 

origin  of  the,  245 
Falte,  vena  cava,  483 
Falx  cerebri,  6*91 
Fat-cells,  development  of,  417 
Femur,  ossification  of  the,  459 
Fenestra  ovalis,  740 

rotunda,  740 
Fentes  branchiales,  263 
FeuiUe  angioplastique,  212 
Feuillet  vasculaire,  341 


Fibres,  ganglionic,  619 

medullary,  616 

muscle,  multiplication  of,  474 

nerve,  origin  of,  616 

osteogenetic,  409 

segmental  or  skeletal  muscle,  470 

smooth-muscle,  origin  of,  417 
Fibrillae,  muscular,  473 
Fibrils,  connective-tissue,  origin  of, 

399 

Fibrin,  canalisirtes,  323 
Fibula,  ossification  of  the,  459 
Filament  epiaxial,  132 
Fissura  calcarina,  697 

parieto-occipitalis,  697 
Fissure,  calloso-marginal,  697 

choroid,  712 

of  Rolando,  698 

splenial,  697 

triradiate,  700 
Fissures,  cerebral,  695 

accessory,  702 

evolution  of,  702 

of  the  frontal  lobe,  700 

of  the  island  of  Reil,  701 

of  the  occipital  lobe,  701 

of  the  parietal  lobe,  701 

of  the  temporal  lobe,  701 

transitory,  702 

Fleischschicht  of  Von  Baer,  167 
Flexure,  dorsal,  313 
Flexures,  cerebral,  600 
Floor  of  the  third  ventricle,  687 
FlUgelleiste  of  the  medulla,  666 
Flilgelplatte,  607 
FlUgelwulst  of  the  medulla,  667 
Fluid,  allantoic,  355 

aniniotic,  337 

chorionic,  318 
Foetal  appendages,  314 

appendages,  origin  of  the,  280 
Foetus,  the,  379 

growth  of,  381 

nutrition  of,  373 

progressive  history  of,  35 

retrogressive  history  of,  28 

transition  from  the  embryo  to, 
391 

see  also  under  Embryo,  Ova,  and 

Ovum 
Follicle,  dental,  origin  of  the,  584 

Grraafian,  development  of  the,  51 
Follicles,  primary,  growth  of  the,  50 


800 


INDEX. 


Fontana's  canals,  726 
Fontaiielles,  the,  465 
Foramen  caecum,  592,  748 

of  Monro,  597,  680,  693 

of  Wirislow,  770 

ovale,  529 
Fore-brain,  178,  595,  610 

median  portion  of  the,  679 
Fore-gat,  origin  of  the,  261 
Formative  yolk,  99 
Fornix,  684 
Fossa  of  Sylvius,  695 
Fourth  ventricle  and  its  roof,  676 
Fovea  cardiaca,  261 
Fretum  Halleri,  523,  525 
Froriep's  ganglion,  656 

law,  469 

Fruchtschmiere,  562 
Fruchtwasser,  337 
Funiculus  amnii,  284,  285 

restiformis,  667 
Funnel,  Miillerian,  245 
Furvhung  of  ova,  93 
Furrow,  dorsal,  173 
Fusszellen,  origin  of,  494 

GANGLIA,  basal,  694 

cephalic,  603 

sensory,  origin  of,  601 

sympathetic,  631 
Granglienleiste,  602 
Ganglion,  acustico-faciale,  644 

ciliary,  640,  642 

cochleare,  645 

Ehrenritter's,  649 

Froriep's,  656 

Gasserian,  642 

geniculi,  645 

glosso-pharyngeal,  648 

hypoglossal,  656 

impar,  632 

intracranial,  645 

jugulare,  652 

mesocephalic,  640 

nodosum,  652 

olfactory,  637 

petrosum,  649 

spiral,  647 

thalamic,  640 

trigeminal,  641 

trochlear,  641 

vagus,  651 

vest ibul  are,  (54o 


Ganglionic  commissure,  603 

fibres,  619 

nerve  cells,  626 

sense-organs,  706 
Ganoids,  concrescence  in,  120 

development  of  the  primitive 
segments  in,  195 

primitive  axis  in,  129 
Gall-bladder,  766 
Gallertgewebe,  329 
Gallertschicht,  329 
Gartner's  canals,  503 
Gasserian  ganglion,  642 
Gastrula  theory,  112 
G-aumentcbsche  of  Selenka,  183 
Gefdsshof,  197 

differentiation  of  the,  272 
GefOssschicht,  167,  212 
Gegenbauer's    theory   of  the  skull, 

469 

Gehirnblfischen,  595 
Gekrose,  244 
Genital  cord,  501 

cord,  formation  of  the,  491 

fold,  247 

mesenchyma,  production  of  the, 
248 

products,  the,  37 

ridge,  230,  244,  251 

tubercle,  516 
Genitalia,  external,  development  of 

the,  516 

Genitalstrang,  501 
Genoblasts,  history  of  the,  39 
Germ-band  theory  of  the  mesoderm, 

156 
Germ-layers,  the,  91,  159 

differentiation  of,  160 

inversion  of,  in  rodents,  141 

law  of  the  unequal  growth  of,  162 

organs  of  the  human  body  de- 
rived from  the  several,  160 

r61e  of,  159 
Germinal  area,  271 

continuity,  87 

disc,  99 

epithelium,  48,  247 

spot  of  the  ovum,  57 

vesicle  of  the  ovum,  57 

wall,  133,  271 
Geschlechtszellen,  249 
Gesichtxkopfbenge,  600 
Gill-arches,  265,  267 


INDEX. 


801 


Gill-clefts,  263 
Girigivse,  578 
Girdle,  pelvic,  455 

shoulder,  453 
(T la nd,  pineal,  (580,  688 

prostatic,  515 

thyroid,  747 
Glands,  161 

Bartholiiii's,  516 

classification  of,  163 

Cowper's,  51(5 

intestinal.  T.V.I 

lachrymal,  m 

lymph-,  development  of,  414 

mammary,  564 

Montgomery's,  565 

mucous,  of  the  stomach,  755 

of  the  skin,  563 

salivary,  590 

sebaceous,  development  of,  562 

sexual,  development  of,  492 

sweat,  563 
U  lobules  de  maturation  parfaite,  81 

tin  debut,  80 

polaires,  64,  65 

prGcoces,  80 

tin-t/iffi,  80 

Globules,  groups  of,  eliminated  from 
the  ovules,  80 

polar,  of  the  ovum,  64 
Glosso-pharyngeal  nerve,  648 
Goette's  theory  of  the  mesoderm,  156 
Graafian  follicle,  development  of  the, 
51 

ovulational    metamorphosis    of 

the,  66 

Grttnzplatte,  671 
Gray  matter,  616 

of  the  medulla  oblongata,  670 

of  the  spinal  cord,  663 
Grenzvene  of  the  placenta,  373 
Groove,  dorsal,  134 

medullary,  134,  173 

primitive,  140 

primitive,  stage  when  it  is  at  its 

maximum,  134 
pulmonary,  773 
Growth,  law  of  unequal,  162 
Grundplatte,  607 
Grundsubstanz,  1(56 
Gubernaculum,  248,  496 
<i inns,  formation  of  the,  578 
Gut,  post-anal,  260 


Gyri  of  the  brain,  695 
Gyrus  arcuatus,  696 

choroideus  anterior  et  posterior, 
877 

dentatus,  696 

fornicatus,  698 

subcallosus,  703 

uncinatus,  696 

HABEXULA.  tecta,  736 
Hahneutritt  of  the  ovum,  99 
Hairs,  development  of,  557 
embryonic,  561 
loss  and  renewal  of,  561 
Il«li'<  iikn'1  in  in  n  itf/.  (500 
Hantelzellen  of  the  ovum,  56 
Harnsack  of  Von  Baer,  296 
Hatschek's  germ-band  theory  of  the 

mesoderm,  156 
Hauptstttck  of  a  spermatozoon,  40, 

41 

Hautfaserblatt  of  the  mesoderm,  152 
Hautschwht  of  Von  Baer,  167 
Head,  coelom  of  the,"  199 
evolution  of  the,  704 
Head-kidney,  231 
Head- process,  128,  134,  140 
Heart,  amniote  mode  of  development 

of  the,  225 

division  into  right  and  left,  528 
endothelium  of  the,  228 
muscles  of  the,  478 
non-muscular  areas  of  the,  527 
origin  of  the,  224 
primitive  mode  of  development 

of  the,  224 

transformations  of  the,  521 
valves  of  the,  532 
walls  of  the,  525 
Hecker's  ovum,  308 
Hemispheres,  cerebral,  690 
Hensen's  knot,  124,  138 
Herd  der  Dotterbildung,  53 
Heredity,  85 

the  essential  factor  in  the  func- 
tion of,  90 

Hernia,  intestinal,  758 
Hexenmilch,  565 
Hillocks  on  the  inner  surface  of  the 

pregnant  uterus,  13 
Hind-brain,  178,  595,  598 
][>uterdarm,  260 
Hinterhirn,  178,  595,  598 


802 


INDEX. 


Hinterseitenstrang  of  the  spinal  cord, 

662 
Hippocampus  major,  692 

minor,  692,  693 
Hirnanhang,  571 
Hirnblasen,  178 
Hirnsichel,  691 
His,  zones  of,  606,  661 

zones  of,  in  the  adult,  666 

zones  of,  in  the  fore-brain,  686,  687 
His'  embryos,  291,  293,  294,  297,  298, 

304 
Historical  references  to — 

concrescence,  117 

epiphysis  cerebri,  689 

ganglionic  fibres,  620 

hypoglossal  ganglion,  657 

hypophysis  cerebri,  575 

kidney,  514 

mammary  glands,  566 

mesentery,  772 

neuromeres,  606 

origin  of  the  decidual  cells,  12 

ossification,  412 

ovulation,  69 

peripheral  nerve-fibres,  622 

placenta,  373 

spermatozoa,  47 

theories  of  the  inesoderm,  153 

theories  of  the  skull,  468 

theory  of  the  germ-layers,  166 

thymus  gland,  747 

thyroid  gland,  751 

Wolffian  bodies,  243 
Holoblastic  ova,  55,  99,  111 
Hornstreif,  695 

Horzellen,  ab-  und  aufsteigende,  734 
HUlle,  serdse,  286 
Humerus,  ossification  of  the,  456 
Humor,  aqueous,  724 

vitreous,  723 
Hyaloid  canal,  723 
Hydatid  of  Morgagni,  503 
Hymen,  biperforate,  cause  of,  505,  507 

development  of,  507 
Hyoid  bars,  445 
Hypochordal  brace  of  the  vertebra, 

425 

Hypoglossal  nerve,  655 
Hypophysis  cerebri,  571 

IDIOPLASMA,  88 
Impregnation,  69 


Incus,  445 

Infundibulum,  687 

InnenschicM  of  the  medullary  wall, 

616 
Inseln  on  the  inner  surface  of  the 

pregnant  uterus,  13 
Insert io  f  urcata  of  the  umbilical  cord, 
361 

velamentosa    of    the    umbilical 

cord,  361 
Intercellular  network,  origin  of,  401 

substance,  disappearance  of,  413 

substance,  hypertrophy  of,  413 
Intersegmental  arteries,  540 
Intervertebral  ligament,  426 
Intervillous  space,  317 
Intestine,  755 

Invaginations  of  the  germ-layers,  161 
Iris,  725 

primitive,  714 
Island  of  Reil,  696 

fissures  of,  701 

JACOBSON'S  organ,  577 

Janosik's  embryo,  300 

Joints,   embryological   classification 

of,  461 
of  the  limbs,  development  of  the, 

460 
Jones'  (Wharton)  ovum,  289 

KARYOKINESIS  of  the  ovum,  94 

Karyosomen,  76 

Kasefirniss,  562 

Keibel's  ovum,  291 

Keilstrttnge,  Burdacli'sche,  661 

Keimcylinder  of  Selenka,  141 

Keimepithel,  48,  247 

Keimhof,  271 

Keimplasma,  87 

Keimscheibe,  99 

Keimstreif,  158 

Keimwall,  133 

Keimzellen  des  Markes,  611 

Kernplatte,  63,  95 

Kernsaft,  62 

Kernspindel,  63 

Kidney,  development  of  the,  507 

head-,  231 

primitive,  231,  235 

shape  of  the,  513 

the  human,  513 
KiemenbGgen,  265,  267 


INDEX. 


803 


Kiemempalten,  263 

Knot,  Hensen's,  124,  138 

Kolbenhaare,  561 

Kolk's   (Schroeder  van  der)  embryo, 

806 

Kollmann's  ova,  289,  293 
K»/,n,,  a*!,,  600 
Knittfnrtx'ttz,  12s,  134,  140 
Kopfkappe,  282 
Kopfkrilmmung,  cordere,  600 

LABIA  inajora,  origin  of  the,  520 

minora,  origin  of  the,  518 
Labyrinth,  7:>7 
Lachrymal  duct,  formation  of,  580 

gland,  7J; 
Lamina  spiralis,  732 

terminalis  -V.»7,  679,  682 
J.itiHjshiiiidil,  hinteres,  671 
Lanugo,  56 1 
Larynx.   17$ 
Lateral  nerve,  654 

Layer,  outer  nuclear,  of  the  cerebel- 
lum, 675 
/,'  <1>  /•//<; tit,  553 
Leg.  skeleton  of  the,  458 
LeibeshGhle,  197 
Lens,  optic,  714 

capsule  of  the,  716,  723 

vascular  tunic  of  the,  716 
Lenticular  zone  of  the  optic  cup,  722 
Leucocytes,  origin  of,  221 
Ligament,    broad,    development    of 

the,  499 
Ligaments,  development  of,  421 

of  the  liver.  765 

Ligamentum  epididymis,  origin  of, 
244 

pectinatum,  726 

spirale,  737 
Ligula,  677 
Limbs,  joints  of  the,  460 

origin  of  vertebrate,  448 

position  of  the,  452 
Limbus  Vieusenii,  529 
Lip-groove.  609 
J.il>l>vnfurche,  609 
Lips,  formation  of  the,  578 
Liquor  amnii,  337 

folliculi,  51 
Liver,  development  of  the,  761 

functions  of  the,  766 

origin  of  the,  268 


Liver,    relation    of,   to    the   septum 
trans versum,  '26'.) 

veins  of,  changes  in  the,  545 
Lobes,  cerebral,  692 

olfactory,  703 
Lobules  of  the  liver,  763 
Lobus  inferior  inedialis,  484 
Lungs,  774 
Lutein  cells,  67 

Lymphatic  vessels,  origin  of,  413 
Lymph-glands,  development  of,  414 

MACULA  ampullae  posterioris,  647 

sacculi,  647 
Maculae  acustica?,  736 
Malleus,  445 
Malpighian  corpuscles,  509 

corpuscles,  differentiation  of  the, 
238 

layer  of  the  epidermis,  550 
Mammals,  allantois  in,  354 

blastodermic  vesicle  in,  135 

concrescence  in,  124 

mesoderm  of,  148 

yolk-sac  of,  349 
Mammary  glands,  564 

evolution  of  the,  565 

literature  of,  566 
Man,  allantois  in,  354 

yolk-sac  of,  349 

Mandible,  development  of  the,  464 
Mandibular  arch,  578 

bars,  444 

muscles,  478 

Mantle  layer  of  the  hemispheres,  691, 
694 

of  the  medullary  wall,  616 

of  the  spinal  cord,  growth  of,  661 
Manubrium,  434 
Margo's  theory  of  the  multiplication 

of  muscle  fibres,  474 
Markcylinder,  vordere,  607 
Markkilgelchen,  687 
Markprisma,  hinteres,  607 
Marksegel,  hinteres,  673,  677 
Markstrdnge,  249 

origin  of  the  follicular  cells  from 

the,  50 

Marrow,  origin  of,  420 
Marsipobranchs,     concrescence    in, 
120 

development    of    the    primitive 
segments  in,  195 


804 


INDEX. 


Marsipobranchs,  primitive  axis  in,  129 
Mass,  relations  of  surface  to,  161 
Mastzellen,  possibly  regressive  stages 

of  fat-cells,  419 
Matrix,  niesenchymal,  ICG 
Maxillary  process,  578 

of  the  first  branchial  arch,  268 
Meatus  auditorius  externus,  738,  740 
Meckel's  cartilage,  445 
Mediastinum,  483 
Medulla  oblongata,  599,  665 

development  of,  607 

dorsal  zone  of  His,  666 

gray  matter,  670 

neuroblasts  of  the  dorsal  zone, 
668 

ventral  zone  of  His,  668 

zones  of  His  in  the  adult,  666 
Medullary  canal,  178 

canal,  evolution  of  the,  179 

cords,  development  of  the,  249 

fibres,  616 

groove,  173 

nerve-cells,  624 

plate,  173 

sheaths,  620 

tube,  widening  of  the,  595 

wall,  layers  of  the,  616 
Membrana  adamant ina,  586 

basilaris,  735 

capsularis  lentis,  717 

capsulo-pupillaris,  717 

eboris,  587 

fauces,  262 

granulosa  of  the  Graafian  folli- 
cle, 52 

limitans  interna,  612,  613 

nictitans,  727 

olfactoria,  710 

pleuro-pericardiaca,  482 

praeformativa,  588 

propria  mesenterii,  772 

propria  of  the  Graafian  follicle,  52 

pupillaris,  717 

serosa,  286 

tectoria,  732 

Membrane  of  Descemet,  725 
Membrane-bones,  461 
Membranes,  basement,  421 

development  of,  421 

lining,  of  the  splanchnocoele,  485 
Menstruation,  changes  in  the  mucosa 
corporis  during,  4 


Meridional  cleavage,  98 
Meriten,  75 
Meroblastic  embryo,  128 

ova,  55,  99,  111 
Merocyten,  352 
Merocytenkerne,  352 
Mesainceboids,  112,  166 
Mesencephalon,    178,    595,    598,   610, 

677 
Mesenchyma,  112,  144 

anterior,  of  the  eye,  723 

condensation  of,  in  the  formation 
of  cartilage,  403 

differentiation  of,  165 

embryonic,  398 

genital,  production  of  the,  248 

intercellular,  differentiation    of, 
399 

organs  of  the  human  body  de- 
rived from  the,  160 

origin  of  the,  153,  207 

tissues  resulting  from  the  differ- 
entiation of  the,  397 
Mesenchymal  cavities,  420 

matrix,  differentiation  of,  399 

portion  of  the  kidney,  508 

tissues,  397 

tissues,  classification  of,  397 
Mesentery,  483,  767 
Mesoblast,  157 
Mesocardium,  228,  483 

laterale,  481 

Mesocephalic  ganglion,  640 
Mesocolon,  769 
Mesoderin,  112,  144 

cceloni  theory  of  the,  155 

differentiation  of  the,  327 

division    of,    into    somatic    and 
splanchnic  layers,  271 

expansion  of  the,  150 

gastral  and  peristomial,  of  Rabl, 
145,  147,  156 

germ-band  theory  of  the,  156 

Goette's  theory  of  the,  156 

Hatschek's  germ-band  theory  of 
the,  156 

of  amphibia,  145 

of  elasmobranchs,  144 

of  mammals,  148 

of  sauropsida,  147 

of  teleosts,  145 

origin  of  the,  144 

parablast  theory  of  the,  153 


INDEX. 


805 


Mesoderm,  peristoimal  and  gastral  of 

Rabl,  145,  147,  150 
primitive  cells  of  the,  149 
Rabl's  theory  of  the,  155 
somatic,  1~>. 
splanchnic.   152 
Theories  of  the,  153 
vertebrate  type  of  origin  of,  148 
Mesodermal  organs    of    the  human 
body,  160 

_:astri  um,  769 


Mesonephros,  231,  235 

Mesorchiuin,  4'.»7 
origin  of,  J  1  1 
Mesothelial  muscles,  470 
Mesothelium,  144 

histogenesis  of  the,  152 

or.u-ans  of  the  human  body  de- 

rived from  the,  160 
origin  of  ova  from,  48 
Mesovarium.  origin  of,  '-2  1  J 
Metacarpal  bones,  ossification  of  the, 

458 
Metagastrula  stage  of  segmentation, 

104 

Met  a  meres,  193 
Metatarsal  bones,  ossification  of  the, 

400 

Metencephalon,  178,  595,  598 
Micropyle  of  the  ovum,  59 
Mirroporus  in  the  head  of  some  sper- 

matozoa, 41 
Microsoma  of  the  segmentation  nu- 

cleus, 93 
J//v/'"vo///'-/t,  76 

Mid-brain,  178,  595,  598,  610,  677 
Mid-gut,  113 
Milrhlini^  564 
Milk  at  birth,  565 
Milk-glands,  564 
Minor's  ova,  296 
MiW-lhirn,  178,  595,  598 
Mitt'  l{>latte,  201 
MitMxtttck   of  a  spermatozoon,  40, 

•-21 
Monro,  foramen  of,  597,  680,  693 

sulcus  of,  680 
Montgomery's  glands,  565 
Morgagni,  hydatid  of,  503 
Mouth  cavity,  development  of  the, 

567 
evolution  of  the  vertebrate,  569 


Mucosa  cervicis  uteri,  24 
Mucosa  corporis  uteri,  3 

changes  in  the  blood-vessels  of 
the,  during  pregnancy,  11 

changes  in  the  glands  of  the,  dur- 
ing pregnancy,  10 

during  menstruation,  4 

during  pregnancy,  0 

in  the  virgin,  3 

post-partum  regeneration  of  the, 

21 

Mucous  tissue,  358,  361,  403 
MOllerian  duct,  230,  244,  253,  503 

funnel,  245 
Mailer's  embryo,  308 
Mundrachenhautt  262 
Mundrachenraum,  568 
Muscle-fibre,  segmental  or  skeletal, 
470 

smooth,  origin  of,  417 
Muscle-plates,  475 
Muscles,  mandibular,  478 

inesothelial,  470 

inyotomic,  477 

of  the  branchial  arches,  477 

of  the  heart,  478 

union  of  nerves  and,  624 
Muscularis  uteri,  1 
Muskelkiiospen,  475 
Muskelspindel,  475 
Mutterkuchen,  364 
Hutterzellen,  44 
Myelencephalon,  599 
Myeloplaxes,  410 
Myocoele  of  Hatschek,  209 
Myotome,  201 
Myotomic  muscles,  477 

Nachhirn,  599 

Nackengrube,   gradual    obliteration 
of,  in  the  embryo  of  forty  days, 
391 
in  embryo  of  thirty-one   days, 

388 

NackenhGcker,  600 
NackenkrUmmung,  600 

in  embryo  of  thirty-one  days,  389 
in  embryos  of  twenty-three  days 

and  above,  385 

Nackeriltinge,  a  measure  of  the  em- 
bryo, 384 
Nagelfeld,  556 
Nagelplatte,  556 


806 


INDEX. 


Nagelzellen  of  the  ovum,  56 
Ntihrplasma,  88 
Ntthrzellen  of  Nagel,  51 
Nail -plate,  556 

Nails,  development  of  the,  554 
Narbe  of  the  ovum,  99 
Nasal  pits,  575 

process,  576 

Nebendarm  in  invertebrates,  187 
Nebenkern  of  Platner.  homology  of, 
46 

sphere  of  attraction  not  improb- 
ably identical  with,  94 
Nebenolive,  671 
Neck-bend,  600 

in  embryo  of   thirty-one   days, 
389 

in  embryos  of  twenty-three  days 

and  above,  385 
Nephridia,  230 
Nephridial  ridge,  230,  251 
Nephrotorne,  201 
Nerve,  abducens,  644 

acoustic,  644 

cochlear,  647 

epibranchialis,  651 

facial,  644 

glosso-pharyngeal,  648 

hypoglossal,  655 

lateral,  654 

oculo-motor,  639 

olfactory,  637 

ophthalmicus  profundus,  642 

optic,  638,  717 

pathetic,  640 

post-treinatic  branch  of  the 
glosso-pharyngeal,  650 

post-trematic    branches    of    the 
vagus,  654 

prse-trematic  branch  of  the  glos- 
so-pharyngeal, 650 

prse-trematic    branches    of   the 
vagus,  654 

spinal  accessory,  654 

thalamic,  640 

trigeminal,  641 

trochlear,  640 

vagus,  650 
Nerve-cells,  development  of,  624 

ganglionic,  626 

medullary,  624 

origin  of,  611 
Nerve-fibres,  origin  of,  616 


Nervenleiste,  602 
Nerves,  branchial,  636 

cervical,  629 

cranial,  633 

growth  of,  622 

origin  of,  622 

spinal,  627 

sympathetic,  630 

union  of  muscles  and,  624 
Nervous  layer  of  the  epidermis,  549 

system,  development  of  the,  593 

system,  sympathetic,  630 
Netzbeutel,  770 

Neugliederung  des  Axenskelets,  hy- 
pothesis of,  423 
Neural  canal,  179 

crest  or  ridge,  601 
Neuralleiste,  601 
Neureiiteric  canal,  188 

canal,  significance  of  the,  191 
Neuroblasts,  611,  613 

of  the  dorsal  zone  of  the  medulla, 

668 
Neuroglia,  origin  of  the,  612 

specialization  of  the,  614 
Neuroglia-layer,  outer,  616,  671 
Neuromeres,  604 

relations  of  the  cranial  nerves  to, 

636 

Neuron,  axis  of  the,  600 
Neuroporus,  177 
Nickhaut,  727 
Nose,  development  of  the,  575 

external,  development  of,  578 
Notochord,  181 

disappearance  of  the,  187 

histogenesis  of  the,  186 

morphology  of  the,  187 

of  teleosts,  184 

origin  of,  from  the  notochordal 
canal,  182 

relations  of,  to  other  parts,  185 

separation    of,    from    the    ento- 
derm,  182 

shape  of  the,  185 
Notochordal  canal,  126 

and  yolk  cavity,  fusion  of,  127 
Nuclear  substance,  the  essential  fac- 
tor in  the  function  of  heredity, 

90 
Nucleus,  segmentation-,  93 

the    organ  of  hereditary  trans- 
mission, 90 


INDEX. 


807 


Nutrition  of  the  foetus,  373 
Nyiuphitt,  development  of  the,  518 

Oberkieferfortsatz,  268,  578 
Obex,  <,;; 
Oculo-motor  nerve,  639 

Odonroblasts,  587 


Oil-globules  of  the  yolk,  54 
(f/Cf-n'fif/ff  Kni'ix-i;  243 
Olfactory  lobes,  703 

membrane,  710 

nerve,  < 

pit,    distinctness    of,    after   the 
twenty-fifth  day,  387 

plates,  575 
Olivary  body,  GT1 
(  MneiituiM,   Hi- 

meshes  of  the,  772 

minus,  7<i~i 

sac  of  the,  483 
(  >«  -spore,  78 
Opi-reuluin,  696 
Opossum,  yolk-sac  of  the,  351 
Optic  chiasma,  688,  718 

cup,  secondary,  713 

examinations,  594 

nerve,  638,  717 

recess  us,  688 

thalami,  686 

vesicles,  710 
Oral  plate,  262 
(  )ruans  of  special  sense,  706 
Ossification,  407 

metaplastic,  408 

neoplastic,  410 
Osteoblasts,  409 
<)ste<  Blasts,  410 
Osthim  primuiu,  529 

secundum,  529 
Otocyst,  origin  of  the,  728 
Otoliths,  : 
Ova,  48 

alecithal,  61 

centrolecithal,  61 

holoblastic,  55,  99,  111 

known  human,  eight  stages  of, 
308 

known  human,  of  the  second  and 

third  weeks,  286 
Ahlfeld's,  289 

Allen  Thomson's,  293,  294,  307 
BiegePs,  289,  308 


Ova,  known  human,  of  the  second 
and  third  weeks,  Breus',  288 

Bruch's,  308 

Chiarugi's,  304 

Coste's,  300 

Ecker's,  307 

Hecker's,  308 

His\  291,  293,  294,  297,  298,  304 

Janosik's,  300 

KeibePs,  291 

Kollmann's,  289,  295 

Minot's,  296 

Milller's,  308 

Reichert's,  287 

Remy's,  303 

Schro'der  van  der  Kolk's,  303 

Schwabe's,  2J«) 

Spree's,  291,  21)5 

Von  Baer's,  296,  307 

Wagner's,  308 

Wharton  Jones',  289 
mesoblastic,  55,  99,  111 
primitive,  48 

primitive,  appearance  of  the,  250 
telolecithal,  61 
see    also    under    Embryo    and 

Foetus. 

Ovary,  development  of  the,  495 
Oviduct,  230,  244,  253 
Ovoblast,  appearance  of  the,  250 
Ovomeriten,  75 
Ovulation,  66 

physiology  of,  68 
Ovum,  48 

amphiaster  of  the,  63 

animal  pole  of  the,  60 

archiamphiaster  of  the,  63 

attachment  of  the,  374 

definition  of,  48 

entrance    of    the    spermatozoon 

into  the,  70 
envelopes  of  the,  57 
full-grown,  before  maturation,  55 
growth  of  the,  49,  50 
human,  of  three  weeks,  32 
karyokinesis  of  the,  94 
maturation  of  the,  61 
nuclear  spindle,  63 
nucleus  and  nucleol%s  of,  57 
polar  globules  of  the,  64 
polarity  of  the,  60 
primordial,  49 
segmentation  of  the,  93 


808 


INDEX. 


Ovum,  vegetative  pole  of  the,  61 
see    also    under    Embryo    and 
Foetus. 

PALATE,  formation  of  the,  579 

Pallium  of  the  hemispheres,  691,  694 

Pancreas,  766 

Paiigenesis,  85 

Papilla,  dental,  587 

Parablast,  120,  153 

Parablast  theory  of  the  mesodenn, 

153 

Parablastic  nuclei  of  the  yolk,  348,352 
Parachordals,  430 
Paradidyinis,  501 
Paraphysis  cerebri,  690 
ParietalhGhle,  151,  197 
Paroo"phoron,  501 
Parovarium,  500 
Parturition,  causes  of,  27 

changes  in  the  decidua  at,  21 
Pathetic  nerve,  640 
Peduncles  of  the  cerebrum,  678 
Pelvic  girdle,  455 
Penis,  development  of  the,  517 
Peptic  cells,  755 

Pericardial  and  pleural  cavities,  sep- 
aration of,  482 
Perionix,  555 
Periotic  capsules,  438 
Perivitelline  space  of  the  ovum,  56 
PflUger^SGhe  Schlduche,  48,  496 
Phalanges,  ossification  of  the,  458,  460 
Pharynx,  743 

origin  of  the,  263 
Pigment-cells,  origin  of,  419 
Pigment  layer  of  the  retina,  721 
Pigment- Strasse,  in  certain  amphib- 
ian ova,  74 
Pineal  gland,  680,  688 
Pits,  nasal,  575 
Pituitary  body,  571 
Placenta,  the,  364 

allantoic,  376 

at  different  periods  of  pregnancy, 
30 

at  full  term,  364 

chorionic,  376 

cotyledons  of,  366 

evolution  of,  376 

foetal  circulation  of,  369 

lobes  of,  366 

maternal  circulation  of,  372 


Placenta,  theory  of,  374,  378 

two  types  of,  376 
Placoid  scales,  582 
Planula,  112 

Plastids,  red,  origin  of,  221 
Plate,  anal,  190 

buccal,  262 

medullary.  173 

nail-,  556 

oral,  262 

sole-,  556 

sub-germinal,  102 

vertebral,  193 
Plates,  blood-,  origin  of,  223 

muscle,  475 

olfactory,  575 

Pleural  and  abdominal  cavities,  sep- 
aration of  the,  484 

and  pericardial  cavities,  separa- 
tion of,  482 

cavities,  expansion  of  the,  483 
Plexus,  choroid,  681 
Plica  choroidea,  610 

semilunaris,  727 
Polar  crown,  formation  of  the,  95 

globules  of  the  ovum,  64 
Polyphyodont,  definition    of    term, 

582 

Pons  Varolii,  672 
Poreuten,  154 

Post-trematic  branch  of  the  glosso- 
pharyngeus,  650 

of  the  vagus,  354 
Pree-cartflage,  404 
Pnecervical  sinus,  744 
Prae-trematic  branch  of  the  glosso- 
pharyngeus,  650 

of  the  vagus,  654 

Pregnancy,  changes  in   the  mucosa 
corporis  during,  6 

decidua  reflexa  at  various  periods 
of,  19 

decidua  serotina  at  the  seventh 
month,  17 

uterus  at  the  fifth  week  of,  31 

uterus  at  the  first  month  of,  13 

uterus  at  the  third  month  of,  30 

uterus  at  the  eighth  month  of,  28 
Primitive  anus,  259 
Primitive  axis,  118 

axis  in  marsipobranchs,  ganoids, 
and  amphibians,  129 

cartilaginous  skull,  434 


INDEX. 


809 


Primitive  chorion,  281 
groove,  140 
groove,  stage  when  it  is  at  its 

maximum.  134 
ova.  appearance  of  the,  250 
segment,  cavity  of  the,  202 


segments,  division  of  the,  201 
ii-nts  <>t'  the  coelom,  192 

streak  in  birds.  1:51 

stivak  in  elasiuohranchs,  130 

streak  in  iiiainn.als.  13!) 

M  ivak,  oriirin  of,  128 

vertebral  how.  -J25 
Pri  muni  i<il<  i^r,  appearance  of  the, 

850 

Priiiun-iliiil  n  it-re  of  Jacobson,  243 
Proamiiion.  I5o.  341 

importance  of,  in  mammals,  284 

Process.  dflltal. 

luaxillary.  578 

na-al.  570 

Processes,  choroid,  122 
Proot'ssus  glnbularis,  576 

infiindihuli.  »',^; 

vaginalis,  496 
Prochondrium,   H>t 
Proctoda'iim.  259 

Projections  of  the  germ-layers,  161 
Proliferation  islands,  320,  320 
Prnlif,  riitiniixiitst  In.  320,  326 
Pronephric  duct.  230,  234.  253 
Pronephros,  231 
Pronucleus,  female.  05 

fusion  of  the  male  and  female,  74 

male,  71 

ProseiKvphalon,  178,  595,  610 
Prostatic  gland,  development  of,  515 
Protoplasm  of  the  ovum,  49 
Protovertebra,  192,  201,  423 
Pseudocoele,  684 
Pulmonary  anlage,  77:5 

aorta  and  arteries,  538 

groove,  77:5 

veins,  formation  of,  547 
Pupil  of  the  eye.  713 
/'///•A-  ///./'  '"'•/"•*  Wfischeni  57 

RABBIT,  yolk-sac  of  the,  351 
Rabl's  theory  of  the  mesoderm,  155 
Rachenhfuit.  262 
Radius,  ossification  of  the,  457 
,  696 


Randfurche,  607 

of  the  spinal  cord,  662 
Randkeim.  154 

Randschleier,  613,  614,  616,  671 
Randvene  of  the  hand  or  foot,  545 
Randwulst,  117 
Randzone,  weisse,  672 
Rathke's  pocket,  571 
RauttitlipiK-,  608 
Recessus  labyrinthi,  736 

laterales  of  the  fourth  ventricle, 
676 

occipitalis,  693 

opticus,  680,  688 

parietalis  dorsalis,  481 

superior  sacci  omenti,  484 

utriculi,  738 

vestibuli,  729 
Reichert's  cartilages,  445 
Reichert's  ovum,  287 
Reil,  island  of,  696 

island  of,  fissures  of,  701 
Reuiy's  embryo,  303 
Repli  pfrini'itl.  517 
Repose,  period  of,  after  fusion  of  the 

pronuclei,  94 
Respiratory  tract,  7?:! 
Rete  Halleri,  501 
Retes  d-Henle,  551 
Retina,  719 

central  artery  of  the,  712 
Retinaculaof  the  Graafian  follicle,  52 
Ribs,  development  of  the,  432 
Rich tungskOrperchen,  65 
Richtunf/Mjtindel,  zweites,  64 
Ridge,  dental,  578 

genital,  230,  244,  251 

nephridial,  230,  251 

neural,  601 

Wolffiaii,  230,  251 
Ridges,  epidermal,  551 

optic,  688 
Riechgrube,  distinctness  of,  after  the 

twenty-fifth  day,  387 
Riechlappen,  703 
Riegel,  677 
Riemchen,  677 
Rodents,  inversion  of  the  germ-layers 

in,  141 

Roland,  gelatinous  substance  of,  002 
Rolando,  fissure  of,  698 
Roof  of  the  fourth  ventricle,  076 
Rosenmiiller,  organ  of,  500 


810 


INDEX. 


RUckenfurche,  173 

RilckenkrUmmung,  313 
Rttckenplatte  of  Remak,  470 
Rilckenrinne,  134 
Rusconi,  anus  of,  121,  129 

SACCULUS,  736 

Saccus  omenti,  484 

Salivary  glands,  development  of,  590 

Samenfttden,  40 

Samenstammzetten,  43 

origin  of,  494 
SammelrVhrchen  from  the  Malpigh- 

ian  corpuscles,  239 
Santorini,  ducts  of,  7G7 
Sarcoplasten,  474 
Sauropsida,  concrescence  in,  122 

development  of  the  primitive  seg- 
ments in,  195 

mesoderm  of,  147 

the  allantois  in,  353 

yolk-sac  of,  347 
Scala  tympani,  735 
Schaltstilck,  662 
Scheitelkrtl mmung,  600 
Scheitelplatte,  570 
Schizocoele,  169 
tSchleimblatt  of  Pander,  167 
Schleimscheide  in  rats  and  mice,  484 
Schlemm,  canal  of,  724 
Schlundspalten,  263 
Schlussplate  of  Winckler,  325 
Schmelzzellen,  586 
Sehulterzungenstrang,  658 
Schwanzdarm,  260 
Schwanzkappe.  283 
tichwerchfell,  development  of  the,  485 
Schwabe's  ovum,  290 
Sclera,  713,  722 
Sclerotome,  202,  205,  423 
Scrotum,  origin  of  the,  520 
Sebaceous  glands,  development  of, 

562 

Seessel's  pocket,  268 
Segmental  sense-organs,  706 
Segmentalbltischen  of  Braun,  237 
ISegmentalorgane,  232 
Segmentalstrtinge,  249 
Segmentation  cavity,  97 
Segmentation  of  the  ovum,  93 

modified,  of  placental  mammals, 
100 

planes  of  division  during,  109 


Segmentation,  primitive  type  of,  96 

vertebrate  type  of,  97 
Segmentation  planes,  relation  of,  to 

the  embryonic  axis,  110 
Segments,  cephalic,  199 

mandibular,  200 

prse-oral,  200 

primitive,  157 

primitive,  cavity  of  the,  202 

primitive,  division  of  the,  201 
Sehhiigelcentrum,  mediane,  687 
Sehstreif,  688 

Seitenfortstttze  of  the  vertebrae,  431 
Seitenleister^  712 
Sella  turcica,  436 
Seminiferous  tubules,  42 
Sense-cells,  special,  709 
Sensory  ganglia,  origin  of  the,  601 
Sense  organs,  706 

evolution  of  the  ganglionic,  709 
Septum  inferius,  530 

intermedium,  530 

lucidum,  684 

pellucidum,  684 

primum  or  superius,  524,  528 

secundum,  529 

spurium,  525,  533 

superius  of  the  auricles,  524,  528 

transversum,     development    o  f 
the,  480 

transversum,  relation  of  the  liver 

to  the,  269 
Sertoli's  column,  39,  42 

column,  origin  of,  494 
Sex,  changes  in  the  uro-genital  sys- 
tem characteristic  of,  491 

nature  of,  84 

theory  of,  77 
Sexual  cells,  249 

cords,  249 

elements,  39 

elements,  the  bringing  together 
of  the,  69 

glands,  development  of  the,  492 

glands,  differentiation  of  the,  251 
Sexuality,  nature  of,  77 

object  of,  83 

origin  of^82 
Sexualstrttnge,  249 
Sharks,  dermal  teeth  of,  581 
Sheaths,  medullary,  620 
Sheep,  yolk-sac  of,  350 
Shoulder-girdle,  453 


INDEX. 


811 


Sinus,  cervical,  057,  744 

cervical,  deepening  of,  in  embryo 

of  twenty-seven  days,  387 
lacteus,  565 
pivi-cervicalis,  744 
pra-cervicalis,    deepening    of,   in 

embryo  of  twenty-seven  days, 

387 

reuniens,  527 
rhomboidalis,  176 
terminalis,  213,  272 
nrogenital.  515 
venosns,  '-275,  525 
Skeleton,  the.  4 '.'•.' 

appendicular,  448 
axial.  4-24 
branchial,  443 
dermal,  461 

development  of,  422 

of  the  arm,   15U 

of  the  l«-g.   168 

of  the  limbs,  448 

relative  importance  of,  as  com- 
pared with  the  other  systems, 
423 

stages  of,  422 
Skin,  the. 

glands  of.  of,:} 
Skull,  development  of  the,  434 

Froriep's  law  concerning,  469 

(legen baner's  theory  of  the,  469 

homologies  of  the  bones  of  the 
human,  4*15 

morphology  of  the,  465 

ossi Mention  of  the,  439 

primary.  -Hit; 

relations  of  primary  and  second- 
ary. 4<  w 

secondary,  4(17 

theories  of  the,  468 

Yicq  d'A/.yr's  theory  of  the,  468 
Srnegma  embryonum,  562 
Xnlih-uhnni  of  the  claw,  550 
Sole-plate  of  the  claw,  556 
Somatic  mesoderm.  15:2 
Somatopleure,  152 

extra-embryonic,  281 
Somites,  mesoblastic,  157,  193 

relation  of  the  limbs  to  the,  451 
Spaces,  interglobular,  in  the  adult 

tooth,  588 

X/nrn(/r-  of  the  vertebra,  425 
Spatium  iiiterseptale,  525 


Spermatoblasts,  43 
Spermatocytes  (daughter-cells),  44 

origin  of,  494 
Spermatogenesis,  42 
Spermatomeriten,  75 
Spermatozoa,  39 

development  of,  39 

historical  note,  47 

human,  42 

mammalian,  40 

of  amphibia,  40 

of  birds,  40 

of  fish,  40 

of  the  rat,  41 

of  reptiles,  40 

vertebrate,  4J) 

Spermatozoon,  entrance  of,  into  the 
ovum,  70 

parts  of,  39 
Speriniduct,  503 
Sphere  of  attraction,  94 
Spheres,  segmentation,  105 
Spinal  accessory  nerve,  654 
Spinal  cord,  658 

blood-vessels  of  the,  664 

central  canal  of,  659 

development  of  the,  607 

dorsal  zone  of  His,  661 

general  growth  of,  658 

gray  matter,  663 

growth  of  the  mantle  layer,  661 

ventral  zones  of  His,  662 

white  matter,  663 
Spinal  nerves,  627 
Spindle,  nuclear,  of  the  ovum,  63 
Spini  vestibuli,  525 
Splanchnic  mesoderm,  152 
Splanchnocoele,  197,  480 

lining  membranes  of  the,  485 
Splanchnopleure,  152 
Spleen,  development  of  the,  415 
Splenial  bones,  461 
Splint  bones,  461 
Spongioblasts,  611,  612 
Spree's  embryos,  291,  295 
Spundzellen  of  the  ovum,  5G 
S-shaped    tubules    of    the    Wolffian 

body,  237,  241 
Stammzellen,  43 
Stammzone,  193 
Stapes,  740 

Sternum,  development  of  the,  434 
Stigma  of  the  Graafiaii  follicle,  52 


812 


INDEX. 


Stirnfortsatz,  576 
Stirnorgan,  690 
Stirnwulst,  569 

Stratum  corneum  of  the  epidermis, 
550 

lucidum  of  the  epidermis,  550 

proligerum,  99 

Streak,  primitive,  origin  of,  128 
StreifenhUgelstiel,  695 
Stria  cornea,  695 
Stomach,  753 
Stoinodseum,  126 
Sub-germinal  cavity,  115 

plate,  102 

Substantia  gelatinosa  Rolandi,  662 
Sulcus  centralis  insulae,  702 

corporis  callosi,  692 

habenulse,  687 

hippocampi,  696 

of  Monro,  680 

olfactorius,  700 

pineal,  687 

rectus,  700 

spiralis,  732 

Supra-renal  capsules,  485 
Surface,  relations  of  mass  to,  161 
Sweat  glands,  563 
Sylvius,  aqueduct  of,  678 

fossa  of,  695 

Sympathetic  nervous  system,  630 
Sy  no  vial  cavities,  development  of,  421 

Tache  embryonnaire,  136 
Taches  laiteuses,  218 
Tail,  origin  of  the,  260 
Tail-fold,  283 
Tail-gut,  260 

Tarsus,  ossification  of  the,  460 
Taste,  organs  of,  710 
Tear  gland,  727 

Teeth,  age  of   development  of  the 
parts  of,  589 

dermal,  of  sharks,  581 

development  of  the,  581 
Tela  choroidea,  681 
Teleosts,  mesoderm  of,  145 

notochord  of,  184 
Telolecithal  ova,  61 
Tendons,  development  of,  421 
Testis,  descent  of  the,  496 

development  of  the,  492 
Thalamencephalon,  596,  679 
Thalami,  optic,  686 


Thalamic  nerve,  640 

Theca  folliculi,  51 

Thelyblast,  78 

Third  ventricle,  floor  of  the,  687 

Thomson's  (Allen)  embryos,  293,  294, 

307 

Thrttnendrttse,  727 
Thr&nennasengang,  580 
Threads,  achromatic,  95 
Thymus,  746 
Thyro-hyoid  bars,  447 
Thyroid  gland,  747 
Tibia,  ossification  of  the,  459 
Tissues,  genesis  of  the,  164 

mesenchymal,  397 
Tcenia,  609 

fossas  rhomboidalis,  677 

semicircularis,  695 
Tongue,  development  of  the,  592 
Tonsils,  745 

Tooth-germs,  amniote,  583 
Totalfurchen,  692 
Touch,  organs  of,  lack  of  knowledge 

concerning,  710 
Trabeculae  cranii,  434 

significance  of  the,  468 
Trachea,  777 
Tractus  intermedius,  668 

solitarius,  667 
Trigeminal  nerve,  641 
Trochlear  nerve,  640 
Tube,  Eustachian,  740 

Fallopian,  504 

Fallopian,  origin  of,  245 
Tubenfalte  of  Braun,  245 
Tubenleiste  of  Mihalkovics,  245 
Tuber  cinereum,  687 
Tubercle,  genital,  516 
Tubercles,  mammillary,  687 
Tuberculum  impar,  306,  592 
Tubules,  seginental,  230,  235 

renal,  509 

seminiferous,  42 

Wolffian,  235 

Wolffian,  multiplication  of  the, 

239 

Tunica  fibrosa  of  the  Graafian  folli- 
cle, 52 

propria  of  the  Graafian  follicle,  52 

vasculosa  lentis,  716 

vasculosa  of  the  primary  follicle, 

51 
'Tween-brain,  596 


INDEX. 


813 


ULXA,  ossification  of  the,  457 
Umbilical  arteries,  541 
Umbilical  cord,  350 

at  birth.  :><;<) 

at  different  periods  of  pregnancy, 
80 

development  of  the,  857 

twisting  of  the,  3G1 
Umbilical  hernia.  75* 
r//t<  /  h<tiit.:>  //;/'  trebe,  553 
Urachus,  515 
rnlnnn.  113 
r  i-i  i>  r.  appearance  of  the,  250 

in  the  embryo,  82 

origin  of,  48 

Ureter,  development  of  the,  514 
Urethra,  origin  of,  517 
r/7,v  ////:<//,//,  82  * 

rnnmul,  113 

Urniere,  ?.\\.  2:',5 
UrnierenblOschen,  237,  240 


Urogenital  apparatus,  homologiesof, 

in  the  two  B6I68,   ;srj 
Urouvnital   organs,  special  histories 

of  the,  492 

Uroireiiital  sinus,  515 
L'n  genital  system,  evolution  of  the, 

251 

fundamental  parts  of  the,  230 

general  history  of  the,  490 

indifferent  stage,  490 

origin  of  the,  230 
Ursegmente,  157,  193 
Urtoirbel,  192,423 
,  196 

of  Remak,  196 
Uterus,  attachment  of  embryo  in,  374 

blood-vessels  of  the,  24 

dcridua  graviditatis,  (>,  26 

decidtia  immstrualis,  4,  26 

decidual  cells,  12 

degeneration  of  tissues  of,  375 

development  of,  504,  506 

eight  months  pregnant,  28 

five  weeks  pregnant,  31 

histology  of  the,  1 

lymphatics  of  the,  25 

masculinus,  503 

mucosa  cervicis,  24 

mucosa  corporis,  3 

muscularis.  1 

one  month  pregnant,  13 


Uterus,  post-partum  regeneration  of 
the  mucosa,  21 

size  of  the,  1 

special  physiology  of  the,  25 

three  months  pregnant,  33 

weight  of  the,  1 
Utriculus,  736 
Uvea,  722 
Uvula,  development  of  the,  580 

VAGIXA,  development  of,  504,  50G 
Ya-'us  nerve,  (55!) 
Valves  of  the  heart,  532 

aortic,  534 

atrioventricular,  533 

semilunar  534 
Valvula  Eustachii,  525,  532 

Thebesii,  525,  532 

vestibuli  sinistra,  525 
Varolian  bend,  599,  601 
Vascular  area,  274 

growth  of,  276 
Vascular  layer,  212 
Vascular  system,  origin  of  the,  229 
Vasof or mative  cells,  218 
Veins,     hepatic,     changes     in     the, 
545 

of  the  hand  and  foot,  changes  in 
the,  545 

omphalo-mesaraic,  215,  275 

primitive,  541 

primitive,  metamorphoses  of  the, 
544 

pulmonary,  547 

Velum  medullare  posticum,  673,  677 
Vena  Arantii,  546 

cava,   inferior,   early  history  of 
the,  543 

portse,  origin  of  the,  546 

the,  213 
Venous  system,   transformations  in 

the,  541 
Ventricle,  primitive  cardiac,  525 

fourth,  and  its  roof,  676 

third,  floor  of  the,  687 
Ventricles,  cardiac,  division  of  the, 

530 
Verbiri dung stilck  of  a  spermatozoon, 

41 

Vermiform  appendix,  758 
Vernix  caseosa,  5(52 

caseosa  found  in  the  meconium, 
338 


814 


INDEX. 


Verschlussplatte  closing  the  gill-cleft. 

207 
Vertebra,  typical  development  of  a, 

424 
Vertebra?,  caudal,  431 

coccygeal,  431 

evolution  of,  429 

occipital,  429 

ossification  of  the,  428 

sacral,  431 
Vertebral  column,  424 

plate.  193 
Vertebrate  type  of  origin  of  the  meso- 

derm,  148 

Verwachsungsbrilcke,  482 
Vesicle,  blastodermic,  105,  see  Blas- 
toderm ic  vesicle. 

chorionic,  317 

chorionic,  fluid  contents  of  the, 
331 

cylindrical,  141     . 
Vesicles,  162 

amnio-cardial,  198 

cerebral,  178,  593 

optic,  710 

primary  cerebral,  595 
Vesicula  germinativa  of  the  ovum, 
57 

prostatica,  504  . 

seminalis,  503 
Vesiculse  cerebrales,  595 
Vessels,  blood,  see  Blood-vessels. 

lymphatic,  development  of,  413 
Vestibule  anale,  516 
Vicq    d'Azyr's  theory  of  the  skull, 

468 
Villi,  chorionic,  318 

chorionic,  outgrowth  of,  375 

placental,  vessels  of  the,  371 
Virgin,  uterine  mucosa  in  the,  3 
Visceral  arches,  265,  267 
Vu-ceralb&gen,  265,  267 
Visceralsp alien,  263 
Vitelline  membrane,  58 

veins,  542 

Vitreous  humor,  723 
Vocal  chords,  778 
Vomer,  development  of  the,  463 
Von  Baer's  ova,  296,  307 
Vorderdarm,  126 

origin  of  the,  261 
Vorderhirn,  178,  596 
Vorhofsblindsack,  737 


Vorknorpel,  404 
Vomierengang,  234 

V-shaped  grains,  95 

WAGXERIAN  spot  of  the  ovum,  57 
Wagner's  embryo,  308 
Wagnei^scher  Fleck,  57 
Wangenplatte,  568 

Weismann's  theory  of  the  multiplica- 
tion of  muscle  fibres,  475 
Wharton's  jelly,  358,  361,  403 
White  matter,  616 

of  the  spinal  cord,  663 
Willis,  accessory  nerve  of,  654 
Winslow,  foramen  of,  483,  770 
\Virbelbogen,  425 
\Yh-belplatte,  193 
ffirbelsaite,  181 
Wirsung,  duct  of,  767 
Wolfflan  body,  231,  235 

historical  note,  243 

resorption  of,  243 

structure  of  the  mature,  240 
Wolffian  duct,  230,  234,  253,  502 
Wolfflan  ridge,  230,  251 
Wolfflan   tubules,  multiplication  of 
the,  239 

XYPHOID  cartilage,  434 

YOLK,  changes  in,  during  the  passage 
of  the  spermatozoon  through 
the  ovic  envelopes,  73 
development  of  the,  49 
formative,  99 

parablastic  nuclei  of,  348,  352 
Yolk-blastopore,  124 
Yolk-cavity,  115 

and  notchordal  canal,  fusion  of, 

127 
Yolk-grains,  first  appearance  of  the, 

53 

Yolk-nucleus,  54 
Yolk-sac,  346 

general  morphology  of  the,  346 
of  mammals,  349 
of  man,  349 
of  sauropsida,  347 
of  sheep,  350 
of  the  opossum,  351 
of  the  rabbit,  351 
separation    of    the   archenteron 
from  the,  255 


INDEX. 


815 


Zahnleiste,  583 

Zahnsack,  origin  of  the,  584 

Zapftrn  of  the  mucosa,  23 

Zellen,  leucocytoide,  420 

Z'-llkiioten,  320 

Zellschicht  of  Langhans,  323 

Zellstreif,  675 

Zirbr/,  688 

Zirbeldrtlse,  688 

Zinn,  zonule  of,  723 

Zona  pectinata,  ?:'..""> 
pellucida,  51,  5'i,  58 
radiata,  51,  53,  58 


Zone,  lenticular,  of  the  optic  cup,  722 
parietal,  of  the  mesoderm,  193 
segmental,  of  the  mesoderm,  193 
segmenting,  101 

Zones  of  His,  606,  661,  602,  686,  687 
in  the  adult,  666,  668 

Zonule  of  Zinn,  723 

Zioerchfellband  der  Urniere,  245 

Zwillingszellen  of  the  ovum,  56 

Zwischenganglion,  648 

Zwischenhirn,  596,  679 

Zwischenrinne,  603 

Zwischenstrang,  601 


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